INFLUENCE OF DIETARY RUMEN-DEGRADABLE PROTEIN ON BACTERIAL-GROWTH IN THE RUMEN OF SHEEP RECEIVING DIFFERENT ENERGY-SOURCES

An experiment with ruminally cannulated sheep was undertaken to determ ine how the dietary energy source affected the ability of rumen fermen tation to respond to pre-formed amino acids compared to non-protein ni trogen in the diet. Four sheep received diets of grass hay (H) or mola ssed sugar beet pulp (BP) supplemented with 9.3 g N kg(-1) DM added ei ther as urea (U) or casein (C) in a 4 X 4 latin square with a factoria l design. Measurements of rumen fermentation, microbial numbers and pu rine excretion were made in the third week of each 21-day period. Rume n total volatile fatty acids concentrations were higher with the BP di ets (130 vs. 91 mM; P < 0.01), and the molar proportion of propionate was lower and that of butyrate was higher with BP, but the nitrogen so urce had no significant effect on volatile fatty acids. Rumen ammonia concentrations were lower with the BPU diet than with HU, and were dec reased when casein replaced urea in both diets (11.1, 19.4, 5.0 and 10 .2 mM for BPU, HU, BPC and HC respectively; P < 0.01). The source of n itrogen had no influence on microbial numbers or yield in the hay diet s, but total viable bacteria more than doubled in response to pre-form ed amino acids with BP (16.9 vs. 7.4 X 10(8) ml(-1); P < 0.01). Microb ial protein flow, calculated from urinary purine excretion, also incre ased in response to casein but only with BP (13.5 vs. 7.4 g N per day) . The rate of loss of diets from nylon bags was much more rapid (P < 0 .01) with BP than with hay, and was unaffected by dietary nitrogen sou rce (P > 0.05). A microbial response to rumen-degradable protein there fore occurred only with the more rapidly degraded BP diet, consistent with the idea that rumen microbial growth responds to pre-formed amino acids only when the energy source is fermented rapidly.