Two early season suppressive anthelmintic programmes, ivermectin given three, eight and 13 weeks after turn out and a morantel sustained release bolus administered at turn out, were compared on a commercial farm. The morantel treated cattle grew significantly faster than the ivermectin treated group during the period of treatment, on average at 0.80 kg/day compared with 0.71 kg/day (P less than 0.01). In the second half of the grazing season (13 to 25 weeks after turn out) the ivermectin treated group grew faster than the morantel treated group although the difference was not statistically significant. Over the entire grazing season there was no significant difference in average growth rate between the morantel treated group which grew at 0.80 kg/day and the ivermectin treated group which grew at 0.77 kg/day. These results were related to pasture larval counts, faecal egg counts and plasma pepsinogen levels throughout the grazing period. It was concluded that the morantel sustained release bolus allowed growing cattle to reach their production potential during the period of treatment. However, its efficacy in maintaining production throughout the grazing season was reduced by the 90 day treatment period which failed to give the level of control of gastrointestinal nematode parasites achieved by the 105 day period of treatment in the ivermectin programme.