Heterogeneity of variance of milk yield among herds and application of adjustment factors for sire and cow evaluation in Kenyan friesian cattle

Abstract

Production and reproduction records of Kenyan Friesian cows obtained at the Kenya Milk Records (KMR) , covering the period 1968 - 1984 , were used to study heterogeneity of variance of 305 - day milk yield and to develop parity and lactation length adjustment factors. Two different methods of multiplicative correction factors and linear regression were used to standardize milk yield to 305 day equivalent. Coefficients of variation and standard deviations across herds were used to test heterogeneity of variance. A fixed effect least squares model was used to analyse the contribution of fixed effects to heterogeneity of variance of milk yield, while lactation length and parity adjustment factors were developed by ratio method. The least squares means for 305M, 305L - day and annual milk yields were 2740.3 ± 10.37 kg, 2813.2 ± 11.08 kg and 2722.8 ± 12.88 kg respectively. Significant heterogeneity of variance of milk yield was found to exist between herds. Year of calving, herd, herd class , parity , calving interval and lactation length affected milk yields significantly and contributed to heterogeneity of variance. Season of calving did not influence milk yield. Age within parity affected 305 day milk yields significantly but not annual milk yield. Average herd standard deviation and coefficient of variation after correcting for the fixed effects were 672.2 Kg and 26.7% respectively, while scaled standard deviations between herds were 180 kg and 3.4% . Use of parity adjustment factors reduced the CV of 305M day milk yield from 24% to 20%, while the amount of variation accounted for by parity reduced from 82% to 23% . It was concluded that heterogeneity of variance of milk yield _ should be accounted for in sire and cow evaluation. The scaling of observations within individual herds by sample standard deviation minimises heterogeneity of variance and increases accuracy of selection. Adjustment factors for lactation length and parity were proposed as possible means of using all lactation records in sire and cow evaluation in Kenya.