Comparing the effectiveness of injectable ivermectin in combination with oral fenbendazole to injectable ivermectin with oral levamisole at reducing strongyle-type fecal egg count in weaned beef calves

The objectives of this study were 1) evaluate combination anthelmintic therapy effect on average daily gain (ADG) in weaned beef calves on pasture, and 2) determine if injectable ivermectin + oral levamisole was noninferior to injectable iver­mectin + oral fenbendazole at reducing strongyle-type fecal egg count (FEC). This study was performed in two phases using weaning-age commercial beef calves housed on pasture. Phase 1 included 74 beef calves weaned in March 2025, while phase 2 included 103 calves weaned in September 2025. In each phase, calves were allocated to 1 of 4 treatment groups based on age, body weight, and FEC at weaning (day 0). Treatment groups in­cluded injectable ivermectin (A), injectable ivermectin + oral levamisole (AP), injectable ivermectin + oral fenbendazole (AS), and no treatment (NO). On day 14 following weaning, calf body weight and FEC were collected, ADG was analyzed using linear mixed models, and non-inferiority analysis was performed us­ing a one-tailed, two-sample t-test. In phase 1, no treatment achieved the target 99% FEC reduction, with combination treat­ments AP and AS achieving mean FEC reductions of 89.8% (90% C.I. 84.5-95.2%) and 77.1% (90% C.I. 65.6-88.6%), respectively, indicating the presence of multiclass anthelmintic resistance. In phase 2, treatments AP and AS achieved mean FEC reduc­tions of 95.4% (90% C.I. 91.7-99%) and 95.9% (90% C.I. 92.8-99%), respectively. In phase 2, AP was noninferior to AS (P = 0.0264). Evidence of ivermectin resistance was present in both phases, with multiclass anthelmintic resistance apparent in phase 1.