The reproductive performance of broodmares holds significant importance within the equine industry. Achieving optimal pregnancy rates is critical for both the profitability of breeding operations and the holistic well-being of mares and foals.

Many different factors can affect mare fertility, with an increasing emphasis being placed on the role of phytoestrogens in the diet. Phytoestrogens are compounds derived from plants that share similar properties with estrogen, the primary female sex hormone. Common horse feeds such as alfalfa and clover hay, as well as soy-based concentrated feeds, can be high in phytoestrogens.

Research shows that phytoestrogens can bind to estrogen receptors in the body, acting either as weak estrogens or anti-estrogens. Suspected links may exist between high levels of phytoestrogens in the diet and various reproductive issues in mares.

Understanding how these compounds influence mare fertility is crucial for horse breeders and those involved in equine health management. This article explores the presence of phytoestrogens within the equine diet and potential implications on the fertility of mares.

Phytoestrogens and Mare Fertility

Phytoestrogens are a diverse group of plant-derived compounds that are structurally and functionally similar to 17ß-estradiol (a type of estrogen). [1]

Estrogens are a group of steroid hormones that play a crucial role in the development and regulation of the female reproductive system and secondary sex characteristics.

Phytoestrogens can exert both estrogenic and anti-estrogenic effects in the body. They are considered endocrine-disrupting chemicals because they can interfere with the body’s natural hormone system.

They are found in a variety of plants, including clover, soybeans, and other legumes, and can enter the equine diet through forage, commercial feeds, and supplements.

The effects of phytoestrogens on animal health and reproduction are complex and not yet fully understood. However, research is ongoing to understand how these chemicals affect the mare’s endocrine system and fertility.

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Sources of Phytoestrogens in Equine Diets

Common sources of phytoestrogens in equine diets include:

  • Clover (especially red and subterranean varieties)
  • Other legumes (e.g. alfalfa)
  • Soybeans and soybean meal
  • Linseed (flaxseed)
  • Certain herbs (e.g. licorice root and hops)
  • Grains and cereals (e.g. oats and barley, though in lower amounts)
  • Fruits and vegetables (e.g. apples and carrots)

These plants contain different types of phytoestrogens, which can vary in their estrogenic potency.

Factors Impacting Concentration

Many factors can influence the levels of phytoestrogens in feedstuffs. Environmental conditions affecting the plant, such as temperature fluctuations, pest pressure, and the availability of water and light, can impact phytoestrogen levels within the plant. [6]

Additionally, processing, heating and fermenting soy-based food can alter the chemical forms of isoflavones and thus alter the overall phytoestrogen exposure and potential estrogenicity of the feed. [7]

Additionally, the processing, heating, and fermenting of soy-based feed can change the chemical forms of phytoestrogens present. These alterations can influence the overall exposure to phytoestrogens and also affect the potential estrogenic effects of the feed. [7]

Types of Phytoestrogens

Based on their chemical structure, phytoestrogens are generally classified into three primary categories: [2]

  • Polyphenols
  • Flavonoids
  • Isoflavonoids

Of these compounds, polyphenols and flavonoids don’t share a close structural similarity with estrogens. However, isoflavonoids do closely resemble the structure of estrogens and have a higher affinity for binding to estrogen receptors. [3]


Isoflavonoids are the most significant phytoestrogen for horses due to their prevalence in equine feed sources and their heightened estrogenic activity.

Isoflavonoids can be further differentiated based on the arrangement or structure of their carbon atoms. This structural variation is significant because it influences how these compounds interact with estrogen receptors.

The three main types of isoflavonoids are: [4]

  • Isoflavones: Includes genistein, daidzein, glycitein, biochanin A, and formononetin. These compounds are present in soybeans and various other legumes.
  • Isoflavans: Includes equol, a metabolite of daidzein, which is found in soybeans and other legumes.
  • Coumestans: Includes coumestrol, which is prevalent in alfalfa and clover.

Phytoestrogens and Fertility in Horses
Adapted from “The Pros and Cons of Phytoestrogens,” by Heather B. Patisaul and Wendy Jefferson, 2010, Frontiers in Neuroendocrinology, 31(4), 400-419.

Inactive vs. Active Forms

Isoflavones in plants are found either in their bioactive aglycone (unconjugated) form or in an inactive glycoside (conjugated) form. The majority of phytoestrogens are ingested in the glycoside form, such as genistin and daidzin. [3][5]

These compounds then undergo a process called hydrolysis in the digestive system, transforming into their bioactive aglycone counterparts, like genistein and equol. This transformation is crucial because the aglycone forms are more readily absorbed by the body. [3][5]

Once converted, these bioactive compounds are absorbed through the small intestine via passive transfer mechanisms, allowing them to enter the circulatory system and exert their effects. [3][5]

This process of conversion and absorption is significant because it determines the extent to which phytoestrogens can exert their biological effects. The inactive glycoside forms are not directly effective until they are transformed into their active aglycone forms.

Binding Affinity

Once phytoestrogens enter the bloodstream, their effects are primarily determined by how strongly they bind to estrogen receptors in the horse’s body.

Some phytoestrogens, for instance, coumestrol and genistein are considered more estrogenic as they are more likely to bind with estrogen receptors compared to daidzein.

This binding ability is crucial as it dictates how effectively phytoestrogens can mimic or block the effects of the body’s natural estrogens.

Phytoestrogens and Livestock Fertility

Research shows that diets with high levels of phytoestrogens can lead to reproductive disorders in livestock animals, such as sheep and cows.

Exposure to phytoestrogens is inevitable in livestock diets as these animals consume large amounts of plant material. The concentration of phytoestrogens varies in different types of feed and forage, with some being particularly high in these compounds.


Cows fed diets high in phytoestrogens have exhibited signs of ovarian dysfunction, leading to cystic ovaries and estrous cycle abnormalities.

This disruption in ovarian function can result in reduced conception rates, increased embryo loss, and even temporary infertility. [8]


In ewes, grazing on phytoestrogen-rich plants can result in long-term fertility issues. While their ovarian function might be less visibly affected compared to cows, ewes can experience permanent infertility due to complications with their reproductive tracts. [8]

This phenomenon has been observed in female sheep grazing plants such as red clover and alfalfa [8] In some geographic areas, such as western Australia, the impact of phytoestrogens on livestock fertility is very evident.

Studies show that ewes grazing on red clover often experience subclinical or undiagnosed infertility. [9] These findings emphasize the intricate interplay between diet, hormones, and reproduction in livestock animals.

Reproductive Effects in Broodmares

The effects of phytoestrogens on fertility in horses remains a subject of ongoing research, with current studies offering conflicting results. The limited scope of studies, small number of horses, and variability in outcomes have made it difficult to establish definitive conclusions.

However, the potential impact of these compounds on equine reproductive health cannot be overlooked. Further research is needed to provide more comprehensive and conclusive recommendations.

Phytoestrogens in Horse Feed

Recent research has investigated the presence of phytoestrogens in horse feed. Historically, studies of phytoestrogens in livestock feeds have focused on fermented feeds. However, there is growing awareness of non-bioactive glycoside isoflavones in plant-based feeds for horses.