Lactation length correction factors for standardization of lactational milk yield in crossbred cattle

A total 484 lactation records of 98 crossbred cows were used to standardize lactation milk yield for lactation length. Actual and standardized milk yields were analyzed by using least squares technique with a model containing effect of sire, season, year of calving and lactation length. Lactation length was classified into five classes with 60 days interval between different classes. The multiplicative correction factors (MCF) were derived by dividing least squares means of base class by the least squares mean of class of interest and the standard correction factors (SCF) were used as per Parekh and Dutt (1995). The standardized milk yield of each observation was derived by multiplying the ratio of respective lactational milk yield and lactation length by 305 days. To standardize the actual milk yield, ratio correction factor (RCF) was used. The standardized milk yields obtained by MCF, SCF and RCF methods were then reanalyzed to observe their validity. The least squares analysis of variance showed that sire, year of calving and lactation length class had significant effect on actual milk yield and standardized milk yield except MCF and RCF methods which showed non-significant effect of lactation length. On the basis of F-ratio of lactation length classes, reduction in repeatability, reduction in phenotypic correlation between lactation length and standardized milk yield and maximum percent reduction in residual variance, it could be concluded that RCF method of milk yield standardization was efficient than other methods for crossbred animals.