Targeted-Selective Treatment of Dairy Cows against Gastrointestinal Nematodes: a Stepwise Decision Making Strategy

In dairy cows, the milk production (MP) response to anthelmintic treatment (AT) is variable, and the decision to treat still depends on how much risk averse farmers or vets are. The objective of this study was to develop a stepwise decision making strategy based on nested criteria associated with an increasing probability of MP response. A randomized controlled trial was conducted at housing in 123 grazing dairy herds (injectable eprinomectin). Monthly individual test-day milk yields were obtained. Linear mixed models were used to assess the effect of AT on MP. In a first step, all herds were included in the analysis. The criterion associated with the highest MP response was identified. In a second step, only herds meeting the first criterion were included in order to identify the criterion associated with the highest MP response in this subset. This was repeated a third time on herds meeting the first 2 criteria. When all the herds were taken into account, the effect of AT was significant but low (+0.3 kg/cow/day). The first discriminating criterion (step 1) was identified as the % of grazed grass in the diet (no MP gain in low-pasturing herds versus +1 kg/cow/day in moderate/high-pasturing herds). Among moderate/high-pasturing herds, the second discriminating criterion (step 2) was the TEC (+0.6 kg/cow/day in high-TEC herds versus +1.4 kg/cow/day in low-TEC herds). Finally, among moderate/high-pasturing AND low-TEC herds, the third discriminating criteria (step 3) was the bulk tank milk Ostertagia ODR (+0.2 versus +1.8 kg/cow/day when ODR < 0.9 and ≥ 0.9, respectively). The 3 criteria identified, when applied sequentially, are associated with increasing expected MP gains and are useful for farmers and vets with different attitudes regarding risk.