Investigation of interaction between genotype and production level in Iranian Holstein dairy cattle using test day records

In this research, milk yield and fat percentage records of Iranian Holstein dairy cattle were used for investigating the existence of genotype by production level interaction via multi-trait random regression model. The records of first lactation were extracted from the database of National Animal Breeding Centerfor cows calved between 2001 and 2010. Herd-years of calving were grouped in low, medium and high production levels based on milk yield and fat percentage. For milk yield, 827295 records from 98136 cows/2146 herds and for fat percentage, 713053 records from 87928 cows/2395 herds remained in the data file. (Co)variance components for milk production traits were estimated by restricted maximum likelihood (REML) method. For milk yield, rank correlations of estimated breeding values for 305-d yield between low and medium production levels and between low and high production levels were 0.77 and 0.78, respectively and for fat percentage, among all levels were higher than 0.90. Genetic correlations between low and high production levels and between low and medium production levels of milk yield in early and late lactation period were less than 0.8; but correlation between medium and high production levels of milk production and between all levels of fat percentage were high. Re-ranking of sires in different production levels led to ratings and genetic correlations lower than 0.8 implying to significant presence between genotype and environment interaction.