Title
Sources of variation in the nutritive value of wheat and rice straws
Author
G.R. Pearce, J.A. Lee, R.J. Simpson and P.T. Doyle
School of Agriculture and Forestry, University of Melbourne, Parkville, Victoria 3052, Australia
Straw quality per se can be improved by:
Animals fed cereal straws alone invariably lose weight, indicating that the nutritive value of these materials is low. However, published data show that there is wide variation in their nutritive characteristics (Table 1). It is not known how much of this variation is due to inherent characteristics of the plant material and how much is due to differences in the proportions of morphological fractions (leaves, stems etc.) arising from different growing conditions, harvesting procedures (particularly height of cutting) and threshing and storage methods. In feeding trials, the composition of the straw actually consumed is affected by feeding practices. Descriptions of the origin and history of straw samples are usually inadequate, so it is not possible to determine the extent to which comparisons of different straws are valid.
Examinations of separate plant fractions indicate that real variation occurs within these fractions (Table 2). It is still difficult, however, to attribute variation to genotype, environmental conditions of plant growth or interactions between these factors. Although differences among cultivars have often been measured (for example, Table 3), it is not possible to state confidently that any one cultivar produces better quality straw than another, let alone to quantitate the magnitude of any apparent superiority. Differences also occur among seasons, even in the same location with similar harvesting procedures. Table 4 shows the variation in in vitro organic matter digestibility (IVOMD) over three seasons of two rice cultivars grown at one location in the Philippines.
Table 1. Ranges in nutritive characteristics of wheat and rice straws.
In assessing the nutritive value of mature, senescent and dead forages a clear distinction should be made between the cell wall, measured as neutral-detergent fibre (NDF), and the cell contents, measured as neutral-detergent solubles (NDS% = 100 - NDF%), because the cell wall is usually slowly and poorly digestible, while the cell contents may be highly and rapidly digestible. In the following discussion the characteristics of the cell wall and the cell contents are dealt with separately and it will be seen that opportunities for manipulation differ for these two components.
With wheat straws, greater emphasis is placed on the stem (internodes) fraction than the leaf (sheath and blade) fraction because the stem comprises a much larger proportion of the straw than leaves (Table 5). More leaf material is lost during harvesting and threshing than stem, and therefore the straw offered to animals contains a larger proportion of stem than indicated in Table 5. In whole rice straw, the proportions of internodes, sheaths and blades tend to be more equal but again more leaf blade material is likely to be lost during harvesting and threshing.
Table 5. Percentage of whole culm weight of wheat and rice straw in internodes, nodes, leaf sheaths and leaf blades.
Table 6. Ranges in the composition of NDF (as % of NDF) in wheat and rice straws.
Studies on the developing plant can provide important information on the types of manipulation that might be used to improve nutritive value and on the critical time periods during which a plant expresses characteristics important in determining straw quality. In the growing and maturing plant, the partitioning of mass into the different morphological fractions follows a generally well defined pattern. Temporal changes in the NDF mass of two stem internodes of wheat (S1, the topmost internode, and S5, a lower one) are shown in Figure 1. The main features were:
Source: G.R. Pearce (unpublished data).
Figure 2 shows changes in the mass of cellulose, hemicellulose and lignin in S2 over the same period. During growth, cellulose, hemicellulose and lignin are all being synthesised in the cell wall but the amount of cellulose deposited is greater than that of hemicellulose or lignin until after anthesis and beyond the point of maximum elongation. Synthesis of all three components stopped at the same time, about 4 weeks after anthesis. Between 20 days before anthesis and 30 days after anthesis, the amount of cellulose increased about 3.5 fold, the amount of hemicellulose about 2 fold and the amount of lignin about 5 fold (Figure 2). The pattern in the other stem segments was similar. This shows that manipulations of cell-wall composition to improve feeding value must focus on this period when rapid changes are occurring. In this case, manipulations would not have any effect if applied and expressed 4 weeks, or later, after anthesis.
Source: G.R. Pearce (unpublished data).
These changes in chemical composition of stem internodes are reflected in in vitro NDF digestibility (NDFD) values (Figure 3). In the upper internodes, the decline in digestibility in the period immediately before anthesis was rapid and dramatic. For example, the NDFD of S1 fell by about 3.6% units per day and continued to decline until after anthesis. The changes in the lower internodes were earlier and slower than in the upper ones.
Source: G.R. Pearce (unpublished data).
Other results (J.A. Lee, unpublished data) have shown that NDFD and the digestibilities of cellulose and hemicellulose in the internodes of wheat decline at similar rates, but the final digestibility of hemicellulose was somewhat higher than that of NDF and cellulose. The similarity of the rates of change in digestibility for both cellulose and hemicellulose suggests that these components were affected similarly and equally by lignification and other factors associated with maturation and loss of digestibility.
The leaf blades and sheaths in the wheat examined by Lee were similar in mass and attained their maximal masses earlier than their corresponding internodes. In LS2 (Figure 4) and LB2 (Figure 5) maximal mass had been attained by anthesis. In LB2 there was an apparent loss of hemicellulose after anthesis, but there was no explanation for this. The NDFD of sheaths remained at about 40% beyond anthesis, while the NDFD of the blades continued to decline steadily but was still 50 to 60% in the straw.
Cell-wall lignification is recognised as a prime cause of declines in digestibility but, often, only poor correlations have been obtained between lignin content and digestibility (e.g. Pearce, 1984). However, these have usually been between the percentage of lignin in the dry matter and dry-matter or organic-matter digestibility of whole straw. When the percentage of lignin in the NDF is correlated with NDFD for individual straw fractions, closer relationships may be obtained. For example the r2 values for internodes, sheaths and blades were 0.94, 0.83 and 0.87, respectively, in exponential decay functions (J.A. Lee, unpublished data). For each relationship, the regression co-efficients were substantially different, indicating that a single regression from the pooled data, as is represented in whole straw, would be associated with wide variation and a lower r2. Thus, the association of lignin with cellulose and hemicellulose is different in the three plant fractions and this must be taken into account.
Source: J.A. Lee (unpublished data).
Source: J.A. Lee (unpublished data).
Few attempts have been made to alter lignin synthesis in growing plants. However, at the University of Melbourne, treatment of annual ryegrass (Lolium rigidum) with gibberellic acid caused stem elongation and increased the proportion of lignin in the cell wall. The S1 contained 13% lignin in the NDF of the straw compared with 8% in untreated plants and the NDFD values were 17 and 26%, respectively. On the other hand, acute copper deficiency reduces lignin synthesis (Graham, 1976; Downes and Turner, 1986).
The relationship between cell-wall composition and digestibility deserves a continuing high research priority.
In senescing plants, fluctuations in NDS content are due mainly to changes in the amount of storage carbohydrates (fructans in wheat and starches in rice). The levels of storage carbohydrates retained in straw are critical to feeding value because of their high digestibility. The effects of changes in NDS in a senescing and dead internode of wheat on the digestibility of the internode are shown in Figures 6 and 7. NDS increased rapidly from shortly before anthesis and peaked about 4 weeks after anthesis (Figure 6). NDS digestibility was high throughout this period (Figure 7), as would be expected of a tissue enriched with soluble carbohydrates. Subsequently, the amount of NDS in the internode and NDS and organic/dry-matter digestibilities declined. Both the amount of NDS and its digestibility approached basal levels at the time of grain maturation.
The changes in the amount of NDS are associated with grain development in the following manner: Immediately after anthesis, photosynthesis are produced in excess of requirements for grain growth because grain development is very slow. Grain mass increases rapidly from about 3 weeks after anthesis, consuming carbohydrates generated by current photosynthesis and, increasingly, from the mobilisation of storage carbohydrates in the culm (Blacklow et al, 1984). The contribution that reserve carbohydrates make to the final grain mass depends upon environmental conditions and perhaps upon genetic factors. If the plant is stressed the reserves will be drawn upon heavily by the grain. For example, in drought-stressed barley Gallagher et al (1975) found that 74% of the grain mass could be attributed to storage carbohydrates, whereas other reports (e.g. Evans and Wardlaw, 1976) have suggested as little as 10%.
The extent to which, and the conditions under which, storage carbohydrates are respired are uncertain. Rawson and Evans (1971) estimated that 30% of the fructans of wheat are used for respiration, but L.C. Incoll (personal communication) found that almost all C14 - labelled fructans in stems are mobilised to the grains, indicating that, at least during grain filling, fructans are most probably not used for respiration. Another possible loss is transport to the lower parts of the plant to support late tillering. In certain crops and particularly under conditions of high fertility and moisture (e.g. irrigation), late tillering can occur. In rice this is called "rattooning" and is sometimes used as a means of obtaining a second grain harvest.
Source: G.R. Pearce (unpublished data).
Source: G.R. Pearce (unpublished data).
NDS exhibits two main levels of digestibility; 90% when the stems are high in NDS and 40-50% when the stems are low in NDS, after they have senesced. It thus seems reasonable to consider NDS as being comprised of two pools of nutrients: (a) the intrinsic nitrogen compounds, carbohydrates, fats and minerals of the cytoplasm, mitochondria, membranes, nucleus, chloroplasts and other organelles, which, as a whole, are apparently less digestible in senescing and dead plant tissues, and (b) the reserve carbohydrates and proteins. Proteins are efficiently mobilised from senescing plant tissues and, except under conditions of high nitrogen fertilizer application (e.g. Roxas et al, 1985), their concentration in straw is low (Darling, 1985). However, considerable amounts of storage carbohydrates, such as fructans in temperate grasses (Ojima and Isawa, 1968) and glucans and starches in rice, may be present in dead crop dry matter.
Different plant fractions do not store carbohydrates uniformly. Stems store considerably more than leaf sheaths, which store more than leaf blades. In wheat, the penultimate internode, S2, accumulates most fructans. In wheat, the upper parts of the culm contain larger proportions of NDS than the lower parts, as might be expected from their age, degree of senescence and proximity to photosynthesising leaves in the crop canopy. In irrigated rice, however, there may be no difference in the NDS content of the upper and lower parts of the straw (Hart and Wanapat, 1985; Winugroho, 1986).
The proportion of NDS in wheat straw ranges from 12% (W.J. Wales, unpublished data) to 41% (Ayres et al, 1976) of the dry matter and in rice straw from 14% (Cheva-Isarakul and Cheva-Isarakul, 1984) to 46% (Roxas et al, 1984). Higher values are associated with higher digestibility because of greater amounts of residual storage carbohydrates. In considerations of straw quality, therefore, the role of the storage carbohydrates is critical: the more storage carbohydrates remaining in straw the higher the digestibility. This has been expressed by Pearce (1984) as a high correlation between NDS% and IVOMD%. The pertinent question, therefore, is under what conditions are residual storage carbohydrate levels high in straw? The answer lies in the interactions between the photosynthetic activity of the plant at critical time periods and the demands of the grain.
The first requirement for high levels of storage carbohydrates is a high level of accumulation during the period around anthesis and for some time afterwards. For wheat in southern Australia this period spans about 40 days, from about 10 days before anthesis to about 30 days afterwards. Storage carbohydrates accumulate during this period if conditions are favourable for photosynthesis, i.e. suitable light and temperature conditions, adequate nutrient and water availability and the absence of disease. Thus, the history of the plant in terms of soil fertility, fertilizer application, spacing of plants and leaf development up to this time may be important.
The full extent to which storage carbohydrate accumulation varies is not known but, under a range of conditions at the University of Melbourne, amounts of NDS in stem segments varied widely. For example, among five different, but related, wheat cultivars grown side-by-side in the same season, NDS content of the S2 at peak accumulation ranged from 114 to 280 mg. The NDS content of S2 of plants grown under normal field conditions peaked at 158 ma, compared with only 60 mg for the same cultivar grown under apparently favourable conditions in pots in a glasshouse (W.J. Wales, unpublished data). Ambient temperatures and rates of growth relative to crop photosynthetic activity may all have an effect on the accumulation of carbohydrate reserves. Under field conditions in Western Australia, M. Nicolas (personal communication) has concluded that wheat cultivars do, however, vary widely in their ability to accumulate fructan reserves.
The second requirement for high levels of storage carbohydrates in straw is a low rate of removal. Normally, developing grains draw on storage carbohydrates to augment the metabolites provided by current photosynthesis. As current photosynthesis declines during senescence the reserves are drawn upon. If current photosynthesis is optimal then the extent to which reserves are used depends largely on the number of grains developing, i.e. the size of the "sink". If there are few grains levels of storage carbohydrates and straw quality are more likely to be high. However, correlation between grain yield and straw quality is often poor (e.g. Erickson et al, 1982), partly because of the variable accumulation and loss of storage carbohydrates.
Where the sink is small, mobilisation of storage carbohydrates may be delayed and reduced if sufficient moisture and nutrients are available for the crop. This may happen under irrigation but in dryland situations seasonal constraints are likely to predominate.
At this stage, it is only possible to conclude that the relationships between the amounts of residual storage carbohydrate in straw and other physiological events involved in yield formation in crops are likely to be complex. However, an understanding of these interactions may identify opportunities for manipulation of carbohydrate reserves so as to achieve high-quality straw for animal production without significant reductions in grain yield and quality.
Selection in relation to physical characteristics of straws
Weston (1985) included texture as a criterion governing acceptability of feeds and Hogan et al (1986) suggested that animals select plant material on the basis of "tenderness." However, such characteristics are difficult to assess because, often, no one feature predominates and several factors interact. In straws, such as wheat (Doyle et al, 1987) and barley (Wahed and Owen, 1986) animals usually show a preference for leaves rather than stems. Where such preference is shown, useful comparisons between many literature reports are almost impossible because of the lack of information on feeding procedures and because of unspecified degrees of selection, especially when straws have been offered in excess of appetite.
In rice straw, selection of leaves in preference to stems may not occur (Doyle et al, 1987), possibly because the leaf blades contain more silica than the stems (Doyle et al, in press), as suggested by Van Soest (1982).
Resistance to particle size reduction
If the plant material is highly resistant to particle size reduction voluntary intake will be reduced because large particles of digesta cannot pass through the reticulo-omasal orifice into the lower digestive tract. Thus, large particles are retained in the reticulo-rumen until they are broken down by rumination or detrition. The comminution of feed particles during chewing also contributes to this process. Lee and Pearce (1984) found that, in five roughages, including barley and oat straw, chewing by cattle reduced about 50% of the material to particles that would pass through a 1 mm screen.
The extent and rate of particle size reduction is not the only mechanism controlling roughage intake and the rate at which digesta leave the reticulo-rumen is not necessarily proportional to feed intake because (a) the amount of digesta in the reticulo-rumen can vary and (b) the rate and extent of fermentation varies according to the available nutrient content of the roughage. Thus, straws of differing composition would be expected to produce differences in fermentation kinetics. However, the full details of the system have not been resolved.
Grinding energy has been used as an index of resistance to particle size reduction. This is the amount of electrical energy required to grind 10 g of feed through a 1 mm screen (Chenost, 1966; Foot and Reed, 1981). In wheat straws, the grinding energy of leaf blades is lower than that of leaf sheaths which is lower than that of stems, but in rice straw the differences may not be as great (Table 8). In experiments with wheat straw (W.J. Wales, unpublished data) and with rice straws (Chanpongsang, 1987), good inverse relationships were obtained between intake by sheep and grinding energy within cereal species but not between species (Table 9). Other factors, including possibly silica content, are thus involved.
Table 8. Grinding energy (J g-1 DM) of wheat and rice straw fractions (averages from three straws).
Sources: W.J. Wales (unpublished data); Chanpongsang (1987).
Eating rate
Eating rate has been used to evaluate herbage quality. Table 9 shows positive relationships between intake and eating rate of wheat and rice straws. These measurements were made with trained sheep and reflected characteristics of the feeds associated with palatability or acceptability. With wheat straw, material with a high proportion of stem is eaten much more slowly than leafy material. Habituation is also a factor, because straws that are eaten relatively slowly in early tests may be eaten more quickly in later tests. This occurred particularly with rice straws in which, it is believed, silica levels may have affected acceptability initially. The specific plant causes of such animal responses are unknown.
Metabolic factors influencing intake of straws
Voluntary intake of highly digestible forages is controlled by metabolic factors or is linked to requirements for maintenance and production. In the case of poorly digestible materials, however, attention has been focused on physical control of intake, particularly distension of, and removal from, the reticulo-rumen. However, the amounts and balance of nutrients supplied by low-quality straws are so limiting that metabolic contributions to the control of intake should not be overlooked.
Doyle et al (1987) concluded that, in view of the relatively small amount of digesta in the reticulo-rumen of animals fed unsupplemented straws, the purely physical control mechanisms do not operate or the levels at which the system is sensitised are lower, due perhaps to nutrient imbalances in the tissues. In either case, an extremely complex set of interactions determines intake levels. The kinetic features of digestion for straws with varying proportions of nutrients and for straws which are supplemented to provide limiting nutrients are variable.
Cell-wall digestibility can be improved by chemical and other treatments but such procedures are too expensive for wide practical application. The alternative, separating material with high digestibility (e.g. leaf blades in wheat) from material with low digestibility (e.g. stems in wheat), is also expensive and suffers from the disadvantage that the less digestible fraction usually forms the largest proportion of a crop residue. Breeding for greater leafiness may also not be attractive because this would lower the harvest index.
The main factor determining the digestibility of cell-wall material is lignin, but the precise limiting role of lignin is still unresolved. The information presented in this paper indicates that lignification effects need to be studied in discrete plant tissues. Identification of periods during which cell-wall digestibility is changing rapidly probably provides the best opportunity to understand the significance of concurrent chemical changes. The thrust of agronomic or genetic manipulations to alter cell wall characteristics must await a clearer understanding of lignin chemistry in relation to other cell wall constituents.
The potential contribution of the cell contents to the nutritive value of straws appears to have been under-estimated in the literature. Not only can straws contain quite large proportions of cell contents, but the amount of storage carbohydrates in the cell contents can vary widely. Because storage carbohydrates are highly digestible (probably 100%), they may have a marked effect on the nutritive value of straws. In this paper, the pattern of accumulation and subsequent removal of solubles in wheat stem internodes has illustrated the means by which varying amounts of storage carbohydrates remain in straws. However, the precise mechanisms involved have not been elucidated. Complex interactions occur between storage carbohydrate metabolism and other physiological processes in the plant, mediated by environmental and genetic factors. Only an understanding of these will permit genetic improvement of straw quality without reducing grain yield and quality.
Factors in straws that impinge directly upon an animal's senses, thus influencing its level of intake, have not been studied in detail, partly because straws have rarely been fed without supplementation. It is clear, however, from the available results that important differences may occur. To date, these have been monitored as differences in features such as grinding energy, eating rate and chemical composition, but alone or collectively they may result in pronounced differences in voluntary intake by animals, often associated with high degrees of selection for certain plant parts. The precise plant features that are responsible and the variation between animals in response to these have not been defined. Again, an understanding of these is necessary so that appropriate manipulations, by genetic or other means, may be approached.
Akin D E. 1982. Microbial breakdown of feed in the digestive tract. In: J B Hacker (ed.), Nutritional limits to animal production from pastures. Proceedings of a symposium held at St. Lucia, Queensland, 24-28 August 1981. Commonwealth Agricultural Bureaux, Farnham Royal, UK.
Alawa J P and Owen E. 1984. The effect of milling and sodium hydroxide treatment on intake and digestibility of wheat straw by sheep. Animal Feed Science and Technology 11:149-157.
Armstrong D G. 1982. Digestion and utilization of energy. In: J B Hacker (ed.), Nutritional limits to animal production from pastures. Proceedings of a symposium held at St. Lucia, Queensland, 24-28 August 1981. Commonwealth Agricultural Bureaux, Farnham Royal, UK.
Ayres J F. McManus W R and McFarlane J D. 1976. The evaluation of forage nutritive quality by microdigestion. Australian Journal of Experimental Agriculture and Animal Husbandry 16:472-477.
Ben-Ghedalia D and Miron J. 1981. Effect of sodium hydroxide, ozone and sulphur dioxide on the composition and in vitro digestibility of wheat straw. Journal of the Science of Food and Agriculture 32:224-228.
Blacklow W M, Darbyshire B and Pheloung P. 1984. Fructans polymerised and depolymerised in the internodes of winter wheat as grain-filling progressed. Plant Science Letters 36:213-218.
Braman W L and Abe R K. 1977. Laboratory and in vivo evaluation of the nutritive value of NaOH-treated wheat straw. Journal of Animal Science 45:496-505.
Chanpongsang S. 1987. Variation in the nutritive value of rice straws. M.Agr.S. thesis, University of Melbourne, Australia.
Chenost M. 1966. Fibrousness of forages: Its determination and its relation to feeding value. In: Proceedings of the Tenth
International Grassland Congress, held in Helsinki, Finland, 7-16 July 1966. Valtioneuvoston Kirjapaino, Helsinki, Finland.
Cheva-Isarakul Boonlom and Cheva-Isarakul Boonserm. 1984. Comparison of the intake and digestibility of different crop residues by sheep, cattle and buffaloes. In: P T Doyle (ed.), The utilization of fibrous agricultural residues as animal feeds. Proceedings of the Third Workshop of the AAFARR Network held in Peradeniya, Sri Lanka, 17-22 April 1983. University of Melbourne, Australia.
CSIRO (Commonwealth Scientific and Industrial Research Organisation). 1982. Composition of animal feedstuffs in Australia. Australian Feeds Information Centre, CSIRO, Canberra, Australia.
Dalling M J. 1985. The physiological basis of nitrogen redistribution during grain filling in cereals. In: J E Harper, R W Howell and L E Schrader (eds), Exploitation of physiological and genetic variability to enhance crop productivity. American Society of Plant Physiologists, Rockville, MD, USA.
Devendra C. 1983. Physical treatment of rice straw for goats and sheep and the response to substitution with variable levels of cassava (Manihot esculenta Crantz), Leucaena (Leucaena leucocephala) and Glyricidia (Glyricidia maculate) forages. MARDI Research Bulletin 11:3:272-290.
Downes G and Turvey N D. 1986. Reduced lignification in Pinus radiata D. Don. Australian Forestry Research 16:371-377.
Doyle P T. Chanpongsang S. Wales W J and Pearce G R. (in press). Variation in the nutritive value of wheat and rice straws. In: R M Dixon (ed.), Ruminant feeding systems utilizing fibrous agricultural residues - 1987. Proceedings of the Seventh Workshop of the AAFARR Network held in Chiang Mai, Thailand, 2-6 June 1987. International Development Program of Australian Universities and Colleges, Canberra, Australia.
Doyle P T. Pearce G R and Djajanegara A. 1987b. Intake and digestion of cereal straws. In: Proceedings of the 4th Animal Science Congress, held in Hamilton, New Zealand. Fourth AAAP Congress.
Erickson D O. Meyer D W and Foster A E. 1982. The effect of genotypes on the feed value of barley straws. Journal of Animal Science 5:1015-1026.
Evans L T and Wardlaw I F. 1976. Aspects of the comparative physiology of grain in cereals. Advances in Agronomy 28:301-359.
Foot J Z and Reed K F M. 1981. The energy consumed in the grinding of herbage and its relationship to voluntary intake of herbage by ruminants. In: J L Wheeler and R D Mochrie (eds), Forage evaluation: Concepts and techniques. Proceedings of a workshop held in Armidale, New South Wales, 27-31 October 1980. American Forage and Grassland Council/CSIRO.
Franklin M C, McInnes P and Briggs P K. 1967. Maintenance of Merino sheep fed varying protein roughages with or without supplement of chopped lucerne hay and wheat. Austalian Journal of Experimental Agriculture and Animal Husbandry 7:202-212.
Gallagher J N. Biscoe B V and Scott R K. 1975. Barley and its environment. V. Stability of grain weight. Journal of Applied Ecology 12:319-336.
Gordon A H. Lomax J A, Dalgarno K and Chesson A. 1985. Preparation and composition of mesophyll, epidermis and fibre cell walls from leaves of perennial ryegrass (Lolium perenne) and Italian ryegrass (Lolium multiflorum). Journal of the Science of Food and Agriculture 36:509-519.
Graham R D. 1976. Physiological aspects of time of application of copper to wheat plants. Journal of Experimental Botany 27:717-724.
Hart F J and Wanapat M. 1985. Comparison of the nutritive value of straw and stubble from rice grown in the north-east of Thailand. In: R M Dixon (ed.), Ruminant feeding systems utilizing fibrous agricultural residues 1985. Proceedings of the Fifth Workshop of the AAFARR Network held in Bogor, Indonesia, 13-17 April 1985. International Development Program of Australian Universities and Colleges, Canberra, Australia.
Hogan J P. Kenny P A and Weston R H. 1986. Factors affecting the intake of feed by grazing animals. In: J M Wheeler, C J Pearson and G E Robards (eds), Temperate pastures, their production, use and management. Proceedings of a Conference held at Leura, New South Wales, December 1985. Australian Wool Corporation/CSIRO, Australia.
Koller B L, Hintz H F. Robertson J B and Van Soest P J. 1978. Comparative cell wall and dry matter digestion in the cecum of the pony and the rumen of the cow using in vitro and nylon bag techniques. Journal of Animal Science 47:209-215.
Lee J A and Pearce G R. 1984. The effectiveness of chewing during eating on particle size reduction of roughages by cattle. Australian Journal of Agricultural Research 35:609-618.
Levy D, Holzer H N and Folman Y. 1977. Chemical processing of wheat straw and cotton byproducts for fattening cattle. 1. Performance of animals receiving the wet material shortly after treatment. Animal Production 25:27-37.
McManus W R and Choung C C. 1976. Studies on forage cell walls. 2. Conditions for alkali treatment of rice straw and rice hulls. Journal of Agricultural Science (Cambridge) 86:453-470.
NRC (National Research Council). 1970. Nutrient requirements of cattle. National Research Council, Washington, DC, USA.
Ojima K and Isawa T. 1968. The variation of carbohydrates in various species of grasses and legumes. Canadian Journal of Botany 46:1507-1511.
Pearce G R. Beard J and Hilliard E. 1979. Variability in the chemical composition of cereal straws and in vitro digestibility with and without sodium hydroxide treatment. Austalian Journal of Experimental Agriculture and Animal Husbandry 19: 350-353.
Pearce G R. 1984. Some characteristics of senescent forages in relation to their digestion by rumen micro-organisms. In: S K Baker, J M Gawthorne, J B Mackintosh and D B Purser (eds), Ruminant physiology - concepts and consequences. Proceedings of a symposium held at the University of Western Australia, 7-10 May 1984. University of Western Australia.
Rawson H M and Evans L T. 1971. The contribution of stem reserves to grain development in a range of wheat cultivars of different height. Australian Journal of Agricultural Research 22:851-863.
Roxas D B. Castillo L S. Obsioma A, Lapitan R M, Momongan V G and Juliano B O. 1984. Chemical composition and in vitro digestibility of straw from different varieties of rice. In: P T Doyle (ed.), The utilization of fibrous agricultural residues as animal feeds. Proceedings of the Third Workshop of the AAFARR Network held in Peradeniya, Sri Lanka, 17-22 April 1983. University of Melbourne, Australia.
Roxas D B. Obsioma A R. Lapitan R M, Castillo L S. Momongan V G and Juliano B O. 1985. The effects of variety of rice, level of nitrogen fertilization and season on the chemical composition and in vitro digestibility of straw. In: P T Doyle (ed.), The utilization of fibrous agricultural residues as animal feeds. Proceedings of the Fourth Workshop of the AAFARR Network held in Khon Kaen Thailand, 10-14 April 1984. International Development Program of Australian Universities and Colleges, Canberra, Australia.
Sannasgala Kshanika and Jayasuriya M C N. 1984. Effects of physiological and morphological characteristics on the chemical composition and in vitro digestibility of different varieties of rice straw. In: P T Doyle (ed.), The utilization of fibrous agricultural residues as animal feeds. Proceedings of the Third Workshop of the AAFARR Network held in Peradeniya, Sri Lanka, 17-22 April 1983. University of Melbourne, Australia.
Van Soest P J. 1982. Nutritional ecology of the ruminant. O and B Books Inc., Oregon, USA.
Vijchulata P and Sanpote S. 1982. Changes in the nutritive value of rice straw after growth of Volvariella volvacea fungus. In: P T Doyle (ed.), The utilization of fibrous agricultural residues as animal feeds. Proceedings of the Second Workshop of the AAFARR Network held in Serdang, Malaysia, 3-7 May 1982. The University of Melbourne Printing Services, Parkville, Australia.
Wahed R A and Owen E. 1986. Comparison of sheep and goats under stall-feeding conditions: Roughage intake and selection. Animal Production 42:89-95.
Weston R H. 1985. The regulation of feed intake in herbage-fed ruminants. Proceedings of the Nutrition Society of Australia 10:55-62.
White L M, Hartman G P and Bergman J W. 1981. In vitro digestibility, crude protein, and phosphorus content of straw of winter wheat, spring wheat, barley, and oat cultivars in eastern Montana. Agronomy Journal 73:117121.
Winugroho M. 1981. Studies on the utilization of cereal straws. M.Agr.S. thesis, University of Melbourne, Australia.
Winugroho M. 1986. Intake and digestibility of the upper and lower fractions of rice straw by sheep and goats. In: R M Dixon (ed.), Ruminant feeding systems utilizing fibrous agricultural residues - 1985. Proceedings of the Fifth Workshop of the AAFARR Network held in Bogo, Indonesia, 13-17 April 1985. International Development Program of Australian Universities and Colleges, Canberra, Australia.
Yoon C S. Choi E S. Oh T K, Lee N H. Kim C W and Kim C S. 1982. Effects of aqueous ammoniatreated rice straw on feed intake, nutritive value and rumen characteristics. Korean Journal of Animal Science 25:613-622.
Yu Yu, Thomas J W and Emery R S. 1975. Estimated nutritive value of treated forages for ruminants. Journal of Animal Science 41:1742-1751.
Pearce: We measured neutral-detergent solubles as 100% minus the neutral-detergent fibre. We measure the NDF before we start and the NDF of the residue and 100-NDF is the indigestible neutral detergent solubles.
Van Soest: Is this after cellulase digestion?
Pearce: Yes, after pepsin-cellulase.
Van Soest: There might be fractions in there that might be digestible by other enzymes. Is that possible?
Pearce: Such as what?
Van Soest: Such as pectins, galactans, soluble hemicelluloses or phenolics.
Pearce: There is not much pectin in wheat straw. How else can we measure the digestibility of neutral-detergent solubles?
Van Soest: The Lucas test on straw indicates that the neutral-detergent solubles have a 90% plus true digestibility and wheat straw follows that relationship in animal digestion trials.
Pearce: Yes, although I am not convinced about that. It is quite plausible that, in the absence of soluble carbohydrates, the cell contents are not completely digestible. Chloroplasts are probably undigestible and membranous materials and nucleus residues are only 20% digestible.
Uden: I think the difference between the animal work and the cellulase work is that you can never tell the digestibility of the neutral-detergent solubles in vivo. A lot of soluble compounds are excreted in the urine. Chesson has found low digestibility of chloroplasts and other organelles in the cell solubles.
Pearce: Yes, it's impossible to get an estimate of digestibility of neutral detergent solubles because of all the material that is added on the way through the digestive tract.
Thomson: You made the comment that you should try to have more cell contents in the straw. How will that affect grain yield?
Pearce: Under favourable conditions the grain does not rely very much on the stored fructans in wheat. I am not suggesting that we need to retain all the fructans, why not use enough for the grain and save the rest? We need to reduce wastage by respiration in the senescing plant. On the other hand, if the plant is stressed during grain development, it might be detrimental.
For further details log on website :
http://www.fao.org/Wairdocs/ILRI/x5495E/x5495e0c.htm
Sources of variation in the nutritive value of wheat and rice straws
Author
G.R. Pearce, J.A. Lee, R.J. Simpson and P.T. Doyle
School of Agriculture and Forestry, University of Melbourne, Parkville, Victoria 3052, Australia
Introduction
The nutritive value of cereal straws can be improved in several ways. Treatment with chemicals, such as alkali, alters the characteristics of straws and renders the cell-wall constituents more susceptible to microbial attack and thus increases straw intake and digestibility. Supplementation with limiting nutrients can also improve straw utilisation. The responses to supplementation are influenced by the characteristics of the straw: response is likely to be better with a good quality straw than with a poor quality one.Straw quality per se can be improved by:
· breeding or genetic engineering;None of these will be readily adopted because grain yield and quality are primary considerations in commercial crop production. Consideration of any form of manipulation of cereal plants requires an understanding of the sources of variation and of the nature and timing of the changes occurring during growth and development of the plant that determine its characteristics at the straw stage. This paper examines these aspects in relation to wheat and rice straws.
· modification of agronomic practices; and
· altering harvesting, threshing and storage methods to optimise straw feeding value.
Variation in the nutritive value of straws
The nutritive value of a feed is determined by (a) the concentration of nutrients in the feed, (b) the amount eaten, (c) the proportion of the nutrients digested and (d) the efficiency with which absorbed nutrients are used. Data on all of these components are rarely available for cereal straws, and indices, such as chemical composition and digestibility values, are commonly used to assess nutritive value. Very few in vivo digestibility experiments have been conducted using cereal straw fed alone (without supplements) and most digestibility measurements are made in vitro.Animals fed cereal straws alone invariably lose weight, indicating that the nutritive value of these materials is low. However, published data show that there is wide variation in their nutritive characteristics (Table 1). It is not known how much of this variation is due to inherent characteristics of the plant material and how much is due to differences in the proportions of morphological fractions (leaves, stems etc.) arising from different growing conditions, harvesting procedures (particularly height of cutting) and threshing and storage methods. In feeding trials, the composition of the straw actually consumed is affected by feeding practices. Descriptions of the origin and history of straw samples are usually inadequate, so it is not possible to determine the extent to which comparisons of different straws are valid.
Examinations of separate plant fractions indicate that real variation occurs within these fractions (Table 2). It is still difficult, however, to attribute variation to genotype, environmental conditions of plant growth or interactions between these factors. Although differences among cultivars have often been measured (for example, Table 3), it is not possible to state confidently that any one cultivar produces better quality straw than another, let alone to quantitate the magnitude of any apparent superiority. Differences also occur among seasons, even in the same location with similar harvesting procedures. Table 4 shows the variation in in vitro organic matter digestibility (IVOMD) over three seasons of two rice cultivars grown at one location in the Philippines.
Table 1. Ranges in nutritive characteristics of wheat and rice straws.
Component
|
Wheat straw
|
Rice straw
|
| Nitrogen (% DM) |
0.241-0.992
|
0.383-1.524
|
| Sulphur (% DM) |
0.045-0.196
|
0.017-0.136
|
| In vitro digestibility (%) |
218-589
| |
| Voluntary intake (kg DM 100 kg-1 LW) |
0.611-1.412
|
1.013-2.714
|
Sources:Table 2. Ranges in in vitro organic matter digestibility (IVOMD) of morphological fractions of wheat and rice straws.
1. Pearce et al (1979)
2. Acock et al (1978)
3. Sannasgala and Jayasuriya (1984)
4. Roxas et al (1985)
5. CSIRO (1982)
6. NRC (1970)
7. McManus and Choung (1976)
8. Braman and Abe (1977)
9. Levy et al (1977)
10. Winugroho (1981)
11. Franklin et al (1967)
12. W.J. Wales (unpublished data)
13. Vijchulata and Sanpote (1982)
14. Devendra (1983)
Fraction
|
IVOMD %
| |
Wheat straw
|
Rice straw
| |
| Stem internode |
21 - 35
|
42 - 77
|
| Leaf sheath |
45 - 63
|
38 - 56
|
| Leaf blade |
58 - 77
|
45 - 60
|
Source: Winugroho (1981).Table 3. Ranges in in vitro digestibility of straw from wheat and rice cultivars.
Type of straw
|
No. of cultivars
|
In vitro digestibility (%)
|
Country/reference
|
| Wheat |
20
|
28 - 42
|
USA/White et al (1981)
|
| Rice |
21
|
44 - 62
|
Australia/Winugroho
(1981) |
Experimental approaches
Special experimental strategies are required to obviate the problems described above. In studies at the University of Melbourne aimed at determining the effect of plant factors on straw quality the following approaches have been developed:1. Plants are harvested whole and dissected into stem internodes, leaf sheaths and leaf blades for separate analysis. The nodes, rachis and glumes are analysed occasionally but are sometimes discarded because they represent minor proportions of the total mass of the plant.2. The topmost stem internode, designated S1, is analysed separately from the second internode, S2, and separately from S3, S4 and so on. In the same way, the leaf sheaths are sub-grouped into LS1, LS2 etc and the leaf blades into LB1, LB2, etc.Table 4. Seasonal variation in IVOMD of straw from two rice cultivars.
3. Samples are taken, usually at weekly intervals, during vegetative growth, maturation and senescence. This permits critical periods of change to be identified in relation to the subsequent feeding value of the straw.
4. The mass of a chemical component in a particular fraction is calculated. When this is plotted against time, meaningful patterns of change can be readily seen.
IVOMD (%)
| |||
1981/821
|
1983/842
| ||
Cultivar
|
Wet season
|
Wet season
|
Dry season
|
| IR 36 |
36
|
53
|
51
|
| IR 42 |
41
|
41
|
47
|
Sources:This overall approach reveals real changes occurring in plants in relation to ontogenetic events and permits the assessment of changes in terms of indices of nutritive value, such as in vitro digestibility. Obviously, in practice and in animal feeding experiments, straw is still used as harvested, but it is useful to monitor feed intake by separating samples of feed offered and refused into morphological fractions to help explain results when selection of dietary components occurs.
1. Roxas et al (1984).
2. Roxas et al (1985).
In assessing the nutritive value of mature, senescent and dead forages a clear distinction should be made between the cell wall, measured as neutral-detergent fibre (NDF), and the cell contents, measured as neutral-detergent solubles (NDS% = 100 - NDF%), because the cell wall is usually slowly and poorly digestible, while the cell contents may be highly and rapidly digestible. In the following discussion the characteristics of the cell wall and the cell contents are dealt with separately and it will be seen that opportunities for manipulation differ for these two components.
With wheat straws, greater emphasis is placed on the stem (internodes) fraction than the leaf (sheath and blade) fraction because the stem comprises a much larger proportion of the straw than leaves (Table 5). More leaf material is lost during harvesting and threshing than stem, and therefore the straw offered to animals contains a larger proportion of stem than indicated in Table 5. In whole rice straw, the proportions of internodes, sheaths and blades tend to be more equal but again more leaf blade material is likely to be lost during harvesting and threshing.
Table 5. Percentage of whole culm weight of wheat and rice straw in internodes, nodes, leaf sheaths and leaf blades.
Wheat straw
|
Rice straw
| |
| Internodes |
50
|
31
|
| Nodes |
8
|
5
|
| Sheaths |
24
|
36
|
| Blades |
18
|
28
|
Source: Winugroho (1981)
Cell wall
The plant cell wall comprises cellulose, hemicellulose, pectin, lignin, minerals and protein. These, with the exception of pectin, are insoluble in neutral-detergent solution. The composition of cell walls of both wheat straw and rice straw is variable (Table 6), although the ranges in cellulose and hemicellulose levels in wheat are relatively small. In a limited number of analyses, Winugroho (1981) found that the ranges in NDF composition in stem internodes, leaf sheaths and leaf blades were usually narrower than in samples of whole straws (Table 7). Thus, much of the variation shown in Table 6 may be due to differences in the proportions of plant morphological fractions, rather than differences in cell wall composition per se. Insufficient data are available in the literature to show the extent to which cell wall composition varies in morphological fractions.Table 6. Ranges in the composition of NDF (as % of NDF) in wheat and rice straws.
Component
|
Wheat straw
|
Rice straw
|
| Cellulose |
50a-56b
|
41g-57g
|
| Hemicellulose |
28c-39d
|
4g-39h
|
| Lignin |
9b-18a
|
8h-18g
|
| Residual ash |
1e-8f
|
8h-38g
|
Sources:This argument may be taken further to consider possible variation in the proportion of different cell types (epidermal, mesophyll, schlerenchyma, parenchyma etc) within a morphological fraction and the extent to which variation occurs in the characteristics of the cell wall of a particular cell type. This has been studied in ryegrass (Lolium) leaves (Gordon et al, 1985) but not in cereal species.
a. Koller et al (1978).
b. Yu et al (1975).
c. Pearce et al (1979).
d. Ayres et al (1976).
e. Alawa and Owen (1984).
f. Ben-Ghedalia and Miron (1981).
g. Roxas et al (1984). h. Yoon et al (1982).
Studies on the developing plant can provide important information on the types of manipulation that might be used to improve nutritive value and on the critical time periods during which a plant expresses characteristics important in determining straw quality. In the growing and maturing plant, the partitioning of mass into the different morphological fractions follows a generally well defined pattern. Temporal changes in the NDF mass of two stem internodes of wheat (S1, the topmost internode, and S5, a lower one) are shown in Figure 1. The main features were:
1. The lower internodes attained maximum NDF mass earlier than the upper ones;2. After the point of maximum elongation, which for S1 was about 15 days after anthesis and for S5 about 20 days before anthesis, mass continued to increase, presumably due to cell wall thickening; andTable 7. Ranges in the composition of NDF (as % of NDF) in stem internodes, leaf sheaths and leaf blades of wheat and rice straws.
3. Mass did not increase beyond about 4 weeks after anthesis.
Component
|
Wheat straw
|
Rice straw
| ||||
Internodes
|
Sheaths
|
Blades
|
Internodes
|
Sheaths
|
Blades
| |
| Cellulose |
50-53
|
48-48
|
42-45
|
43-58
|
49-50
|
37-41
|
| Hemicellulose |
30-31
|
32-36
|
22-30
|
21-39
|
21-29
|
22-25
|
| Lignin |
15-17
|
8-9
|
8-9
|
11-13
|
7-10
|
7-8
|
| Residual ash |
2-3
|
7-12
|
15-28
|
7-7
|
15-20
|
26-33
|
Source: Winugroho (1981).
Figure 1. Changes in mass of NDF in two internodes (51, 55) of wheat.
Figure 2 shows changes in the mass of cellulose, hemicellulose and lignin in S2 over the same period. During growth, cellulose, hemicellulose and lignin are all being synthesised in the cell wall but the amount of cellulose deposited is greater than that of hemicellulose or lignin until after anthesis and beyond the point of maximum elongation. Synthesis of all three components stopped at the same time, about 4 weeks after anthesis. Between 20 days before anthesis and 30 days after anthesis, the amount of cellulose increased about 3.5 fold, the amount of hemicellulose about 2 fold and the amount of lignin about 5 fold (Figure 2). The pattern in the other stem segments was similar. This shows that manipulations of cell-wall composition to improve feeding value must focus on this period when rapid changes are occurring. In this case, manipulations would not have any effect if applied and expressed 4 weeks, or later, after anthesis.
Figure 2. Changes in mass of dry matter, NDF, cellulose, hemicellulose and lignin in S2 of wheat.
These changes in chemical composition of stem internodes are reflected in in vitro NDF digestibility (NDFD) values (Figure 3). In the upper internodes, the decline in digestibility in the period immediately before anthesis was rapid and dramatic. For example, the NDFD of S1 fell by about 3.6% units per day and continued to decline until after anthesis. The changes in the lower internodes were earlier and slower than in the upper ones.
Figure 3. Changes in NDF digestibility in three internodes (S1, S3, S5) of wheat
Other results (J.A. Lee, unpublished data) have shown that NDFD and the digestibilities of cellulose and hemicellulose in the internodes of wheat decline at similar rates, but the final digestibility of hemicellulose was somewhat higher than that of NDF and cellulose. The similarity of the rates of change in digestibility for both cellulose and hemicellulose suggests that these components were affected similarly and equally by lignification and other factors associated with maturation and loss of digestibility.
The leaf blades and sheaths in the wheat examined by Lee were similar in mass and attained their maximal masses earlier than their corresponding internodes. In LS2 (Figure 4) and LB2 (Figure 5) maximal mass had been attained by anthesis. In LB2 there was an apparent loss of hemicellulose after anthesis, but there was no explanation for this. The NDFD of sheaths remained at about 40% beyond anthesis, while the NDFD of the blades continued to decline steadily but was still 50 to 60% in the straw.
Cell-wall lignification is recognised as a prime cause of declines in digestibility but, often, only poor correlations have been obtained between lignin content and digestibility (e.g. Pearce, 1984). However, these have usually been between the percentage of lignin in the dry matter and dry-matter or organic-matter digestibility of whole straw. When the percentage of lignin in the NDF is correlated with NDFD for individual straw fractions, closer relationships may be obtained. For example the r2 values for internodes, sheaths and blades were 0.94, 0.83 and 0.87, respectively, in exponential decay functions (J.A. Lee, unpublished data). For each relationship, the regression co-efficients were substantially different, indicating that a single regression from the pooled data, as is represented in whole straw, would be associated with wide variation and a lower r2. Thus, the association of lignin with cellulose and hemicellulose is different in the three plant fractions and this must be taken into account.
Figure 4. Changes in mass of NDF, cellulose, hemicellulose and lignin in leaf sheath 2 of wheat.
Figure 5. Changes in mass of NDF, cellulose, hemicellulose and lignin in leaf blade 2 of wheat.
Few attempts have been made to alter lignin synthesis in growing plants. However, at the University of Melbourne, treatment of annual ryegrass (Lolium rigidum) with gibberellic acid caused stem elongation and increased the proportion of lignin in the cell wall. The S1 contained 13% lignin in the NDF of the straw compared with 8% in untreated plants and the NDFD values were 17 and 26%, respectively. On the other hand, acute copper deficiency reduces lignin synthesis (Graham, 1976; Downes and Turner, 1986).
The relationship between cell-wall composition and digestibility deserves a continuing high research priority.
Cell contents
The cell contents comprise proteins, peptides and other nitrogen-containing compounds, carbohydrates, fats and minerals. These are removed effectively from plant tissues by neutral-detergent solution, except for starch which often requires further digestion with amylase for complete removal.In senescing plants, fluctuations in NDS content are due mainly to changes in the amount of storage carbohydrates (fructans in wheat and starches in rice). The levels of storage carbohydrates retained in straw are critical to feeding value because of their high digestibility. The effects of changes in NDS in a senescing and dead internode of wheat on the digestibility of the internode are shown in Figures 6 and 7. NDS increased rapidly from shortly before anthesis and peaked about 4 weeks after anthesis (Figure 6). NDS digestibility was high throughout this period (Figure 7), as would be expected of a tissue enriched with soluble carbohydrates. Subsequently, the amount of NDS in the internode and NDS and organic/dry-matter digestibilities declined. Both the amount of NDS and its digestibility approached basal levels at the time of grain maturation.
The changes in the amount of NDS are associated with grain development in the following manner: Immediately after anthesis, photosynthesis are produced in excess of requirements for grain growth because grain development is very slow. Grain mass increases rapidly from about 3 weeks after anthesis, consuming carbohydrates generated by current photosynthesis and, increasingly, from the mobilisation of storage carbohydrates in the culm (Blacklow et al, 1984). The contribution that reserve carbohydrates make to the final grain mass depends upon environmental conditions and perhaps upon genetic factors. If the plant is stressed the reserves will be drawn upon heavily by the grain. For example, in drought-stressed barley Gallagher et al (1975) found that 74% of the grain mass could be attributed to storage carbohydrates, whereas other reports (e.g. Evans and Wardlaw, 1976) have suggested as little as 10%.
The extent to which, and the conditions under which, storage carbohydrates are respired are uncertain. Rawson and Evans (1971) estimated that 30% of the fructans of wheat are used for respiration, but L.C. Incoll (personal communication) found that almost all C14 - labelled fructans in stems are mobilised to the grains, indicating that, at least during grain filling, fructans are most probably not used for respiration. Another possible loss is transport to the lower parts of the plant to support late tillering. In certain crops and particularly under conditions of high fertility and moisture (e.g. irrigation), late tillering can occur. In rice this is called "rattooning" and is sometimes used as a means of obtaining a second grain harvest.
Figure 6. Changes in mass of dry matter and neutral-detergent solubles in 52 of wheat.
Figure 7. Changes in IVOMD and NDS digestibility in S2 of wheat
NDS exhibits two main levels of digestibility; 90% when the stems are high in NDS and 40-50% when the stems are low in NDS, after they have senesced. It thus seems reasonable to consider NDS as being comprised of two pools of nutrients: (a) the intrinsic nitrogen compounds, carbohydrates, fats and minerals of the cytoplasm, mitochondria, membranes, nucleus, chloroplasts and other organelles, which, as a whole, are apparently less digestible in senescing and dead plant tissues, and (b) the reserve carbohydrates and proteins. Proteins are efficiently mobilised from senescing plant tissues and, except under conditions of high nitrogen fertilizer application (e.g. Roxas et al, 1985), their concentration in straw is low (Darling, 1985). However, considerable amounts of storage carbohydrates, such as fructans in temperate grasses (Ojima and Isawa, 1968) and glucans and starches in rice, may be present in dead crop dry matter.
Different plant fractions do not store carbohydrates uniformly. Stems store considerably more than leaf sheaths, which store more than leaf blades. In wheat, the penultimate internode, S2, accumulates most fructans. In wheat, the upper parts of the culm contain larger proportions of NDS than the lower parts, as might be expected from their age, degree of senescence and proximity to photosynthesising leaves in the crop canopy. In irrigated rice, however, there may be no difference in the NDS content of the upper and lower parts of the straw (Hart and Wanapat, 1985; Winugroho, 1986).
The proportion of NDS in wheat straw ranges from 12% (W.J. Wales, unpublished data) to 41% (Ayres et al, 1976) of the dry matter and in rice straw from 14% (Cheva-Isarakul and Cheva-Isarakul, 1984) to 46% (Roxas et al, 1984). Higher values are associated with higher digestibility because of greater amounts of residual storage carbohydrates. In considerations of straw quality, therefore, the role of the storage carbohydrates is critical: the more storage carbohydrates remaining in straw the higher the digestibility. This has been expressed by Pearce (1984) as a high correlation between NDS% and IVOMD%. The pertinent question, therefore, is under what conditions are residual storage carbohydrate levels high in straw? The answer lies in the interactions between the photosynthetic activity of the plant at critical time periods and the demands of the grain.
The first requirement for high levels of storage carbohydrates is a high level of accumulation during the period around anthesis and for some time afterwards. For wheat in southern Australia this period spans about 40 days, from about 10 days before anthesis to about 30 days afterwards. Storage carbohydrates accumulate during this period if conditions are favourable for photosynthesis, i.e. suitable light and temperature conditions, adequate nutrient and water availability and the absence of disease. Thus, the history of the plant in terms of soil fertility, fertilizer application, spacing of plants and leaf development up to this time may be important.
The full extent to which storage carbohydrate accumulation varies is not known but, under a range of conditions at the University of Melbourne, amounts of NDS in stem segments varied widely. For example, among five different, but related, wheat cultivars grown side-by-side in the same season, NDS content of the S2 at peak accumulation ranged from 114 to 280 mg. The NDS content of S2 of plants grown under normal field conditions peaked at 158 ma, compared with only 60 mg for the same cultivar grown under apparently favourable conditions in pots in a glasshouse (W.J. Wales, unpublished data). Ambient temperatures and rates of growth relative to crop photosynthetic activity may all have an effect on the accumulation of carbohydrate reserves. Under field conditions in Western Australia, M. Nicolas (personal communication) has concluded that wheat cultivars do, however, vary widely in their ability to accumulate fructan reserves.
The second requirement for high levels of storage carbohydrates in straw is a low rate of removal. Normally, developing grains draw on storage carbohydrates to augment the metabolites provided by current photosynthesis. As current photosynthesis declines during senescence the reserves are drawn upon. If current photosynthesis is optimal then the extent to which reserves are used depends largely on the number of grains developing, i.e. the size of the "sink". If there are few grains levels of storage carbohydrates and straw quality are more likely to be high. However, correlation between grain yield and straw quality is often poor (e.g. Erickson et al, 1982), partly because of the variable accumulation and loss of storage carbohydrates.
Where the sink is small, mobilisation of storage carbohydrates may be delayed and reduced if sufficient moisture and nutrients are available for the crop. This may happen under irrigation but in dryland situations seasonal constraints are likely to predominate.
At this stage, it is only possible to conclude that the relationships between the amounts of residual storage carbohydrate in straw and other physiological events involved in yield formation in crops are likely to be complex. However, an understanding of these interactions may identify opportunities for manipulation of carbohydrate reserves so as to achieve high-quality straw for animal production without significant reductions in grain yield and quality.
Animal factors interacting directly with straw characteristics
Straw intake and digestibility in ruminants are influenced by straw characteristics (including chemical composition, morphological and anatomical features, physical nature and palatability); by feeding conditions (including the amount offered and the frequency of feeding); and by animal characteristics (including species/genotype, liveweight, age, body condition, type and level of production and disease). Extremes of temperature and humidity and social interactions between animals may also affect intake. Reviews on herbage digestibility (e.g. Akin, 1982) and intake (e.g. Armstrong, 1982) have discussed the principles involved, but without specific reference to straws. A major limitation is the small number of experiments in which animals have been fed straw alone; the diet has usually been supplemented with nitrogen and minerals and, often, energy. The following discussion is limited to specific aspects that are particularly relevant to straws.Selection in relation to physical characteristics of straws
Weston (1985) included texture as a criterion governing acceptability of feeds and Hogan et al (1986) suggested that animals select plant material on the basis of "tenderness." However, such characteristics are difficult to assess because, often, no one feature predominates and several factors interact. In straws, such as wheat (Doyle et al, 1987) and barley (Wahed and Owen, 1986) animals usually show a preference for leaves rather than stems. Where such preference is shown, useful comparisons between many literature reports are almost impossible because of the lack of information on feeding procedures and because of unspecified degrees of selection, especially when straws have been offered in excess of appetite.
In rice straw, selection of leaves in preference to stems may not occur (Doyle et al, 1987), possibly because the leaf blades contain more silica than the stems (Doyle et al, in press), as suggested by Van Soest (1982).
Resistance to particle size reduction
If the plant material is highly resistant to particle size reduction voluntary intake will be reduced because large particles of digesta cannot pass through the reticulo-omasal orifice into the lower digestive tract. Thus, large particles are retained in the reticulo-rumen until they are broken down by rumination or detrition. The comminution of feed particles during chewing also contributes to this process. Lee and Pearce (1984) found that, in five roughages, including barley and oat straw, chewing by cattle reduced about 50% of the material to particles that would pass through a 1 mm screen.
The extent and rate of particle size reduction is not the only mechanism controlling roughage intake and the rate at which digesta leave the reticulo-rumen is not necessarily proportional to feed intake because (a) the amount of digesta in the reticulo-rumen can vary and (b) the rate and extent of fermentation varies according to the available nutrient content of the roughage. Thus, straws of differing composition would be expected to produce differences in fermentation kinetics. However, the full details of the system have not been resolved.
Grinding energy has been used as an index of resistance to particle size reduction. This is the amount of electrical energy required to grind 10 g of feed through a 1 mm screen (Chenost, 1966; Foot and Reed, 1981). In wheat straws, the grinding energy of leaf blades is lower than that of leaf sheaths which is lower than that of stems, but in rice straw the differences may not be as great (Table 8). In experiments with wheat straw (W.J. Wales, unpublished data) and with rice straws (Chanpongsang, 1987), good inverse relationships were obtained between intake by sheep and grinding energy within cereal species but not between species (Table 9). Other factors, including possibly silica content, are thus involved.
Table 8. Grinding energy (J g-1 DM) of wheat and rice straw fractions (averages from three straws).
Fraction
|
Wheat straw
|
Rice straw
|
| Leaf blade |
83
|
100
|
| Leaf sheath |
122
|
103
|
| Stem |
213
|
147
|
Segments were chopped into 2 cm lengths prior to grinding.Table 9. Intake, grinding energy and rate of eating for three wheat and three rice straws.
Source: Doyle et al (in press).
Measurement
|
Wheat straw
|
Rice straw
| ||||
1
|
2
|
3
|
1
|
2
|
3
| |
| Intake (g OM d-1) |
617
|
484
|
303
|
492
|
383
|
369
|
| Grinding energy(J g-1 DM) |
138
|
161
|
195
|
95
|
109
|
107
|
| Rate of eating(g air dry h-1) |
516
|
434
|
287
|
386
|
309
|
269
|
Eating rate
Eating rate has been used to evaluate herbage quality. Table 9 shows positive relationships between intake and eating rate of wheat and rice straws. These measurements were made with trained sheep and reflected characteristics of the feeds associated with palatability or acceptability. With wheat straw, material with a high proportion of stem is eaten much more slowly than leafy material. Habituation is also a factor, because straws that are eaten relatively slowly in early tests may be eaten more quickly in later tests. This occurred particularly with rice straws in which, it is believed, silica levels may have affected acceptability initially. The specific plant causes of such animal responses are unknown.
Metabolic factors influencing intake of straws
Voluntary intake of highly digestible forages is controlled by metabolic factors or is linked to requirements for maintenance and production. In the case of poorly digestible materials, however, attention has been focused on physical control of intake, particularly distension of, and removal from, the reticulo-rumen. However, the amounts and balance of nutrients supplied by low-quality straws are so limiting that metabolic contributions to the control of intake should not be overlooked.
Doyle et al (1987) concluded that, in view of the relatively small amount of digesta in the reticulo-rumen of animals fed unsupplemented straws, the purely physical control mechanisms do not operate or the levels at which the system is sensitised are lower, due perhaps to nutrient imbalances in the tissues. In either case, an extremely complex set of interactions determines intake levels. The kinetic features of digestion for straws with varying proportions of nutrients and for straws which are supplemented to provide limiting nutrients are variable.
Conclusions
In this paper, attention has been directed towards the characteristics of the cell wall and cell contents as they determine the nutritive value of cereal straws, and reference has been made to some other factors directly affecting intake and digestion of straws by animals. Because of differences between morphological fractions of plants, only limited information can be obtained from assessments of whole straws. Different harvesting, threshing and feeding practices, which affect the proportions of the main morphological fractions, will determine the nutritive value of the straw actually consumed by animals.Cell-wall digestibility can be improved by chemical and other treatments but such procedures are too expensive for wide practical application. The alternative, separating material with high digestibility (e.g. leaf blades in wheat) from material with low digestibility (e.g. stems in wheat), is also expensive and suffers from the disadvantage that the less digestible fraction usually forms the largest proportion of a crop residue. Breeding for greater leafiness may also not be attractive because this would lower the harvest index.
The main factor determining the digestibility of cell-wall material is lignin, but the precise limiting role of lignin is still unresolved. The information presented in this paper indicates that lignification effects need to be studied in discrete plant tissues. Identification of periods during which cell-wall digestibility is changing rapidly probably provides the best opportunity to understand the significance of concurrent chemical changes. The thrust of agronomic or genetic manipulations to alter cell wall characteristics must await a clearer understanding of lignin chemistry in relation to other cell wall constituents.
The potential contribution of the cell contents to the nutritive value of straws appears to have been under-estimated in the literature. Not only can straws contain quite large proportions of cell contents, but the amount of storage carbohydrates in the cell contents can vary widely. Because storage carbohydrates are highly digestible (probably 100%), they may have a marked effect on the nutritive value of straws. In this paper, the pattern of accumulation and subsequent removal of solubles in wheat stem internodes has illustrated the means by which varying amounts of storage carbohydrates remain in straws. However, the precise mechanisms involved have not been elucidated. Complex interactions occur between storage carbohydrate metabolism and other physiological processes in the plant, mediated by environmental and genetic factors. Only an understanding of these will permit genetic improvement of straw quality without reducing grain yield and quality.
Factors in straws that impinge directly upon an animal's senses, thus influencing its level of intake, have not been studied in detail, partly because straws have rarely been fed without supplementation. It is clear, however, from the available results that important differences may occur. To date, these have been monitored as differences in features such as grinding energy, eating rate and chemical composition, but alone or collectively they may result in pronounced differences in voluntary intake by animals, often associated with high degrees of selection for certain plant parts. The precise plant features that are responsible and the variation between animals in response to these have not been defined. Again, an understanding of these is necessary so that appropriate manipulations, by genetic or other means, may be approached.
Acknowledgements
Much of the research conducted in this area at the University of Melbourne was funded by the Australian Wool Corporation through the Wool Research Trust Fund, the Australian Meat and Livestock Research and Development Corporation, the Australian Centre for International Agricultural Research and The University of Melbourne.References
Acock C W, Ward J K, Rush I G and Klopfenstein T J. 1978. Effect of location, variety and maturity on characteristics of wheat straw. Journal of Animal Science 47: Supplement 1:327 (Abstract).Akin D E. 1982. Microbial breakdown of feed in the digestive tract. In: J B Hacker (ed.), Nutritional limits to animal production from pastures. Proceedings of a symposium held at St. Lucia, Queensland, 24-28 August 1981. Commonwealth Agricultural Bureaux, Farnham Royal, UK.
Alawa J P and Owen E. 1984. The effect of milling and sodium hydroxide treatment on intake and digestibility of wheat straw by sheep. Animal Feed Science and Technology 11:149-157.
Armstrong D G. 1982. Digestion and utilization of energy. In: J B Hacker (ed.), Nutritional limits to animal production from pastures. Proceedings of a symposium held at St. Lucia, Queensland, 24-28 August 1981. Commonwealth Agricultural Bureaux, Farnham Royal, UK.
Ayres J F. McManus W R and McFarlane J D. 1976. The evaluation of forage nutritive quality by microdigestion. Australian Journal of Experimental Agriculture and Animal Husbandry 16:472-477.
Ben-Ghedalia D and Miron J. 1981. Effect of sodium hydroxide, ozone and sulphur dioxide on the composition and in vitro digestibility of wheat straw. Journal of the Science of Food and Agriculture 32:224-228.
Blacklow W M, Darbyshire B and Pheloung P. 1984. Fructans polymerised and depolymerised in the internodes of winter wheat as grain-filling progressed. Plant Science Letters 36:213-218.
Braman W L and Abe R K. 1977. Laboratory and in vivo evaluation of the nutritive value of NaOH-treated wheat straw. Journal of Animal Science 45:496-505.
Chanpongsang S. 1987. Variation in the nutritive value of rice straws. M.Agr.S. thesis, University of Melbourne, Australia.
Chenost M. 1966. Fibrousness of forages: Its determination and its relation to feeding value. In: Proceedings of the Tenth
International Grassland Congress, held in Helsinki, Finland, 7-16 July 1966. Valtioneuvoston Kirjapaino, Helsinki, Finland.
Cheva-Isarakul Boonlom and Cheva-Isarakul Boonserm. 1984. Comparison of the intake and digestibility of different crop residues by sheep, cattle and buffaloes. In: P T Doyle (ed.), The utilization of fibrous agricultural residues as animal feeds. Proceedings of the Third Workshop of the AAFARR Network held in Peradeniya, Sri Lanka, 17-22 April 1983. University of Melbourne, Australia.
CSIRO (Commonwealth Scientific and Industrial Research Organisation). 1982. Composition of animal feedstuffs in Australia. Australian Feeds Information Centre, CSIRO, Canberra, Australia.
Dalling M J. 1985. The physiological basis of nitrogen redistribution during grain filling in cereals. In: J E Harper, R W Howell and L E Schrader (eds), Exploitation of physiological and genetic variability to enhance crop productivity. American Society of Plant Physiologists, Rockville, MD, USA.
Devendra C. 1983. Physical treatment of rice straw for goats and sheep and the response to substitution with variable levels of cassava (Manihot esculenta Crantz), Leucaena (Leucaena leucocephala) and Glyricidia (Glyricidia maculate) forages. MARDI Research Bulletin 11:3:272-290.
Downes G and Turvey N D. 1986. Reduced lignification in Pinus radiata D. Don. Australian Forestry Research 16:371-377.
Doyle P T. Chanpongsang S. Wales W J and Pearce G R. (in press). Variation in the nutritive value of wheat and rice straws. In: R M Dixon (ed.), Ruminant feeding systems utilizing fibrous agricultural residues - 1987. Proceedings of the Seventh Workshop of the AAFARR Network held in Chiang Mai, Thailand, 2-6 June 1987. International Development Program of Australian Universities and Colleges, Canberra, Australia.
Doyle P T. Pearce G R and Djajanegara A. 1987b. Intake and digestion of cereal straws. In: Proceedings of the 4th Animal Science Congress, held in Hamilton, New Zealand. Fourth AAAP Congress.
Erickson D O. Meyer D W and Foster A E. 1982. The effect of genotypes on the feed value of barley straws. Journal of Animal Science 5:1015-1026.
Evans L T and Wardlaw I F. 1976. Aspects of the comparative physiology of grain in cereals. Advances in Agronomy 28:301-359.
Foot J Z and Reed K F M. 1981. The energy consumed in the grinding of herbage and its relationship to voluntary intake of herbage by ruminants. In: J L Wheeler and R D Mochrie (eds), Forage evaluation: Concepts and techniques. Proceedings of a workshop held in Armidale, New South Wales, 27-31 October 1980. American Forage and Grassland Council/CSIRO.
Franklin M C, McInnes P and Briggs P K. 1967. Maintenance of Merino sheep fed varying protein roughages with or without supplement of chopped lucerne hay and wheat. Austalian Journal of Experimental Agriculture and Animal Husbandry 7:202-212.
Gallagher J N. Biscoe B V and Scott R K. 1975. Barley and its environment. V. Stability of grain weight. Journal of Applied Ecology 12:319-336.
Gordon A H. Lomax J A, Dalgarno K and Chesson A. 1985. Preparation and composition of mesophyll, epidermis and fibre cell walls from leaves of perennial ryegrass (Lolium perenne) and Italian ryegrass (Lolium multiflorum). Journal of the Science of Food and Agriculture 36:509-519.
Graham R D. 1976. Physiological aspects of time of application of copper to wheat plants. Journal of Experimental Botany 27:717-724.
Hart F J and Wanapat M. 1985. Comparison of the nutritive value of straw and stubble from rice grown in the north-east of Thailand. In: R M Dixon (ed.), Ruminant feeding systems utilizing fibrous agricultural residues 1985. Proceedings of the Fifth Workshop of the AAFARR Network held in Bogor, Indonesia, 13-17 April 1985. International Development Program of Australian Universities and Colleges, Canberra, Australia.
Hogan J P. Kenny P A and Weston R H. 1986. Factors affecting the intake of feed by grazing animals. In: J M Wheeler, C J Pearson and G E Robards (eds), Temperate pastures, their production, use and management. Proceedings of a Conference held at Leura, New South Wales, December 1985. Australian Wool Corporation/CSIRO, Australia.
Koller B L, Hintz H F. Robertson J B and Van Soest P J. 1978. Comparative cell wall and dry matter digestion in the cecum of the pony and the rumen of the cow using in vitro and nylon bag techniques. Journal of Animal Science 47:209-215.
Lee J A and Pearce G R. 1984. The effectiveness of chewing during eating on particle size reduction of roughages by cattle. Australian Journal of Agricultural Research 35:609-618.
Levy D, Holzer H N and Folman Y. 1977. Chemical processing of wheat straw and cotton byproducts for fattening cattle. 1. Performance of animals receiving the wet material shortly after treatment. Animal Production 25:27-37.
McManus W R and Choung C C. 1976. Studies on forage cell walls. 2. Conditions for alkali treatment of rice straw and rice hulls. Journal of Agricultural Science (Cambridge) 86:453-470.
NRC (National Research Council). 1970. Nutrient requirements of cattle. National Research Council, Washington, DC, USA.
Ojima K and Isawa T. 1968. The variation of carbohydrates in various species of grasses and legumes. Canadian Journal of Botany 46:1507-1511.
Pearce G R. Beard J and Hilliard E. 1979. Variability in the chemical composition of cereal straws and in vitro digestibility with and without sodium hydroxide treatment. Austalian Journal of Experimental Agriculture and Animal Husbandry 19: 350-353.
Pearce G R. 1984. Some characteristics of senescent forages in relation to their digestion by rumen micro-organisms. In: S K Baker, J M Gawthorne, J B Mackintosh and D B Purser (eds), Ruminant physiology - concepts and consequences. Proceedings of a symposium held at the University of Western Australia, 7-10 May 1984. University of Western Australia.
Rawson H M and Evans L T. 1971. The contribution of stem reserves to grain development in a range of wheat cultivars of different height. Australian Journal of Agricultural Research 22:851-863.
Roxas D B. Castillo L S. Obsioma A, Lapitan R M, Momongan V G and Juliano B O. 1984. Chemical composition and in vitro digestibility of straw from different varieties of rice. In: P T Doyle (ed.), The utilization of fibrous agricultural residues as animal feeds. Proceedings of the Third Workshop of the AAFARR Network held in Peradeniya, Sri Lanka, 17-22 April 1983. University of Melbourne, Australia.
Roxas D B. Obsioma A R. Lapitan R M, Castillo L S. Momongan V G and Juliano B O. 1985. The effects of variety of rice, level of nitrogen fertilization and season on the chemical composition and in vitro digestibility of straw. In: P T Doyle (ed.), The utilization of fibrous agricultural residues as animal feeds. Proceedings of the Fourth Workshop of the AAFARR Network held in Khon Kaen Thailand, 10-14 April 1984. International Development Program of Australian Universities and Colleges, Canberra, Australia.
Sannasgala Kshanika and Jayasuriya M C N. 1984. Effects of physiological and morphological characteristics on the chemical composition and in vitro digestibility of different varieties of rice straw. In: P T Doyle (ed.), The utilization of fibrous agricultural residues as animal feeds. Proceedings of the Third Workshop of the AAFARR Network held in Peradeniya, Sri Lanka, 17-22 April 1983. University of Melbourne, Australia.
Van Soest P J. 1982. Nutritional ecology of the ruminant. O and B Books Inc., Oregon, USA.
Vijchulata P and Sanpote S. 1982. Changes in the nutritive value of rice straw after growth of Volvariella volvacea fungus. In: P T Doyle (ed.), The utilization of fibrous agricultural residues as animal feeds. Proceedings of the Second Workshop of the AAFARR Network held in Serdang, Malaysia, 3-7 May 1982. The University of Melbourne Printing Services, Parkville, Australia.
Wahed R A and Owen E. 1986. Comparison of sheep and goats under stall-feeding conditions: Roughage intake and selection. Animal Production 42:89-95.
Weston R H. 1985. The regulation of feed intake in herbage-fed ruminants. Proceedings of the Nutrition Society of Australia 10:55-62.
White L M, Hartman G P and Bergman J W. 1981. In vitro digestibility, crude protein, and phosphorus content of straw of winter wheat, spring wheat, barley, and oat cultivars in eastern Montana. Agronomy Journal 73:117121.
Winugroho M. 1981. Studies on the utilization of cereal straws. M.Agr.S. thesis, University of Melbourne, Australia.
Winugroho M. 1986. Intake and digestibility of the upper and lower fractions of rice straw by sheep and goats. In: R M Dixon (ed.), Ruminant feeding systems utilizing fibrous agricultural residues - 1985. Proceedings of the Fifth Workshop of the AAFARR Network held in Bogo, Indonesia, 13-17 April 1985. International Development Program of Australian Universities and Colleges, Canberra, Australia.
Yoon C S. Choi E S. Oh T K, Lee N H. Kim C W and Kim C S. 1982. Effects of aqueous ammoniatreated rice straw on feed intake, nutritive value and rumen characteristics. Korean Journal of Animal Science 25:613-622.
Yu Yu, Thomas J W and Emery R S. 1975. Estimated nutritive value of treated forages for ruminants. Journal of Animal Science 41:1742-1751.
Discussion
Van Soest: I am pleased to see the relationship between the cell walls and the lignin. How did you measure the digestibility of the neutral-detergent solubles and what is the indigestible fraction?Pearce: We measured neutral-detergent solubles as 100% minus the neutral-detergent fibre. We measure the NDF before we start and the NDF of the residue and 100-NDF is the indigestible neutral detergent solubles.
Van Soest: Is this after cellulase digestion?
Pearce: Yes, after pepsin-cellulase.
Van Soest: There might be fractions in there that might be digestible by other enzymes. Is that possible?
Pearce: Such as what?
Van Soest: Such as pectins, galactans, soluble hemicelluloses or phenolics.
Pearce: There is not much pectin in wheat straw. How else can we measure the digestibility of neutral-detergent solubles?
Van Soest: The Lucas test on straw indicates that the neutral-detergent solubles have a 90% plus true digestibility and wheat straw follows that relationship in animal digestion trials.
Pearce: Yes, although I am not convinced about that. It is quite plausible that, in the absence of soluble carbohydrates, the cell contents are not completely digestible. Chloroplasts are probably undigestible and membranous materials and nucleus residues are only 20% digestible.
Uden: I think the difference between the animal work and the cellulase work is that you can never tell the digestibility of the neutral-detergent solubles in vivo. A lot of soluble compounds are excreted in the urine. Chesson has found low digestibility of chloroplasts and other organelles in the cell solubles.
Pearce: Yes, it's impossible to get an estimate of digestibility of neutral detergent solubles because of all the material that is added on the way through the digestive tract.
Thomson: You made the comment that you should try to have more cell contents in the straw. How will that affect grain yield?
Pearce: Under favourable conditions the grain does not rely very much on the stored fructans in wheat. I am not suggesting that we need to retain all the fructans, why not use enough for the grain and save the rest? We need to reduce wastage by respiration in the senescing plant. On the other hand, if the plant is stressed during grain development, it might be detrimental.
For further details log on website :
http://www.fao.org/Wairdocs/ILRI/x5495E/x5495e0c.htm
No comments:
Post a Comment