Fact or Fiction: "A Shot to Make Mares Cycle"
by Dr. Jim and Lynda McCall

Conspiring to produce a pregnancy, the mare owner phoned with the plan.  "I'm going to have my vet give my mare a shot so she'll be ready to breed when I drop her off.  I'll return the following weekend to pick her up."

If we had a dollar each time we have received the same set of instructions from a mare owner...

Attempts to help Mother Nature entice the sexual receptivity of broodmares did not just begin with the onset of modern pharmacology.  In the past, horse breeders used to say things like "If you force open the mare inside, she'll come on and breed" or "If a mare isn't cycling, all you need to do is flush out her womb with mild salty water and she'll breed".

It is interesting that both the old and new comments about how to encourage the mare to cycle center around the hormone prostaglandin.  The early physical manipulations most likely stimulated the mare's uterus to produce prostaglandin.  Today's magic injection bullet which is commercially marketed under several trade names is also a form of prostaglandin generically referred to as  F2 alpha prostaglandin.

First discovered in the early 1950's, this hormone came into vogue in the late 1960's as a cure-all.  Research revealed that prostaglandin is important in maintaining the estrus cycle of the mare.  Produced in the uterus, it's primary action is to lyse or kill the corpus luteum - (CL).

After the release of the egg, a corpus luteum is formed at the site of ovulation.  The function of the CL is to produce the hormone progesterone which will help maintain the impending  pregnancy.  If a pregnancy does not occur,  the body is notified by  the uterus by producing  prostaglandin. Prostaglandin then attacks and kills the CL which allows the mare to begin another reproductive cycle.

Commercial production of F2 alpha prostaglandin allows horse breeders to, theoretically,  shorten the cycle by killing the CL and coaxing the  barren mare back into heat.  This is especially useful in those situation where the mare retains her CL longer than normal therefore delaying  her cycle.

Having spent all those years in higher education reading the research and experimenting with F2 alpha, it has been our practice to use the drug only when a CL was present on the ovaries.  Yet today it is common practice to give an injection of F2 alpha  prostaglandin to bring a mare into heat regardless of the structures of her ovary.

In an attempt to judge the effectiveness of "giving that shot to make'em breed", we looked at the records of 47 mares.  The mares were divided into two groups. the first of which consisted of 10 cycling mares for whom we knew the last ovulation date.  An injection of F2 alpha was given three to seven days after the last egg was released.  All of the mares came into standing heat and ovulated.  The average time from injection to standing heat was five days.  The average time from injection to egg release was eight days.

The second group consisted of 37 mares for whom we had no information on the date of their last ovulation.  Of this group, 54% responded to the injection by having a standing heat and ovulation, with averages similar to the first group: five days to standing heat. eight days till ovulation. Two mares from this group, or 5.4%  had a standing heat but did not release an egg during that cycle.  The remaining 40%, 15 mares, did not respond to the injection by having a heat or an ovulation within three weeks of the injection.

These figures again prove that  if you are going to nudge Mother Nature you had better sharpen your stick. Giving  the "magic bullet" without information about the presence of a CL is a shot in the dark. Expect it to work only about half the time.

copyright  Dr Jim and Lynda McCall

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