This is significant
because racing and other forms of competition, such as halter
classes at Quarter Horse, Arabian, Paint, Appaloosa, and other
shows, all divide entries by age. It would be difficult for a
Thoroughbred foal born late in the year to be able to make the Run
for the Roses, for example, when it is only two-plus years of age
chronologically while its counterparts are three-plus years.
The same would be true at the annual Thoroughbred sales. Potential
buyers, generally speaking, are looking for young horses born
early in the year so that training can begin and the horses can be
racing when two years old.
Then there is the matter of racing and showing fillies and mares.
During the spring and summer months, their reproductive systems
want to operate in normal rhythm with Nature. This, however, can
be counter-productive to success on the track and in the show
ring.
"Whenever I'm doing a pre-purchase examination of a performance
mare, I always give the prospective owner my 'female speech," says
Dave Beckman, DVM, a practitioner from Anchorage, Ky. "How she
will react when in estrus is not something I can objectively
evaluate during a pre-purchase exam, so I make certain they know
that there are some mares that, when they come into heat, are a
definite behavioral problem and they will not perform well, be it
in a race, jumping, eventing, or whatever. It is like a big
rolling ball of hormones within their reproductive system and you
don't know what to expect.
"Now, there are some mares that humiliate me. I give that speech,
the person buys the mare, and she never causes them a problem.
There are a handful of great, athletic mares out there that will
perform at their peak without Regumate or anything else.
"However, in my experience, the majority of them are some sort of
problem during the heat period."
There also is the matter of the stallion to consider. If the
breeding farm is able to control ovulation in the mares being
bred, it is far easier to manage the stallion, especially if he
happens to have fertility problems. Having mares come into estrus
at regularly spaced intervals can be an important assist in
maintaining his fertility.
The same can be true for stallions which are being used in an
artificial insemination program. In many cases, several mares can
be bred with sperm from a single ejaculate if they all are in
estrus at the time the stallion is collected.
Then, too, there is the matter of embryo transfer. The donor and
recipient mares must be on the same page, reproductively speaking,
if the transfer is to be a success.
What all of this boils down to is that in modern horse breeding,
regulating estrus has become a necessity in many instances.
Before one discusses regulating an important part of the horse's
system, it would be wise to review just what it is that is being
regulated and how.
The
Reproductive Cycle
We
start with the basic premise that the key to reproduction is
light. We can talk about the balmy days of spring, the arrival of
green grass, and the gentle touch of soft winds and warm rains,
but what matters the most is light. Because of this knowledge,
horsemen literally can trick Nature into moving out of its natural
rhythm with the administration of artificial light.
More about that later. First, let's take a look at the role light
plays in jump-starting the reproductive system of both male and
female.
The mare's reproductive activity is described as being "seasonally
polyestrous." Basically, this means that she has a reproductive
season and a non-reproductive season. Both are controlled by
light. The non-reproductive season, known as anestrus, comes
during the fall and winter when there is little light. The
reproductive season begins in the spring and continues through the
summer when there is a great deal of light.
During anestrus, the mare will not respond to a stallion's
attention and her ovaries become reduced in size and are inactive.
During the reproductive season, the mare's attitude changes, at
least on certain days, along with hormonal activity within her
reproductive system. During the reproductive season, the mare will
experience a series of estrous cycles. During this period of
sexual activity, the ovaries swell up to the size of tennis balls
and become active, rather than dormant. These cycles will repeat
themselves at 21- to 23-day intervals until she becomes pregnant
or until she reverts to anestrus as light fails and late fall and
winter arrive.
The above is true if man does not interfere. However, as
mentioned, artificial light can be used to stimulate an earlier
onset of the reproductive season and drugs can be administered
that can shorten her cycle and dictate when she will ovulate.
When a mare is operating in rhythm with nature, without human
interference, there will be peak times for both conception and
anestrus. While there are two basic reproductive seasons, each of
these two can be split once again, making four in all. As already
mentioned, the natural breeding season occurs during spring and
summer, with the highest efficiency coinciding with the longest
day of the year--June 24. During this period, nearly 100% of mares
will be cycling.
Conversely, the anestrus season is at its peak during the winter
months, coinciding with days when there is relatively little
light. During this period, only a small percentage of mares will
cycle and ovulate.
The other two cycles are transitional stages that occur prior to
the active season and just before anestrus. During those times,
mares generally are erratic in their cyclic and sexual behavior.
The estrous cycle during the active months is controlled by the
interaction of various hormones within the body. However, it all
starts with the eye, which allows the entry of light.
As
days get longer, the mare's brain records the increased amount of
light. This stimulates the hypothalamus gland located within
tissues of the mid-brain. The hypothalamus starts the reproductive
system by producing gonadotropic-releasing hormone (GnRH). When
GnRH is secreted in the proper quantity, the pituitary gland,
located at the base of the brain, is stimulated. The pituitary is
attached to the hypothalamus by a stalk containing both blood
vessels and nerves, which serve as its pathway for communication
with the hypothalamus.
When stimulated, the pituitary gland secretes two hormones that
affect the ovaries. The first hormone is known as follicle
stimulating hormone (FSH). This hormone moves through the
bloodstream to the ovaries, where it stimulates development of one
or more follicles.
The now-developing follicles in the ovaries, when they reach the
stage where they are 20 to 25 millimeters in diameter, secrete
estrogen. The estrogen has several effects, including affecting
behavioral centers in the brain, stimulating estrual activity,
affecting the cervix by allowing relaxation for entrance of
spermatozoa into the uterus, stimulating the smooth muscles in the
mare's reproductive tract for increased contractions to transport
sperm and ovum, and, causing the pituitary gland to inhibit
further secretion of FSH and stimulate the release of the second
gonadotropic hormone--luteinizing hormone (LH).
LH
facilitates maturation and ovulation of the growing, egg-bearing
follicle.
Ovulation occurs when the mature egg leaves the follicle and
begins its trip through the oviduct, generally late in estrus.
Once ovulation has occurred, the luteal phase of the estrous cycle
is ushered in.
In
the wake of ovulation, the estrogen level falls and the remains of
the ovulated follicle are converted to form a corpus luteum (CL)
or yellow body. Luteal cells secrete the hormone progesterone,
which has as its task the shutting down of secretion of the estrus-stimulating
hormones and thus setting the stage for maintaining a pregnancy.
Because of its role in the reproductive system, progesterone (in a
synthetic state) becomes highly important in artificially
manipulating the estrous cycle.
During a normal estrous cycle, the first task for progesterone is
to subdue the actively contracting reproductive tract and to
tighten and close the relaxed and open cervix. At the same time,
progesterone inhibits the secretion of FSH and LH from the
pituitary. When this has been accomplished, the mare goes into a
state of diestrus, which means she is no longer in heat and is no
longer receptive to the stallion.
Under natural circumstances, what happens next is dependent on
whether the mare has become pregnant. If she did not conceive, the
uterus will remain under the influence of progesterone for 12 to
14 days, then changes will occur.
If
no embryo is present in the uterus at the end of that time, the
uterine endometrium will secrete the hormone prostaglandin. This
hormone will destroy the corpus luteum, which is producing
progesterone. With the corpus luteum destroyed, no progesterone is
produced. Without progesterone as an inhibitor, the level of
follicle stimulating hormone (FSH) rises and the cycle starts all
over.
Manipulating the Cycle
The most basic procedure involved with controlling the estrous
cycle in a mare is light. It is basic and simple because all it
involves is utilizing a 200-watt lightbulb.
Martha M. Vogelsang, PhD, of Texas A&M University, says that light
usage can bring benefits, but there is at least one side
effect--shedding--that could pose a problem unless dealt with.
"Currently, the simplest and most effective management protocol
for bringing mares into heat prior to the natural breeding season
is the use of extended day length," said Vogelsang. "By imposing
an artificially long day on mares beginning in late November or
early December, the hormonal mechanisms that control estrous
cyclicity are stimulated such that mares will begin to ovulate in
mid- to late February (rather than March or April).
"Although different lighting schedules have been studied, a
lighting program that supplies 16 hours of light to eight hours of
dark seems to provide a consistent response. The additional light
can be provided by placing mares in stalls or paddocks where there
is sufficient light in the afternoon, and maintaining the light
artificially until approximately 11 p.m. The amount of light
necessary to elicit photo periodic stimulation is at least three
foot-candles at the level of the horse's eye. A 200-watt
incandescent bulb provides adequate light in the average stall.
"It is important for the mare owner to understand that extending
the day length does not provide an immediate response, nor does it
eliminate the transitional period between anestrus and the
ovulatory season. A behavioral response can be seen 30 to 60 days
after the program is initiated, with the first ovulation occurring
60 to 90 days after the lighting program begins.
"It should also be noted that exposure to extended day length
stimulates shedding. Depending on the climate, mares in extended
day length programs may need shelter and/or increased nutrient
intake during this period."
Although not as frequently mentioned as a key element in
controlling the estrous cycle, nutrition can play a pivotal role,
as can exercise.
Scott Bennett, DVM, a practitioner who also operates an equine
hospital in Shelbyville, Ky., and focuses much of his practice on
breeding problems, believes that both exercise and diet can play
significant roles in the estrous cycle.
Mares can be put under lights to stimulate an earlier onset of the
cycle, but if they receive no exercise and either are too fat or
too thin, light therapy alone might do little.
"I
like to see these mares get exercise," says Bennett. "I believe a
stall is a horse's worst enemy. I like to see mares turned out
during the day where they can exercise and keep themselves fairly
fit, and brought into the barn at night and put under lights. A
lot of times, I will just group them in a shed under lights."
He
also believes there must be a middle ground between a mare which
is too fat and one which is too thin. The way to control a horse's
condition, he says, is to feed by weight: "Feed horses
individually and feed by weight, not by coffee can."
Mares which are too fat, he says, should be placed in a dry lot
where their intake can be closely monitored. Those which are too
thin should be fed an increased ration.
Diet can be especially important to the senior mare, according to
results of a study conducted by Elaine M. Carnevale, DVM, MS, PhD,
which was presented during the annual AAEP convention in 1996.
The study was based on the premise that normally mares 19 years of
age and older ovulate two weeks later than mares 13 years of age
and younger. Another premise was that more cycles per conception
were required for the older mares.
The objectives of her study were to compare the effects of two
diets on young and old mares for time to first ovulation of the
year and to compare circulating concentrations of insulin, free
fatty acids, and cortisol.
Involved in the study were eight mares ranging in age from three
to 15 years, and 10 mares which were 19 years of age and older.
The mares were primarily of Standardbred and Thoroughbred lineage.
At
the beginning of the study, young and old mares randomly were
divided into two groups. One group received a commercial ration
designed specifically for older horses at the rate of 10 pounds
per day, and the other group received 10 pounds of oats per day.
Both groups also were fed hay that was a mixture of orchard grass
and alfalfa.
The results were significant for the older mares. Carnevale
reported this as a conclusion:
"Old mares fed a highly digestible, nutrient-dense diet ovulated
approximately two weeks earlier in the spring than old mares fed
an isocaloric control. Type of feed did not affect time to
ovulation for younger mares. Old mares fed the special diet
ovulated on approximately the same mean dates as young mares, in
contrast to a later ovulation date for old mares fed the control
diet.
"Concentrations of insulin and free fatty acids were higher in old
horses receiving a nutrient-dense diet. Differences in effects of
diet between young and old mares were probably caused by the
reduced capabilities of old mares to digest and assimilate feed,
resulting in a functional nutrient deficiency. From these data, it
is apparent that older horses need to be maintained on a feed
program that supplies them with the additional nutrients that they
need for reproductive performance."
Before we leave the matter of light, nutrition, and exercise, it
should be noted that what is good for the goose is good for the
gander. Research has shown that the use of lights also can
stimulate the stallion to early reproductive capability when
combined with a proper diet and exercise. (Caution should be used,
however, because some studies have indicated that stallions
"started" earlier in the year from light therapy tend to "shut
down" earlier in the year.)
While the use of light, plus appropriate diet and exercise, can
influence the estrous cycle, they are basically non-invasive. In a
sense, their influence on the cycle is indirect.
By
contrast, the administration of certain drugs or hormones has a
direct effect.
Early
Season Chemical Ovulation Control
One of the most popular forms of treatment involves the
administration of altrenogest, a synthetic form of progesterone
that is marketed under the trade name Regumate. Its basic function
is the same as that of progesterone during a normal cycle--shut
down sexual activity until nature can determine whether a
pregnancy has occurred.
In-depth research on manipulation of estrous cycles has been
carried on at Colorado State University. While the CSU program
prefers using altrenogest in controlling the estrous cycle,
researchers point out that there are alternatives, such as
injections of progesterone in oil.
That being said, let's take a look at how CSU prepares its mares
for manipulation of the estrous cycle.
The researchers begin with light. Starting on Dec. 1, mares are
provided 16 hours of light per day. After 60 days of exposure to
light, each mare is fed one milligram of altrenogest per 100
pounds of body weight daily for 15 days. The mare is kept on this
regimen for 15 days, then the altrenogest is removed.
Research at CSU has demonstrated that the use of lights and the
15-day administration of altrenogest is effective in establishing
normal estrous cycles early in the year.
In
a study headed by E. L. Squires, PhD, 34 mares were split into
experiment and control groups. One group of 17 received the above
described light and altrenogest treatment, and the other 17
received neither. All 17 of the mares under lights and
administered altrenogest returned to estrus within six days
post-treatment, compared to only seven of the 17 control mares.
If
one wants to "short cycle" a mare and shorten the above treatment
period from 15 to nine days, prostaglandins are used. The
administration of altrenogest or progesterone in oil for nine
days, plus an injection of prostaglandin (the hormone that works
to bring a mare into estrus) on day nine, has proven to be an
effective combination for estrus synchronization, says Squires.
Unfortunately, he adds, the corpus luteum is easier to control
than follicular growth. Therefore, there is no guarantee when
ovulation will occur.
Enter two more hormones--human chorionic gonadotrophin (hCG) and
gonadotropin releasing hormone (GnRH). Both can be utilized to
hasten ovulation and make it more predictable.
First, hCG. One of the early studies carried out involving hCG was
at Colorado State when Carnevale attended the school as a graduate
student seeking her doctorate. The study, however, concentrated on
ovulation in mares in the spring transitional phase of the cycle
rather than those already in the receptive stage.
In
the study, 38 light horse mares were involved. Beginning on Feb.
1, the mares were presented daily to a stallion to detect estrous
behavior. Follicular activity was monitored at four-day intervals
via ultrasonography. When follicular development reached a
particular stage, some of the mares were administered hCG and
others were not.
The mares administered hCG ovulated earlier than the controls.
This led Carnevale to report the following:
"It was concluded that administration of hCG is an effective
method to hasten time to ovulation in transitional mares. Human
chorionic gonadotrophin could be used to minimize time, expense,
and stallion use during the extended transitional estrous period
by reducing time from recruitment of a dominant follicle to
ovulation. Diestrus progesterone concentrations were not affected
by treating mares with hCG, suggesting adequate luteal function
for sustained pregnancy."
This study at CSU was followed by one that took a look at
administering GnRH for hastening ovulation in transitional mares.
In
this study, 45 non-lactating, seasonally anestrus light horse
mares were assigned randomly to one of three groups, with 15 in
each group. One group was an untreated control. The second group
was given an injection of the GnRH agonist buserelin at 12-hour
intervals. The third group had GnRH administered via an implant
that was designed to release GnRH over a 28-day period. The
implants were placed beneath the skin of the neck.
The results revealed that GnRH agonist-treated mares, receiving
the hormone as either an injection or an implant, experienced
hastened ovulation when compared with the non-treated control
mares.
Seven of 15 mares in Group 2 responded to twice-daily injections
of GnRH agonist and ovulated between Day 10 and Day 25 after
initiation of treatment. Similarly, nine mares given an implant of
GnRH agonist (Group 3) ovulated between Day 4 and Day 30. None of
the control mares ovulated within 30 days of initiation of
treatment.
The researchers concluded the following:
"A
GnRH agonist hastened the initial ovulation of the breeding season
in mares. The ability to administer GnRH agonist via a
subcutaneous implant has practical applications for early foal
production."
Cycling Mares and Chemical Ovulation Control
Now for a look at using GnRH on mares which already are in the
midst their receptive estrous cycles.
At
the 1994 American Association of Equine Practitioners meeting,
Patrick J. Meyers, DVM, MS, Diplomate ACT, of Ontario, Canada,
reported on a study involving the administration of the GnRH
analog deslorelin.
(Dorland's
Illustrated Medical Dictionary describes an analog as being a
chemical compound with a structure similar to that of another, but
differing from it in respect to a certain component, while an
agonist is described as being a drug that has affinity for and
stimulates physiologic activity at cell receptors normally
stimulated by normally occurring substances.)
Meyers had this to say in introducing the results of the study:
"There is considerable variation between mares and sometimes
within the same mare in the length of estrus, the interval from
onset of estrus to ovulation, and the size of the follicle(s) at
ovulation. This accounts for difficulties in controlling ovulation
when attempting to time insemination accurately with imminent
ovulation. The ability to control ovulation accurately and
reliably in mares plays a pivotal role in maximizing our
reproductive management of both stallions and mares.
"A
single intramuscular or intravenous injection (2,500 IU) of human
chorionic gonadotrophin (hCG) at the appropriate time during
estrus has resulted in ovulation within 24 to 48 hours and a
shortening of the estrous period compared with untreated controls.
Disadvantages of hCG include some refractory responses from
antibodies formed against this foreign protein.
"The purpose of the studies summarized here was to determine the
efficacy (ovulation response) of a highly potent GnRH analog,
deslorelin, delivered in a novel slow-release implant, in
accelerating and ensuring ovulation within 24 to 48 hours of
administration in cyclic estrous mares."
Over a three-year study period, subcutaneous implants, delivering
either zero milligrams or 2.2 milligrams of deslorelin, were
administered to 967 estrous mares at several different locations
in Australia, Canada, Germany, Sweden, and the United States in
double-blind multicenter clinical trials. Standardbred,
Thoroughbred, Quarter Horse, Saddlebred, and Arabian lactating and
non-lactating mares were included. Horses were managed under
prevailing conditions and practices. A minimum body condition
score of 6 was a prerequisite for entry into the trial to insure
that the mare was on an appropriate nutritional diet.
Once estrus was established, the reproductive tract of each mare
was examined by palpation or ultrasonography. When a lead follicle
became 30 millimeters or more in diameter, implants that contained
either deslorelin or a placebo were inserted just beneath the skin
of the neck.
The results revealed that in all geographic locations, there was a
shortening of the interval to ovulation of 38.4 hours. The mares
treated with deslorelin--a total of 566--ovulated at an average of
47.9 hours after treatment. The mares treated with the placebo--a
total of 401--ovulated 86.2 hours after treatment.
The percentage of mares ovulating within 48 hours after treatment
was 88.6% for those administered deslorelin and 31.6% for the
placebo-treated mares.
Meyers offered the following conclusion:
"Combined studies involving 566 treatments with deslorelin and 401
placebo implants strongly support the claim that treatment of
estrous mares with a lead follicle of 30 millimeters (or more) in
diameter with the GnRH analog deslorelin, delivered in a
slow-release biocompatible implant, causes acceleration of
ovulation and ensures that 80% of the treated mares will ovulate
within 48 hours after treatment, reducing the time to ovulation by
55%.
"Treatment with deslorelin had no adverse effects on pregnancy
rates, early embryonic loss rates, abortion rates, or foal
vitality. Local reactions varied between locations, always were
slight, disappeared mostly within three days, and never required
treatment of any kind."
Thus, we can conclude, administration of hormones can help bring a
mare into estrus and can hasten time to ovulation whether she is
in the estrous cycle or in the transitional period. Not to be
overlooked in the overall scheme of things, however, are the use
of light, nutrition, and exercise.
The above involves the mare which we are breeding or want to
breed. What about the performance mare where we want to suppress
the estrous cycle?
Suppressing the Cycle
There are three basic ways to get the job done, says Beckman, with
the safest being the administration of altrenogest. The other two
involve implants that have not been approved in the United States,
and spaying.
Regumate (altrenogest) appears to be the safest way to go when
seeking to prevent a mare from coming into heat, says Beckman,
although it does have a downside--it is expensive. Generally
speaking, one can expect to pay three dollars per day for Regumate
that is administered orally, either by top-dressing feed or by
squirting it directly into the horse's mouth with a syringe.
It
takes Regumate a bit longer to act on the hormonal system than
does a shot of progesterone. "You need to get the mare on it five
or six days before the event or race, then keep her on it
throughout the competition for it to prevent her from cycling,"
Beckman says.
Some trainers administer Regumate continuously throughout a
season, while others will remove it if the mare has a downtime of
three weeks or more between performances. Normally, Beckman says,
about nine days after Regumate is no longer administered, the mare
will cycle.
If
the owner plans on using the mare in the broodmare band in the
future, Beckman believes, the on-again, off-again approach to
Regumate administration would be preferred to keeping her on it
constantly.
It
can be argued that man should not battle Nature in the equine
reproductive world, but as long as certain competitions are based
on age and as long as some females have erratic behavior when in
estrus, it will remain an economic necessity.
