Effect of stocking rate, grazing system, and fenbendazole treatment on subclinical parasitism in dairy heifers.

Average daily weight and fecal parasitic egg count were compared among sixty 7- to 15-month-old Holstein heifers randomly assigned to 8 groups. The following variables were examined: grazing system (continuous vs rotational), stocking rate (3.5 vs 5 heifers/ha), and treatment [control vs dosing with a 10% suspension of fenbendazole (5 mg/kg of body weight) given orally 21 and 49 days after heifers were placed on pasture]. Two heifers from each group were euthanatized 4 weeks after the completion of the 148-day trial to determine the number of parasitic larvae and adults in the abomasum and intestine. Heifers treated with fenbendazole had significantly (P less than 0.001) fewer fecal ova and fewer inhibited abomasal larvae (P less than 0.05) than did controls. Mean total weight gain and mean daily rate of gain of fenbendazole-dosed heifers were significantly (P less than 0.01) increased by 17.3 kg or 0.12 kg/day, respectively, compared with those of nontreated heifers. Seemingly, stocking rate and grazing systems had no marked effect on subclinical parasitism in heifers.