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Sire effects on antibodies to nematode parasites in grazing dairy cows

Abstract A study was carried out on New Zealand dairy cows at pasture, to test for evidence of genetic differences in immunological response to nematode parasites. Nine widely used Holstein‐Friesian artificial insemination bulls, with daughters in many herds, were evaluated for nematode antibodies by sampling milk of their daughters in mid lactation in each of 20 North Island herds. One milk sample was taken from each cow (ranging from 4 to 11 years of age), during a routine herd test in the period from mid November 2000 to early February 2001. Assays were undertaken subsequently on pooled samples of skim milk (0.2 ml per cow; up to 20 cows per sire x herd) to assess antibodies to both the infective third larval (L3) and adult parasitic stages of Cooperia oncophora and Ostertagia ostertagi. Sire effects were significant for all four antibody types (P < 0.001). For O. ostertagi L3, there was a 1.20‐fold range in the mean antibody levels of the sire groups; corresponding proportional ranges for the other three antibody types were from 1.22 to 1.23. The correlations of sire means among the four antibody types were high, averaging 0.81 (range 0.65–0.99). These results show that significant sire effects on anti‐parasite antibody levels were present in mature lactating dairy cows at pasture in New Zealand, with repeatabilities of sire means across herds of 0.29–0.39, and that sire effects on antibody levels to two different nematode parasite species were highly correlated (for antibodies to either the L3 or the adult stages).