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A randomized clinical study was conducted in a total of 45 commercial dairy farms in France (14 farms), Germany (28 farms) and the UK (3 farms) to evaluate the effect of an anthelmintic treatment on milk yield in the subsequent lactation. A total of 1287 animals with suspected exposure to Ostertagia ostertagi were included in the study. Animals were treated during the dry period (7-77days before parturition) with moxidectin pour-on (Cydectin(®) 0.5% Pour-On, Zoetis; 638 animals) or left untreated (649 animals) according to a randomized block design. Animals were either heifers (n=296) or multiparous cows (n=991). The milk production was monitored at regular intervals after treatment up to 335days after lactation, and analysed using a general linear mixed model with the milk production as outcome variable and several random effects. The effect on milk yield after anthelmintic treatment over the whole subsequent lactation varied from no effect (-0.43kg/day; P=0.35) to an increase of milk yield with 2.35kg/day (P=0.01), depending on the study region and parity of the cows. Lactation curve analysis suggested that the treatment effect was mainly caused by a slower decay of the milk production in the treated animals compared to untreated animals. The present study highlights the beneficial effect of a topical treatment with moxidectin before parturition on milk yield in the subsequent lactation, as well as the importance of a careful evaluation of nematode exposure risk based on local grazing management practices to guide and target production-based anthelmintic treatment decisions.