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Gestation Length (GL)

Available for all breeds, Gestation Length measures variation in gestation length in number of days.

Benefits of Trait

Introduced in August 2017, genetic evaluations for males and genomic predictions of both sexes are provided for gestation length (GL) as a trait expressed in days.

Gestation Length Trait

For service sires, the PTA’s represent the influence the service sire is expected to have on the number of days his mates (the cow carrying his calf) carry his calves during their pregnancies.

The evaluation model defines GL as a trait of the service sire because maternal effects on GL are small compared to direct effects of sire, dam, and Mendelian sampling of the calf.

Number of days

For example, females bred to a Holstein bull with a GL PTA of +4 days are expected to carry their calves about 281 days, or four days longer than the breed average (277 days). A Holstein bull with a GL PTA of -5 days would have mates expected to be pregnant for only 272 days. The difference between these examples is extreme at nine days.

BreedMean GL for CowsMean GL for HeifersMean GL for the
current base year (2010)
Brown Swiss287.2287.5286
Milking Shorthorn281.1279.3279

August 8, 2017

Ayrshire, Brown Swiss, Guernsey, Holstein (B&W, R&W), Jersey, Milking Shorthorn, and crossbred animals that have usable genotypes. The all-breed model includes crossbred matings and breed effect of dam. The all-breed model includes crossbred matings and breed effect of dam.

Note there is a breed variance in GL, as shown in this table.

BreedMean GL for CowsMean GL for HeifersMean GL for the
current base year (2010)
Brown Swiss287.2287.5286
Milking Shorthorn281.1279.3279

There are no immediate plans to incorporate GL into merit indices. Research by Norman et al. showed cows with intermediate GLs also have the most favorable same or subsequent-lactation performance for several other economically-important traits (i.e. calving ease, stillbirth, current and subsequent lactation milk and milk component yields, and longevity).

It is suggested that producers continue to rely primarily on the composite economic index with the specific choice dependent on the farm’s milk payment situation and management system. Producers might consider bypassing those service bulls having long predictions for gestation length as one means to manage calvings.

Estimated for heifers at 48%

BreedAverage Reliability for GL of
Young Genotyped Bulls by Breed
Brown Swiss33%
Milking ShorthornGenomic evaluations not published

38% with PTA for daughter calving ease, 24%-29% with PTAs for yield and productive life.

Collected through Dairy Herd Information (DHI) affiliates. The edited GL data included a total of 12.4 million conceptions and calving dates of 6.8 million cows.

To predict the GL of a calf, add the predicted transmitting ability (PTA) of the sire plus the PTA of the dam (or half the PTA of the calf’s maternal grandsire if the dam is not genotyped) plus the GL mean for the breed and breed combination.

The standard deviation (variation) for GL PTA is 1.4 days. Because 1 and 2 standard deviations normally include 68% and 95% of observations, respectively, we assume about 68% of bulls will have a GL PTA between -1.4 and +1.4 days while 95% of the bulls will range from -2.8 to +2.8 days.

GL PTAs range from 5.6 days shorter to 6.4 days longer than average, in the population of Holstein bulls born since 1995 and ≥90% reliability. The range of Brown Swiss and Jerseys was slightly smaller, as fewer bulls have predictions.

Pre-release testing indicates the active AI Holstein sires (520 bulls as of April 2017) range from -5 days to +4 days, with the average at about 0.0 days.

Future Development

It is possible that a second GL trait could be provided in the future, showing how many days the daughters of each sire are expected to carry their calves. The first trait – influence of the service sire on gestation length of his mates – was priority because it has a larger genetic influence, nearly twice the influence of expression through the bulls’ daughters.

Related Publications
Potential consequences of selection to change gestation length on performance of Holstein cows. Norman et al, 2011
Genetic and environmental factors that affect gestation length in dairy cattle. Norman et al, 2009
Genetic evaluation of gestation length as a trait of the service sire. Wright and VanRaden, 2017

Information last updated August 8, 2017.