Genetic and environmental factors that affect gestation length H. D. Norman, J. R. Wright, M. T. Kuhn, S. M. Hubbard,* and J. B. Cole Animal Improvement Programs Laboratory, Agricultural Research Service, USDA, Beltsville, MD Abstr. T73 INTRODUCTION GL is interval from conception to subsequent parturition Factors sometimes reported to be related to GL:  Breed  Calf gender  Milk yield  Calving age  Multiple births  Cow’s sire  Parity  Stillbirth  Service sire  Season  Dystocia Objective is to document effects of genetic and environmental factors on GL for current US dairy population DATA Lactation, reproductive, and dystocia records from USDA national dairy database Inseminations after February 1998 that resulted in parturitions before January 2006 Gestations ≤15 d different from breed mode Conceptions (no.): CONCLUSIONS Conception year had almost no effect on GL Calves conceived during autumn are born about 2 d earlier than calves conceived during spring For single-calf births, females are born 1 to 2 d earlier than males Twins are born about 5 d earlier than single calves Within parity, younger animals calve sooner GL decreases slightly ( ~ 0.3 d/100 DIM for Holsteins and Jerseys) as lactation length increases GL increases slightly ( ~ 0.1 d/1,000 kg milk for Holsteins and Jerseys) as milk yield increases Holstein service-sire GL heritability (34%) similar to that assumed for yield traits Holstein cow sire GL heritability (12%) similar to that for productive life, service-sire calving ease, and service- sire stillbirth More accurate prediction of calving date should aid herd managers in meeting management and health requirements of pregnant cows RESULTS (continued) Conception season effect: Parturition code effect: Age-within-parity effect: Lactation length effect for cows: INTERPRETIVE SUMMARY Information on >9 million gestations of US dairy cows was examined. Mean gestation length (GL) was ~ 280 d for Holsteins and Jerseys but as high as 288 d for Brown Swiss. Shorter GL was associated with young cows, autumn conceptions, multiple births, and heifer calves. More accurate prediction of calving date may aid dairy producers in managing maternity operations more efficiently. BreedHeifersCows Ayrshire71135,611 Brown Swiss1,70471,517 AI service sire1,24822,824 Guernsey65038,505 Holstein360,6838,494,57 3 AI service sire311,5515,245,25 1 Jersey14,187618,492 AI service sire10,887239,324 Milking Shorthorn608,022 Red & White34818,176 ANALYSIS GL =calf conception year (4 heifer groups, 8 cow groups) + calf conception herd-year (absorbed) + calf conception month (Jan., Feb., etc.) + calf parturition code (8 groups based on multiple birth status and gender) + age within parity at conception (5 heifer groups, 17 cow groups) + lactation length (7 groups) + milk yield (5 groups) + service sire (random) + cow sire (random) + cow (random) + residual (random) Separate analyses by breed (no crossbreds for Brown Swiss, Holstein, and Jersey) RESULTS GL mean ± SD (d): Conception year effect for cows: BreedHeifersCows Ayrshire ± ± 5.5 Brown Swiss ± ± 6.2 AI service sire ± ± 5.8 Guernsey ± ± 5.6 Holstein ± ± 5.7 AI service sire ± ± 5.3 Jersey ± ± 5.2 AI service sire ± ± 4.9 Milking Shorthorn ± ± 6.8 Red & White ± ± 6.2 RESULTS (continued) Milk yield effect for cows: Holstein heritability estimates based on all heifers and cows:  Service sire:34%  Cow sire:12%