Digestibility is a very useful index of the energy content of ruminant feeds, but cheaper and quicker laboratory methods are required as an alternative to the ultimate measure of in vivo digestibility using animals. These methods involve either prediction of digestibility from chemical composition or in vitro and in situ simulation of the digestion process. This review presents a range of chemical and in vitro techniques for predicting digestibility, together with an assessment of their advantages and limitations, particularly the degree to which they account for the sources of variation in in vivo digestibility in ruminants. In situ digestion of feed samples in the actual rumen environment is probably the most accurate of the non in vivo procedures, but is not suitable for routine application. Thein vitro gas production technique offers no advantages in prediction of total tract digestibility, but is useful for screening cereal grains for rate of starch hydrolysis in the rumen. The preferred procedure for routine laboratory prediction of digestibility is the pepsin-cellulase technique, provided amylase is included or high temperature digestion is used for samples high in starch content. Prediction of digestibility from chemical composition is not recommended. The optical technique of near infrared reflectance spectroscopy can be calibrated against any of the methods outlined in this review, and is unsurpassed in terms of speed and repeatability. Direct NIR prediction of in vivo digestibility is also possible, but is limited by the lack of adequate numbers of feed samples with known in vivo values. Future work should be aimed at filling this gap and also improving the accuracy of laboratory methods for predicting the digestibility of low quality feeds.
The total water-soluble and water-insoluble non-starch polysaccharides (NSP) were determined in seven soyabean meals processed in Australia, three soyabean meals processed in the U.S.A., and one sunflower meal processed in Australia. Sunflower meal had a higher content of total NSP than any of the soyabean meals, due mainly to increased concentrations of cellulose and xylose in the insoluble NSP and uronic acids in the soluble NSP. Galactose and fucose concentrations were much greater in the insoluble NSP of soyabean meals. Soyabean meals processed in the U.S.A. had lower concentrations of total NSP, cellulose and xylose than equivalent meals processed in Australia. Broiler chickens fed diets containing soyabean meal as the sole dietary protein concentrate grew significantly more poorly than broilers fed isoenergetic and isonitrogenous diets in which approximately 25% of the soyabean meal was replaced with sunflower meal. There was a significant (P < 0.05) negative correlation (r2 = 0.57) between the water-soluble xylose content of the soyabean meals and the improvement in growth observed with sunflower meal supplementation. Multi-enzyme preparations designed to act on soyabean meal NSP substrates failed to induce any improvement in the growth of broilers fed soyabean meal diets. The poor performance of broilers fed the soyabean meal diets did not appear to be related to inadequate processing or to the NSP content or composition of the soyabean meals. Measurement of free sugars in the supernatant of the digesta in the ileum indicated that the stachyose derived from the oligosaccharides of soyabean meals appeared to exert an antinutritive effect when soyabean meal was present at high concentrations as the sole protein concentrate in broiler diets.
Two studies using broiler chicks and one using adult White Leghorn roosters were conducted to determine the influence of stachyose and raffinose (alpha-galactosides of sucrose) present in soybean meal (SBM) on the nutritional value of the meal. In Experiment 1, the addition of four levels (0, .05, .10, or .20 g/kg) of alpha-galactosidase with and without 1 g/kg of invertase to a corn-SBM diet had no effect on weight gain, feed efficiency, protein digestibility, or the digestible energy value of the feed when fed to broiler chicks. However, both enzymes decreased (P < .001) dietary AMEn. In Experiment 2, ethanol extraction and incubation of SBM with alpha-galactosidase decreased the concentrations of the alpha-galactosides of sucrose in SBM from 6.59 to .81 and 1.43%, respectively. However, when broiler chicks were fed semi-purified diets containing SBM, ethanol-extracted SBM, water-incubated SBM, or water plus alpha-galactosidase-incubated SBM, no improvements in weight gain, feed efficiency, or apparent protein digestibility were observed. There was also no improvement in TMEn when the above meals were precision fed to adult White Leghorn roosters (Experiment 3). These results indicate that the removal of up to approximately 90% of the alpha-galactosides of sucrose has no beneficial effect on the nutritional value of SBM for chickens.
Broiler chickens fed diets containing soyabean meal as the sole dietary protein concentrate consistently showed poor growth in a number of feeding trials. Soyabean meal processed in Australia gave inferior performance to soyabean meal processed in the U.S.A. Growth was improved at similar dietary energy and protein concentrations by replacing approximately 25% of the soyabean meal with either sunflower meal, cottonseed meal or rapeseed meal but not by supplementing the soyabean meal diet with lysine, methionine and cystine. The responses of broilers to the soyabean meal diet were unaffected by the cereal component of the diet and the beneficial responses observed when sunflower meal replaced part of the soyabean meal were not related to the amino acid composition or digestibility of the diets.
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