Composition of bacteria harvested from the liquid and solid fractions of the rumen of sheep as influenced by feed intake

Abstract

A study was conducted to determine the effect of the feed intake on the chemical composition of bacteria associated with the solid (solid-associated bacteria; SAB) and liquid (liquid-associated bacteria; LAB) fractions of rumen digesta, the digestive passage kinetics and their relationships. Whole rumen contents were sampled after a period of continuous infusion of 15NH3 from four ruminally-cannulated wethers provided successively with a hay–concentrate diet (2: 1 w/w on a DM basis) at two rates of feed intake: 40 and 80 g DM/kg body weight0·75. SAB had a higher content of organic matter and total lipids (P < 0·001) and a similar N content as compared with LAB. The concentration of purines and 15N was lower (P = 0·011 and P < 0·001 respectively) in SAB than LAB, whereas the opposite was observed for the concentration of amino acids (mg/g DM; P = 0·031). An increase in feed intake produced an increase in the N (P = 0·034) and purine (P = 0·066) concentrations in bacteria and a decrease (P = 0·033) in their amino acid concentrations. Significant increases of rumen outflow rates of liquid and particles were also observed with increased feed intake. Rates of rumen outflow showed positive and negative linear relationships (P < 0·001) with the purine: N ratio and the proportion of amino acid on total N of bacteria respectively. SAB contained significantly higher proportions of leucine, isoleucine, lysine and phenylalanine and lower proportions of alanine, methionine and valine than LAB. The increase in feed intake also induced significant changes in the amino acid profile of bacteria, increasing arginine and methionine and decreasing alanine and glycine proportions. Results show that the outflow rate of rumen contents is a major factor in determining the proportion of nucleic acids and protein in rumen bacteria and explains some of the differences observed between LAB and SAB.