Effects of forage level on the comparative feeding value of supplemental fat in growing-finishing diets for feedlot cattle.

Ninety-six crossbred steers (316 kg) were used in a 135-d growth-performance trial to evaluate the comparative feeding value of yellow rease (0 vs 6%) in 10% forage vs 30% forage rowing-finishing diets. There were interactions between forage level and supplemental fat on ADG (P < 10), DM conversion (P < .05), diet NE (P < .10), longissimus muscle area (P < .01), fat thickness (P < 10), and percentage of retail yield (P < .01). Supplemental fat increased (8.5%; P < .10) marbling score, sufficient to move the average carcass grade from high Select to low Choice. With the low-forage diet supplemental fat did not affect (P > .10) ADG, but decreased (P < .01) retail yield (2.3%) and longissimus muscle area (5.6%). With the high-forage diet supplemental fat increased ADG (13.3%; P < .05) and longissimus muscle area (7.1%; P < .01) but did not affect (P > .10) retail yield. Differences in carcass fat were small and not affected by treatment (P > .10). The NEm and NEg values of yellow grease were 3.55 and 2.65 Mcal/kg, respectively, for the low-forage diet and 5.71 and 4.65 Mcal/kg, respectively, for the high-forage diet. Treatment effects on characteristics of ruminal and total tract digestion were evaluated using four Holstein steers (233 kg) with cannulas in the rumen and proximal duodenum. There were no interactions (P > .10) between forage level and supplemental fat on ruminal and total tract digestibility of OM, ADF, starch, lipid, and gross energy. Postruminal lipid digestibility averaged 65.5% and was not affected (P > .10) by forage level. The ME value of yellow grease was the same (6.94 Mcal/kg) for both the low- and high-forage diets. There was an interaction between supplemental fat and forage level on nonammonia N flow to the small intestine (P < .10), percentage of ruminal escape feed N (P < .05), postruminal N digestion (P < .05), and methane production (P < .10). With the low-forage diet, fat supplementation did not affect (P > .10) ruminal degradation for feed N and methane production. With the high-forage diet fat supplementation decreased ruminal degradation of feed N (19.0%; P < .05) and methane production (14.8%; P < .05). We concluded that supplementation of a 30% forage (alfalfa hay) finishing diet with 6% yellow grease will permit growth-performance similar to that of steers fed a 10% forage diet without supplemental fat. The improved performance may be attributed to increased diet energy density and positive associative effects on protein flow to the small intestine and decreased ruminal methane production.