DIGESTA FLOW AND RUMEN MICROBIAL PROTEIN-PRODUCTION IN EWES GRAZING PERENNIAL RYEGRASS

Herbage organic matter (OM) intakes and the abomasal flows of OM, non-ammonia nitrogen (NAN) and microbial nitrogen (MN) were estimated in ewes grazing perennial ryegrass during lactation in spring/summer (Group 1) or in autumn after the end of lactation (Group 2). On average, ewes in Group 1 consumed 804 g OM/day more than ewes in Group 2, while their daily abomasal flows of OM, NAN and MN were, respectively, 372, 28.8 and 26.7 g/day more than those in Group 2 ewes. The regression equations relating the flows of digesta or digesta components to OM intake were significantly different between the two groups of ewes. Rumen ammonia and volatile fatty acid levels were significantly higher in Group 1 ewes, though ewes in Group 2 had significantly higher molar proportions of acetate and significantly lower proportions of butyrate and especially propionate. Ewes in Group 1 had significantly faster CrEDTA fractional disappearance rates (FDR) (0.1474 h(-1)) than ewes in Group 2 (0.0847 h(-1)). The efficiency of microbial protein synthesis in Group 1 ewes was consistent with published values and was almost twice that estimated in Group 2 ewes. Results are presented to suggest that the observed differences between the two groups or ewes are not due to an effect of lactation, but to differences in the composition of the consumed herbage in autumn compared with spring/summer. It is argued that the observed differences in rumen fermentation pattern between the two groups of ewes reflect underlying differences in the water-soluble carbohydrate content of the consumed herbage. The inclusion, in multiple regression analyses, of either rumen propionate or rumen acetate:propionate ratio, together with OM intake explained approximately 0.80-0.85 of the variance in NAN or MN flows, reduced residual standard deviations compared with fitting OM intake alone, and removed the difference between the two groups of ewes. Inclusion of the CrEDTA FDR contributed no further explanation of the variance in the above flows.