The performance of 98 Merino weaner lambs at two stocking rates (55 and 68 lambs/ha), with access to either no supplementation, 250 g molasses meal/lamb/day or 250 g molasses meal plus 35 mg ionophore (lasalocid)/lamb/day, with or without a growth implant (zeranol) was examined on irrigated Italian ryegrass pasture. The lambs grazed the pasture from August to November for approximately 90 days, in a six camp rotational grazing system over two grazing seasons. Stocking rate, energy supplementation and the growth implants exerted a significant (p ≤ 0.05) effect on livemass gain and the carcass grade achieved by the lambs over all treatments. The lambs at the light stocking rate gained 1 kg (p ≤ 0.05) more than those at the heavy stocking rate. Lambs which received molasses meal gained 2.7 kg (p ≤ 0.01) more than the unsupplemented lambs and 1.8 kg (p ≤ 0.05) less than those which received molasses meal and the ionophore.