Effect of Variation in Supplement Inclusion Rate and Laidlomycin Propionate on Growth Performance of Feedlot Cattle

One hundred twelve yearling steers (364±3.4 kg) were used in a 123-d finishing trial to evaluate the interaction of laidlomycin propionate and variation in supplement inclusion rate on growth performance and carcass characteristics. Two levels of laidlomycin propionate (0 vs. 10 mg/kg) and two levels of variation in daily supplement (12 g urea, 18 g limestone, 2 g MgO₂ and 5 g trace mineralized salt) inclusion rate (0 vs 20% coefficient of variation) were evaluated in a 2 × 2 factorial arrangement. Variation was produced by altering the inclusion rate of the supplement portion of the diet to achieve a 20% CV in supplement concentration of the complete mixed, as follows: on d 1 and 5 of each week the diet contained 125% of the specified amount of supplement; on d 2, 4 and 6 the diet contained 100% of the specified amount of supplement; on d 3 and 7 the diet contained 75% of the specified amount of supplement. There were no treatment interactions (P>0.20). Variation in supplement inclusion rate decreased ADG (8%, P<0.05), gain efficiency (G:F; 5%, P<0.01), dietary NE_m (2.3%, P<0.01) and dietary NE_g (3.3%, P<0.01). Supplemental laidlomycin tended to increase ADG (6%, P=0.19), but did not affect (P>0.20) gain efficiency or dietary NE. There were no treatment effects (P>0.20) on carcass characteristics. It is concluded that variation in supplement inclusion may depress growth performance of feedlot cattle, attributable to reduction in both energetic efficiency and DMI.