Anthelmintic resistance against gastrointestinal nematodes in beef cattle: effect on liveweight gain of grazing livestock in Mexico

This study aimed to determine the efficacy and parasite resistance of levamisole (LV) and ivermectin (IVM) in beef cattle naturally infected with gastrointestinal nematodes, as well as the effect on the liveweight gain in a tropical wet region of Oaxaca, Mexico. From November 2019 to January 2020, sixtysix grazing calves were randomly allocated into three groups of twenty-two animals each, treated with LV or IVM or an untreated control group (day 0). Feces were collected 1 day before treatment and 15 days after treatment. The liveweight gain from each animal was recorded at days 0, 15, 30 and 45 post treatment. The LV group presented the highest reduction of eggs per gram (EPG) of feces, followed by the IVM group. Resistance to IVM was detected, although LV resistance was also suspected. The IVM group had significantly higher effective treatment at 93.5%, resulting in an increase (P<0.05) of liveweight gain of 16.1kg, followed by the LV group (92.4%) with 17.1kg, compared to the untreated control group. A significant (P < 0.05) negative correlation was observed between EPG and weight gain for the LV (r = -0.46) and IVM groups (r = -0.32). LV and IVM showed a lack of efficacy against gastrointestinal nematodes, as well as an adequate capacity for EPG reduction but with IVM resistance and detrimental effects on growth performance in grazing beef cattle.