THE INFLUENCE OF THE BASAL DIET ON THE EFFECTS OF YEAST CULTURE ON RUMINAL FERMENTATION AND DIGESTIBILITY IN STEERS

The effects of dietary inclusion of yeast culture (Yea-Sacc) on rumen fermentation and digestibility in vivo were examined. In a Latin squar e experiment with 30-day periods, four rumen fistulated steers were of fered 0 or 10 g yeast culture per animal daily in a low quality (4 kg barley straw plus 2.5 kg barley/soyabean meal mixture per day) (L) or high quality (2 kg barley straw plus 8 kg barley/soyabean meal mixture per day) (H) diet. The diets were offered in two feeds at 08:00 h and 16:00 h with no refusals. Rumen samples were withdrawn on 2 days per period at 08:00 (before feeding), 12:00, 16:00, 20:00 and 24:00 h. Mea n rumen pH, liquid outflow rate, molar acetate proportion and the acet ate to propionate ratio were lower (P < 0.001) and the concentrations of total volatile fatty acids (VFA), ammonia and bacterial nitrogen an d molar proportions of butyrate and isoacids were higher (P < 0.001) o n Diet H. Rumen ammonia concentration was not affected when yeast was included in Diet L but was reduced (P < 0.05) when yeast was added to Diet H. Inclusion of yeast culture in Diet L decreased (P < 0.05) VFA 4 h after the second feed, but inclusion of yeast culture in Diet H in creased (P < 0.1) VFA subsequent to the second feed. Degradability of cotton was lower on Diet H, but unaffected by yeast culture. In a repl icated Latin square experiment with 34-day periods, 12 non-fistulated steers were offered the experimental diets. Digestibility of dry matte r, organic matter and crude protein (CP) were higher (P < 0.001) and a cid detergent fibre lower (P < 0.05) on Diet H. Digestibility of CP wa s decreased (P < 0.05) when yeast was included in Diet L, but not affe cted when yeast was added to Diet H. It is concluded that dietary incl usion of yeast culture had a small influence on rumen fermentation par ameters and in vivo digestibility, but that its effect on nitrogen met abolism appeared to be dependent on the nitrogen content of the basal diet.