In sacco degradation characteristics of organic matter, neutral detergent fibre and crude protein of fresh grass fertilized with different amounts of nitrogen

Abstract

Thirty composite grass samples, containing mainly Lolium perenne, were taken at three measuring periods in the spring and summer of 1991 and 1992 and two in the autumn of 1992 and 1993 from plots fertilized with 150, 300 and 450 kg N ha −1 year −1. In each period × N plot combination, grass samples were taken at a dry matter (DM)-yield of 1500 to 2000 kg ha −1 over four consecutive days and subsequently combined to one composite sample. These samples were incubated in rumen and intestinal cannulated lactating cows using the nylon-bag and mobile-nylon bag techniques to study the effects of N fertilizer on rumen degradation of organic matter (OM), crude protein (CP) and neutral detergent fibre (NDF) and on the intestinal digestibility of ruminal escaped CP (DRE CP). Nitrogen fertilizer increased CP content and decreased the content of DM and sugar whereas NDF content and in vitro digestibility (d OM) were not markedly influenced. The NDF content and d OM were more related to differences in age of the grass than to differences in the amounts of N applied. The content of neutral detergent insoluble N (NDIN) was increased significantly at higher rates of N fertilization. However, this effect on NDIN was difficult to interpret because of the fact that NDIN was highly influenced by the method of sample preparation. The N fertilizer had no effect on the content of acid detergent insoluble N (ADIN) which was negligible compared to the level of NDIN. The washable OM fraction was not influenced by N fertilizer whereas the washable CP fraction increased significantly. The results showed a decrease of the undegradable OM, CP and NDF fractions by using more N fertilizer. The rate of degradation of potentially degradable, but insoluble OM, CP and NDF fractions increased significantly at higher levels of N fertilization. As a consequence, more N fertilizer decreased the fraction of ruminal escaped CP (RE CP) significantly. However, during the whole season the total amount of RE CP entering the small intestine was increased although the increase was more pronounced in spring and early summer than in late summer and autumn. Only in autumn small intestine digestibility of RE CP tended to be higher at higher rates of N fertilization.