Corn oil supplementation to steers grazing endophyte-free tall fescue. I. Effects on in vivo digestibility, performance, and carcass traits

Eighteen Angus steers (438 [+ or -] 4 kg of BW) were supplemented with varying levels of corn oil (0 g/kg of BW, none; 0.75 g/kg of BW, MED; or 1.5 g/ kg of BW, HI) on rotationally stocked, endophyte-free tall fescue to determine the effect of supplemental oil level on in vivo digestibility, intake, performance, and carcass traits. Pelleted cottonseed hulls were used as a carrier for the oil supplements, and all supplements were offered to steers using Calan gate feeders for individual intake determination. On d 49, each steer was dosed with a controlled-release capsule containing chromium sesquioxide, and fecal samples were obtained 12 d later over a 7-d period to estimate fecal output that, with forage, supplement, and fecal indigestible NDF concentration, was used to estimate DMI and in vivo total diet digestibility. Steers were slaughtered at the end of the 116-d grazing period, and carcass data were collected at 24 h postmortem. Total fatty acid intake linearly increased with corn oil supplementation, and forage DMI, total DMI, and total DE intake were lin early decreased (P < 0.01). The decrease in total DMI was reflected in forage substitution rates greater (P [less that or equal to] 0.01) than 1, with a trend (P = 0.09) for a greater substitution rate in HI than in MED. In vivo DM, OM, and NDF digestibility were linearly decreased (P < 0.01) by corn oil supplementation. Average daily gain and final BW tended (P = 0.09) to increase linearly in response to oil level. Oil conversion (0.36 kg of BW gain/ kg of corn oil) was greater (P[less that or equal to]0.05) than zero and did not differ (P = 0.15) between MED and HI. Dressing percent (P = 0.09), carcass weight (P = 0.01), and carcass backfat thickness (P = 0.01) increased linearly with oil supplementation. No treatment effect was observed for carcass LM area, KPH percentage, marbling score, or yield grade (P > 0.10). Oil supplementation to grazing steers linearly reduced forage DMI intake; however, animal performance was maintained and tended to be greater for oil-supplemented cattle. Oil supplementation increased carcass fat thickness and weight without altering other carcass quality parameters. Key words: beef, carcass, corn oil supplementation, digestibility, forage