Dose-response relationship of the ionophore tetronasin in growing-finishing cattle.

The dose-response relationship of the ionophore tetronasin was defined in growing-finishing cattle (84 heifers and 84 steers). Cattle were allotted by sex and weight replication (light and heavy) to 24 pens. Six dosages (D) of tetronasin (0, 3.7, 7.3, 11.0, 14.7 and 18.3 mg/kg, dry matter [DM] basis) were fed. Diets containing 40, 25 and 10% roughage were fed for 84, 56 and either 56 (heavy replication) or 91 d (light replication), respectively. Average starting weights were 242 and 270 kg and ending weights were 433 and 480 kg for heifers and steers, respectively. No interactions existed for ionophore with sex (P greater than .2) over the trial. Ionophore treatment did not affect overall average daily gain (.94 kg/d; P greater than .2). Daily DM intake (kg/d) decreased linearly as the level of tetronasin increased (8.4-.042D (dosage), r2 = .83, P less than .001). Gain efficiency (gain/feed, kg/100 kg DM) increased curvilinearly as ionophore dosage increased (11 + .16D - .0058D2, r2 = .77, P less than .001). Solving the first derivative indicated an optimum dose for gain efficiency improvement of 13.8 mg/kg. The gain efficiency response was greatest the first 84 d, less the next 56 d and minimal the last 56 or 91 d (20, 7 and 2% improvement for D = 14.7 mg/kg, respectively). Tetronasin was found to improve gain efficiency by about 10% over control at an optimum dosage of 13.8 mg/kg.