Comparison of two genetic evaluation methods of Iranian Holsteins using unadjusted and adjusted milk records based on 305-d lactation

In dairy cattle genetic evaluation, the use of real records as well as accurate statistical model are of great importance.
The main objective of the present research was to compare two methods of genetic evaluation of Holstein cows for milk yield. The records were unadjusted (for days in milk) and adjusted (for 305-days) milk yields.
A total of 332,160 records obtained from 332,160 first-parity Iranian Holsteins in 2,318 herds and collected from 1990 to 2010 were utilized. Animal models were used in which the effects of herd, year and month of calving, cow type (crossbred or pure), inseminated sperm (internal, external or unknown), covariables of lactation length (only for unadjusted milk trait), calving age, as well as additive genetic random effect of the cows were included. DMU software was applied to fit the models.
Heritability of unadjusted and adjusted milk traits were 0.23 (SE=0.004) and 0.25 (SE=0.004), respectively and significantly different from each other. For the bulls, correlation between breeding values obtained from two methods considerably increased as the number of their daughters increased. The magnitude of genetic trends in two methods were found to be 22.18 (for unadjusted milk) and 18.5 (for adjusted milk) kg per year which were not significantly different from each other.
With respect to the advantage of real records, it is suggested that unadjusted milk records to be used in an animal model (with covariable of lactation length) for predicting breeding value of Iranian Holstein cattle.