THE EFFECT OF PARTIAL REPLACEMENT OF GRASS-SILAGE WITH FRESH GRASS ONRUMEN FERMENTATION CHARACTERISTICS AND RUMEN OUTFLOW RATES IN CATTLE

An experiment was designed to examine the effects of partially replaci ng extensively fermented grass silage with varying proportions of fres h grass (0, 0.33, 0.67 and 1.00) on rumen fermentation, degradation of dry matter (DM) and rate of outflow of liquid and particulate phases from the rumen with four mature Limousin steers. The fresh grass had a higher pH and water-soluble carbohydrate and lower ammonia-N and lact ic acid concentrations than the silage. Partial replacement of silage with fresh grass resulted in a reduction in rumen ammonia concentratio n, and in the proportion of rumen propionate. i-butyrate and n-valerat e and an increase in the proportion of rumen acetate and in both the p articulate and liquid outflow rates from the rumen. These changes in r umen fermentation parameters could account for increases in animal per formance in situations in which grass silage is partially replaced wit h fresh grass.