Heritability of Johne’s disease and survival data from Romney
and Merino sheep
S.M. HICKEY, C.A. MORRIS, J.L. DOBBIE AND D.E. LAKE
AgResearch, Ruakura Research Centre, PB 3123, Hamilton, New
Zealand.
NZSAP 2003 Abstract No. 44
Proceedings of the New Zealand Society of Animal Production
63: 179-182
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.
Keywords: NZSAPAB;
Johne's disease; sheep; heritability; survival
Last Updated 8/07/2004
