Sire by flock-year interactions for body weight in Poll Dorset sheep

The performance of sires across flocks and years are likely to vary due to factors such as interactions with the environment, differences in ewe genotypes, errors in recording, non-random mating, and preferential treatment of progeny. Research in other breeds and species has shown that these sire by flock-year interactions typically account for less than 5% of the phenotypic variance. This paper examines the significance of sire by flock-year effects for weight traits in Poll Dorset sheep. The results demonstrate that while the sire by flock-year effects explained between 2 and 4% of the phenotypic variance, they significantly improved the fit of the model and resulted in a direct-maternal genetic correlation closer to zero. However, heritabilities were reduced significantly by up to 50%, indicating that sire by flock-year effects may be removing too much genetic variation for traits with maternal effects. On balance however, it seems advisable to include sire by flock-year interaction effects in the across flock evaluation.