Incidence and heritability of black wool spots in Romney sheep

The occurrence of detected black wool spots (BWS) and identities of affected animals in a New Zealand Romney flock have been routinely recorded since 1984. Incidence and pedigree data from 1984 to 1994 (122 094 animals, with 100 672 born between 1984 and 1994) were used to estimate the heritability of detected BWS by weaning (BWSW) and yearling ages (BWSY). The binary data were analysed with a nonlinear threshold model using two procedures: animal model marginal maximum likelihood (MML) and animal model Method R (MAPR). For both, the statistical model contained the fixed effect of contemporary group (year-mob) and a random individual additive genetic effect. Over the 10-year period, an average of 4.1 and 5.6% of animals were culled for BWSW and BWSY, respectively. Heritability estimates of BWSW using MML and MAPR methodology were 0.00 and 0.070 +/- 0.018, respectively. For BWSY, heritability estimates were 0.056 and 0.072 +/- 0.014 for MML and MAPR methodology, respectively.