The value of bulk milk ELISA Ostertagia antibody titres as indicators of milk production response to anthelmintic treatment in the dry period.

An experiment was done to study whether estimation of the herd infection level, by assessing Ostertagia antibodies in bulk milk samples can serve to predict the effect of anthelmintic treatment on milk production. Bulk milk samples were collected from 134 farms at monthly intervals on three occasions prior to the start of the study. The ELISA titres to Ostertagia found in October at the end of the grazing season served as a basis to select 16 and 18 farms as having high and low levels of parasitism respectively. Heifers and cows within each herd were ranked by expected calving date, paired and randomly allocated to be treated with ivermectin or a placebo. Records of milk production and composition were collected for all the trial animals. The response to treatment expressed as the 305 day corrected milk yield of anthelmintic treated animals minus that of placebo treated animals was not statistically significant and amounted to 78 kg for multiparous cows and 124 kg for heifers. For cows as well as heifers the response to treatment was larger in the high antibody level herds than in low antibody level herds, but these differences also lacked statistical significance.