Feeding Mares Soy: Is It Okay?

Feeding soy to horses continues to be a concern for some horse owners. This has become a rather popular topic, as many owners worry about soy contributing to their horse’s allergies. We previously discussed the topic of soy allergies in horses. Of the horses suffering from true food allergies, it is thought that less than 5% are caused by soy. Another concern about soy is that it may contribute to mare infertility. Here, the data is not as definitive, but does give some insight to whether soy can influence a mare’s reproductive ability. 

Can Feeding Horse Feeds with Soy Impact Mare Fertility?

First, it is important to note that there is very limited research in horses concerning soy and any impacts it may have on a mare’s fertility. In fact, most mares are quite fertile and generally do not have too much difficulty in getting pregnant, usually within 1 to 2 estrous cycles. However, there are some problematic mares that do have difficulty in getting pregnant and some are worried that soy may be a contributing factor. This leads to the question of how soy can impact mare fertility. How can it impact any female mammal’s fertility? The answer is quite complex and is why scientists are beginning to investigate this question.

Soy contains compounds called phytoestrogens. Phytoestrogens are “estrogen like,” and in excess, they can compete with natural estrogen in the body. By competing, phytoestrogens act like an endocrine disruptor, where they replace natural estrogens, which can negatively impact female fertility.

We do know that soy and other feedstuffs like alfalfa and clover, which are routinely fed to horses, do have elevated levels of phytoestrogens in them as compared to other feed ingredients. Research out of the University of Florida established the levels of phytoestrogens in the following equine feedstuffs:

  • Grain-based commercial horse feeds with soybean meal (crude protein 14%) were high in the phytoestrogens genistein and daidzein.
  • Alfalfa hay had elevated levels of the phytoestrogens genistein and coumestrol.
  • Grass hays, perennial peanut and Coastal bermudagrass did not have any measurable phytoestrogens.

The research team in Florida then took their research a step further to see if these phytoestrogens were then actually absorbed in the bloodstream of the mares after being fed a commercial horse feed with 14% crude protein. They even measured the levels of phytoestrogens of suckling foals in their study to see if any were passed on in the mare’s milk. They found:

  • Mares consuming a commercial horse feed with soybean meal had measurable levels of the phytoestrogens genistein and equol. The finding of equol was interesting and noted in the study because daidzein is converted to equol in the blood.
  • Suckling foals were found to have measurable levels of genistein in their blood.

The research out of Florida is interesting, as it was able to develop a reliable technique of evaluating phytoestrogens in equine feedstuffs and in the blood serum of horses for future research.  However, the study did not evaluate any reproductive parameters in these horses or if having elevated levels of phytoestrogens in the blood of mares and foals had any negative consequences to their health.

What Research is There on the Impact of Soy on Mare Fertility?

There is very little research to support the idea that feeding soy has any negative impacts on mare fertility. This is mainly because, with our current understanding of mare reproduction, any fertility issues are usually due to the age of the mare (i.e., age related infertility) or mares having physiological issues in their reproductive tracts (i.e., uterine infections). However, this does not mean soy might not be contributing to infertility in some mares. We just do not have any significant data to support that claim.

There is one study that showed phytoestrogens can have some negative impacts on mare reproduction.  In 2013, a group of scientists out of Portugal reported that some mares fed a diet of clover-alfalfa haylage did experience some reproductive abnormalities. As noted, legume plants, like alfalfa and clover, tend to be high in phytoestrogens. In this study, the reproductive abnormalities only subsided when the researchers stopped feeding the haylage. These observations are also supported in what has been found in other species, such as sheep and cattle. They, too, suffer from reproductive abnormalities when fed diets high in phytoestrogens from clover and alfalfa. The level of phytoestrogens these mares were consuming from haylage was significantly higher than a horse would consume eating a typical growth or mare and foal horse feed fed at label recommended rates.

Does this mean you should stop feeding mares alfalfa or pull them from pastures with clover? The short answer is no. While the study conducted in Portugal is interesting, it is one study on one small set of mares. Thus, more research is needed before making any such recommendations.

Another study followed the reproductive track record of 12 mares over two years. In year one, they were fed a “high” phytoestrogen diet, based on soy and alfalfa, and year two, they were fed a “low” phytoestrogen diet that excluded these ingredients. The researchers did observe some minor changes in the year one mares’ cycles, but there was no difference in conception rates between years one and two.

Overall, there is no research in horses that concludes that soy has any impact on mare fertility. With our current understanding of feeding soy, the only recommendation for mares to be switched to a soy-free diet would be in cases where no other causes of infertility could be identified. This would be a last-ditch effort to get the mare pregnant. Thankfully, there are soy-free options in the Tribute® line of feeds that could be considered:

Take Home Message

All in all, there are many causes of infertility in mares. With no data currently available to support the theory that feeding soy is harmful to a mare’s reproductive ability, there is no need to worry about her health. However, if your mare is having difficulty getting pregnant, then it is best to consult with your veterinarian or an equine reproductive specialist. They can then conduct a breeding soundness exam to help identify any issues and help best guide you.


Adkin, A. (2018). The characterization of phytoestrogens in equine feeds and serum. PhD Dissertation. University of Florida, Gainesville.

Ferreira-Dias, G. et al. (2013). Coumestrol and its metabolite in mares' plasma after ingestion of phytoestrogen-rich plants: potent endocrine disruptors inducing infertility. Theriogenology 80: 684-692.

Samocha, M. B. (2017). The effects of a high-phytoestrogen diet on equine estrous cycles and fertility using domestic mares as models for captive exotic ungulates. University of California, Davis.

Article By: Nicole Rambo, Ph.D.
Back to news