Breeding the Older Mare, PT 3

Welcome back to our series on breeding the Older Mare! If you had a chance to catch MSL’s blog on February 26, we discussed contamination and infection during breeding, endometritis, poor anatomical conformation and past foaling traumas, plus other circumstances that may plague the older broodmare.

It takes a lot for a broodmare to gain matriarch status in the performance horse breeding world. Firstly, many owners today want horses that come from proven lines – both on the top and bottom sides of their pedigrees. Secondly, many owners would like to see the dam side with credentials: for instance, has the dam showed and won money or titles herself? And what about the granddam? Has the granddam also proved herself in the showpen? Lastly, is the dam also a “proven producer”? Meaning that her foals are additionally winning money and proving themselves in the show pen.

Once a breeder has this golden combination in a specific mare, I certainly can understand why they would want to continue breeding her – even if she is getting up there in age. After all, Somethingroyal, the dam of Secretariat, was 18 when she foaled Secretariat. Penny Chenery, her owner, continued to have her bred until she was 24, probably in the hopes that she would produce another amazing racer. And Natalma, the dam of Northern Dancer (Kentucky Derby winner and leading sire), had her last foal at age 24.


So today, let’s talk about the natural process of aging in the mare and what can be done to help her maintain a pregnancy.

As it is with Mother Nature, infertility is another of the challenges that can plague older mares. As a mare enters her late teens and early 20s, her reproductive chances begin to decline and this may partly be because of the older eggs/oocytes she is producing. Older eggs may be unable to undergo fertilization or may develop abnormally. Or they may tend to undergo early embryonic death because the chromosomes may have defects in them and the embryo does not form naturally. So you get a reduction in fertility even if the mare does conceive.

Then of course, the next hurdle to get over is keeping your older mare pregnant once she has been confirmed “in foal.” If an embryo leaves the oviduct and enters the uterus around Day 6 or 7, it needs a healthy environment in which to survive. Therefore, a poor uterine environment or poor uterine defense mechanisms (as can happen in the older mare due to wear and tear to the uterine lining) will affect good placental attachment.


Hormones also play a huge role in maintaining pregnancy. As ovulation occurs, the estrogen level begins falls and the remains of the ovulated follicle are converted to form a corpus luteum (CL). The luteal cells then are responsible for secreting the hormone progesterone. And progesterone’s “job” is to shut down the estrus-stimulating hormones and to set the stage for maintaining a pregnancy. This includes subduing the actively contracting reproductive tract and to tighten and close the relaxed and open cervix. It also prohibits the secretion of Follicle Stimulating Hormone and luteinizing hormone from the pituitary gland, and puts the mare “out of heat.”

Unfortunately, in some mares (and often, older mares) the correct amount of progesterone cannot be secreted and when that happens the other hormones swing into action to terminate the pregnancy and bring the mare back into heat. Luckily, progesterone therapy can be administered to the mare throughout the pregnancy to help her maintain it. However, this is another economic factor breeders will have to take into consideration.

The next period of concern is around Days 35 to 40 post-ovulation, when the placental formation and attachment begin. If the uterine lining (endometrium) has significant pathology, the placenta may not form enough of an attachment to allow the fetus to survive. But the good news is, once the mare gets to her halfway point in the pregnancy, she will likely carry to term because the placenta/endometrial unit has a capability for compensating for slight abnormalities.

Tomorrow, in our last installment of Breeding the Older Mare, My Stable Life will talk about optimum care for aging broodmares and possible solutions for pregnancy. See ya then!

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