Title General discussion - Session 3 The effect of genotype and environment on the nutritive value of crop residues
McDowell: The four papers presented in this session emphasised methodology and genetic and environmental effects on the nutritive value of crop residues. We should focus our discussion on how the methods relate to specific types of residue rather than on the relative merits of the different methods. We heard yesterday about the quantity and potential of crop residues but very little on the effect of environment on the nutritive value of crop residues.
Berhane: Has any high-yielding variety been rejected because of low straw quality?
Ørskov: No, I do not think so because it is only in the last few years that we have realised that there are large differences in straw nutritive value among varieties.
Fussell: In India, a high-yielding millet variety, WC75, had a better acceptance because it had better straw quality.
Gupta: WC75 is improved material from an introduction from Nigeria. We just heard that pearl millet in West Africa, especially Niger, is not accepted by cattle because of poor quality of the stems. However, similar material is used by farmers in India as stored cattle feed. This could be due to cultural differences and not because of the crop. We should not generalise that millet is not a good crop for feeding animals on crop residues.
Reed: The difference between West Africa and India in the use of millet crop residue may be related to cultural differences, but we cannot rule out the possibility of differences in nutritive value. In groundnuts, there is a large difference in the incidence of foliar diseases and their effects on the nutritive value of the crop residue. ICRISAT is breeding for resistance to foliar diseases in groundnuts and this would increase the quality of crop residue by reducing leaf loss. Groundnuts normally retain leaves in the absence of foliar diseases and the nutritive value of the crop residue can be very high.
McAllan: Reed showed differences in digestibility of sorghum grown at different sites and suggested that temperature was important. Barry, in Australia, grew lotus in two areas and the tannin content was markedly higher in the lotus grown on low-fertility soil. Has any work been done on the effect of soil fertility in sorghum?
Reed: I cannot eliminate soil fertility as a factor, although we applied N and P at moderate levels at both sites. Trace elements or soil pH could have had an effect. Certainly temperature and soil effects are important and lead to genotype by environment interactions.
McAllan: It is not surprising that we find large variation in nutritive value within a species if all these conditions are not controlled. How can you breed for something you do not know how to control?
Reed: Pigmentation is a genotypic characteristic. Some genotypes respond by producing greater pigmentation, others do not respond. The non-bird resistant varieties do not produce the pigmentation and the difference between the sites was very low.
Pearce: Plants respond to differences in soil fertility by altering their rate of growth. Slow-growing plants tend to accumulate more secondary metabolites than fast-growing plants. Could this have caused the differences observed by Barry?
McAllan: No, they were absolute effects. The lotus were harvested at maturity.
Pearce: They can be harvested at maturity and still have different rates of growth at a critical point in time.
Nordblom: I want to respond to the question of whether any varieties have been rejected on the grounds of residue quality. In Egypt, a traditional wheat variety is not being replaced by new, improved varieties because of lower quantity and quality of their crop residue. They were improved in terms of the breeders' objective of higher grain yield but rejected by farmers because of quality and quantity of crop residue.
Thomson: We have seen considerable contrast among the four papers presented during this session in the approach used, from the very animal-oriented approach we use at ICARDA to the much more detailed approach of Dr. Pearce, looking at individual components. At ICARDA we will continue the animal work and introduce the laboratory work. I feel it is important to have a solid animal input in this research. We should consider national programmes and universities in addition to international centres when discus-sing research methods. Animal evaluations are often more appropriate where complex and expensive materials are difficult to obtain.
Yilma: I think the search for better quality in crop residues will be possible for the plant breeder as long as it does not compromise agronomic and yield attributes. We have seen that there is variation among cultivars and crop species in straw quality and traits that determine straw quality such as morphology. We have to determine the difference between local cultivars and improved cultivars in quality of crop residue. We need more information on why farmers grow a particular cultivar and the importance they place on feeding crop residue. Can simple and specific criteria, either laboratory or animal, be established that can be incorporated into a breeding programme for looking at straw quality? Alter natively, can we describe an ideal ecotype in terms of morphological proportions, height, maturity, tillering etc? These are things that a plant breeder would ask.
Van Soest: A particular set of factors will be unique to each plant species. We need to determine the limiting factors in relationship to the desired nutritive value of the plant.
McDowell: It seems that the plant breeder is looking for a recommendation on phenotypic characteristics of the plant that can be selected to meet animal needs. On the other side, there are the laboratory techniques that indicate that this may be misleading. We need a close integration between the plant and animal scientist so that we can begin to develop an indexing system that could also include cost factors for evaluating crop varieties. Much more effort is required to develop these indices, including research on the effects of preservation and time of harvest. Dr. Pearce, have you looked at the effects of early harvest and artificial drying on preserving the solubles in residue fractions?
Pearce: We have thought of this but grain drying under our conditions is too expensive. Grain harvest and the length of the period between grain maturity and harvest is determined by weather conditions.
McDowell: Would you recommend picking up the straw the day after combine-harvesting and bailing it to reduce respiration and maintain higher levels of fructans?
Pearce: No, by the time the plant has obtained that low a moisture content it is dead and respiration is finished. But this is not the case for irrigated rice, which is still green and actively growing after harvest.
Ørskov: Reed made a good point, that we need to use different criteria for each objective. We have to consider yield of different parts of the plant and whether animals are allowed to feed selectively. The message to the plant breeder is that there is no golden answer that will apply to everything. We have to be flexible and think about why we are using the method.
Schildkamp: Crop residues are used primarily for maintaining animals. Animal nutritionists should not necessarily look for the highest nutritive value but rather stress that crop residues should not fall below a certain standard below which animals cannot use the material for maintenance.
For further details log on website :
http://www.fao.org/Wairdocs/ILRI/x5495E/x5495e0e.htm#TopOfPage
McDowell: The four papers presented in this session emphasised methodology and genetic and environmental effects on the nutritive value of crop residues. We should focus our discussion on how the methods relate to specific types of residue rather than on the relative merits of the different methods. We heard yesterday about the quantity and potential of crop residues but very little on the effect of environment on the nutritive value of crop residues.
Berhane: Has any high-yielding variety been rejected because of low straw quality?
Ørskov: No, I do not think so because it is only in the last few years that we have realised that there are large differences in straw nutritive value among varieties.
Fussell: In India, a high-yielding millet variety, WC75, had a better acceptance because it had better straw quality.
Gupta: WC75 is improved material from an introduction from Nigeria. We just heard that pearl millet in West Africa, especially Niger, is not accepted by cattle because of poor quality of the stems. However, similar material is used by farmers in India as stored cattle feed. This could be due to cultural differences and not because of the crop. We should not generalise that millet is not a good crop for feeding animals on crop residues.
Reed: The difference between West Africa and India in the use of millet crop residue may be related to cultural differences, but we cannot rule out the possibility of differences in nutritive value. In groundnuts, there is a large difference in the incidence of foliar diseases and their effects on the nutritive value of the crop residue. ICRISAT is breeding for resistance to foliar diseases in groundnuts and this would increase the quality of crop residue by reducing leaf loss. Groundnuts normally retain leaves in the absence of foliar diseases and the nutritive value of the crop residue can be very high.
McAllan: Reed showed differences in digestibility of sorghum grown at different sites and suggested that temperature was important. Barry, in Australia, grew lotus in two areas and the tannin content was markedly higher in the lotus grown on low-fertility soil. Has any work been done on the effect of soil fertility in sorghum?
Reed: I cannot eliminate soil fertility as a factor, although we applied N and P at moderate levels at both sites. Trace elements or soil pH could have had an effect. Certainly temperature and soil effects are important and lead to genotype by environment interactions.
McAllan: It is not surprising that we find large variation in nutritive value within a species if all these conditions are not controlled. How can you breed for something you do not know how to control?
Reed: Pigmentation is a genotypic characteristic. Some genotypes respond by producing greater pigmentation, others do not respond. The non-bird resistant varieties do not produce the pigmentation and the difference between the sites was very low.
Pearce: Plants respond to differences in soil fertility by altering their rate of growth. Slow-growing plants tend to accumulate more secondary metabolites than fast-growing plants. Could this have caused the differences observed by Barry?
McAllan: No, they were absolute effects. The lotus were harvested at maturity.
Pearce: They can be harvested at maturity and still have different rates of growth at a critical point in time.
Nordblom: I want to respond to the question of whether any varieties have been rejected on the grounds of residue quality. In Egypt, a traditional wheat variety is not being replaced by new, improved varieties because of lower quantity and quality of their crop residue. They were improved in terms of the breeders' objective of higher grain yield but rejected by farmers because of quality and quantity of crop residue.
Thomson: We have seen considerable contrast among the four papers presented during this session in the approach used, from the very animal-oriented approach we use at ICARDA to the much more detailed approach of Dr. Pearce, looking at individual components. At ICARDA we will continue the animal work and introduce the laboratory work. I feel it is important to have a solid animal input in this research. We should consider national programmes and universities in addition to international centres when discus-sing research methods. Animal evaluations are often more appropriate where complex and expensive materials are difficult to obtain.
Yilma: I think the search for better quality in crop residues will be possible for the plant breeder as long as it does not compromise agronomic and yield attributes. We have seen that there is variation among cultivars and crop species in straw quality and traits that determine straw quality such as morphology. We have to determine the difference between local cultivars and improved cultivars in quality of crop residue. We need more information on why farmers grow a particular cultivar and the importance they place on feeding crop residue. Can simple and specific criteria, either laboratory or animal, be established that can be incorporated into a breeding programme for looking at straw quality? Alter natively, can we describe an ideal ecotype in terms of morphological proportions, height, maturity, tillering etc? These are things that a plant breeder would ask.
Van Soest: A particular set of factors will be unique to each plant species. We need to determine the limiting factors in relationship to the desired nutritive value of the plant.
McDowell: It seems that the plant breeder is looking for a recommendation on phenotypic characteristics of the plant that can be selected to meet animal needs. On the other side, there are the laboratory techniques that indicate that this may be misleading. We need a close integration between the plant and animal scientist so that we can begin to develop an indexing system that could also include cost factors for evaluating crop varieties. Much more effort is required to develop these indices, including research on the effects of preservation and time of harvest. Dr. Pearce, have you looked at the effects of early harvest and artificial drying on preserving the solubles in residue fractions?
Pearce: We have thought of this but grain drying under our conditions is too expensive. Grain harvest and the length of the period between grain maturity and harvest is determined by weather conditions.
McDowell: Would you recommend picking up the straw the day after combine-harvesting and bailing it to reduce respiration and maintain higher levels of fructans?
Pearce: No, by the time the plant has obtained that low a moisture content it is dead and respiration is finished. But this is not the case for irrigated rice, which is still green and actively growing after harvest.
Ørskov: Reed made a good point, that we need to use different criteria for each objective. We have to consider yield of different parts of the plant and whether animals are allowed to feed selectively. The message to the plant breeder is that there is no golden answer that will apply to everything. We have to be flexible and think about why we are using the method.
Schildkamp: Crop residues are used primarily for maintaining animals. Animal nutritionists should not necessarily look for the highest nutritive value but rather stress that crop residues should not fall below a certain standard below which animals cannot use the material for maintenance.
For further details log on website :
http://www.fao.org/Wairdocs/ILRI/x5495E/x5495e0e.htm#TopOfPage
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