To investigate future tools for selective treatment for gastrointestinal nematodes (GIN) in adult dairy cows, we evaluated herd and individual variation factors of the evolution of milk production (MP) after treatment. A field trial involving 20 pastured dairy herds in Western France was conducted in autumn 2010 and autumn 2011. In each herd, lactating cows were randomly allocated to a treatment group (fenbendazole) (533 cows), or a control group (544 cows). Daily cow MP was recorded from 2 weeks before until 14 weeks after treatment. Individual serum anti-Ostertagia antibody levels (expressed as ODR), pepsinogen levels, egg output, and bulk tank milk ODR were measured at time of treatment. Moreover, in each herd, information regarding heifers’ grazing history was collected to assess the time of effective contact (TEC, expressed in months) with GIN larvae before the first calving. TCE was expected to reflect the development of immunity against GIN. MP data were analyzed using linear mixed models (herd and cow as random effects). The overall treatment effect was significant but slight (maximum=+0.86 Kg/d on week 6 after treatment). Cows from low-TEC versus high-TEC (>8 months) herds, particularly when the bulk tank milk ODR was high, showed the best response to treatment; however, bulk tank milk ODR taken into account alone did not appear as a significant variation factor. Cows from herd where the percentage of positive egg count was >22.6% responded better than those from herd where it was lower. Primiparous cows, cows with days in milk (DIM)<200 at time of treatment, and cows with low individual ODR (<0.38) responded better than multiparous cows, cows with DIM>200, and cows with higher ODR respectively. These results highlight promising key criteria for selective treatment for GIN in dairy cows, particularly those reflecting a low level of immunity against GIN