Heat
Detection vs. Ovulation Prediction
by
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.
