Efficiency of different age correction factors for milk yield in Holstein Friesian halfbred cows

Multiplicative, standard and ratio correction factors were developed and compared for precorrection of milk yield for age at calving using the data on 412 lactation records of Holstein Friesian halfbred cows. Age at calving was classified in to nine different classes with an interval of 14 months between classes. The least squares analysis of variance was carried out to study the influence of genetic and non-genetic factors on lactation milk yield and the analytical model included sire as random effect and season, year of calving and age at calving as fixed effects. The multiplicative correction factors (MCF) were derived by dividing least squares means of base class by the least squares mean of class of interest. The ratio correction factor (RCF) was derived by multiplying the ratio of milk yield and age at calving of individual with standard age at first calving by 1020 days and standard inter calving period as 365 days in cumulative pattern for subsequent lactations. The standardized milk yield by Standard Correction Factors (SCF), MCF, and RCF methods were reanalyzed to test their validity. Results indicated that MCF and SCF methods expressed nearly same coefficient of variation. The age at calving class had non-significant effect on actual milk yield and standardized milk yield by MCF and RCF, except SCF method. F-ratio of age at calving for standardized milk yield by MCF were almost zero. The heritability for actual milk yield and standardized milk yield by MCF, SCF and RCF methods were 0.67 ± 0.44, 0.46 ± 0.33, 0.85 ± 0.52 and 0.99 ± 0.57, respectively. The repeatability estimates of actual milk yield was found to be 0.95 ± 0.04 and the respective values for standardized milk yield by MCF, SCF and RCF methods were reduced and found to be 0.50 ± 0.20, 0.89 ± 0.08 and 0.86 ± 0.09, respectively.