Estimation of genetic and phenotypic parameters for milk fat to protein ratio of Holstein dairy cattle in Iran using random regression model

The aim of this research was to estimate (co) variance components, heritability, genetic and phenotypic correlations between different stages of lactation for milk fat to protein ratio (FPR) of Holstein dairy cattle in Iran using a random regression model. The data used included 1302984 test day records of 149440 cows in 307 herds that were calved during the years of 1996 to 2017. Contemporary groups herd-month of recording (herd test day (HTD)), the age at the calving and lactation curve were considered as fixed effects in the model. Additive genetic and permanent environment effects were fitted by the Legendre’s orthogonal polynomial with forth order. The results showed that FPR had an adequate genetic variation, but due to the high environmental variations, the estimates of heritability were low in different stages of lactation and varied from 0.06 to 0.08. Genetic correlations of FPR in different stages of lactation were in the rage of 0.24 to 0.99. The genetic correlations between first and second stage was high (0.84); but its decreased dramatically with other stages and ranged from 0.24 to 0.52. However, the genetic correlation between the other lactation stages was gradual decreased with increasing the interval between them. The phenotypic correlations between lactation stages were lower than the genetic correlation and were observed in the range of 0.02-0.25. Genetic improvement is possible for FPR because of genetic variation; however, more daughters are needed for each sire to achieve appropriate accuracy of breeding values due to low heritability.