Comparative Feeding Value of Supplemental Fat in Finishing Diets for Feedlot Steers Supplemented with and Without Monensin

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Abstract
Two comparative slaughter trials and a metabolism trial were conducted. Treatments consisted of: 1) 0 fat, 0 monensin; 2) 4% yellow grease, 0 monensin; 3) 0 fat, 33 mg/kg monensin and 4) 4% yellow grease, 33 mg/kg monensin. Trial 1 involved 104 crossbred beef steers (267 kg) in a 140-d comparative slaughter trial. There were no interactions (P>.20) between supplemental fat and monensin on steer performance. Monensin supplementation decreased rate of weight gain (P<.10) and feed intake (P<.05) with no effect on energy value of the diet (P>.20). Fat supplementation increased (P<.01) rate of weight gain 12.5% and increased the net energy for maintenance (NEm) and net energy for gain (NEg) value of the diet 8.5 and 9.4%, respectively. Trial 2 involved 154 Holstein steers (290 kg) in a 94-d comparative slaughter trial. There were no interactions between supplemental fat and monensin (P>.20). Monensin supplementation did not affect rate or composition of gain (P>.20), but supplementation reduced (P<.05) feed intake and feed required per unit weight gain 3.6%. Fat supplementation increased (P<.01) fat and energy gain 12.5 and 10.3%, respectively, and the NEm and NEg content of the diet 7.5 and 8.4%, respectively. Trial 3 utilized four crossbred beef steers (220 kg) with cannulas in the rumen, proximal duodenum and distal ileum. There were no interactions between supplemental fat and monensin with respect to site of digestion (P>.20). Supplemental fat did not affect (P>.20) organic matter, starch, fiber or N digestion. Intestinal digestibility of fat averaged 77.3%. Monensin increased (P<.10) intestinal digestibility of fat 7.4%. There were negative associative effects between supplemental fat and monensin on ruminal acetate:propionate ratios and estimated methane production. It was concluded that the feeding value of feed fat is underestimated in tables of feed standards currently in use, and that the net effects of monensin on these estimates are additive.