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Abstract :

In first grazing season calves (FGSC), the anthelmintic (AH) treatments used to control the negative impact of gastrointestinal nematodes (GIN) on growth must be rationalized to preserve their long-term efficacy. The aim of this PhD thesis was to develop and assess targeted selective treatment (TST) strategies based on growth in FGSC, in order to preserve GIN populations in refugia i.e. not exposed to AH, and thus delay the emergence of AH resistance. Firstly, the relation growth/GIN infection at housing was demonstrated in different environments which allowed identifying groups, and within groups, the most infected animals. Then, a tree treatment decision at housing was proposed combing grazing management indicators to identify the groups at risk, and several average daily weight gain (ADWG) thresholds to identify, within groups, the animals suffering the most of infection. A TST strategy based on mid- season mean ADWG was assessed in field survey in comparison with whole group treatment (WT). No significant difference, in terms of growth and GIN infection, was observed at housing between the TST and the WT groups. Lastly, the veterinarians’ behaviors and perceptions about the control of GIN in dairy cattle farming, including a more rational AH management, were assessed. The veterinarians recognize the need to consider the sustainability of the AH treatment but identify serval obstacles
as the development of advices and the availability of simple, reliable and inexpensive tools.
The results of this thesis show that it is possible to target the use of AH in FGSC basing on individual and group indicators.

Key words :

Heifer, gastrointestinal nematodes, average daily weight gain, anthelmintic, targeted selective treatment, grazing management, veterinarian