Control of nematodes in dairy heifers by prophylactic treatments with albendazole in the spring.

: The effect of 2 prophylactic anthelmintic treatments with albendazole (7.5 mg/kg) was evaluated in 2 groups of Holstein dairy replacement heifers treated 3 and 6 weeks after turnout to spring pasture on May 2, 1983. The lightweight group consisted of twelve 7- to 9-month-old heifers with a mean initial weight of 201.1 kg. The heavyweight group consisted of ten 11- to 15-month-old heifers with a mean weight of 319.4 kg. Within each group, the heifers were paired by initial weight, and one member of each pair was randomly assigned to the treatment group. Each of the 4 resulting groups grazed separate, contaminated pastures until winter housing on Oct 31, 1983. The albendazole-treated, lightweight heifers had bodyweight gains 28.9% greater than that of the control heifers at the end of 23 weeks (P less than 0.001). There was no significant difference in gain for the treated heavyweight heifers, which had a lower feeding rate/kg of bodyweight than did the lightweight heifers. At the end of the study, there was a sevenfold difference in mean pasture L3 counts between treatment (1,303 L3/kg) and control (9,153 L3/kg) replicates. It was concluded that it is desirable to treat all heifers in northern regions during their first spring at pasture. This strategy would likely provide substantial economic returns to the farmer in years with a wet summer, which would favor early migration of L3 to herbage, but would provide smaller returns in dry seasons, which would delay this pattern of migration.