Seasonal control of gastrointestinal parasites among dairy heifers using two strategically timed treatments of fenbendazole

Abstract

Sixty heifers aged 7-18 months, in their first grazing season, were randomly assigned to four groups: treated, continuous grazing management, 16 head; control, continuous grazing, 16 head; treated, rotational grazing, 14 head; control, rotational grazing, 14 head. Treated animals received fenbendazole (5 mg/kg orally) on days 22 and 50 after turnout and controls received a placebo. Monitoring was carried out on weight gains, faecal egg counts, herbage larvae and botanical composition of pastures. Effective grazing density of the continuously grazing groups was 3.2 animals/ha. Each rotationally grazed group was allotted one of ten 0.4-ha. paddocks, each paddock being grazed 1-6 days depending on available forage. There were no significant differences between treated and control heifers in daily gains on either continuous or rotational pastures during 140 days after treatment. Heifers on continuously grazed pasture gained more than on rotational pasture. Faecal egg counts from rotationally grazed groups showed significant differences between treated and control groups on days 78 and 106. There were no significant differences in egg counts between continuously grazed groups. There were significant differences in numbers of infective larvae recovered after day 100, with most collected from rotational paddocks. Samples for botanical analysis from continuous pastures contained more than 50% white clover, whereas the rotational paddocks never had more than 15%. The results can be interpreted from two propositions: first, the level of parasitism was too low to affect weight, faecal egg count or herbage larvae; second, the accumulation of high levels of pasture larvae in the system of rotational grazing occurred too late to affect weight gains.