Effects of a High-Phytoestrogen Diet on Equine Estrous Cycles and Fertility Using Domestic Mares (Equus caballus) as Models for Wild Ungulates
American Association of Zoo Veterinarians Conference 2017
Mollie Samocha1, MS; Troy Tollefson2, PhD; Nicole Sharpe3, PhD; Bruce W. Christensen4, DVM, MS, DACT
1Animal Biology Graduate Group, University of California, Davis, CA, USA; 2Mazuri Exotic Animal Nutrition, Gray Summit, MO, USA; 3College of Biological Sciences Dean’s Office, University of California, Davis, CA, USA; 4Population Health and Reproduction, School of Veterinary Medicine, University of California, Davis, CA, USA


Phytoestrogens are plant-based estrogenic compounds commonly found in zoological and domestic animal feeds composed of alfalfa, soy, and clover.4,6 Dietary phytoestrogens may play various roles in the infertility of several ungulate species.1-3,5,7 The reproductive health of domestic equids (Equus caballus) fed a commercial soy- and alfalfa-based exotic herbivore diet was investigated.a Twelve mares were fed the treatment diet February–November 2015 (T1) and subsequently fed a control diet February–November 2016 (C1). Mares were examined via transrectal palpation and ultrasound for estrous cycle and reproductive tract evaluations, and blood samples were collected weekly to obtain serum progesterone and estradiol concentrations. Toward the end of each trial, mares were artificially inseminated to evaluate fertility. It was hypothesized that T1 mares would exhibit abnormal cycle and phase lengths, lower progesterone values, higher estradiol values, and reduced conception rates compared to C1 mares. Results showed that T1 mares experienced significantly more short luteal phases (p=0.015), higher average estradiol (p<0.01), and lower average progesterone preceding the expected period of luteolysis (nonoverlapping 95% CI of trendlines). T1 mares also tended to exhibit fewer long cycles (p=0.075) and more incidences of abnormal cervical tone and uterine edema during the expected luteal phase (p=0.061). No difference in conception rates was found (p=0.946). These results suggest that phytoestrogen-rich diets may alter key reproductive hormone concentrations and influence estrous cycle dynamics yet have no apparent effect on conception rates in horses. Further investigation into long term, gestational, and developmental exposure to phytoestrogens is necessary to better understand their relationship to equine fertility.


a. Mazuri® ADF-16 Herbivore Pellets, Purina Mills LLC, Gray Summit, MO, USA


The authors would like to thank the Theriogenology service at the William R. Pritchard Veterinary Medical Teaching Hospital and the University of California, Davis Center for Equine Health for their assistance in the care of these animals. Authors are also pleased to thank Mazuri® Exotic Animal Nutrition for providing the feed and funding for this project.

Literature Cited

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Speaker Information
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Mollie Samocha, MS
Animal Biology Graduate Group
University of California
Davis, CA, USA

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