Digestion kinetics of protein feed ingredients and their impact on nitrogen utilization in growing pigs

The velocity of nitrogen release impacts nitrogen utilization efficiency and animal growth. Yet the crucial junctures linking nitrogen release and utilization from diverse feed ingredients to digestion kinetic parameters remain obscure. This study aimed to evaluate and subdivide in vitro protein digestion characteristics (such as release rate of free amino acids [FAA]) of three commonly used protein feed ingredients and to correlate the characteristics with nitrogen utilization in vivo. Ten soybean meals (SBM), eleven cottonseed meals (CM), and nine rapeseed meals (RM) were selected for in vitro study. After predigested for 120 min, the characteristics were evaluated within the small intestine stage. The release rate of total FAA from CM was significantly lower than SBM or RM during the fast-release phase (0 to 20 min). The total FAA release rate in SBM was substantially lower than others during the slow-release phase (20 to 140 min). In the in vivo experiment, cannulated growing barrows (<i>n</i> = 24; BW = 43.2 ± 2.8 kg) were allocated to 4 treatments (6 replicates for each treatment and 1 pig for each replicate). Three diets, each containing an equal concentration of crude protein (CP) supplied by either SBM, CM, or RM, fulfilled the requirements for essential amino acids. Nitrogen-free control diet was also included. There were strong correlations between plasma urea nitrogen (PUN) and the release rate of total FAA in both the fast- and slow-release phases (<i>P</i> < 0.01). In summary, differences in in vitro digestion kinetic characteristics among SBM, CM, and RM were determined. It is believed that separating the total small intestine phase of total FAA release into the fast- and slow-release phases as the evaluation standard of protein feed ingredients and diets might effectively reveal their character and can be related to the PUN concentration of pigs.