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 square experiment with 30-day periods, four rumen fistulated steers were offered 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 Rumen samples were withdrawn on 2 days per period at 08:00 (before feeding), 12:00, 16:00, 20:00 and 24:00 h. Mean rumen pH, liquid outflow rate, molar acetate proportion and the acetate to propionate ratio were lower (P < 0.001) and the concentrations of total volatile fatty acids (VFA), ammonia and bacterial nitrogen and molar proportions of butyrate and isoacids were higher (P < 0.001) on 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 increased (P < 0.1) VFA subsequent to the second feed. Degradability of cotton was lower on Diet H, but unaffected by yeast culture. In a replicated Latin square experiment with 34-day periods, 12 non-fistulated steers were offered the experimental diets. Digestibility of dry matter, organic matter and crude protein (CP) were higher (P < 0.001) and acid detergent fibre lower (P < 0.05) on Diet H. Digestibility of CP was decreased (P < 0.05) when yeast was included in Diet L, but not affected when yeast was added to Diet H. It is concluded that dietary inclusion of yeast culture had a small influence on rumen fermentation parameters and in vivo digestibility, but that its effect on nitrogen metabolism appeared to be dependent on the nitrogen content of the basal diet.