Diapositiva 1

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Horse Breeding
Paul R EarlFacultad de Ciencias
Biológicas Universidad Autónoma de Nuevo León San
Nicolás, NL 66451, Mexico
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In most herbivores the male is responsible for
the protection of the herd against predators.
Also, the stallion will fend off competing males,
and mares in heat (estrus) will fend off other
mares from a stallion. Spring is favorable for
births. Lets pick April for birth. Then if
gestation is about 3 months as in sheep, breeding
took place in say December. If gestation is 11
months as in horses, breeding took place in April
or even in June. Then we have short and long day
animals. Goats breed in winter to have kids in
spring, just as horses breed in spring to have
foals in spring. Also, summer in one hemisphere
is winter in the other.
Light duration controls the annual sexual cycle
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After the equinox which is 21 September, as days
get shorter goats are turned on sexually and
horses off. Horses begin to react on 21 December
as the days begin to get longerand so do many
other animals.Do chickens lay more eggs with
more hours of light ? Which hormones are involved
with ovulation ? These climatologic effects can
lead to considerations of the biological clock.
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Detecting heat (estrus) by using a mares
responsiveness to a stallion is known as teasing.
It is the most important technique in horse
breeding. Mares are most successfully bred at
maximum receptivity, and this is at ovulation.
Rectal palpataion of the uterus and its
ultrasonics are 2 supplementary techniques.
When ovulation absolutely correlates with the
time of sexual intercourse with the stallion,
one cover will do. Otherwise, returns make
mating much more costly.
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Here are some suggestions1/ External
genital examination of stallion and mare, 2/
Breeding suitability examination of the stallion,
3/ Palpatation per rectum of complete mare
reproductive tract, 4/ Ultrasonographic
examination of mare reproductive tract (pregnancy
diagnosis via palpation and ultrasonography) and
5/ natural cover or artificial insemination with
cool fresh shipped or deep frozen semen.
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HormonesThe hypothalamus
secretes gonadotropin-releasing hormone (GnRH),
which stimulates the anterior pituitary to
secrete Luteinizing Hormone (LH) The hypothalamus
secretes gonadotropin-releasing hormone (GnRH),
which stimulates the anterior pituitary to
secrete LH and Follicle Stimulating Hormone
(FSH). These hormones cause the growth and
development of ovarian follicles. At the time of
estrus, there is a massive release of luteinizing
hormone that triggers ovulation, called the
preovulatory surge.
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A corpus luteum (yellow body) develops from the
remains of the follicle after the egg has been
released. Once it has developed, it produces the
hormone progesterone. It acts to inhibit GnRH
release thereby reducing pituitary FSH secretion
and thus prevents follicle maturation. As long as
progesterone circulates in blood, estrus is
prevented, e. g., pregnancy is so maintained if
conception occurred.
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Regumate is synthetic progesterone named
altrenogest given orally in vegetable oil. Each
ml contains 2.2 mg altrenogest. Dosage is 1 ml/50
kg body weight for 10 consecutive days. Regumate
can be used for 1/ inducing an ovulatory estrus,
for inducing normal cyclical ovarian activity, 2/
Treatment of the mare in lactation anestrus,
3/for suppression of estrus either during
prolonged estrus or in normally cycling mares and
4/ to control the cycle of breeding mares to
allow efficient use of the stallion. The majority
of mares will return to estrus within 8 days
after the last oral dose of Regumate and will
ovulate between 713 days later.
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Edema begins 7-8 days before ovulation, peaking
at 24-48 hours before ovulation and declining
within 2 days of ovulation. Edema is found on
both the day before and at ovulation with a 7.5
MHz ultrasound transducer.The hormones most
commonly used to manipulate the mare's estrus
cycle are progesterone and its analogues like
altrenogest (Regumate). Estradiol is responsible
for estrous behaviour in mares, and estrogen
concentrations during estrus correlate well with
sexual behavior and grossly observable changes in
the reproductive tract. In contrast, progesterone
inhibits estrus behaviour. Its rise in
concentration leads into the next estrus cycle.
Steps towards successful fertilization then
foaling include
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For prostaglandin to affect an estrous cycle, it
requires an active corpus luteum (CL).
Progesterone started on the day of parturition,
at regular dosage levels (150 mg/day IM for
progesterone or 0.44 mg/kg per day orally for
Regumate) for a period of 8 days, will usually
result in the mare entering estrus 3 days after
treatment ends.Progesterone and estradiol-17
beta combined is an injectable product given at
the dosage level of 150 mg progesterone, 10 mg
estradiol 17 beta per day IM.
Estrogen and prostaglandin
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1/ A healthy ova (egg).2/ Ova live 12 hours so
must be exposed to sperm within this time.3/
Most sperm live 3-5 days in the mare that must
contact the egg in the oviduct (tube connecting
ovary uterus) at this time.4/ Oviduct must be
clear to allow passage of ova sperm.5/
Fertilization must take place within the
oviduct.6/ The oviduct environment must be
suitable for egg, sperm embryo.7/ Suitable
uterine environment for embryo to implant on its
wall must be available.
Successful breeding accounts for these factors
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Testes. A stallion's testicles each weigh 225-300
grams, averaging 4-5 cm long, 6-7 cm high and 5
cm wide. The testes produce sperm. Epididymis.
The epididymis is attached to the testicle. The
epididymis serves to transport spermatozoa out of
the testes, concentrate it by absorption of water
and provide a place for it to mature and be
stored. Vas Deferens. The vas deferens, or
spermatic cord, transports sperm from the tail of
the epididymis to the urethra.
The male reproductive tract
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The female reproductive tractOvaries
Mares typically have long erratic estrous periods
in January, February and March. During the
physiological breeding season-April through
August-a large follicle develops during each
estrous period and ruptures toward the latter
part of the heat period, releasing an egg
(ovum). The collapsed follicle is replaced by the
CL which produces the hormone progesterone. If
the mare does not become pregnant, the CL will
have a lifespan of 12-15 days. If the mare
becomes pregnant, several large follicles may
develop on either ovary of the mare. These
follicles may rupture or luteinize to form
accessory or secondary CL as supplementary
sources of progesterone.
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The anovulatory season is a period of sexual
quiescence extending from the last ovulation of
the breeding season to the first ovulation of the
next breeding season. Follicular activity is
minimal December-February (Northern Hemisphere)
with an increase in activity from March to May.
During anestrus the ovaries are usually small and
hard. Estrous cycle length and duration of
the heat period is longest during the autumn
months and shortest between April and October.
The number of estrous periods is lowest in winter
months. The number of small follicles (lt20 mm)
increases slowly during January to March. In the
latter part of March, the number of small
follicles decreases with a concomitant rise in
the number of large follicles (gt30 mm).
The Estrous Cycle
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The pituitary hormones involved in the estrous
cycle are the gonadotropins follicle-stimulating
hormone (FSH) and luteinizing hormone (LH) and
estrogen. Estrogen stimulates 5-7 days heat in
mares. LH causes the ovarian follicle to mature
and ovulate. After ovulation the follicle is
replaced by the CL which secretes progesterone
needed to maintain pregnancy up to day 100. Both
FSH and LH are under control of the
gonadotropin-releasing hormone (GnRH), which is
secreted by the hypothalamus gland of the
pituitary. Seasonal anestrus is caused by a
change in the feedback mechanism between the
ovary and the hypothalamus involving a signaling
system and cytokinines like inhibin and activin.
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The estrous cycle is divided into 2 periods. The
follicular phase is the period during which there
is rapid folliclular growth. The mare displays
behavioral estrus for 5-7 days, then ovulation.
The luteal phase begins with ovulation. During
diestrus, the 15-17 days in between, the CL is
formed and progesterone is produced by it. If the
mare conceives, the integrity of the CL is
maintained. Progesterone is secreted to maintain
pregnancy for 40-50 days. Afterwards, a second
rise of progesterone begins due to the formation
of secondary CLs.
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At the beginning of the menstrual cycle, anterior
pituitary gonadotrophic cells release FSH. This
FSH travels to the ovary where it binds to
immature oocytes, initiating their maturation.
Such maturing oocytes or follicles make estrogen,
which gives negative feedback to the pituitary to
inhibit further FSH release. After about 14 days,
one oocyte matures and releases a final burst of
estrogen that causes the pituitary gonadotrophs
to secrete both FSH and LH (luteinizing hormone).
This "spike" of LH/FSH induces follicle rupture,
leading to a decline in estrogen production.
Cells in the ovary that contributed to earlier
follicle maturation and now remain following
ovulation develop into a structure called the
corpus luteum.
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Management procedures include1/ teasing, 2/
rectal palpation, 3/ ultrasound examination, 4/
artificial insemination and 5/ artificial
lighting. Teasing and estrous behavior record
keeping are the most important parts of a
successful breeding program. Positive signs of
estrus include posturing, tail raising,
urination and eversion of the vulvar labia
(winking the clitoris). Mares not in heat may
kick, bite, swish their tail and pin their ears
back when they think they are being assaulted.
Even when they know you well, they may take shots
at you when going in and out of heat !
Breeding management proceedures
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Separate records should include the year, the
name, color and age of the mare, and the farm
number. Assign individual mares a numbered neck
band. Neck bands help in quick and accurate
identification of each mare. Records should also
identify the stallion the mare is booked to, the
mares owner and previous breeding results. Of
course records are kept on stallions including
fees paid which can be big business.
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Advantages of artificial insemination can
include 1/ Decreased transport
costs, 2/ The stallion can be collected and his
ejaculates frozen and 3/ There have been no
limitations on the storage time of frozen semen.
The disadvantages include 1/ Wide individual
freezing variability, 2/ The techniques are not
standardized, 3/ After thawing, frozen semen has
a shorter life span in the mare than fresh semen,
4/ The conception rates of artificial
insemination with frozen semen are lower than
with fresh semen. Conception rates of 60-65 can
be reached.
Sperm and artificial insemination
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A ghost used in stallion collection substituting
for a jump mare.
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1/ Volume 25-100 ml, 2/ Concentration 30-800
million spermatozoa/ml, 3/ Progressive motility
60-100 , 4/ Morphology 70-80 normal and pH
7.4-7.6.Let us say that semen has been diluted
and deep frozen as in liquid nitrogen into 200
straws. Cost for managing this ejaculate is say
600. Then each straw costs 3.Embryo transfer
and testtube growth of oocytes (ova) has been
performed with very low success levels. A mare
could have 25 plus foals by transfers
(transplants) and nuclear transplants can be
performed as in cloning.
A stallion is considered fertile when thesemen
evaluation meets the following criteria