Control of internal parasites in dairy replacement heifers by two treatments in the spring

Eleven dairy replacement heifers (mean body weight 262 kg) were given two treatments with levamisole in the spring, 3 and 6 weeks after they had been moved from confinement to permanent pastures, at which time they were 5 to 12 months old. Treated heifers gained more weight than a group of 11 control heifers of the same mean body weight, and outweighed them by mean values of 9.2, 19.7, 31.0 and 39.5 kg at the end of July, August, September and October, respectively. Control heifers had highest egg counts (47-51 eggs/g) from July to September, but there was a 2-month delay before treated heifers attained peak concentrations (42 eggs/g). This delay occurred at a critical time of the year (July, August), when environmental conditions were favourable for the rapid development of eggs to infective larvae. When the egg count for treated heifers peaked at the end of September, environmental conditions were no longer favourable for optimal development of free-living stages..