Effect of dietary free lipid on anaerobic fungi and digestion in the rumen of sheep

Six 2-year-old Border Leicester Merino wethers were offered diets containing 75% mixed wheaten and lucerne hays (2:1), 10% rolled barley, and 15% unprotected and protected cottonseed kernel supplement mixed to provide 2.1, 4.0, or 5.9% free lipid in a total lipid content of 7% of the dry matter. Rumen anaerobic fungi were undetectable with 5.9% free lipid but were present at normal levels with 4 and 2.1% free lipid. In association with the absence of anaerobic fungi, protozoal numbers tended to be lower but bacterial counts increased. Lignin degradation was substantially reduced (by 45–60%) and digestion of hemicelluloses (by 7%) and cellulose (by 9%) was also reduced. As rumen digestion of hemicelluloses was reduced by 40% and of cellulose by 14%, digestion in the large intestine partially compensated for the effect of free lipid on rumen digestion. Acetate:propionate ratios decreased as free lipid content increased but the calculated reduction in methane emissions suggested that they only partly compensated for the reduction in energy digestion caused by the free lipid. It was concluded that there is a threshold level, >4% but <6%, for polyunsaturated free lipid above which rumen anaerobic fungi cannot be maintained and fibre digestion will be reduced.