Effect of avoparcin on rumen fermentation and duodenal nutrient flow in sheep.

: In a digestive-physiological experiment series, the effect exerted by avoparcin on rumen fermentation and on the ruminal digestion of nutrients was studied in wethers provided with rumen and duodenal cannulas, as a function of the composition of feed as substrate. Three control (I, II, III) groups containing different amounts of rumen degradable protein (RDP) and nonstructural carbohydrate (NSC) were formed (composition of diet: group I, 74% RDP and 38% NSC; group II, 57% RDP and 32% NSC; group III, 48% RDP and 23% NSC). The feeding of control diets was followed by the administration of experimental diets containing avoparcin (groups I+A, II+A, and III+A). The dose of avoparcin was 0.75 mg/kg body weight. Irrespective of the RDP and NSC content of the feed, avoparcin reduced the molar ratio of acetic acid and increased that of propionic acid, decreased the acetic acid/propionic acid ratio, and increased the molar ratio of isobutyric acid. Ammonia concentration of the ruminal fluid was significantly lower in group I+A than in the corresponding control group (I). Avoparcin supplementation of diet III improved the apparent digestibility of organic matter from 52.9% to 56.4%. When added to a ration of high NSC and RDP content (I), avoparcin decreased the true digestibility of organic matter from 77.0 to 72.5%. Compared to diet III as well as to diets II and III, avoparcin significantly increased the ruminal degradation of cellulose and hemicellulose, respectively. Avoparcin supplementation of the diet significantly decreased the microbial N content of the duodenal chymus irrespective of the NSC and RDP content of the diet. In group I+A, the amount of dietary N passed from the rumen into the duodenum in 24 h was significantly higher (7.1 g/day vs. 2.7 g/day). In wethers fed the diet of the lowest NSC and RDP content (III), avoparcin supplementation (III+A) increased the apparent digestibility of N in the rumen. In contrast, in wethers fed diets of higher RDP and NSC content (I and II) the true ruminal digestibility of N decreased. Irrespective of the RDP and NSC content of the diet, avoparcin supplementation significantly reduced the efficiency of microbial protein synthesis. The enhanced propionic fermentation induced by the administration of avoparcin allows more efficient utilization of the dietary energy. The higher ratio of undergraded, bypass protein reaching the duodenum provides the animal with a protein source degraded and utilized directly in the small intestine. The results support the observation that avoparcin increases the body weight gain of animals during fattening.