Weight gain based-targeted selective treatments for gastrointestinal nematode control: a field trial in a large-scale cattle farm

Abstract

Parasite control in grazing ruminants throughtargeted selective anthelmintic treatments(TST) has been proposed as a novel and sustainablestrategy to delay the development ofanthelmintic resistance by preserving nematodepopulations in refugia. TST involves treatingonly those individuals likely to benefit mostfrom anthelmintic intervention. In the presentstudy, we evaluated a weight gain-based TSTapproach for the control of gastrointestinal nematodes(GIN) in grazing cattle predominantlyexposed to Haemonchus spp. and Cooperiaspp. A total of 189 recently weaned beef calveswere enrolled in the trial. All animals receiveda levamisole treatment at the beginning of thestudy (baseline treatment). From March to July,calves were monitored monthly and treatedonly if their weight gain fell below the predetermined minimum threshold for that period. Treatments consisted of drug combinations (abamectin + oxfendazole or ivermectin + levamisole) with >99% efficacy, as established by a prior fecal egg count reduction test (FECRT). Most calves required only a single treatment during the study; five animals were never treated, as their growth rates consistently exceeded the threshold. Significant differences in average daily weight gain were observed between treated and untreated animals. Notably, treated calves with initially lower growth rates exhibited a marked improvement in weight gain following treatment. Their post-treatment gains were significantly higher than both their own pre-treatment values. In contrast, daily weight gain in untreated calves remained relatively stable, suggesting limited impact of GIN parasitism in this subgroup. These findings indicate that the growth response to anthelmintic treatment is most pronounced in animals with suboptimal pre-treatment performance, supporting the effectiveness of weight gain-based TST in selectively targeting individuals most in need of intervention.