Animal Production in Australia Vol. 15 EFFECT OF SEASON, STOCKING RATE, GRAIN SUPPLEMENTATION AND ANTHELMINTIC ON GROWTH OF FRIESIAN WEANERS GRAZING TROPICAL GRASS PASTURES R.J. MOSS* D.V. KERR* I.K, BUCHANAN* and P.K. O'ROURKE** SUMMARY Forty-eight Friesian weaners grazed irrigated Setaria anceps cv. Kazungula pastures for 12 months. Treatments were (i) 8.25 weaners/ha and 0.5 kg maize/head/day; (ii) 8.25 weaners/ha and 1.0 kg/head/day; (iii) 10.75 weaners/ha and 1.0 kg/head/day; (iv) 10.75 weaners/ha and 1.5 kg/head/day. Half the calves in each treatment were treated with anthelmintic at four weekly intervals. The increased level of maize increased growth of calves. An increase in stocking rate reduced weight gain/weaner by 8%, though this decrease was reduced if a concurrent increase was made in the level of maize supplementation. Anthelmintic treatment increased growth with the greatest response in younger animals. During the wet season liveweight gains in all treatments were low and mortalities high. INTRODUCTION In north Queensland, dairy replacements are reared on tropical pastures. These pastures are capable of producing high dry matter yields but their digestibility is low. High stocking rates permit greater utilization of pastures, but concentrate supplementation may be. necessary to maintain intake of digestible energy by young animals. Calf losses on commercial dairy farms can be high, particularly during the wet season. In this experiment the effects of stocking rate and level of grain supplementation on growth of dairy weaners grazing irrigated tropical grass pastures were studied. The limitations to liveweight gain caused by internal parasites (nematodes) and by seasonal conditions were also investigated. MATERIALS AND METHODS The experiment was conducted at Ayr Research Station, situated in a tropical coastal area of predominantly summer rainfall (1092 mm annual average) 90 km south of Townsville, Queensland. Summers are hot and humid and winters cool and dry. Mean maximum and minimum temperatures are 25OC and ll°C respectively in July and 33OC and 23OC in January. Pasture was Setaria anceps cv. Kazungula, fertilized annually with superphosphate (43 kg P/ha/annum) and muriate of potash (62 kg K/ha/annum). Nitrogen fertilizer was applied as urea in equal dressings every six weeks (336 kg N/ha/annum). Pastures were spray irrigated and set stocked. Forty-eight Friesian weaners (32 female, 16 male castrates) aged between two and seven months were used in a replicated randomized block of four main treatments: 480