Effects of supplemental soybean oil level on in vitro digestion and performance of prepubertal beef heifers

In vitro digestion and growth studies were conducted to evaluate the effects of level of soybean oil inclusion in forage-based diets. In Exp. 1, diets were bromegrass hay (H), bromegrass hay and corn-soybean meal supplement (C), C with 3% added soybean oil (O3), and C with 6% added soybean (O6). Diets containing supplements were formulated to be isonitrogenous and isocaloric. Treatment means were compared using a single-degree-of-freedom contrast (H vs C, O3, and O6) and orthogonal polynomial contrasts within diets C, O3, and O6. Diet H had the lowest (P = .0003) IVDMD and a linear decline (P = .0001) in IVDMD was observed from C to O6, but 24-h IVDMD disappearance was greatest (P = .001; quadratic) for O3. Total VFA increased from C to O3 and then decreased from O3 to O6 (quadratic; P = .001), and acetate:propionate ratio decreased linearly (P = .0001) from C to O6. Changes in long-chain fatty acids reflected biohydrogenation by ruminal microbes; however, only 18:3 was hydrogenated to the same extent across all diets. In Exp. 2, 36 Angus x Gelbvieh heifers (260.0 +/- 6.0 kg initial BW) were individually fed C, O3, or O6 as mixed rations for 104 d. Diets were formulated to be isonitrogenous and provide ADG of .91 kg. Feed efficiency and ADG was greatest (P < .02; quadratic) for O3 heifers. Serum NEFA increased linearly (P = .02) and serum glucose (P = .02), cholesterol (P = .002), and GH (P = .04) showed a quadratic response to level of dietary soybean oil. Plasma proportions of 16:0, 16:1, 18:0, and 18:1 increased quadratically (P < .03), and 18:2 increased linearly (P < .001) from C to O6. In Exp. 3, 42 Angus x Gelbvieh heifers (288.7 +/- 6.6 kg initial BW) were divided into six pens (two pens/treatment) in a randomized complete block designed experiment. Rations were delivered as hay plus a top-dressed supplement (C, O3, or O6). Heifers fed O3 conceived 10 d earlier (quadratic; P = .06) than heifers fed C and O6. Other production estimates did not differ (P > or = .10) among dietary treatments. Inclusion of soybean oil at 3% of a forage-based diet increased total VFA, many blood metabolites, ADG, and feed efficiency, and it decreased time to conception. Adding soybean oil as 3% of a forage-based diet is an acceptable feeding strategy for developing beef heifers.