Four silages were prepared from perennial ryegrass: two unwilted, ensiled either without (UWN) or with formic acid additive (UWA) and two wilted for either 19 (WS) or 43 h (WL) prior to ensiling. The influence of conservation method on digestion in the rumen and the flow of organic mater (OM) and nitrogen (N) to the small intestine was studied using young steers fitted with rumen and duodenal T-piece cannulae. The silages were well preserved and of similar chemical composition despite a 1·5-fold increase in dry matter (DM) following wilting. The unwilted silages had a higher concentration of fermentation acids and a lower concentration of soluble N in the DM. Polyester bag studies showed that significantly more DM was potentially degradable but the degradation rate was slower in the rumen of steers fed UWA and WS silages (P < 0·05), however, rate of passage was not significantly different for the four treatments. Conservation method had little effect on post-feeding rumen ammonia concentration but feeding unwilted silages caused a greater depression in post-feeding rumen pH (P < 0·05). Wilting lowered N digestibility in the whole tract (P < 0·05), but had no effect on apparent digestion or degradability of N in the rumen as measured by the polyester bag method. Apparent OM digestibility in the whole tract was lower in steers given wilted rather than unwilted silages (P < 0·05), but disappearance of apparently digestible OM in the rumen was greatest for the 19 h wilted silage (WS, P < 0·05). It is concluded that, provided unwilted silage is well preserved, the use of formic acid has little effect on digestion of silage by steers and the effects of wilting are confined to a reduction in whole tract digestibility. © 1990, British Society of Animal Science. All rights reserved.
