Genotype-environment interaction for age at first calving in Limousine and Charolais cattle raised in Italy, employing reaction norm model

The genotypes express differentiated responses depending on the environmental conditions and this can cause a re-ordering of the genotype in the environmental gradient. Therefore, our objective was to evaluate the genotype-environment interaction (GxE) for age at first calving (AFC) in Limousine and Charolais cattle raised in Italy, using reaction norm model (RNM). The data comprises information from 35,255 Limousine and 7,339 Charolais animals, born between 1999 and 2017. The standard animal model (AM), which ignores the G x E interaction, and the hierarchical models of reaction norms with homogeneous and heterogeneous residual variances with one and two steps (MHNRHO1P, MHNRHO2P, MHNRHE1P, and MHNRHE2P) were used for the analysis. In both breeds, the MHNRHOP1 model presented the best fit. In favorable environments (lower AFC), there was less phenotypic variation; the AFC expression of the animals was close and this may have caused lower estimates of additive genetic effects ( σ A 2 ) and consequently lower heritabilities (h²). In Charolais, the h² for the animal model (AM) was 0.13, whereas for MHNRHOP1 the value varied throughout the environmental gradient from 0.06 to 0.13, for low and high AFC environments, respectively. In Limousine breed, a lower h² (0.06) was observed in the animal model (AM), while in the MHNRHOP1 model it varied from 0.00 to 0.15. In both breeds, bulls with higher genetic merits for one environment were not for another, demonstrating the existence of the GxE. In Charolais and Limousine breeds, more than 93.75% and 87.72% of the bulls presented extremely robust or robust genotypes, respectively. Although most bulls are robust, there are bulls reactive to the environment, which can lead to incorrect selection of breeders. The results suggest that the genotype-environment interaction should be considered in the selection for AFC in both breeds raised in Italy.