Main Factors Affecting Milk Yield Traits of Holstein Cattle Bred under The Conditions of Different Operations, and Essential Parameters to Selection
I. Effects of Environment and Heredity on the Shape of Lactation Curve in Holsteins

The log-transformed gamma (ln(yn)=ln(a)+bln(n)-cn) and inverse polynomial (n/yn= A0 + A1n + A2n²) functions fitted monthly test day milk yields explained 68% and 96% of variation respectively. Analysis of variance indicated significant (P<0.05) effects of breeding region, calving year, calving season, parity, service period and age on the lactation curve and milk production traits. Peak yield and 2X305 day-milk yield were lower in summer calvers, but persistensi was higher in summer and fall calvers. The highest peak and 2X305 day-milk yield were reached in the second and third lactations. Cows that conceived shortly after calving had lower 2X305 day-milk yield.

Heritability estimates were ln(a) 0.011, (b) 0.31, (c) 0.164, peak yield 0.067, days to peak 0.033, persistensi (s) 0.071, 2X305 day-milk yield 0.095, A0 0.085, A1 0.146, A2 0.206. Genetic correlations indicated that selection for faster rate of increase to peak would result in the faster rate of decrease in the milk yield after peak. Keywords : Cattle, Holstein, milk, Lactation curve, persistensi