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By Darcy Vial
USDA FSA Oregon via Wikimedia Commons |
Written by Tom Brink
Calving ease is especially
important in first-calf females, and is therefore something we pay close
attention to when breeding virgin heifers. In fact, calving ease considerations
usually rank first on the list when selecting bulls to use on heifers.
Red Angus bulls often see
service on heifers thus, mapping the relationship between calving ease EPDs
(CED) and unassisted births is a worthwhile task. The better Red Angus breeders
understand this relationship, the better selection and mating decisions they
can make for themselves and their customers.
The curved line shown in the
chart below was statistically derived from tens of thousands of Red Angus
calving records stored in the RAAA database. All of the calvings are from first
parity females bred to bulls ranging from -8 to 20 for CED. This line captures
the “average” or “typical” experience calving heifers as the sires’ CED goes
from the low end to the high end of the Red Angus bell curve.
As expected, bulls with a
higher CED produce a greater number of unassisted births. Conversely, bulls
with low single digit or negative CEDs generally cause more calving difficulty.
The shape of the curve also provides useful perspective. Note that moving from
-8 to 0 for CED results in a sharp increase in unassisted births. That’s
obviously a good thing. It’s also why we don’t see many heifers bred to
negative CED bulls.
Unassisted births improve
further when moving from a CED of 0 up to the 8-10 area. This improvement is
significant in magnitude, but the incremental benefit of each additional
one-point increase in CED is now becoming smaller as the law of diminishing
returns is taking effect.
Moving up the curve from a
10 to 15 CED provides a bit more benefit but the incremental advantage is now
very small. Over 15, the line flattens out completely, which means there is no
practical difference in unassisted births for bulls having CEDs ranging from 15
to 20 or above. Unless a group of heifers are small in size, have
inadequate pelvic area, and/or are underdeveloped at the time of calving, we
would not expect any difference in calving difficulty between a bull with a CED
of 14 or 15 versus one whose CED is 19 or 20. In that zone, the law of
diminishing returns is in full control, stamping out any more benefit
completely.
As a final thought, don’t forget that EPD accuracies matter a great deal
when making mating decisions in which calving ease is critical. It may in some
situations be better to use a high-accuracy bull with a lower but still
acceptable CED instead of one with a higher CED that is unproven with low
accuracy.
Decker's Take Home Message
It is so easy to want to select for maximums. We need to remind ourselves to select for optimums.
The Missouri Show-Me-Select Replacement Heifer Program
requires Red Angus service sires to have a CED EPD greater than 8. Artificial insemination (AI) sires must have an EPD accuracy greater than 0.6.
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