How Can Diet Affect Mare Behavior?

Phytoestrogens in horse diets might have physiological effects on mares. An equine nutritionist explains how and why.

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bay horse pinning ears, grumpy
Your veterinarian might suggest adding Regu-Mate to your mare’s diet to help reduce behavior problems. | Getty Images

Q: I have a mare that is on Regu-Mate (altrenogest) for mareish behavior and consumes a forage-based diet that includes ingredients like soybean oil, alfalfa pellets, and ground flax. She seems to have some behavioral issues in the early spring and late fall, despite being on Regu-Mate. I have heard that phytoestrogens in these and other ingredients can counteract the active ingredient in Regu-Mate, but I haven’t been able to find anything that supports or explains this. Do phytoestrogens in feed counteract the effects of Regu-Mate, and should ingredients with phytoestrogens be avoided if you are feeding a mare that is also on Regu-Mate?

A: Mares can be amazing, yet some can be challenging at times. To get everyone on the same page let’s start with discussing phytoestrogens because they’re more complicated than they appear at first glance.

Phytoestrogens are a family of polyphenolic plant compounds that bind to estrogen receptors and have been shown to inhibit synthesis, secretion, transport, and binding of endogenous (coming from within the body) estrogens. Exposure to phytoestrogens in horse diets is inevitable due to the consumption of large amounts of plant materials. Legume plants such as soy, lupine, clover, and alfalfa are generally highest in phytoestrogen content, but certain phytoestrogens are also found in flaxseeds, rice, and wheat. The amount and specific phytoestrogen compounds present are extremely variable across various plants, stage of maturity, plant disease state, and growing conditions and locations. 

To date, more than 8,000 polyphenolic compounds have been identified. Flavonoids, isoflavonoids, and coumestans are the major types of phytoestrogens found in common equine diets. Lignans, which are structural cell wall components of plant cells, are also common in livestock diets but are a less studied group of phytoestrogens. To add more complexity, there are multiple compounds within each phytoestrogen type and they vary in bioavailability, how they are metabolized, physiological effects, and potency.

Fermentation of plant material changes the form of some phytoestrogens from nonbioactive to bioactive. This could partially explain why differences in digestive tract architecture between livestock species, specifically ruminants (sheep and cattle) versus nonruminants (horses), contribute to variation in physiological effects of phytoestrogens.  Sheep are reportedly more sensitive to phytoestrogen consumption than cattle, and horses appear to be least sensitive. 

In a 2013 Portuguese study conducted on a small population of horses, researchers reported that plasma concentrations of the phytoestrogen coumestrol were elevated and infertility was induced in mares consuming an alfalfa-based haylage. However, this was a limited study and there is currently very little research reporting whether naturally occurring phytoestrogens in typical horse rations would have a negative impact on reproductive performance in mares. 

Due to the lack of sound science clearly identifying effects of naturally occurring phytoestrogens in normal horse diets on mare reproduction or behavior, it seems prudent to focus on choosing a well-balanced diet that provides optimal nutrition to support the age and activity level of your horse. I find no studies directly looking at interactions between dietary phytoestrogens and Regu-Mate and would suggest working with your veterinarian to determine what seasonal changes might be happening with your mare that could be negatively affecting her behavior.

References/links below:

Adkin, Angie. The Characterization of Phytoestrogens in Equine Feeds and Serum. 2018. University of Florida, PhD dissertation

Kirkland, B,. Wickens, C., Vineyard, K. and Adkin, A.  Let’s talk about Horses + Soy.  2022.


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Written by:

Karen Davison, PhD, director of equine technical solutions for Purina Animal Nutrition, earned her Master of Science and PhD degrees in equine nutrition from Texas A&M University. Davison’s research included some of the early work investigating the use of added fat in horse diets. She spent eight years as an associate horse specialist with Texas Agricultural Extension Service, developing and teaching youth and adult education programs, prior to joining Purina in 1993. Davison has guest-lectured at universities and veterinary schools, is published in scientific research journals and magazines, has authored book chapters, and presented at regional and national veterinary meetings on equine nutrition topics. She and her family are involved with training and competing in the cutting and rodeo performance horse industries.

5 Responses

  1. It is possible that a mare in seasonal anoestrus ie totally inactive ovaries not producing either oestrogen or progesterone, could be influences to display signs of oestrus when fed sufficient phyto-oestrogenic material or even less likely enhanced her oestrous behaviour when in true oestrus. However when in dioestrus and her brain is dominated by progesterone or when fed the progestogen ‘Regumate’, even a large ‘dose’ of oestrogen (oestradiol) will not over-ride the behaviour induced by progesterone/Regumate
    Ref Silva, Newcombe and Cuervo-Arango Effect of treatment with oestradiol benzoate on oestrous expresion ……
    Animals 2023,13:1718. pp150-163

  2. I hope all mares have been evaluated for polycystic ovary problems. I had a mare that turned real mean to where she would lunge over gate to get you and you had to carry a whip to keep her off you in the pasture. She was like a mean stud. I raised this mare from birth. I finally bred her and you talk about a 360 change. She was like the foal I raised…until after the colt had nursed for 4 months. Then it was back to the I’m going to kill you behavior. Wish I had of had a really good vet back then. They just said she was mental.

  3. Burks, the supplement Yvonne is referring to is: for behavior modification in mares, chasteberry is also a relatively common dietary supplement for horses with equine Cushing’s disease, even though preliminary equine research with chasteberry extract has not demonstrated any improvement in symptoms as compared to treatment with pergolide mesylate and other remedies. I feed my ornery mare Mare Magic and have done so for a few years. I do believe it improves her surly mood.

  4. I had good results feeding my mare chase tree berry to help regulate her hormones. It took the edge off her aggressive behavior.

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