AVOPARCIN AND MONENSIN EFFECTS ON DIGESTIVE FUNCTION IN COWS FED A HIGH FORAGE DIET

Three non-lactating cows with ruminal, duodenal, and ileal cannulas we re used in a 3 x 3 Latin square design to evaluate the effect of oral avoparcin and monensin on fibre, starch, and nitrogen (N) digestion. C ows were fed three high forage diets; the control diet had no additive , and the others contained 33 mg kg(-1) DM of either avoparcin or mone nsin. Neither additive affected apparent total tract digestibilities o f organic matter, fibre, starch or N. Avoparcin and monensin reduced t he ruminal breakdown of fibre, starch and dietary N. Efficiency of bac terial N synthesis was unchanged, and passage of bacterial N to the sm all intestine was not modified. Compared with the control, additives c aused a greater proportion of feed starch, N, and total amino acid to be digested in the small intestine than in the rumen. Diets containing avoparcin or monensin increased availability of total essential amino acid in the small intestine compared with that of control diets. Post -ruminal digestion of fibre was greater for diets supplemented with an tibiotics, because the cows compensated digestion in the lower foresto mach.