Abstract

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.

Endnotes

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

Acknowledgments

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

1.  Adams NR. Detection of the effects of phytoestrogens on sheep and cattle. J Anim Sci. 1995;73(5):1509–1515.

2.  Ferreira-Dias G, Botelho M, Zagrajczuk A, Rebordão M, Galvão A, Bravo PP, Piotrowska-Tomala K, Szóstek AZ, Wiczkowski W, Piskula M, Fradinho MJ, Skarzynski DJ. Coumestrol and its metabolite in mares’ plasma after ingestion of phytoestrogen-rich plants: Potent endocrine disruptors inducing infertility. Theriogenology. 2013;80(6):684–692.

3.  Ferris R, Christensen BW, Dujovne GA, Scott CJ. Effect of a phytoestrogen-rich diet on the mare (Equus caballus) estrous cycle: a model for exotic Perissodactyla. In: Proceedings from the American Association of Zoo Veterinarians. 2016:305–306.

4.  Hughes CL, Jr. Phytochemical mimicry of reproductive hormones and modulation of herbivore fertility by phytoestrogens. Environ Health Perspect. 1988;78:171.

5.  Tubbs C, Hartig P, Cardon M, Varga N, Milnes M. Activation of southern white rhinoceros (Ceratotherium simum simum) estrogen receptors by phytoestrogens: potential role in the reproductive failure of captive-born females? Endocrinology. 2012;153(3):1444–1452.

6.  Tubbs CW, Moley LA, Ivy JA, Metrione LC, LaClaire S, Felton RG, Durrant BS, Milnes MR. Estrogenicity of captive southern white rhinoceros diets and their association with fertility. Gen Comp Endocrinol. 2016;238:32–38.

7.  Wocławek-Potocka I, Mannelli C, Boruszewska D, Kowalczyk-Zieba I, Waśniewski T, Skarżyński DJ. Diverse effects of phytoestrogens on the reproductive performance: cow as a model. Int J Endocrinol. 2013;2013:1–15.