H.D. Norman,1 J.R. Wright, M.T., Kuhn, S.M. Hubbard, J.B. Cole, and P.M. VanRaden
Animal Improvement Programs Laboratory, Agricultural Research Service, USDA, Beltsville, MD 20705-2350
2007 J. Dairy Sci. (?)
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Genetic and environmental factors that might affect gestation length (GL) were investigated. Data from over 9 million parturitions from 1999 through 2006 for 7 dairy breeds were assembled from lactation, reproduction, and dystocia records from across the United States. Effects examined were year, herd-year, month, and age within parity of conception; parturition code (gender and multiple-birth status); lactation length and standardized milk yield of cow; service sire; cow sire; and cow. All effects were fixed except for service sire, cow sire, and cow. Mean GL for heifers and cows, respectively, were 277.8 and 279.4 d for Holsteins, 278.4 and 280.0 d for Jerseys, 279.3 and 281.1 d for Milking Shorthorns, 281.6 and 281.7 d for Ayrshires, 284.8 and 285.7 d for Guernseys, and 287.2 and 287.5 d for Brown Swiss. Estimated standard deviations of GL were greatly affected by data restrictions but were near 5 to 6 d for most breeds. Year differences in GL were extremely small, but month effects were moderate. For Holstein cows, GL was 2.0 d shorter for gestations from October conceptions than for those from January and February conceptions; 4.7 and 5.6 d shorter for multiple births of the same gender than for single-birth females and males, respectively; and 0.8 d longer for lactations of <250 d than for lactations of >500 d; 0.6 d shorter for standardized yield of <8,000 kg than for yield of >14,000 kg. Heritability estimates for GL derived from parities 2 to 5 were 33 to 36% for service sire and 7 to 12% for cow sire; corresponding heritability from parity 1 were 46 to 47% and 10 to 12%. Estimates of GL genetic correlation between service sire and cow sire were 0.73, 0.79, 0.85, and 0.84 for Holstein heifers, Holstein cows, Jersey heifers and Jersey cows, respectively. More accurate prediction of calving date could help herd managers meet management requirements of pregnant animals and administer better health care during high risk phases of animals' lives.
(Key words: gestation length, breed, twinning)