Fatty acid ruminal metabolism and digestibility in cows fed perennial ryegrass

The 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.