U.S. genetic evaluations for gestation length (GL) will be provided in the weekly and monthly evaluations beginning August 8. For service sires, predicted transmitting abilities (PTAs) of the gestation length will become available routinely thanks to the efforts of Jan Wright and Paul VanRaden of USDA’s Animal Genomic and Improvement Laboratory (AGIL)1. In addition, a PTA for GL will be provided for any bull or cow that has been genotyped.
Gestation length evaluations will be provided for all breeds, including crossbreds that have usable genotypes, to show the influence each service sire is expected to have on the number of days his mates carry their calves during their pregnancies. The PTAs represent the influence of the service sire, and these were developed in preference to a second potential GL trait representing the influence the bull has on his daughters. This choice was made because the PTAs selected for distribution have a larger genetic influence – nearly twice as much as that expressed through the bulls’ daughters. In the future, the second GL trait may be provided also, showing how many days the daughters of each sire are expected to carry their calves.
Dairy farmers focusing on grazing or seasonal calving have requested genetic evaluations for GL because they believe having these, along with highly fertile animals, will allow them to concentrate parturitions into a couple of months. Failure to achieve high conception rate in a spring calving herd means abundant pasture is unavailable when needed most. Other advantages of the GL evaluations, regardless of operation style. are to help pinpoint expected calving dates, schedule dry dates and manage the maternity pen. It is expected that one primary use of GL will be to choose service sires for cows that didn’t conceive on their first service. Breeds vary in GL along with difference for age and season, so predictions of calving date can be improved considerably over what is normally provided.
Over 1.8 million genotypes and 12.4 million reproductive records from 6.8 million DHIA cows born since 1990 were used in research to produce GL evaluations for over 70 million animals, feasible because of relationship with their sires. The decision has not yet been made, but it is possible that more female GL PTAs could be available as early as December 2017.
There are no immediate plans to incorporate gestation length into CDCB’s lifetime net merit indices. Previous research by Norman et al.2 showed those cows having intermediate GLs also have the most favorable same or subsequent-lactation performance for several other economically-important traits. The traits examined included calving ease, stillbirth, the current and subsequent lactation milk and milk component yields, and longevity.
Genetics have a large influence on GL, with the heritability estimate for heifers being 48%. For Holstein bulls born since 1995 and having Reliabilities of at least 90% GL PTAs range from 5.6 days shorter to 6.4 days longer than average. The range was slightly smaller for Brown Swiss and Jerseys, not surprising due to fewer bulls with predictions. Preliminary testing indicates that the current active AI Holstein sires (520 bulls) could range from -5 days to +4 days, with the average at about 0.0 days.
The averages of GLs by breed from recent studies for cows and heifers are shown in Table 1, demonstrating that breeds clearly differ in lactation length (up to 10 days) and again confirming a significant genetic component to GL. Perhaps most interesting, there is evidence – based on large numbers – that GL has decreased somewhat in recent years, particularly in Holsteins.
|Breed||Mean GL for cows3||Mean GL for heifers3||Mean GL for the base year (2010)4|
Interpreting Gestation Length Evaluations
Now let’s review how one can interpret these PTAs. 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 (277 ̶- 5 days). The difference between these examples is extreme at nine days. Because the standard deviation (variation) for GL PTA is 1.4 days and 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.
Thanks to genomic testing, accuracies (Reliabilities) of the evaluations for new traits are higher than those for the traits provided prior to 2008, and genomics has raised the accuracy for the traits introduced earlier as well. The young genotyped bulls will have Reliabilities for GL averaging 28% for Guernsey, 33% for Brown Swiss, 35% for Ayrshire, 54% for Jersey and 65% for Holstein.
As this new trait becomes available, it is suggested that producers continue to rely primarily on a composite economic index like Lifetime Net Merit, Cheese Merit, Fluid Merit or Grazing Merit with that choice dependent on the farm’s milk payment situation and management system. Nevertheless, producers might consider bypassing those service bulls having long predictions for gestation length as a means of keeping calving frequencies in sync with nutritional supplies.
The arrival of evaluations for new traits has accelerated and is likely to continue. After bull rankings became widely available in various published forms for milk and fat yields throughout the early 1900’s, it wasn’t until 1977 and 1978 that evaluations for protein and accurate evaluations for type were available. Predicted transmitting abilities for new traits is quickly becoming the expected normal with several new traits introduced in recent years and expected into the future.