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Mel
DeJarnette
Select Sires Reproduction Specialist
I
recently had the opportunity to spend a week riding
with several Select Sires technicians across central
Wisconsin. It was an eye-opening experience for me in
more ways than one. It was a pleasure to spend some
time with these dedicated employees and to see first
hand some of the problems they deal with on a day-to-day
basis. One area that surprised (and troubled) me was
the number of cows being bred based on some secondary
sign of heat followed by the farmer telling the technician
to "Palpate her and see what you think."
When
a technician or veterinarian palpates an animal for
signs of estrus, he is basically feeling for tone in
the uterus in response to rising estrogen levels in
the circulatory system. Any mucous that can be discharged
and ovarian status (presence of follicle and/or CL)
are additional clues to estrous diagnosis.
However,
these symptoms (along with a red swollen vulva, mounting
cows not in heat, excessive bellowing, nervous or restless
behavior, and many others) are all secondary signs of
estrus and are less unreliable as predictors for proper
timing of insemination compared to the primary sign
"standing to be mounted."
Ovulation
Prediction Programs
Perhaps
the wrong term was coined when breeding programs for
cattle were first developed many years ago. Instead
of calling them "Heat Detection Programs,"
we should have called them "Ovulation Prediction
Programs." In order to achieve maximum fertilization
rates, cows must be inseminated at such a time that
ovulation (release of the egg from the ovary) occurs
when viable sperm cells are present in the oviduct.
Thus, in order to properly time inseminations, we must
have a reasonably accurate prediction of when ovulation
will occur.
The
term "heat detection" implies that a herdsman
must watch for signs of estrus. Signs of estrus, however,
are of limited value if the herdsman does not consciously
attempt to predict time of ovulation and then inseminate
animals based on that prediction.
Figure
1 below displays a time scale scenario of the physiological
events occurring proximal to estrus and ovulation in
the cow. As estrogen levels rise in the circulatory
system, animals will begin to show many of the secondary
signs of estrus. Eventually estrogen reaches a threshold
level and triggers two events simultaneously; 1) standing
heat and 2) ovulation.
Figure 1. Time scale synopsis of physiological
events occurring proximal to estrus, ovulation and proper
insemination timing.
Standing
heat varies in length and intensity from cow to cow
but on the average last about 10-12 hours.
Although
ovulation is triggered by the same hormonal mechanism
that causes the cow to enter standing heat, it does
not actually occur for another 25-30 hours.
Secondary
signs of estrus, including uterine tone, are stimulated
by small increases in estrogen levels. In some cases,
secondary signs may often be observed as much as 48
hours prior to standing heat. However, in others, they
may occur only a few hours prior to standing heat. Absence
of secondary signs of estrus are a good indication that
animals should not be bred. However, since they may
occur two days before, the day of and a day or two after
standing heat, presence of secondary signs (again including
uterine tone) are indicators that an animal is under
the influence of estrogen but does not yield any reliable
prediction of ovulation.
Accurate
timing of insemination is only accomplished by knowing
initial time of "standing heat. Inseminating animals
10-12 hours after first observing standing heat will
allow the viable life span of spermatozoa (20-24 hours)
to overlap with ovulation and the viable life to the
egg (6-8 hours). Inseminating cows based on secondary
signs of heat will often result in many animals being
inseminated too early and the sperm life is depleted
prior to ovulation.
I
realize that in many instances animals must be bred
strictly on secondary signs of estrus. This is particularly
true during the winter months in many northern states
when cows are kept in stanchions or tie stalls because
there is a "Nor'Easter" bearing down on the
farm. However, whenever possible and practical, cows
should be allowed every opportunity to display mounting
behavior.
Breeding
cows based on secondary signs, including "palpate
her and see what you think," is the exception and
not the rule in herds with high levels of reproductive
efficiency.
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