Equine Reproduction

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 North America. By placing a barren or maiden mare “under lights” (using a 200 watt light bulb in a stall is most commonly used) to make sure that they receive at least 16 hours of light per day for a period of about 6 weeks, will cause that mare to go through her transitional phase before the breeding season starts.

If these mares are placed under lights by December 1st, by the middle 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, and have mares foaling as early as January 1 the following year.

Click here to take the Quiz