A highly palatable feedstuff, soybean meal is characterised by a high protein content (from 43 to 53%) and a low crude fibre content (less than 3% for the dehulled soybean meals). It has a very good amino acid balance and contains high amounts of lysine, tryptophane, threonine and isoleucine, which are often lacking in cereal grains. However, the concentration of cystine and methionine are suboptimal for monogastric animals, and methionine supplementation is necessary (McDonald et al., 2002). Amino acid digestibility is also very high (more than 90% for lysine in pigs and poultry) (Sauvant et al., 2004).
Soybean meal contains oligosaccharides such as raffinose and stachyose that cannot be digested by monogastric animals, due to the lack of a specific endogenous alpha-galactosidase. Raffinose and stachyose can cause flatulence and diarrhoea that may increase the digesta passage rate, and decrease digestion and absorption of dietary nutrients. In poultry, these oligosaccharides have been shown to decrease nitrogen-corrected true metabolizable energy, fibre digestion, and transit time (Parsons et al., 2000; Coon et al., 1990; Rackis, 1975 and Reddy, 1984 cited by Zuo et al., 1996). Low-oligosaccharide soybean meals are now available.
About 60-70% of phosphorus in soybean meal is bound to phytic acid, which is nutritionally unavailable to monogastric animals and reduces the availability of P and other minerals (Wilcox et al., 2000). Supplementation with inorganic phosphorus is required, and the addition of phytase may alleviate the problem. Low-phytate soybeans are under development but their productivity is still low (Waldroup et al., 2008).
Soybean meal is a poor source of B vitamins and lack of B vitamin supplementation in soybean meal-based diets may cause reproductive and performance problems in sows, older pigs and hens (McDonald et al., 2002).