Genotype-by-environment interaction on genetic relationships between lactation persistency and conception measures in Japanese Holstein cows.

The objective of this study was to examine the genetic relationships between first-lactation persistency and four measures of conception accounting for heterogeneous genetic (co)variances over a wide range of milk production and fertility management environments. The combination of genotype and management environment conditions (genotype-by-environment (GE) interaction) was studied by using a reaction norm model. Data were obtained on 421,999 cows from 9296 herds for first parity and 257,815 cows from 8318 herds for second parity. Conception status during the first two parities from calving to 70 days in milk (DIM) and 180 DIM were evaluated, leading to a total of four conception measures. Under poor reproductive management, the genetic correlations between first parity lactation persistency and conception to 70 DIM were 0.408 and 0.242 for the first and second parities, respectively, compared with 0.178 and 0.116 for the respective genetic correlations between lactation persistency and conception to 180 DIM. These positive correlations indicate that increased persistency would help to improve conception to 70 DIM or 180 DIM. The genetic correlation between persistency and conception to 180 DIM during the first parity was −0.211 under a high level of reproductive management and moderately high milk production, suggesting that conception to 180 DIM for low-persistency cows of first parity was increased by improving reproductive management under this situation. In contrast, the genetic correlation between persistency and conception to 180 DIM in second parity was 0.351 under high management of milk production and reproduction. The genetic correlations between lactation persistency and conception to 70 DIM or 180 DIM for the first two parities ranged from negative to positive, depending on the combination of genotype and management environment conditions. Therefore, it is important to consider GE interaction when combining lactation persistency, conception measures, and milk yield to select for sustainable milk production systems. [ABSTRACT FROM AUTHOR]