Is the Famacha© system always the best criterion for targeted selective treatment for the control of haemonchosis in growing lambs?

Targeted selective treatment (TST) aims to aid in the control of gastrointestinal nematodes in small ruminants, identifying animals that require treatment with the purpose of preserving the Refugia population and delaying the buildup of anthelmintic (AH) resistance. The objective of this study was to evaluate the effectiveness of both productive and clinical indicators in the selective treatment of gastrointestinal parasites in growing lambs. Two experiments were carried out between 2015 and 2016. Experiment 1: lambs (n = 70) exclusively raised on pasture were evaluated from the ages of 60 days to 180 days old. Experiment 2: lambs (n = 48) raised on pasture and fed concentrated commercial supplementation were evaluated from the ages of 60 days to 150 days old. Parasitological (eggs per gram of feces - EPG), productive (average daily weight gain - DWG), and clinical (hematocrit - Ht and FAMACHA© system - F) parameters were measured every 14 days. The lambs were divided into 4 experimental groups with different criteria for AH treatment: the control group (CG), was treated with AH every 30 days; the FAMACHA© group (FG), in which lambs classified as F3, F4, and F5 were treated with AH; Daily weight gain group (DWGG), in which lambs with average DWG ≤ the average DWG of the CG minus one standard deviation were treated with AH; FAMACHA© + daily weight gain group (FDWGG), in which lambs classified as F3, F4 and F5 and/or lambs with average DWG ≤ the average DWG of the CG minus one standard deviation were treated with AH. In experiment 1, no significant differences in the general means of DWG and Ht (p > 0.05) were observed between experimental groups, although the lowest mean EPG was observed in the CG (p < 0.05). In experiment 2, the lowest mean EPG (p < 0.05) was also observed in the CG (1044.5 EPG), and the highest mean (p < 0.05) in the FG (4845.5 EPG). There were differences in the mean DWG of the CG (198.4 g) and FDWGG (200.0 g), which were both higher (p < 0.05) than that of the FG (90.5 g), and similar (p > 0.05) to the DWGG (151.8 g). In both experiments, considering the total mean number of AH treatments, FG had the lowest number of lambs treated with AH (p < 0.05), with the DWG and FDWGG being similar to each other (p > 0.05). Even when Haemonchus is the major parasite, it is not recommended to use the FAMACHA© system as an exclusive criterion for TST in growing lambs. The DWG productive criterion can be used effectively in the TST of growing lambs.