Abstract:
Proc. Amt. Sot. Anim. Prod. 1994 Vol. 20 RUMEN AMMONIA AND BACTERIAL PROTEIN KINETICS IN THE DAIRY COW W.J. WALES and D.W. DELLOW Dept of Agriculture, Kyabram Dairy Research Centre, ISIA, Kyabram, Vic. 3620 Determining the efficiency of utilisation of herbage protein and the value of supplements requires an understanding of rumen ammonia and bacterial protein kinetics. This information is not available for dairy cows eating fresh herbage. Rumen ammonia and bacterial protein kinetics were estimated by reference to a continuous intraruminal infusion of *'N-NH&l (Nolan and Leng 1972) and chemical analyses of rumen contents in 6 lactating cows in a crossover design. Cows were fed either (on a dry matter basis), (i) white clover (WC), (ii) 60% white clover and 40% maize silage (WC+MS) or (iii) 60% white clover, 30% maize silage and 10% maize grain (WC+MS+MG). Table 1. N intake (g/day), digestible OM intake (kg/day), net ammonia production (g N/day), proportion of bacterial non-ammonia-N (NAN) from ammonia and rumen pool sizes of OM (kg) and components of nonammonia-N (g) Digestible organic matter intakes on the 2 mixed diets were higher, but N intakes were lower. Net ammonia production were similar for all diets, but as a proportion of N intake it was lowest in the WC fed cows (60%) and highest in the WC+MS fed cows (74%). The proportion of bacterial non-ammonia-N (NAN) derived from ammonia-N de fzovo ranged from 67 to 76%. By difference, 24 to 33% of this protein arose directly from dietary amino acids and peptides. Also, when the cows were fed WC, the major proportion of rumen digesta NAN was undigested dietary protein (60%) but when fed WC+MS or WC+MS+MG the bacterial NAN proportion was higher (57% and 59% of rumen NAN, respectively). This is largely due to the higher concentration of crude protein in the WC diet and indicated that a higher level of dietary protein is likely to bypass the rumen fermentation in cows fed WC alone. There was no difference in the ratio of bacterial protein:OM present in the rumen. This was constant at II%, suggesting little difference in bacterial protein outflow from the rumen. The cows fed WC produced less NH$nit intake of N and had a higher rumen content of undigested dietary N reflecting a higher protein concentration in the diet. As supplements were offered, bacterial protein became a more important source of rumen NAN. NOLAN, J.V. and LENG, R.A. (1972). Br. J. Nutr. 27: 177-94. 377
