CORRECTION FACTORS FOR ADJUSTING LACTATION MILK-YIELD FOR LACTATION LENGTH IN SAHIWAL

A total of 1 310 lactation records of Sahiwal cows were used to develop additive and multiplicative correction factors for lactation length. Lactation length was classified into 14 classes with an interval of 30 days between different classes. The mathematical model for least-square analysis and for deriving correction factors in addition to lactation length also included the fixed effects of farm, season and period of calving and age at calving as covariable (linear and quadratic). Different criteria used to compare the effectiveness of these two types of correction factors suggested that the additive and multiplicative correction factors were equally good. But the multiplicative correction factors retained the same coefficient of variation as that of unadjusted milk yield in different classes of lactation length. For deriving the multiplicative correction factors, the environmental effects are assumed to be related to an individual production and therefore the multiplicative correction factors may be more appropriate for adjusting the milk production records.