Gastrointestinal nematode parasites in small ruminants pose a major challenge to global agriculture. In most cases, their control depends on the availability of anthelmintics, which must be used in a manner that achieves a balance between ensuring adequate productivity and minimising the impact of inevitable selection for drug resistance. In this observational field study, the liveweight gains of lambs grazed on putative heavily infective larval contaminated pasture were compared after effective or ineffective anthelmintic drug treatments. Moxidectin, monepantel, and a derquantel-abamectin combination achieved 100% post treatment efficacies in faecal egg count reduction tests, while the efficacy of levamisole against Teladorsagia circumcincta was only 33%. During the 14 days after treatment, lambs treated with moxidectin, monepantel, and a derquantel-abamectin combination, gained about 0.3 kg, 0.5 kg, and 0.7 kg, respectively, more than those treated with levamisole, generally supporting the economic investment in use of effective drugs. These observations serve to highlight the manner in which effective anthelmintic drugs are used most efficiently with the primary objective of minimising subsequent exposure of naïve lambs to high levels of infective larval challenge, as opposed to the more common practice of simply treating animals grazing on heavily contaminated pastures.