AVOPARCIN AND MONENSIN EFFECTS ON THE DIGESTION OF NUTRIENTS IN DAIRY-COWS FED A MIXED DIET

An in vivo study was carried out to evaluate the effect of avoparcin on fiber, starch and nitrogen (N) digestion compared with monensin. Three dairy lactating cows with ruminal, duodenal and ileal cannulas were used in a 3 x 3 Latin square design. Cows were fed three diets; the control diet had no additive, and the others contained 33 ppm of either avoparcin or monensin-sodium. There were no treatment effects on ruminal pH and total volatile fatty acid levels; molar proportion of propionate tended to be higher (27.9-29.9 vs. 23.7%) and of butyrate tended to be lower (9.9-12.9 vs. 15.4%) for the antibiotic-supplemented diets. Also, both additives decreased ruminal ammonia N concentration (77.2-105.6 vs. 164.1 mg L(-1)). The supply of avoparcin or monensin reduced the ruminal breakdown of fiber (36.1 and 37.1 vs. 45.9%), starch (57.4 and 54.7 vs. 63.6%), and dietary N (59.5 and 53.6 vs. 67.3%); efficiency of bacterial protein synthesis was unchanged. Passage of bacterial N to the small intestine was not modified; flows of fiber, starch, nonammonia N and dietary N were improved by the both additives. In comparison with the control, monensin caused a greater quantity of glucose (2100 vs. 1500 g d(-1)) and total amino acids (AA) (1381 vs. 1047 g d(-1)) to be digested in the small intestine; avoparcin had a less pronounced effect on the intestinal absorption of glucose (1700 g d(-1)) and AA (1152 g d(-1)). Only monensin increased the plasma glucose levels (3.6 vs. 3.1 mM). Both additives did not affect total tract digestibilities of organic matter, fiber, starch, and N. Postruminal digestion of fiber was greater (16.0 vs. 7.3%) for antibiotic-supplemented diets, compensating for the lower forestomach digestion. In comparison with monensin dietary avoparcin had a less pronounced effect on the alteration of nutrient digestion.