Potential consequences of selection to change gestation length on performance of Holstein cows

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 also assessed by examining correlated changes in milk and fitness traits. Holstein bulls with ≥ 300 calvings between 1998 and 2005 were stratified into the following 7 groups using predicted transmitting ability (PTA) for service sire GL: <-3.00, -3.00 to -2.01, …, 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 between 2006 and 2009 were segregated by the service sire PTA GL groups (group had 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 dates to account for time opportunity among mates. 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 also examined 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 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. Relationships between GL and subsequent performance for milk yield and fitness traits were examined using 9 phenotypic cow GL groups: ≤ 271, 272-273, …, 284-285, and ≥ 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 also examined 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. Little carryover occurred from GL on the dam to the other traits observed on the offspring when examined a generation later.