Eight years of health records for ovine Johne’s disease (OJD) from a long-term breeding trial with Romneys and Merinos at AgResearch`s Tokanui Station were analysed, to study the quantitative genetics of animal resistance/susceptibility to OJD. Necropsy records were used in preference to serology to determine OJD status. A total of 2348 Romney and 1297 `Merino` ewe records (5 groups: two purebred Merino groups (Australian imported superfine and NZ purebred Merino), two Merino x Romney halfbred groups and a superfine Merino back-cross) were analysed for OJD and survival, comprising animals born between 1971 and 1978. Few males were retained past two years of age, so disease and survival data on males were excluded. Romney ewes had a mean lifetime incidence of OJD of 3.49% (range, by year of birth, 2.11 to 5.00%), whilst the mean for Merino ewes was 4.78% (range, by year of birth, 0.0 to 7.74%). The breed contrast approached significance (P<0.051). The mean age at OJD incidence was 3.44 ± 0.09 years for Romneys, compared with a mean age for surviving females in the sample of 5.04 ± 0.02 years (P<0.001). Corresponding means for Merinos were 3.38 ± 0.13 and 5.00 ± 0.04 years, respectively (P<0.001). The heritability of lifetime incidence of OJD, considered as a binomial trait and using animal-model restricted maximum likelihood methods, was estimated as 0.07 ± 0.14 and 0.18 ± 0.11 for Romneys and Merinos respectively, giving an overall value (pooled within breeds) of 0.14 ± 0.09. Adjusting for Merino group in the relationship matrix (rather than as a fixed effect) led to a heritability for Merinos of 0.24 ± 0.09. These estimates may be biased upwards by `vertical transmission` of OJD organisms from dam to offspring. The heritability of age at death/culling had values of 0.11 ± 0.04 and 0.16 ± 0.05 for Romneys and Merinos respectively, with a pooled value of 0.13 ± 0.03. With the low heritability estimates for OJD in Romneys it is concluded that traditional breeding methods to reduce the incidence would be slow or unsuccessful; however, for Merinos, selection could begin to reduce the incidence, but ranking later generations may become difficult.
Proceedings of the New Zealand Society of Animal Production, Volume 63, Queenstown, 179-182, 2003
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