Download Diapositiva 1

Horse Breeding
Paul R Earl
Facultad de Ciencias Biológicas
Universidad Autónoma de Nuevo León
San Nicolás, NL 66451, Mexico
Light duration controls the annual
sexual cycle
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. Let’s 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.
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.
Detecting heat (estrus) by using a mare’s
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.
Here are some suggestions:
1/ 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.
Hormones
The hypothalamus secretes gonadotropinreleasing 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.
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.
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 7–13 days
later.
Steps towards successful fertilization
then foaling include:
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.
Estrogen and prostaglandin
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.
Successful breeding accounts
for these factors:
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.
The male reproductive tract
Testes. A stallion's testicles each weigh 225300 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 female reproductive tract
Ovaries: 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.
The Estrous Cycle
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 (<20 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
(>30 mm).
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.
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.
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.
Breeding management proceedures
Management procedures include:1/ 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 !
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 mare’s owner
and previous breeding results. Of course records are kept on
stallions including fees paid which can be big business.
Sperm and artificial insemination
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.
A ghost used in stallion collection
substituting for a jump mare.
A stallion is considered fertile when the
semen evaluation meets the following criteria:
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.