|Miller, Robert -|
Submitted to: Journal of Dairy Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: October 19, 2010
Publication Date: February 1, 2011
Citation: Norman, H.D., Wright, J.R., Miller, R.H. 2011. Potential consequences of selection on gestation length on Holstein performance. Journal of Dairy Science. 94(2):1005-1010. Interpretive Summary: The study was to verify the accuracy of predictions of gestation length for service sires and determine the consequence of change in gestation length on other traits. Evidence was clear that gestation length can be changed by selection of bulls. Intermediate gestation length was optimum for subsequent productive life, days open, calving ease, stillbirth, and culling. Results indicated that neither shortening nor increasing the mean for gestation length in the Holstein population provided much overall benefit.
Technical Abstract: Genetic evaluations for gestation length (GL) for Holstein service sires were studied to determine their effectiveness in predicting GL in an independent data set. Consequences of selection on GL were assessed also by examining correlated changes in milk and fitness traits. Holstein bulls, each with 300 calvings between January 1998 and December 2005, were stratified into the following 7 groups according to predicted transmitting ability (PTA) for service-sire GL: <-3.00, -3.00 to -2.01, -2.00 to -1.01, … 1.00 to 1.99, and >=2.00 d. An independent set of 261,598 first-parity cows mated later to the same bulls and calving from January 2006 through November 2009 were segregated by the service-sire PTA GL groups (group size of 8,317 to 73,324 gestations), and these mates’ GL were examined to determine effectiveness of service-sire PTA GL. The model included fixed effects for herd-year and service-sire group, plus covariates for conception date and conception date squared to eliminate differences in time opportunity among mates (of benefit in some traits, e.g., productive life). Mean GL for mates by service-sire group (from lowest to highest PTA GL) were 275.3, 276.5, 277.8, 278.6, 279.5, 280.6, and 281.7 d. Thus, service-sire PTA GL was effective in identifying bulls that modified GL. Subsequent yield and fitness traits were examined also for the (independent) mates with the same service-sire groups. Intermediate service-sire PTA GL was optimal for yield traits and days open; performance for productive life, calving ease, stillbirth and culling generally became less favorable as service-sire PTA GL increased. A second examination was made by replacing service-sire PTA GL groups in the model with phenotypic cow GL groups. Phenotypic relationships between GL and subsequent performance for milk yield and fitness traits were examined using 9 phenotypic cow GL groups: <=272, 273-274, 275-276, … 284-285, >=286 d. Performance generally improved for subsequent lactation yield as cow GL increased; however, intermediate GL was optimal for productive life, calving ease, stillbirth, culling and days open. Results indicated that neither shortening nor increasing the mean for GL in the Holstein breed provided much overall benefit when all traits were considered. The same traits examined in the cows for the correlated effect from various GL were examined also in their offspring to determine whether the GL producing the calf had any influence on these same traits when the offspring reached their own productive period. There was virtually no carryover influence from GL on the dam to the other traits observed on the offspring when examined a generation later.