Effect of selection for milk yield on lactation curve parameters in Jerseys

Daily milk weights were collected for daughters of bulls selected for high predicted difference for milk (selection line) and unselected controls between 1970 and 1984. Cows with records with less than 280 days in production were excluded. Production from day one through seven and beyond day 400 were excluded. Lactation curves for 383 selection and 153 control cows in first lactation cows were predicted from daily milk weights and regression coefficients of lactation curve parameters were used in subsequent analyses. Regression coefficients were analyzed with a model that included line, sire (line), month of calving, year of calving, month of calving * year of calving, and age at calving was a covariate. Line and age at calving did not affect parameter estimates, initial yield (a), prepeak slope (b), and postpeak slope (c). Month of calving, year of calving, and month of calving * year of calving significantly affected all lactation curve parameters. Initial milk yield approached significance for sire (line). Multivariate analysis was performed on lactation curve parameters, b and c, that determine shape. Line was not significantly different. Least square means for factors affecting lactation (total milk production, initial milk production, peak milk production, day of peak production, total days in lactation, and age at calving) were obtained using same model as univariate and multivariate analysis, excluding age at calving. Line significantly affected total milk production, peak milk production, and initial milk production. Selection cows tended to peak later. but day of peak was not different significantly. Lactation curves for selection line cows were scaled upward due to higher initial yields. Line did not alter lactation curve parameters or shape.