Effect of rumen degradation on intestinal digestion of nitrogen of 15N-labelled rapeseed meal and straw measured by the mobile-bag method in cows

Abstract

SUMMARYRape (Brassica campestris) was labelled, during cultivation, with 15N and then prepared into rapeseed meal (RSM) and rapeseed straw (RSS). Three non-lactating cows, equipped with ruminal and Tshaped duodenal cannulae, were used to incubate both feeds in the rumen for various periods of time in mobile bags contained within larger mesh bags. Some of the mobile bags were then inserted into the duodenum and collected from the faeces. The effects of rumen incubation time and contamination by non-feed N during the rumen incubation and subsequent intestinal digestion on the intestinal feed N digestion values so obtained were assessed. The intestinal digestion of intact RSM nitrogen was also measured in vivo by dosing directly into the duodenum and measured as the net loss of excess 15N not recovered in the faeces.With RSM, microbial contamination did not affect the disappearance values of feed N from mobile bags in the rumen, the difference in the disappearance between 15N and total N (NT) being negligible. However, the decrease in the disappearance from the mobile bags in the intestine with increasing rumen incubation time tended to be smaller with 15N than with NT, resulting in significantly increasing differences between 15N and NT with increasing rumen incubation time. The average in vivo net loss within the intestine of 15N of labelled RSM was much lower (50·0%) than the 15N disappearance from the mobile bags (80·9%).With RSS, the disappearance values of 15N and NT from the mobile bags in the rumen were different. The disappearance of 15N was always higher than that of NT, and it also increased significantly more with increasing incubation time. The disappearance of 15N from the mobile bags within the intestine decreased with increasing rumen incubation time, but that of NT remained rather constant.It is concluded that the intestinal N digestion of a feed may vary significantly depending on the degree of preceding rumen degradation. Therefore, constant intestinal digestion coefficients may not be applied in the modern protein evaluation systems. With protein-rich feeds, such as RSM, the influence of non-feed N is small and does not interfere with the estimation of intestinal N digestibility by the mobile-bag method. With fibrous feeds with low N, such as RSS, intestinal N digestion values are seriously confounded by non-feed N. The present study with RSM did not confirm the correspondence between the value obtained by the mobile-bag method and that in vivo.