Equine Reproduction
Dr.
Jack Sales, DVM
LESSON THREE
Manipulation of
Estrus in the Mare
As the study of equine reproduction advances,
more is learned about the mare and stallion reproductive anatomy and
physiology. These findings often lead to techniques that can be applied
directly to the equine breeding industry. This chapter deals with the practical
approaches that can be utilized to enhance the success of a breeding program by
use of certain hormones or other techniques that make it easier and more
efficient to get a mare in foal and allow her to deliver a healthy foal 340
days later.
Assignment: Read Chapter 3 Manipulation of Estrus in the Mare, p.17-30
in Manual of Equine Reproduction.
We will revisit the chart of hormones from or
previous chapter as a beginning review of this lesson.
Hormones Associated with Reproduction
|
Hormone |
Primary
Production site |
Target organ |
Effect |
Therapeutic use |
|
Estrogen - Heat hormone |
Ovary (ovarian follicle) |
Uterus, cervix, and external genetalia of mare |
Expression of estrus. Female
sex characteristics, genital tract changes |
Enhances estrus (heat) but may interfere
with normal cycling. |
|
Progesterone -Pregnancy Hormone |
Corpus luteum of ovary (CL)
also the placenta during pregnancy |
Uterus Mammary gland |
Maintains pregnancy |
Prevent abortion. Synchronize
or suppress estrus (regumate) |
|
Prostaglandin |
Uterus |
Corpus Luteum (CL) |
Luteolysis.(CL
regression) |
Synchronize estrus. Treat
persistent CL. |
|
Gonadotropin releasing hormone
(GnRH) |
Hypothalamus |
Pituitary gland (master gland) |
LH and FSH release |
|
|
Follicle Stimulating Hormone(FSH) |
Pituitary gland |
Ovary-mares Seminiferous tubules-male |
Growth of follicle-mare
spermatogenesis-male |
Same as effect |
|
Luteinizing Hormone (LH) |
Pituitary gland |
Mature follicle–mare Leydig cells –male |
Follicle maturation and
ovulation Stimulates testosterone
production |
Same as effect. (gonadotropin(HCG) |
|
Oxytocin |
Pituitary gland |
Uterus , mammary gland |
Uterine contraction, milk let
down |
Retained placenta elimination,
induce abortion or foaling |
|
Prolactin |
Pituitary gland |
Mammary gland |
Milk production |
|
|
Relaxin |
Ovary, placenta |
Pelvic tissues |
Relaxation of pelvic area for
foaling |
|
Of the main hormones listed above, only certain ones
are used therapeutically as an aid to a more efficient management of broodmares
and stallions on a working breeding farm. The following is a summary of common
hormonal use on mare’s and fillies on the average breeding farm:
Estrogen (the “heat” hormone) is rarely helpful in the broodmare,
and actually is usually detrimental to the mare’s normal cycling patterns. It
is sometimes used in spayed mares that may be used as “jump” mares in an
Artificial Insemination program. The use of this hormone in these mares would
cause the mare to continuously show symptoms of heat, and be receptive to the
stallion at all times. The most common
form of estrogen used is a synthetic injectable form.
Progesterone (the
pregnancy hormone) is occasionally found to be
in low
levels in mares that have a history of early embryonic death or early
or late
term abortion. This hormone is used on this type of mare in an
attempt to prevent the loss of the embryo or
fetus. The most popular and
most
available synthetic progestin is Regumate (a liquid
product). Other
uses
for this type of
hormone include, synchronization of estrus, attempts
to
regulate or shorten the transitional period and blocking of the heat or
estrus
behavior in fillies and mares in competition. These uses are detailed
in
your text reading.
Prostaglandin use on an average breeding operation is usually wide
and
varied. Injectable forms of a synthetic
prostaglandin can be used to bring
a mare
into estrus (heat) if they are in their diestrus phase, shorten cycles, rid the
uterus of an embryo or twin embryos, synchronize a group of mares’ heat cycles
and possibly induce parturition (foaling). These uses are also detailed in your
reading assignment. The product to the left is a commonly used synthetic
prostaglandin.
Gonadotropin Releasing Hormone (GnRH) is found in synthetic form as an injectable. The most common use of this hormone is for
the final maturation and ovulation of a primary follicle. (Basically used for
its Lutenizing Hormone releasing properties).
Follicle Stimulation Hormone (FSH) is available as an injectable, but has no practical uses
in the broodmare.
Leutinizing Hormone (LH) is available as HCG (human chorionic gonadotropin) which
is synthesized from pregnant human tissue. This is
used similarly to GnRH, in that it will cause
final maturization and ovulation
in a mare in the later phase of her estrus
(heat) period within 12 to 48 hours
of injection. It is inexpensive and probably
used most often for this purpose.
Oxytocin is also available in injectable form, is quite inexpensive
and is very helpful
in a breeding farm program. It can be used to treat a
mare for retained placenta, assist in milk
letdown for the recently foaling
mare and is also the most dependable drug
used for induction of foaling.
Prolactin and Relaxin would rarely be used on a commercial breeding farm.
One of the most important manipulations
of estrus in the mare for the modern breeding program does not require drugs at
all.
The use of artificial
lights, in order to schedule the transitional phase of the maiden or barren
mare, is very helpful for the breeding operations in
If these mares are placed
under lights in early December, by the end of January or the first of February,
they should be through with their transitional phase and be having regular
estrous cycles. Mares are not normally fertile during their transitional phase
and the transitional phase without the use of artificial lights usually comes
around February and early March. By utilizing this procedure most breeding
farms are able to begin breeding as early as February 1, and have mares foaling
as early as January 1 the following year.