Postruminal digestion and absorption of nitrogenous components.

Digesta containing microbial protein, undegraded (bypass) feed protein, some free amino acids and oligopeptides, and microbial nucleic acids passes from the rumen-reticulum through the omasum to the lower gut (abomasum and small intestine). The rate of neutralization of abomasal digesta flow is slower in ruminants than in nonruminants, and activation and peak activity of the pancreatic proteases is delayed to the mid jejunum. The overall apparent digestibility in the small intestine of proteins and microbial or exogenous nuclei acids is 60-70% and 75-85% respectively. Absorption mechanisms in ruminants for protein and nucleic acid digestion end products appear similar to those described in nonruminants, but the highest rate of amino acid uptake was found in the mid to lower ileum. Essential amino acids (Met, THr, Val, Ile, Leu, Phe, Lys, His, Arg) are absorbed faster than nonessential amino acids (Gly, Glu, Asp, Ser, Pro, Cys, Tyr). After feeding, plasma amino acids remain unchanged or decline in a mature ruminant, while in nursing lambs, plasma free amino acids levels increase after a meal.