Effect of supplementary energy and protein feeding on the true digestion of grass-silage organic matter, cell walls and nitrogen estimated by the combined synthetic-fibre-bag method in cows

Four ruminally and duodenally cannulated non-lactating Finnish Ayrshire cows were used in a balanced 4 × 4 Latin square to study the effect of different concentrate supplements on the true partial and total-tract digestion (TTD) of grass silage, estimated by using the combined rumen-bag-intestinal-bag method. The cows were fed, at maintenance level, grass silage alone or supplemented with good-quality ground barley, ground barley and rapeseed meal, or ground barley and soybean meal. The determination of the proportion of grass silage degraded in the rumen (RD) was based on disappearance of feeds from nylon bags during the rumen incubation as a function of time, using the outflow rate of k = 0.0625. The intestinal digestion (ID) was estimated by the mobile-bag method with the residues that resisted degradation during the 16-h rumen incubation. Combination of these two was calculated to provide the TTD. Concentrate supplementation always caused a clear and consistent decline in rumen degradation and TTD of organic matter (OM), neutral detergent fibre (NDF) and Kjeldahl-N of grass silage but had no real influence on its ID. The type of concentrate, however, had only little effect. The average TTD of NDF was 16% lower than that of OM, but TTD of N was always very much higher than the respective value for OM. The results indicate that concentrate supplementation decreases the total-tract digestion of OM, cell walls and nitrogen of grass silage owing to impaired ruminal degradation. The combined bag method appears a convenient tool to provide digestion coefficients close to the true feed digestion of the individual feeds. Key words: Grass silage, nylon bag, mobile bag, combined bag, ruminal degradation, intestinal digestion, true digestion