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

Genotype by environment interaction for milk yield was investigated by analyzing 55,162 mature equivalent, first lactation records of daughters from 1339 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 standard deviation was used to define non-overlapping high (> or = 1600 kg) and low (< or = 1300 kg) Mexican environments and a low (< or = 1025 kg) US environment. Variance components across Mexican environments were about 40% less than those of the US environment. Genetic correlation coefficients between milk yield in various Mexican environments and all US environments ranged from 0.60 to 0.71 and were different from unity (P < 0.001). Genetic correlation coefficients with low environment in the US ranged between 0.69 and 0.93; the largest correlation 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 genotype by environment interaction. A significant rank change in estimated breeding values (EBV) of sires in Mexican environments relative to the US was another indicator of genotype 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 from all data in the US. Against EBV estimated from the low environment in the US they were 0.57 and 0.83. The US low environment was a better predictor of performance in Mexican environments.