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This study explores the interaction between genetic potential for growth in Merino lambs and their birth type (BT) or rearing type (RT). Data on birthweight (BWT), weaning weight (WWT), post-weaning weight (PWWT), scan fat (PFAT) and eye muscle depth (PEMD) were used from 3920 single and 4492 twin-born lambs from 285 sires and 5279 dams. Univariate analysis showed a significant sire × BT interaction accounting for 1.59% and 2.49% of the phenotypic variation for BWT and WWT, respectively, and no significant effect for PWWT, PFAT and PEMD. Sire × RT interaction effects were much smaller and only significant for PEMD. Bivariate analysis indicated that the genetic correlation (rg) between trait expression in lambs born and reared as singles versus those born and reared as twins were high for BWT, WWT, PWWT (0.91 ± 0.02 – 0.96 ± 0.01), whereas rg for PFAT and PEMD were lower (0.81 ± 0.03 and 0.86 ± 0.02). The rg between traits expressed in lambs born and reared as singles versus those born as twins but reared as singles were lower: 0.77 ± 0.08, 0.88 ± 0.03, 0.66 ± 0.06 and 0.61 ± 0.08 for WWT, PWWT, PFAT and PEMD, respectively. A different RT only affected the expression of breeding values for PFAT and PEMD (rg 0.62 ± 0.04 and 0.47 ± 0.03, respectively). This study showed genotype × environment interaction for BWT and WWT (sire × BT interaction) and for PEMD (sire by RT interaction). However, sires' breeding value of a model that accounts for sire × BT interaction provides a very similar ranking of sires compared with a model that ignores it, implying that there is no need to correct for the effect in models for genetic evaluation. This study explored whether breeding values for bodyweight are equally expressed in lambs with different birth or rearing type. The correlation between breeding values for early growth of lamb when expressed in single or twin birth types was less than one whereas different rearing type influenced expression of breeding value at late growth. However, this interaction between genotype and birth or rearing type did not affect ranking of sire's breeding value in these two different environments, implying that there is no need to account for the effect of genetic evaluation. [ABSTRACT FROM AUTHOR]