Responses to controlling gastrointestinal parasitism in dairy cattle

Sixty-eight Holstein heifers, paired on milk production potential and season of birth, were randomly assigned within pairs to a control or treatment group to evaluate the effect of raising replacement heifers on a prophylactic anthelmintic regimen. The anthelmintic, thiabendazole (6.6 g/100 kg body weight), was administered orally as a paste at 2.5, 3.5, 6, 9, and 14 mo of age and at parturition. The control group received no thiabendazole. Fecal samples were collected from both groups at the time of thiabendazole treatment. Days to first service were 457.1 and 457.8 for the control and treatment groups. There were no differences in services per conception or days to first calving. Nematode eggs per 5 g of feces were similar at 2.5 mo (.6 and .8), 3.5 mo (38.6 and 22.4), 6 mo (40.2 and 43.1), 9 mo (39.0 and 22.4), 14 mo (55.2 and 22.4), or parturition (16.5 and .4). There were no differences in body weight changes over the first 18 mo of age. Subsequent first lactations were similar (9422 and 8710 kg 305-d mature equivalent, for twice daily milking) as was actual milk production for the first 120 d of lactation. Under these environmental conditions and apparently low parasitism in this herd, thiabendazole did not substantially influence nematode egg count and had no effects on body weight gains or milk production.