An investigation into heterogeneity of variance for milk and fat yields of Holstein cows in Brazilian herd environments

Heterogeneity of variance in Brazilian herd environments was studied using first-lactation 305-day mature equivalent (ME) milk and fat records of Holstein cows. Herds were divided into two categories, according to low or high herd-year phenotypic standard deviation for ME milk (HYSD). There were 330 sires with daughter records in both HYSD categories. Components of (co)variance, heritability, and genetic correlations for milk and fat yields were estimated using a sire model from bivariate analyses with a restricted maximum likelihood (REML) derivative-free algorithm. Sire and residual variances for milk yield in low HYSD herds were 79 and 57% of those obtained in high HYSD herd. For fat yield they were 67 and 60%, respectively. Heritabilities for milk and fat yields in low HYSD herds were larger (0.30 and 0.22) than in high HYSD herds (0.23 and 0.20). Genetic correlation between expression in low and high HYSD herds was 0.997 for milk yield and 0.985 for fat yield. Expected correlated response in low HYSD herds based on sires selected on half-sister information from high HYSD was 0.89 kg/kg for milk and 0.80 kg/kg for fat yield. Genetic evaluations in Brazil need to account for heterogeneity of variances to increase the accuracy of evaluations and the selection efficiency for milk and fat yields of Holstein cows. Selection response will be lower in low variance herds than in high variance herds because of reduced differences in daughter response and among breeding values of sires in low HYSD herds. Genetic investments in sire selection to improve production are more likely to be successful in high HYSD herds than in low HYSD Brazilian herds.

Equity of elite cow status across states

Cows with superior genetic merit, based on an economic index for milk, fat, and yields (milk-fat-protein dollars), have been designated as elite by USDA. Because of the concern that adjustment for heterogeneous variance may have resulted in inequitable state representation for elite cows, the percentages of US cows with elite status were compared by state using evaluations of registered Holstein cows from 1990, 1991, and 1997. The numbers and percentages of eligible cows and the proportions of those cows that were designated as elite were determined by state from May 1997 evaluations of 772,302 registered and 1,499,729 grade Holstein cows; means and standard deviations for milk-fat-protein dollars were computed. Correlations were computed among the number of cows that were eligible for elite status, the number of cows that were designated as elite, the percentage of eligible cows that were designated as elite, and the mean and the standard deviation for milk-fat-protein dollars. Models were examined to for ability to predict the percentage of elite cows by state from mean and standard deviation for milk-fat-protein dollars. The number of elite cows for a state was highly correlated to the number of cows that were eligible for elite status. States with > 1.0% of eligible cows designated as elite had mean milk-fat-protein dollars that were higher than the US mean of $44, but standard deviations were equal to or slightly lower than the US standard deviation of $71. The mean value for milk-fat-protein dollars was associated with the state percentage of elite cows, but variation of the index was not related. However, the standard deviation for milk-fat-protein dollars was important in explaining the percentage of elite cows if the model also contained the mean value. Differences in the variation of lactation records across states or adjustments for those differences did not appear to cause inequity in designating elite status. Corresponding results for grade cows supported findings for registered cows.

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.

Method and effect of adjustment for heterogeneous variance

Lactation records were standardized for differing genetic and error variances across herds and over time based on phenotypic variance for each herd-year-parity group. Each herd-year-parity phenotypic variance estimate was combined with those of adjacent years and regressed toward a region-year-parity variance. Heritability was assumed to be .25 at mean variance within year and to range from .2 for herds with smallest phenotypic SD to .3 for herds with largest phenotypic SD. Lactation deviations from management group mean were adjusted by ratio of base genetic SD to genetic SD estimated from heritability and phenotypic SD. The base was defined as 1987 calvings for first parity and 1988 calvings for later parities. Records were weighted according to heritability by multiplying lactation length weight by herd error weight defined as ratio of base error variance to error variance in the adjusted record. Estimated genetic trend for milk increased by nearly 5 kg/yr for Holsteins with this adjustment, which caused predicted breeding values of oldest animals to be lower by about 100 kg. Most correlations of parent and progeny information were slightly higher with adjusted data. Cows in high variance herds were most likely to have large reductions in their evaluations. Adjustment for heterogeneous variance was implemented in July 1991 for national evaluations for yield traits.

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

In 1967, the Jersey herd at the Dairy Experiment Station, Lewisburg, TN was divided into two groups on the basis of ancestry, type, and breeding value for milk as part of a project to determine effects of single trait selection for high milk yield on yield and correlated traits. Control group was mated randomly to 20 unproven young sires selected randomly from those available from breeding studs in 1967. Selection group was mated to sires selected solely on the basis of their high transmitting ability for milk. Selection sires were selected at intervals and used for 4 yr. By the end of the project (1984), lactation information was available on 672 daughters (520 selection and 152 control) of 37 bulls (17 selection and 20 control). Differences in breeding values for milk, fat, and fat test as calculated from the PTA reported in the July 1989 USDA genetic evaluations and differences in first lactation mature equivalent production of milk, fat, fat test, and 4% FCM were examined. Linear mixed models were used for all analyses and contained the fixed effects group, generation within group, and year. Sires were random, nested within group, and used to test for group differences. Groups differed for all traits. Selection was superior to control in breeding value for milk and fat (828 and 31 kg) and for production of milk, fat, and 4% FCM (1066, 42, and 1061 kg). Control was superior to selection in breeding value and production fat test (.15 and .12%). Group differences existed within generation class for all yield traits but not for fat percentage. Realized response closely matched or exceeded expected response as estimated from pedigree information.

Genotype by environment interaction for Holstein milk yield in Colombia, Mexico, and Puerto Rico

Components of (co)variance and genetic parameters were estimated by REML procedures from first lactation mature equivalent Holstein milk records from 54,604 Colombian, Mexican, and Puerto Rican cows and 198,079 US cows. The objective was to determine the cause of heterogeneous daughter response to sire selection for milk yield between the regions. Data from Latin America were partitioned by country and by herd-year SD class for milk to obtain five joint analyses between the US and Latin America, low herd-year SD, high herd-year SD, Colombia, and Mexico. Sire and residual variances for milk were 41 and 29% smaller in Latin America than in the US, 47 and 58% smaller for low than for high herd-year SD, and 31 and 49% smaller for Colombia than for Mexico. Resultant heritabilities ranged from .20 to .29. Genetic correlations for milk yield between the US and Latin America, low and high herd-year SD, Colombia, and Mexico were .91, .82, .89, .78, and .90. Expected correlated responses for milk in Latin America, low and high herd-year SD, Colombia, and Mexico were 70, 53, 79, 56, and 78% of the direct response in the US. The scaling effects of heterogeneous variance resulted in smaller daughter milk responses in Latin America compared with the US even when herd-year SD was similar.

Commercial feasibility of embryo transfer technology: a case study

The owner of a 500-cow Holstein herd requested economic assessment of his embryo transfer program. Actual net economic benefit was determined from marginal cost and present value of lifetime milk predicted from first lactations of 24 cows produced by embryo transfer compared with those of their 51 contemporaries sired by artificial insemination. Actual average pregnancy rate was 60% by embryo transfer or AI. An average of 5.5 transferable embryos was obtained per collection, which produced 1.37 cows in first lactation. Additional milk from embryo transfer cows was from more intensive selection of sires. Additional milk from the donor cows did not differ from zero. Actual marginal cost of a replacement by embryo transfer was +215, but it would have been +200 with same +25/unit semen price as contemporaries. If donors had been from the elite 5% for transmitting ability in milk, present value of gain in milk (5% real interest rate) ignoring additional feed costs would have been less than the cost of embryo transfer. For management and costs similar to this case, embryo transfer is not economically justified for producers earning income primarily from the sale of milk.

Effects of persistency and production on the genetic parameters of milk and fat yield in Israeli-Holsteins

Herd-years of Israeli-Holsteins were stratified into three groups by two criteria: mean annualized milk yield [365 (total lactations yield/calving interval)] and mean persistency, estimated as the ratio of daily milk production at the 5th mo postpartum to daily production during the postpartum peak period. The latter was taken as an indication of the relative environmental stress on the cow. Primi- and multiparous cows were analyzed separately. Genetic parameters were estimated for annualized milk and fat production at each production and persistency group. Heritabilities increased with a rise in production for both primiparous and multiparous cows, but the effect was greater for multiparous cows. Even though persistency and production were correlated, no clear trends were evident for stratification by persistency; thus, a relationship between stress and heritability was not established. Genetic correlations among stratification groups were between .7 and .9 for persistency and between .6 and .86 for production; thus, sire x environmental interaction was greater for production than for persistency stratification. Production in a given year can be used as a criterion for selecting herds to test progeny of young sires in the following year.

Comparison of heritability estimates from daughter on dam regression with three models to account for production level of dam

Three models were used to estimate heritabilities for milk yields at different production levels and for different years as twice the regression of daughter residual effects on dam residual effects. The denominator is the residual mean square for dams. The numerator is the difference between the residual term for sum of dam's and daughter's records and sum of residual terms for records of dams and daughters. Model 1 included sire of daughter and herd-year-season of daughters only. Model 2 included sire of daughter, herd-year-season of dam, and herd-year-season of daughter. Model 3 included sire of daughter and herd-year-season of dam and herd-year-season of daughter combination. The weighted mean estimates for each method were, respectively, .35, .38, .38 for milk production and .61, .67, .67 for fat test. Yearly time trends in heritability were slightly positive for both milk production and fat test. Standard errors of heritability estimates from model 1 were 40 to 50% smaller than those from models 2 and 3 due to the smaller number of effects in the model. Estimates for model 2 from low to high production levels averaged .30, .38, .38, and .42 for milk yield and .64, .68, .67, and .71 for fat test.

Genetic methods of improving dairy cattle for the south: a review and prospects from Regional Project S-49

Objectives of Southern Regional Research Project S-49, comprising eight contributing experiment stations, are to determine direct response to selection in milk and the resultant correlated responses in nonyield traits, to define and evaluate selection criteria for total economic merit or profit, to quantify genetic and environmental components of underlying physiological characters responsible for genetic gains in economically important traits, to determine interactions of genotype by environment, and to estimate genetic values for males and females. More than 150 scientific articles from S-49 have been published in journals from 1975 to 1985. These results have contributed in a major way in identifying issues and understanding the genetic control of cow performance and economics of genetic improvement programs.