Interaction between milk yield of Holstein cows in Mexico and the United States

Genotype by environment interaction for milk yield was investigated by anal yzing 55,162 mature equivalent, first lactation records of daughters from 1 339 Holstein sires in Mexico and 499,401 daughters from 663 Holstein sires in the northeastern US. There were 474 US sires in common. Herd-year standa rd deviation was used to define non-overlapping high (greater than or equal to 1600 kg) and low (less than or equal to 1300 kg) Mexican environments a nd a low (less than or equal to 1025 kg) US environment. Variance component s across Mexican environments were about 40% less than those of the US envi ronment. Genetic correlation coefficients between milk yield in various Mex ican environments and all US environments ranged from 0.60 to 0.71 and were different from unity (P < 0.001). Genetic correlation coefficients with lo w environment in the US ranged between 0.69 and 0.93; the largest correlati on was between the low US and high Mexico environments. Both reductions in the size of genetic variance in Mexican environments relative to the US and genetic correlation coefficients less than unity were indicative of genoty pe by environment interaction. A significant rank change in estimated breed ing values (EBV) of sires in Mexican environments relative to the US was an other indicator of genotype by environment interaction. Regression coeffici ents of EBV of a sire estimated from daughters performances in low and high environments in Mexico were 0.46 and 0.62 against EBV of sires estimated f rom all data in the US. Against EBV estimated from the low environment in t he US they were 0.57 and 0.83. The US low environment was a better predicto r of performance in Mexican environments.