H.D. Norman*, J.L. Wright, and R.H. Miller
Animal Improvement Programs Laboratory, Agricultural Research Service, USDA, Beltsville, MD
2010 J. Dairy Sci. (?)
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To determine if genetic evaluations of gestation length (GL) were reliable and repeatable, Holstein bulls used to develop those evaluations were stratified into 7 groups according to predicted transmitting ability (PTA) for service-sire GL based on calvings from 1998 through 2004: <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 later mated to the same bulls were grouped by the service-sire PTA GL groups (group size of 8,317 to 73,324 gestations), and their GL were examined to determine effectiveness of PTA GL. The model included fixed effects for service-sire group, conception date, conception date squared, and herd-year. Mean GL for mates by service-sire group (from lowest to highest PTA) were 275.3, 276.4, 277.7, 278.7, 279.6, 280.7, and 281.8 d. Yield in the subsequent lactation was also examined for mates by service-sire group. Least squares differences by service-sire group were -84, -79, -14, 15, 28, 50, and 0 kg for standardized milk yield; -2, -3, -1, 1, 1, 1, and 0 kg for fat yield; and -2, -2, 0, 1, 1, 1, and 0 kg for protein yield. The relationship between PTA for service-sire GL and yield appeared to be curvilinear as yield decreased for the >2.0 d PTA group. Phenotypic relationship between GL and standardized yield in the subsequent lactation also was examined using 9 cow GL groups: <275, 276, 277, … 282, and >283 d. Group size ranged from 17,493 to 64,876 gestations. The model included fixed effects for GL group, conception date, conception date squared, and herd-year. Least squares differences by GL group were -528, -287, -218, -194, -128, -122, -90, -90, and 0 kg for standardized milk yield; -19, -10, -8, -7, -5, -4, -4, -3, and 0 kg for fat yield, and -11, -6, -4, -3, -2, -2, -2, -2, and 0 kg for protein yield. Cows mated to bulls with PTA for longer service-sire GL and cows with longer GL phenotypes were more productive in the lactation following gestation.
(Key words: gestation length, genetic evaluation, lactation yield)