Effects of multiple oral administrations of fenbendazole on growth and fecal nematodes infection of early-weaned beef calves grazing perennial, warm-season or annual, cool-season grasses

Two experiments evaluated growth and total fecal egg count (FEC) of early-weaned calves receiving multiple or a single oral drench of fenbendazole while grazing warm- or cool-season grasses. On d 0, Brangus crossbred calves (n = 64 and 56 in Exp. 1 and 2, respectively) were allocated into bahiagrass or ryegrass pastures (6 to 8 pastures per forage system; 4 calves per pasture). Thereafter, 2 of 4 calves within each pasture (Exp. 1) or all calves in each pasture (Exp. 2) received oral administration of fenbendazole (5 mg/kg of BW) every 28 d from d 0 to 84 (MULT) or once on d 56 (CTRL). Overall ADG from d 0 to 84 increased (P = 0.01; Exp. 1) or tended to increase (P = 0.11; Exp. 2) for MULT versus CTRL calves. In Exp. 1, FEC on d 28 and 56 was less (P ≤ 0.05) for MULT versus CTRL, but the reduction on FEC was greater for MULT calves grazing ryegrass versus bahiagrass (P < 0.001). Fenbendazole treatment on d 56 reduced FEC on d 84 versus 56, but FEC remained greater for CTRL calves grazing bahiagrass than all calves grazing ryegrass (P ≤ 0.03; Exp. 1). In Exp. 2, FEC of MULT calves decreased on d 28 and 56 (P ≤ 0.10) and achieved similar values in d 84 than CTRL calves (P = 0.93). Therefore, monthly oral administrations of fenbendazole improved ADG of early-weaned calves, regardless of forage system, compared with a single, strategic administration on d 56 after weaning.