Objective: In first grazing season calves, the anthelmintic treatments used to control the negative impact of gastrointestinal nematodes (GIN) on growth must be rationalized to preserve their long-term efficacy. In this way, the suitability of a single mid-season targeted selective treatment (TST) for gastrointestinal nematodes control, based on flexible average daily weight gain (ADWG) thresholds, was investigated in 23 groups of first grazing season calves. Materials and methods: Each group was divided in two sub-groups and animals randomly allocated to one of these two sub-groups intended for two different mid-season anthelmintic treatment strategies: (1) a treatment of all calves (whole-group treatment-WT) or (2) a targeted selective weight gain-based treatment (TST). TST was based of those animals showing an individual pre-treatment ADWG inferior to the mean pre-treatment ADWG of the corresponding WT sub-group. Anthelmintic treatment (levamisole 7.5 mg/kg body-weight) was performed 3 to 4 months after turnout. At housing, two parasitological parameters (the anti-Ostertagia ostertagi antibody level and the pepsinogen level) and a clinical parameter (the breech soiling score) were assessed at individual level in each group. Results: Among TST sub-groups, the ADWG thresholds for treatment varied from 338 to 941 g/day and the percentage of treated animals from 28% to 75%. Pre- and post-treatment ADWG as well as parasitological and clinical parameters measured at housing were similar between TST and WT sub-groups. In the 6 highest GIN exposed groups (mean pepsinogen level ≥2.5 U Tyr), the average effect of treatment on post-treatment ADWG was the highest and estimated up to 14 kg after a grazing duration of 4 months. In contrast, in 6 other groups showing mean anti-Ostertagia ostertagi antibody level <0.7 in untreated animals, no effect of treatment was seen suggesting an absence of production losses related to a low level of infection. Conclusion: This study highlighted the suitability of a convenient mid-season TST strategy for first grazing season calves, based on the use of flexible thresholds of ADWG, allowing similar growth compared with a whole-group treatment while keeping a nematode population in refugia through heifers left untreated.