Effect of lasalocid on feedlot performance and energy partitioning in cattle.

Two experiments were conducted to quantify the effects of lasalocid on the performance and energy partitioning of cattle fed a 90% concentrate, barley-based diet. Experiment 1 utilized 100 Hereford heifers (initial weight 308 kg) fed diets containing no added ionophore, lasalocid added at 24, 36 or 54 mg/kg dry matter (DM), or monensin added at 33 mg/kg DM. In the 98-d trial, lasalocid-fed heifers gained an average of 1.35 kg daily, whereas control heifers gained 1.24 kg (P = .12). Heifers fed lasalocid (36 or 54 mg/kg DM), monensin, and the control diet required 6.2, 6.5 and 6.9 kg DM/kg gain, respectively (P greater than .1). Ionophores had no influence on carcass quality. In Exp. 2, four steers (616 kg) were fed the control or lasalocid diet (36 mg/kg DM) at daily feeding levels of 21, 44, 67 and 89 g DM/kg body weight.75; fecal, urinary, methane and heat losses were measured by total collection and indirect calorimetry methods. The proportion of digestible energy lost as methane averaged 7.5% for steers fed the control diet and 7.1% for the steers fed lasalocid (P less than .1). Lasalocid improved (P less than .05) the metabolizable energy (ME) density of the diet by 8, 8 and 5% at the 21, 44 and 67 g DM feeding levels. There was no difference (P greater than .1) between diets in ME density at the 89 g DM feeding level. The net energy for maintenance (NEm) value of the diet was increased (P less than .05) by 10 to 21% with lasalocid, whereas the net energy for gain (NEg) value was not affected. Average heat productions of the steers were increased (P less than .05) by 7% with lasalocid. The ME requirement for maintenance was estimated at 84 and 81 kcal/kg body weight.75 from linear regressions of energy retention and ME intake above maintenance for the control and lasalocid diets, respectively. Corresponding estimates using a semilog-linear method were 90 and 92. No differences (P greater than .1) in blood concentrations of insulin, glucagon or growth hormone were observed with the ionophore treatment. It was concluded that the main method by which lasalocid improved feed conversion was by increasing the ME density of the diet.