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.