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  • Q:

    I am feeding barren and lactating mares a “competition cube” feed at the rate of 5.5 lb (2.5 kg) a day along with 3 lb (1.3 kg) of soy hulls for filler, along with free-fed haylage. The mares were hard to get in foal last year, and I am wondering if the soy hulls were the cause of that. Could this be a problem?

  • A:

    Phytoestrogens or isoflavones are estrogen-like compounds found in legumes such as clover, alfalfa (lucerne), and soybeans. These compounds have received attention recently due to their potential to cause endocrine dysregulation and reproductive effects such as infertility.

    Environmental factors and stage of plant growth can affect phytoestrogen concentrations and ultimately dietary phytoestrogen intakes. Studies in horses have shown a linear relationship between dietary and plasma phytoestrogens levels, indicating the importance for proper pasture and forage management and production.

    Recent work has suggested that one form of isoflavones called coumestans (coumestrol) in alfalfa may exert more potent effects than isoflavanoids found in soybeans and clovers. The hormone-like activity of these compounds may result in estrogenic or anti-estrogenic activity. Studies have identified disruptions in reproductive efficiency in sheep, cattle, and horses fed legume-based diets, but specific guidelines or recommendations for safe levels of these plant phytoestrogens have not been determined for breeding horses. Keep in mind, alfalfa is the primary forage of many successful breeding operations worldwide.

    Based on the diet information, I would not place blame on the soybean hulls for causing the reproductive issues, especially as the hull itself is likely to have a low phytoestrogen content compared to the whole soybean. Research in cattle found that the soybean hulls contained four times fewer isoflavones than soybean meal.

    I would also be interested to know more about the haylage offered to the mares. Is it a grass-clover mix or grass-lucerne mix? These forage types may contribute more phytoestrogen to the mares’ diet than the amount of soybean hulls being fed. Also, were the horses on pasture during the breeding season, and was there more clover than usual in the fields?

    Grass hay is thought to have the lowest phytoestrogen content of common forages and may be necessary for horses that have a history of reproductive problems or infertility. If you’d like to replace some or all of the soybean hulls for an alternative forage source, then feeding sugar beet, grass-based forage pellets, or chaff would be reasonable.

    As a final note, there are many causes of subfertility in mares, so consultation with a veterinary specialist or theriogenologist might uncover the best way to manage these mares.

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