L’élevage des veaux laitiers sous nourrices en agriculture biologique : description des pratiques et étude épidémiologique de la cryptosporidiose et des strongyloses digestives et respiratoires

The rearing of dairy calves with nurse cows has been developing in France since 2010 but remains poorly documented, particularly with respect to its impact on calf health. The first objective of the thesis was to describe this system and the perception that farmers have of it. The second objective was the epidemiological study of two major parasitic diseases of calves: cryptosporidiosis occurring in the neonatal period and digestive and respiratory strongylosis during the first two grazing seasons (GS1 and GS2). In 20 dairy farms rearing calves with nurse cows, farmer interviews were conducted and calves were monitored for parasites for two years. After a phase with the dam and an optional artificial feeding phase, the calves are fostered by a nurse cow at 8 days old, turned out at 1 month old andare weaned between 4 and 10 months old. Farmers perceive that this rearing system improves the welfare, growth and health of the calves. Cryptosporidium oocyst excretion, although generally low, was more frequent when environmental contamination was increased. In GS1, gastrointestinal nematode infection was low because of a dilution effect due to nurse cows and respiratory strongyles, present in the majority of the groups, caused few clinical signs. In GS2, exposure to gastrointestinal nematode was higher, especially in heifers that had not grazed much in GS1 with few calves per nurse cows, which suggests a lack of immunity acquisition in these groups. Further studies are needed to measure the production impact of these infections in heifers during GS2