Effects of Excessive Supplemental Fat on Feedlot Cattle Growth Performance and Digestive Function

Abstract

Ninety medium-framed crossbred steers (245 kg) were used in a 205-d feeding trial to evaluate the influence of 4, 8, and 12% supplemental fat (tallow soap stock) on feedlot growth performance. Increasing level of fat supplementation depressed ADG (P < .01), DM conversion (P < .01), and dietary NE (P < .05). Depressions in DM conversion and dietary NE were more marked at the 12% level of fat supplementation. There was a quadratic effect (P < .01) of level of fat supplementation on DM intake, with intake being higher for 12% supplemental fat than for 8% supplemental fat. Observed:expected diet NEm was 1.00 with 4% supplemental fat and declined (P < .01) to .86 with 12% supplemental fat. The NEm of tallow soap stock declined in a near rectilinear fashion from 5.65 Mcal/kg at the 4% level of supplementation to 2.85 Mcal/kg at the 12% level of supplementation. Six Holstein steers (296 kg) with T cannulas in the rumen and proximal duodenum were used to evaluate treatment effects on digestive function. Ruminal digestion of OM (P < .01), feed N (P < .01), and ADF (P < .05) decreased linearly with increasing level of fat supplementation. Most (55%) of the decline in ruminal OM digestion could be directly attributed to ruminal indigestibility of dietary fat. Increasing level of fat supplementation did not influence (P > .10) postruminal digestion of OM, ADF, starch, and N. Postruminal digestion of lipid decreased (P < .01) with increasing level of fat supplementation. Total lipid intake was a better predictor of postruminal lipid digestibility than percentage supplemental fat. True digestibility of supplemental tallow soap stock at the 8 and 12% levels of supplementation averaged 63 and 40%, respectively. Total tract OM digestion declined linearly (P<.01) with increasing level of supplemental fat. This decline was due to differences in lipid digestion. There were no associative effects of supplemental fat on total tract digestion of ADF, starch, and N. It is concluded that optimal growth performance response to supplemental fat will be obtained when levels of total lipid intake are less than 1.6 g/kg body weight. Detrimental effects of excessive supplemental fat are primarily associated with depressed intestinal lipid digestibility.