Exposure to pasture borne nematodes affects individual milk yield in Swedish dairy herds.

The association between exposure to gastrointestinal nematode (GIN) parasitism, indicated by elevated individual optical density ratio (ODR) to Ostertagia ostertagi, and individual milk production was analysed in dairy cows. The multivariable analysis also accounted for other sources of variation than exposure to GIN parasitism. Intra- and inter-herd variation in parasite exposure was also evaluated. Organic and conventional herds located in south-east Sweden (13 herds per system) were visited during the housing period 2009-2010. Levels of all major pasture-borne helminths in Sweden (O. ostertagi, Fasciola hepatica and Dictyocaulus viviparus) were determined in bulk tank milk, cow milk and serum (only for O. ostertagi). Their specific antibodies were detected using three different ELISAs (Svanova Biotech) and the optical density of the sample was expressed as ODR. Positive cases of Dictyocaulus and Fasciola were seldom found. Variation in ODR in serum samples for Ostertagia was higher within herds than between herds. Correlations between ODR for Ostertagia in serum and milk parameters were significant and negative. In the multivariable analysis, two models were performed separating data from primiparous and multiparous cows. Results from mixed models showed that daily milk yield was significantly influenced by Ostertagia ODR for multiparous cows. No interactions were found between Ostertagia ODR values and herd type.