Fatty acid ruminal metabolism and digestibility in cows fed perennial ryegrass

Abstract

AbstractThe ruminal metabolism and intestinal digestibility of fatty acids (FA) was studied in four mid‐lactation dairy cows fitted with ruminal and duodenal cannulae. The cows received fresh perennial ryegrass as sole feed. Two grasses were compared, differing in the soluble carbohydrate/nitrogen ratio (SC+/N– and SC–/N+ ratios). This ratio was modified by the combination of three factors: age of regrowth, N fertilisation and hour of cutting. The two grasses contained 12.5 and 20.7% of crude protein, and 24.6 and 13.7% of SC, respectively. The SC+/N– grass contained less FA [1.82 vs. 2.49% of dry matter (DM)] and a lower percentage of linolenic acid (60.9 vs. 65.6% of FA). As DM intake did not differ between treatments, FA intake was higher for the SC–/N+ treatment. The duodenal composition of FA revealed numerous isomers of 18:1 and conjugated and non‐conjugated isomers of 18:2. SC–/N+ treatment resulted in a higher duodenal flow of stearic, linoleic, and linolenic acids and of several intermediates of linolenic acid hydrogenation. This was a consequence of differences in intake. Intestinal digestibility of FA from both grasses was high (around 90%). The proportions of FA in plasma did not reflect with accuracy the differences in duodenal FA resulting from differences in ruminal metabolism.