CDCB Connection July 2017
Gestation Length: In-depth Review
U.S. genetic evaluations for gestation
length (GL) will be provided in the weekly, monthly, and tri-annual 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 a combination of 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%, PTAs for GL 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 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 |
Ayrshire |
281.7 |
281.6 |
281 |
Brown Swiss |
287.2 |
287.5 |
286 |
Guernsey |
284.8 |
285.7 |
284 |
Holstein |
279.0 |
277.8 |
277 |
Jersey |
280.0 |
278.4 |
278 |
Milking Shorthorn |
281.1 |
279.3 |
279 |
Interpreting Gestation
Length Evaluations
Now
let's review how one can interpret these PTAs. Females bred to a Holstein bull
with a PTA for GL 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
PTA for GL of -5 days would have mates expected to be pregnant for only 272 days
(272 = 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.
Article prepared by Duane Norman, Technical Advisor &
Industry Liaison, Council on Dairy Cattle Breeding (CDCB)
Photo courtesy of GENEX.
1U.S.
Department of Agriculture, Agricultural Research Service, Animal Genomics and
Improvement Laboratory.
2Norman,
H.D., J.R. Wright, and R.H. Miller. 2011. Potential consequences of selection
to change gestation length on performance of Holstein cows. J. Dairy Sci. 94
(2):1005-1010.
3Norman,
H.D., J.R. Wright, M.T. Kuhn, S.M. Hubbard, J.B. Cole, and P.M. VanRaden. 2009. Genetic and environmental factors that
affect gestation length in dairy cattle. J. Dairy Sci. 92 (5):2259-2269.
4Wright,
J.R., and P.M. VanRaden. 2017. Genetic evaluation of
gestation length as a trait of the service sire. J. Dairy Sci. 100,
Suppl.2:42.v