Genetic trends for milk yield of Jerseys and correlated changes in productive and reproductive performance

Estimates of genetic trends in 24 measures of milk and constituent yields, somatic cell counts, and reproduction were obtained from 935 records of 374 Jerseys in a single herd. Data were obtained from a designed project for s ingle-trait selection from 1969 through 1987. One line was subjected to sel ection solely for milk yield and included 259 cows; an unselected control l ine included 115 cows. Estimates of trends were based on differences in lin ear phenotypic trends between lines for first lactations, all lactations, a nd for 305-d and total records. The genetic changes in milk yield for these four data sets were 1.22 to 1.48%/yr (36.8 to 41.0 kg per cow yr) and 0.54 to 1.64%/yr for five constituent yields. Except for the percentages of min erals plus lactose, all constituent percentages decreased by 0.05 to 0.60%/ yr. The ratios of protein to fat and solids-not-fat to fat increased 0.30 t o 0.54%/yr, respectively. The number of services required per conception in creased (0.17%) in first parity records and in all data (0.69%). The interv als from parturition to first estrus and from parturition to first service decreased in first lactation (1.19 and 0.82%) annually but increased (1.25 and 0.01%) in all data. Age of heifers at first estrus decreased by 0.44% a nnually. Most of the five measures of somatic cells decreased in first lact ations but increased for all data. Estimates of realized genetic correlatio ns of 14 measures of constituent yield and composition (four correlations e ach) agreed well with values expected from the literature. The results quan tified change in milk yield, constituent yields and percentages, reproducti ve performance, and somatic cell counts in a single herd and should prove u seful in the development of selection programs for dairy cattle.