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 nutrie nts 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 pr otein (RDP) and nonstructural carbohydrate (NSC) were formed (composit ion of diet: group I, 74% RDP and 38% NSC; group II, 57% RDP and 32% N SC; group III, 48% RDP and 23% NSC). The feeding of control diets was followed by the administration of experimental diets containing avopar cin (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, avop arcin reduced the molar ratio of acetic acid and increased that of pro pionic acid, decreased the acetic acid / propionic acid ratio, and inc reased the molar ratio of isobutyric acid. Ammonia concentration of th e ruminal fluid was significantly lower in group I+A than in the corre sponding control group (I). Avoparcin supplementation of diet III impr oved 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 dec reased the true digestibility of organic matter from 77.0 to 72.5%. Co mpared to diet III as well as to diets II and III, avoparcin significa ntly increased the ruminal degradation of cellulose and hemicellulose, respectively. Avoparcin supplementation of the diet significantly dec reased 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 dieta ry 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 lowe st NSC and RDP content (III), avoparcin supplementation (III+A) increa sed the apparent digestibility of N in the rumen. In contrast, in weth ers fed diets of higher RDP and NSC content (I and II) the true rumina l digestibility of N decreased. Irrespective of the RDP and NSC conten t of the diet, avoparcin supplementation significantly reduced the eff iciency of microbial protein synthesis. The enhanced propionic ferment ation induced by the administration of avoparcin allows more efficient utilization of the dietary energy. The higher ratio of undegraded, by pass protein reaching the duodenum provides the animal with a protein source degraded and utilized directly in the small intestine. The resu lts support the observation that avoparcin increases the body weight g ain of animals during fattening.