The importance of genotype by environment interaction in genetic analysis of milk yield in Holstein cows

Efficiency of a breeding program affected by accuracy of genetic parameters and appropriate genetic model analysis is important to increase the accuracy of estimated genetic parameters. In an appropriate model all significant fixed and random effects are included. Previous studies show that genotype by environment interaction is an important random effect for genetic analysis of milk production in dairy cattle. Changes in the relative performance of genotypes (sires) across different environments play an important role in dairy production systems, especially in countries relay on imported genetic material (sperms). Genotype by environment interaction arises when the performance of the different genotypes is not equally influenced by the different environments. Then, ignoring the interaction, can led to biased estimates of genetic parameters and reduce the accuracy of selection by changing the ranking of the animals. Therefore, considering this interaction in a model may improve genetic gain and selection accuracy. This study was carried out to investigate the effect of genotype by environment interaction in genetic analysis of milk yield of Holstein dairy cows.
In this research, 90315 records of first lactation of the dairy cattle were used. The data were collected from 1996 to 2008 by the Iranian center of Animal Breeding and Animal Production Improvement which are from 19 different provinces, 188 herds and originated from 145 sires and 72857 dams. The milk yield records were corrected for 2 times milking frequency and mature body weight. Genotype by environment interaction can only be identified, if at least two different environments are considered. Environment can be defined as a unit, but also as a continuous value of the environment. In this study, the sires were defined as different genotypes. Province, herd, birth year and calving year were used as the criteria to define different environments. Then sire by province, sire by herd, sire by birth year and sire by calving year were considered as the genotype by environment interaction effects. To investigate the importance of these effects on genetic analysis of the milk yield, they were added to a base model where only additive genetic effect was fitted as the random effect. Log likelihood ratio test was used for the model comparison. Univariate animal model was used for analyzing the both dataset including the milk yield records corrected for 2 times milking frequency and for mature body weight.
In this research, log likelihood value of the base model for genetic analysis of the milk yield corrected for 2 times milking frequency and for mature body weight was significantly improved when the effects of sire by province, sire by herd, sire by birth year or sire by calving year were added. Accordingly, in both dataset, 0.001, 0.014, 0.001 and 0.002 percent of the phenotypic variance was due to the effects of sire by province, sire by herd, sire by birth year and sire by calving year, respectively. Then these effects were important for genetic analysis of milk yield (P This research highlights the importance of genotype by environment interaction in Iranian dairy cattle. Our results indicated that genotype by environment interaction is not important for genetic evaluation of the sires when herd, province, birth year or calving year are used to define the environment. Therefore, the genotype by environment interaction does not change the bulls ranking in the different environments.