Direct response in yield and correlated response in components accompanying selection for milk yield in Jerseys

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 single-trait selection from 1969 through 1987. One line was subjected to selection solely for milk yield and included 259 cows; an unselected control line included 115 cows. Estimates of trends were based on differences in linear phenotypic trends between lines for first lactations, all lactations, and 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 minerals 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 to 0.54%/yr, respectively. The number of services required per conception increased (0.17%) in first parity records and in all data (0.69%). The intervals 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% annually. Most of the five measures of somatic cells decreased in first lactations but increased for all data. Estimates of realized genetic correlations of 14 measures of constituent yield and composition (four correlations each) agreed well with values expected from the literature. The results quantified change in milk yield, constituent yields and percentages, reproductive performance, and somatic cell counts in a single herd and should prove useful in the development of selection programs for dairy cattle.

Realized versus expected gains in milk and fat production of Holstein cattle, considering the effects of days open

The objective of this study was to determine whether differences in first lactation production, unadjusted or adjusted for days open, were correctly predicted by pedigree estimates for two lines of Holstein cattle. Data on 875 cows from two selection lines were collected from 1970 to 1988. Lines were created by mating foundation females of high or low pedigree merit to sires selected for high or average PTA milk. Both lines were managed identically to minimize environmental differences. The number of days open was analyzed with a fixed effects model containing year, season, interaction of year and season, sire line, foundation group, and interaction of sire line and foundation groups. The high milk line had significantly more days open than did the average line. Adjusted and unadjusted records for milk and fat were analyzed with the model described previously, plus the interaction of sire line and year. Least squares means and estimates for mean parent average were used to calculate realized and expected differences in production between lines within and across years. Adjustment of records for days open reduced estimates of realized gain, but not significantly. Regressions of realized gain on expected gain indicated that expected gains were matched or exceeded by unadjusted or adjusted realized gain.

Deaths and disposals in lines selected for milk yield and control lines of Jersey cattle

Deaths and disposals of Jerseys from birth to 120 d of first lactation during a selection experiment were summarized. Cows were randomly assigned to the high milk yield and young sire lines. Service sires were bulls with the highest estimated breeding value for milk for the high milk line and a collection of all young unproven bulls available in AI in 1967. Cattle that were unsuitable for conditions of normal herd management were candidates for culling. Culling of uncalved heifers and of lactating cows was also permitted to maintain group sizes. Culling for herd reduction was random. Culling for low milk yield was permitted after cows reached 120 d of first lactation. Death and disposal data were analyzed through 120 d of first lactation. By 120 d of first lactation, 29.6% of females born to the young sire line had died or been involuntarily removed compared with 29.8% born to the high milk yield line. Deaths were 54.8% of total losses for the high milk yield line compared with 58.4% for the young sire line. Stillbirths were a larger proportion of total losses for animals in the high milk yield line (21.4%) than for those in the young sire line (15.3%). The most frequent cause of death was scours and pneumonia during 1st mo of life for animals in both lines. Selection of sires for high milk yield does not decrease total survival rates through the early first lactation.

Correlated responses in reproduction accompanying selection for milk yield in Jerseys

Reproductive traits of heifers and primiparous cows from a long-term selection project were analyzed to determine correlated response to single-trait selection for milk yield. Data were from 1056 daughters (765 selection, 291 control) of 37 bulls (17 selection, 20 control). Traits in heifers were ages at first observed estrus and at first breeding, services to conception, interval from first service to conception, and length of first gestation. Traits in primiparous cows were ages at first calving and at first breeding, after calving; services to conception; length of second gestation; and intervals from calving to first observed estrus, to first breeding, and to conception, from first service to conception, and from first to second calving. Analyses for services to conception in heifers and primiparous cows were categorical using models containing genetic group and generation. Analyses of other traits were by linear mixed models using fixed effects of genetic group, generation within group, and year-season of birth. Sires were assumed random and nested within genetic group. The mean square for sires within group was used to test for group differences. No significant differences were found between genetic groups in traits measured in heifers; however, the interval from first service to conception approached significance (control superior). In primiparous cows, differences between genetic groups were significant for the intervals of calving to first breeding and calving to conception and for length of second gestation (control superior). For other traits, reproductive performance of the control was better but not significantly different from that of the selected group. Reproductive performance should be monitored during selection for high milk yield.

Correlated response in growth and body measurements accompanying selection for milk yield in Jerseys

Growth and body measurements from a long-term selection project were analyzed to determine correlated responses to single-trait selection for milk yield. Data were from 1056 daughters (765 selection, 291 control) of 37 bulls (17 selection, 20 control) of 37 bulls (17 selection, 20 control) and included BW and measures of heart girth, chest depth, wither height, and length from withers to pins and from withers to hooks taken at 6 mo, 15 mo, first calving, end of first lactation, and maturity. Other data were birth weight, change in measurements and weights from first calving to end of first lactation, monthly rate of gain from 1 to 13 mo of age, and age reaching breeding weight (250 kg). Principal component scores were calculated from standardized measurements at each age. The first three principal components has meaning (size, length vs. girth, and height vs. girth). All analyses used linear mixed models with fixed effects of genetic group, generation within group, year-season of birth or calving, parity of dam, and birth status (multiple or single birth). Sires were assumed to be random and nested within genetic group. Mean squares for sires was used to test for group differences. Generation did not differ in any analysis and was removed from all models. Selection cows were heavier, larger in some measurements, and had greater overall size at 6 mo of age. Selection cows had greater monthly rate of gain and attained breeding weight at an earlier age. Genetic groups did not differ for any other measurement or weight. Control cows gained more weight and increased more in some measurements between first calving and end of first lactation. Selection for milk yield did not result in an undesirable correlated response in an growth or body measurement.