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

Eleven dairy replacement heifers (mean body weight, 262.0 kg) were given two treatments with levamisole in the spring to reduce worm egg output and consequent pasture contamination with infective nematode larvae during the summer/autumn grazing season. The treatments were given 3 weeks and 76 weeks after the heifers were moved from confinement to permanent pastures in may 1979, at which time they were 5 to 12 months old. Treated heifers gained significantly (P < 0.01) more weight than a group of 11 control heifers (mean body weight, 262.4 kg) 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 favorable 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 favorable for optimal development of free-living stages.