Digestibility of Nitrogen and Amino Acids in Soybean Products for Pigs

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Abstract
The apparent digestibilities of nutrients in six soybean products were determined at the end of the small intestine and over the total digestive tract of barrows. In trial 1, 44 and 48.5% protein soybean meal (44 SBM1, 48 SBM) and a 50% protein soybean flour (SF) were evaluated. In trial 2, another 44% soybean meal (44 SBM2), extruded whole soybeans (EWB) and an extruded mixture (EM) consisting of 44% soybean meal and whole soybeans were evaluated. Both trials were conducted as replicated 3 × 3 Latin squares. All diets were corn starch-based with the soybean products providing .70% lysine. In trial 1, only small differences in N and amino acid digestibilities were found among the soybean products. Measured at the end of the small intestine, digestibilities of most amino acids were higher (P<.05) for SF than for 48 SBM, while 44 SBM1 had intermediate digestibilities. The greatest difference for the essential amino acids occurred for lysine between SF (87.5%) and 48 SBM (83.5%). In trial 2, EWB had lower (P<.05) digestibilities of N and all essential amino acids at the end of the small intestine than did 44 SBM2 or EM. Trypsin inhibitor content was high for EWB, indicating inadequate heat treatment during processing. In general, 44 SBM2 had higher N and amino acid digestibilities than EM, but differences were small. In trial 1, 48 SBM and in trial 2, EWB, were the most degraded in the large intestine. These products also had lower apparent biological value, supporting the contention that N and amino acids disappearing in the large intestine have little nutritional value for the pig, and that digestibilities determined at the end of the small intestine more accurately measure differences in amino acid absorption among feedstuffs than total tract digestibilities. At the end of the small intestine, apparent digestibilities of lysine and methionine for all products (excluding EWB) averaged 86 and 89%; threonine (75%) and tryptophan (80%) were the least digestible essential amino acids.

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