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Abstract :

In adult dairy cows, the anthelmintic treatments used to control the negative impact of gastrointestinal nematodes (GIN) on milk production (MP) must be rationalized, in order to preserve their long-term efficacy. The aim of this PhD thesis was to generate knowledge underlying the development of targeted selective treatment strategies. These strategies address the two key questions “which cows have to be treated?” (selective treatment) and “when do we have to treat?” (targeted treatment).
This work was conducted in pasturing dairy herds, in the North-West of France. First of all, the effect on MP of a fall anthelmintic treatment, applied at housing, has been investigated, and individual and herd-level indicators associated with the post-treatment MP response were identified. Then the sensitivity, specificity and predictive values of the combinations of indicators usable to select the cows that would benefit from treatment at housing were assessed. The effect of anthelmintic treatment administered at housing was positive but slight, and was variable at herd and cow levels. In the context of this study, in order to determine which cows should be treated, operational and specific indicators have been identified, which discriminate on the one hand the herds where the treatment could be profitable and on the other hand the cows within such herds. Particularly, to better target these herds, i) the time of effective contact with GIN larve before the first calving (TEC), and ii) the anti-Ostertagia antibody level in the bulk tank milk turned out to be two indicators to be considered simultaneously. Finally, the effect on MP of a spring anthelmintic treatment, applied 1.5 to 2 months after turn out, has been studied. During this season, anthelmintic treatment had a marked detrimental effect on MP. Treatment for GIN at the beginning of the grazing season should therefore be discouraged in adult dairy cattle.
The results of this work show that whole herd blanket anthelmintic treatments for securing MP are not appropriate. A rationalization is possible in adult dairy cattle, with easy-to-use indicators, preserving refugia parasite populations and thus delaying the emergence of anthelmintic resistance.

Key words :

Cattle, parasite, gastro-intestinal nematodes, milk production, anthelmintics, control strategies, targeted selective treatment, pasture