Kelli Retallick, Genetic Service Director with Angus Genetics Inc. recently shared this informative piece on calving ease.
Calving ease is a desired trait. Most would argue it is not
only desired but necessary, as calving difficulty can lower
calf survivability and extend post-partum intervals for cows,
which then lowers breed-back rates. Expected Progeny
Differences (EPDs) can be utilized to help manage this risk.
Often, producers ask, “Can we have too much calving
ease?” To answer, it is important to reflect on the basis of
the calving ease argument, the build of the EPD and what
to expect when using these tools. Knowing this, cow/calf
producers – both seedstock and commercial alike – can
make informed decisions about their own herds.
Using the tool
Calving Ease Direct, or CED, is the
most effective tool when deciding
which bulls to mate to first-calf
heifers. Expressed as a probability
percentage, CED aims to predict the
percentage of unassisted births a bull
will produce when mated to heifers.
Let’s compare two bulls. Bull A has
a CED EPD of +2, and bull B has a
CED EPD of +7. When mating these
two bulls to similar groups of heifers,
phenotypically and genetically, one
would expect, on average, bull B
to produce 5% more unassisted
births than bull A. While no one can
indefinitely state the perfect cut?off
to be used across the industry, producers can rely on the information
available to them to make the best
decisions. It gets even simpler
when producers can rely on past
records to benchmark the amount
of calving difficulty experienced to
understand where selection pressure
should be placed.
Behind the EPD
Calving ease scores collected by
breeders are utilized to predict CED.
These scores range from 1-5, where
1 would indicate a birth with no
assistance. For Angus cattle, only
scores reported on first-calf heifers
are used in the prediction of the CED
EPD. Mature female scores, while
they can be reported, are not used
in the national cattle evaluation as
not enough variation, or differences
among reported scores, exists to add
value to CED predictions.
Birth weight is used as a correlated
trait in the calving ease evaluation.
The correlation, or strength of
relationship, between calving ease
and birth weight is -0.65 which is a moderately strong, negative
relationship. A negative correlation
suggests as one trait goes up
the other goes down. Therefore,
in most cases, as calving ease
increases, birth weight tends to
trend downward. When focusing
on decreasing calving difficulty in
first-calf heifers, it is most effective
to focus on CED EPDs as this is the
economically relevant trait. On the
other hand, if the focus is to strictly
increase or decrease birth weight,
the BW EPD is the tool of choice and
most effective to influence changes
on actual birth weights of calves.
These scores and weights are then
evaluated together in a threshold
model, but it is important to
understand in a threshold model
an underlying assumption is made
about the amount of existing calving
difficulty. Therefore, if the tool is
predicting the decreased number
of assisted births in a population,
a percentage of assisted births is
assumed. In the case of the Angus
CED EPD, this threshold model was
designed to help lower the level of
assisted births present in a mixed
breed commercial cowherd, which
in most cases would have a higher
incidence of calving difficulty than
a purebred registered Angus herd.
This is why patterns in the data
representing the percentage of
assisted and unassisted births of the
purebred Angus herd are not always
completely linear when tracking up
or down the CED scale. Figure 1
breaks out the percentage of assisted
births recorded for each CED EPD
possessed by the sire when bred to a
breed average Angus female. As the
CED EPD of the sire increases, the
number of assisted births decreases.
However, the decrease may not be
by an entire percentage point as
expected. The reason? Mating Angus
sires to Angus females who are breed
average for both calving ease direct
and maternal is not the expected
industry average mating.
In a recent survey conducted by
the American Angus Association,
producers reported treating calving
ease as a completely separate trait
using independent culling levels
when making genetic decisions.
Independent culling is a selection
strategy stating an individual animal
will be culled if it does not meet the
specific requirement of a single trait,
regardless of the levels of other traits.
Using this method of independent
culling, while efficient in making
genetic change in that specific
trait, can lead to drawbacks which
can affect herds in the long term.
Producers setting specific thresholds
on any trait, ignoring a multi-trait
approach, can lead to eliminating
bulls for falling just under this
culling threshold even though they
may possess other characteristics
valuable to the operation.
The bottom line – not all cowherds
are created equal. What can be
utilized safely in one herd may cause
problems in another. Knowing CED is
designed to be a heifer mating tool,
using different bulls to mate first-calf
heifers and mature cows may be
logical. The best advice may be to
understand the cowherd these bulls
will be utilized in.
As a cattle producer, you know your
cowherd better than anyone. If the
bulls used on first-calf heifers have
averaged a +6 for the CED EPD for
several years and you have yet to
pull a calf, then a +6 CED bull is a
safe option. On the other hand, if
your bulls have averaged +10 CED
and you have repeatedly pulled a
higher percentage of calves from
first-calf heifers than desired, then
more calving ease may be necessary.
Understanding the relationships in
your herd is crucial to the overall
success of the genetic program.
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