Genotype by Birth or Rearing Type Interaction in Merino Sheep

This study explores if there is an interaction between the genetic potential for growth in Merino lambs and their birth (BT) or rearing (RT) type. Data consisted of 3,920 singles and 4,492 twins which were the progeny of 285 sires and 5,279 dams. We found a significant sire by BT interaction with the effect accounting for 1.59% and 2.49% of the phenotypic variation for birth weight (BWT) and weaning weight (WWT), respectively. The effect was not significant for post weaning weight (PWWT), scanned fat (SF) and eye muscle depth (EMD) with sire by BT effects accounting for less than 1 % of the variation in these traits. Sire by RT interaction effects were much smaller and not significant for WWT, PWWT and SF, but accounted for 1.83% of the variation in EMD, which was significant. A bivariate analysis treating phenotypes when expressed in singles and twins as two different traits resulted in genetic correlation estimates significantly lower than one, with BT having a larger effect on genotype expression than RT.