Towards targeted selective treatment for gastrointestinal nematodes in pastured dairy cattle: role of herd indicators and individual indicators

The objective of this study was to evaluate, in adult dairy cows, the evolution of milk production after treatment for gastrointestinal nematode (GIN) at housing, and to identify herd and individual variation factors. A field trial involving 20 pastured Holstein 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, Panacur®) (533 cows), or a control group (544 cows). Daily cow milk production data were recorded from 2 weeks before until 14 weeks after treatment. Individual serum anti-Ostertagia antibody levels (expressed as ODR), pepsinogen levels and egg output, as well as bulk tank milk ODR, were measured once, at time of treatment. Moreover, in each herd, information regarding heifers’ grazing and anthelmintic treatment history was collected to assess the time of effective contact (TEC) with GIN larvae before the first calving. Milk production 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 herd where the percentage of positive egg count was > 22.6% responded better than those from herd where it was lower. Cows from low-TEC versus high-TEC herds also showed a better response, particularly when the bulk tank milk ODR was high; however, bulk tank milk ODR taken into account alone did not appear as a significant variation factor of the treatment effect. Primiparous cows, cows with days in milk (DIM) < 200 at time of treatment, and cows with low individual ODR (<0.38) responded better to treatment 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.