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 t hat 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-vacc enic acid (18:1(trans-11)). Five mature ewes (66.5 +/- 12.8 kg) fitted with ruminal and duodenal cannulas were used in a 5 x 5 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 soyb ean 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 allotment s at 0630 and 1830. After 14 d, Cr2O3 (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 < 0.01) with dietary forage . Duodenal flow of 18:1(cis-9) and 18:2(cis-9,12) decreased (P < 0.001) but duodenal flow of 18:3(cis9,12,15) increased (P < 0.01) with increased diet ary forage. Biohydrogenation of dietary unsaturated fatty acids increased ( P < 0.001) as dietary forage increased, which was concomitant with increase d ruminal pH. Duodenal flow of 18:2(cis-9,trans-11) increased linearly (P < 0.01) with increased dietary forage but increased abruptly when forage was fed at 45.8%. Duodenal flow of the trans-10, cis-12 and cis-10, cis-12 CLA isomers decreased as dietary forage increased, but flow tended to increase on the highest-forage diet, resulting in both linear (P < 0.01) and quadra tic (P < 0.01) effects. Duodenal flow of 18:1(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 inc reased to 7.29 g/d on the highest-forage diet (quadratic, P < 0.1). Duodena l flow of 18:1(trans-11) was 27- to 69-fold greater than flow of CLA. We co nclude that when ewes were fed a 6% crude fat diet duodenal flows of dietar y fatty acids changed incrementally as dietary forage was increased, wherea s changes in flows of CLA isomers seemed to be more abrupt. Biohydrogenatio n changes were gradual with diet, suggesting a gradual shift in ruminal mic robial populations with increasing forage. Finally, the highest-concentrate diet supported the greatest duodenal flows of dietary unsaturated fatty ac ids, as well as the highest flow of 18:1(trans-11).