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-perfo rmance trial to evaluate the comparative feeding value of yellow greas e (0 vs 6%) in 10% forage vs 30% forage growing-finishing diets. There were interactions between forage level and supplemental fat on ADG (P < .10), DM conversion (P < .05), diet NE (P < .10), longissimus muscl e area (P < .01), fat thickness (P < .10), and percentage of retail yi eld (P < .01). Supplemental fat increased (8.5%; P < .10) marbling sco re, sufficient to move the average carcass grade from high Select to l ow Choice. With the low-forage diet supplemental fat did not affect (P > .10) ADG, but decreased (P < .01) retail yield (2.3%) and longissim us muscle area (5.6%). With the high-forage diet supplemental fat incr eased 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 NE(m) and NE(g ) values of yellow grease were 3.55 and 2.65 Mcal/kg, respectively, fo r 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 in teractions (P > .10) between forage level and supplemental fat on rumi nal and total tract digestibility of OM, ADF, starch, lipid, and gross energy. Postruminal lipid digestibility averaged 65.5% and was not af fected (P > .10) by forage level. The ME value of yellow grease was th e same (6.94 Mcal/kg) for both the low- and high-forage diets. There w as an interaction between supplemental fat and forage level on nonammo nia N flow to the small intestine (P < .10), percentage of ruminal esc ape feed N (P < .05), postruminal N digestion (P < .05), and methane p roduction (P < .10). With the low-forage diet, fat supplementation did not affect (P > .10) ruminal degradation for feed N and methane produ ction. 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-performan ce similar to that of steers fed a 10% forage diet without supplementa l fat. The improved performance may be attributed to increased diet en ergy density and positive associative effects on protein flow to the s mall intestine and decreased ruminal methane production.