Effects of avoparcin on ruminal propionate production and amino acid degradation in sheep fed high and low fiber diets.

The effect of the antibiotic, avoparcin, on ruminal propionate production and amino acid degradation was investigated by using four rumen fistulated wethers. Wethers were fed each of four diets during 28-day periods in a 4 X 4 Latin square design. Diets were high fiber (HF), high fiber plus 50 ppm avoparcin (HF-AVO), low fiber (LF) and low fiber plus 50 ppm avoparcin (LF-AVO). Total digestible energy intake (1.25 X maintenance) and crude protein intake for each animal were similar for all diets. Propionate production was determined each period from all animals by using a single injection of [1-14C]propionate and polyethylene glycol. Avoparcin decreased total volatile fatty acid concentration, increased ruminal pH and molar proportions of propionate on both HF and LF diets. Daily propionate production and pool turnover rate increased in sheep fed LF-AVO. Similar changes were noted in HF-AVO animals, although they were not statistically significant. Avoparcin decreased ruminal ammonia (50 vs. 45 mg/100 ml) and increased alpha amino nitrogen (8.6 vs. 14.3 mM) in LF-fed sheep. In vitro studies demonstrated a decreased degradation of crystalline amino acids by mixed populations of rumen microorganisms from sheep receiving LF diets containing avoparcin. Avoparcin appears to modify rumen fermentation by increasing propionate production and inhibiting protein or amino acid degradation.