Effect of forage:concentrate ratio on ruminal digestion and duodenal flow of fatty acids in ewes

The objective of this study was to determine the forage:concentrate ratio that would provide the greatest duodenal flow of unsaturated fatty acids in ewes supplemented with soybean oil and to determine how diets differing in forage content affect flow of conjugated linoleic acid (CLA) and trans-vaccenic acid ([18:1.sup.trans-11]). Five mature ewes (66.5 [+ or -] 12.8 kg) fitted with ruminal and duodenal cannulas were used in a 5 x5 Latin square experiment. Diets were isonitrogenous and included bromegrass hay, cracked corn, corn gluten meal, urea, and limestone. Dietary fat was adjusted to 6% with soybean oil. Five ratios of forage:concentrate (18.4:81.6, 32.2:67.8, 45.8:54.2, 59.4:40.6, and 72.9:27.1) were fed at 1.3% of BW daily in equal allotments at 0630 and 1830. After 14 d, [Cr.sub.2][O.sub.3] (2.5 g) was dosed at each feeding for 7 d and ruminal, duodenal, and fecal collections were taken for the next 3 d. Duodenal flow of 18:0 increased linearly (P [is less than] 0.01) with dietary forage. Duodenal flow of [18:1.sup.cis-9] and [18:2.sup.cis-9,12] decreased (P [is less than] 0.001) but duodenal flow of [18:3.sup.cis-9,12,15] increased (P [is less than] 0.01) with increased dietary forage. Biohydrogenation of dietary unsaturated fatty acids increased (F [is less than] 0.001) as dietary forage increased, which was concomitant with increased ruminal pH. Duodenal flow of [18:2.sup.cis-9,trans-11] increased linearly (P [is less than] 0.01) with increased dietary forage but increased abruptly when forage was fed at 45.8%. Duodenal flow of the trans10, cis-12 and els-lO, cis-12 CLA isomers decreased as dietary forage increased, but flow tended to increase on the highest-forage diet, resulting in both linear (P [is less than] 0.01) and quadratic (P [is less than] 0.01) effects. Duodenal flow of [18:1.sup.trans-11] decreased from 8.28 g/d on the 18.4% forage diet to 5.47 g/d on the 59.4% forage diet then increased to 7.29 g/d on the highest-forage diet (quadratic, P [is less than] 0.1). Duodenal flow of [18:1.sup.trans-11] was 27- to 69-fold greater than flow of CLA. We conclude that when ewes were fed a 6% crude fat diet duodenal flows of dietary fatty acids changed incrementally as dietary forage was increased, whereas changes in flows of CLA isomers seemed to be more abrupt. Biohydrogenation changes were gradual with diet, suggesting a gradual shift in ruminal microbial populations with increasing forage. Finally, the highest-concentrate diet supported the greatest duodenal flows of dietary unsaturated fatty acids, as well as the highest flow of [18:1.sup.trans-11]. Key Words: Concentrates, Fatty acids, Forage, Linoleic Acid, Sheep