Effect of yeast culture on rumen fermentation, microbial protein flow from the rumen and live-weight gain in bulls given high cereal diets

Four groups each of eight Limousin X Friesian bulls, average weight 344 kg, were offered individually ad libitum diets comprising: 920 g rolled barley, 50 g cane molasses, 15 g urea and 15 g minerals/vitamins mix per kg or 867 . 5 g rolled barley, 50 g cane molasses, 7 . 5 g urea, 60 g soya-bean meal and 15 g minerals/vitamins mix per kg, plus or minus yeast culture (YC, 1 . 5 kg/t) (all weights fresh weights), until slaughter (460 kg). Rumen samples were obtained by stomach tube. YC decreased rumen ammonia concentrations (116 . 7, 56 . 0, 74 . 3, 45 . 7 (s.e.d. 19 . 40) mg/l for barley/urea, barley/urea/YC, barley/soya-bean meal and barley/soya-bean meal/YC, respectively). This was associated with increases (P <0 . 05) in both the total number of bacteria (2 . 3, 4 . 2, 3 . 5, 6 . 8 (s.e.d. 1 . 06) X 10(9) per ml, respectively) and the protein content of rumen fluid (7 . 7, 9 . 6, 8 . 7, 9 . 0 (s.e.d. 0 . 59) g/l, respectively). However, the flow of microbial protein from the rumen as estimated from the appearance of purine derivatives in the urine was unaffected (131, 130, 129, 147 (s.e.d. 12 . 6) g N per day, respectively). Thus although the animals responded to the undegradable rumen protein supplied by the soya-bean meal there was no significant response in live-weight gain with YC (1 . 28, 1 . 31, 1 . 42, 1 . 48 (s.e.d. 0 . 081) kg/day, respectively).