Biohydrogenation and digestion of long chain fatty acids in steers fed on Lolium perenne bred for elevated levels of water-soluble carbohydrate

Grass is a rich source of $\alpha$-linolenic acid (18:3$n$-3) and in this study the effects on ruminal fatty acid metabolism of feeding beef steers zero-grazed Lolium perenne containing elevated levels of water-soluble carbohydrate (WSC) were investigated. Eight Hereford $\times$ Friesian steers were offered ad libitum access to one of two varieties of Lolium perenne, Ba11353, high WSC (HS) or AberElan, intermediate WSC (experimental control) harvested at different times of the day (14:00 and 10:00 h, respectively) to accentuate WSC differentials. The grass was zero-grazed and fed for 21 days, after which the animals were offered ad libitum grass silage for 14 days to provide a covariate intake. The dry matter (202 vs. 167 g per kg fresh weight), WSC (243 vs. 161 g per kg DM), total fatty acids (21.4 vs. 17.9 g per kg DM) and proportion of 18:3$n$-3 (0.54 vs. 0.43) were greater and fibre content was lower (251 vs. 296 g ADF per kg DM) for HS compared with the control. DM intake and intake of total fatty acids and 18:3$n$-3 was higher for HS (9.3 vs. 6.7 kg per d; 201 vs. 117 and 108.5 vs. 51.3 g per d, respectively). There was a trend ($P < 0.1$) for the flow of 18:3$n$-3 at the duodenal to be higher on HS (8.5 vs. 5.7 g per d) but surprisingly there was no significant difference in the flows of 18:0 or 18:1 trans (58.5 vs. 48.8 and 11.1 vs. 9.1 g per d, respectively). This may be attributed to the net flows of fatty acids across the rumen (duodenal flow - intake) which were positive on the control and negative on the HS. Biohydrogenation of 18:2$n$-6 and 18:3$n$-3 was not different between treatments and averaged 79.9 and 90.5%, respectively. Intestinal absorption as a proportion of duodenal flow of all the fatty acids were high ranging from 0.70 for 12:0 to 0.96 for 18:1 trans. In conclusion, treatment HS a Lolium perenne bred for elevated levels of WSC had higher total fatty acids and a higher proportion of the beneficial fatty acid 18:3$n$-3 compared to a control. The higher DM intakes achieved when feeding the treatment HS along with the greater content of 18:3$n$-3 resulted in a trend for greater intakes of this fatty acid and flow to and absorption from the small intestine. Biohydrogénation et digestion des acides gras chez le bouvillon recevant du ray-grass anglais sélectionné pour une teneur élevée en glucides solubles. L'herbe est une source importante d'acide $\alpha$-linolénique (18:3$n$-3). Dans cet essai, on a étudié sur des bouvillons les ef