Potentials of milk performance data as indicator for targeted selective treatment in Lacaune sheep

Anthelmintic resistance is a major threat in farming of small ruminants worldwide. One approach to slow down the development of anthelmintic resistance is targeted selective treatment (TST), where a part of animals is left unexposed to anthelmintic treatment and thus providing refugia for susceptible parasites. Closely linked to the successful implementation of TST is the identification of animals in need of treatment. In dairy goats it has been proposed to use milk yield as TST indicator, focussing treatment on high yielding dairy goats. In dairy sheep the relation between milk performance and infection of gastrointestinal nematodes (GIN) is not yet well known. The aim of this study was therefore to investigate the general relation between milk yield and GIN infection in a Swiss Lacaune dairy sheep subpopulation and, based on this, to evaluate milk yield data as a potential TST indicator in dairy sheep. A field study was performed including 1,159 lactating Lacaune dairy ewes on 15 dairy sheep farms in Switzerland. All ewes were sampled once between August and December 2019, when they had exceeded the 70th day of lactation. For each ewe the nematode egg excretion per gram faeces (EPG) was determined and individual milk performance data was obtained closely time-related to the date of faecal sampling. Coprocultures of pooled faecal samples were conducted to determine the proportion of Haemonchus contortus on farm level. A linear mixed model revealed that EPG increased significantly with increasing daily milk yield (P<0.01), indicating high yielding ewes to be less resistant to GIN infections than low yielding ewes. The effect was most pronounced in the earlier stage of lactation, but remained within a moderate range. The other included fixed effects milk protein content, lactation day, lactation number and the proportion of H. contortus did not show significant relations with EPG. The results suggest the possibility of using milk yield data as TST indicator in dairy sheep. In the frame of H2020 project SMARTER no. 772787.