Scent's Influence on Breeding Stallions

Scent's Influence on Breeding Stallions

The flehmen response seems to be a precursor to libido stimulation, Guillaume said.

Photo: iStock

For years, breeders have been putting mare’s urine on breeding dummies to help boost stallions’ libido. But science hasn’t backed the efficacy of this breeding shed trick until now.

To encourage stallions to be more interested in breeding during the winter (the breeding off-season), French researchers recently looked into this “old breeders’ tale.” They found that stallions in their study were quicker to ejaculate when mounting a dummy if they had the scent of urine from mares in heat on their nostrils, compared to that of mares out of heat.

“Our study suggests that only the urine of mares in heat is effective in stimulating a stallion’s libido during semen collection in the off-season,” said Daniel Guillaume, PhD, of the French Agricultural Research Institute’s Division of Physiology of Reproduction and Behavior Department of the Neuroendocrinology of Sexual Interactions and Behavior. He presented his group’s work at the 42nd French Equine Research Day, held March 17 in Paris.

Reaction to the smell of urine—regardless of the horse it comes from—appears to be a response related to testosterone, Guillaume said. The team’s study stallions showed a flehmen response (upper lip curl) when presented with urine from mares in or out of heat, geldings, and even other stallions.

The study geldings, however, showed very little or no flehmen response to any of the urine, he said.

To reach their conclusions, Guillaume and colleagues held experiments to test the effects of odors on horses’ sexual stimulation. In the first experiment, they presented various odors to 13 breeding stallions and 10 sport horse geldings. The tested odors included urine from mares in heat, mares not in heat, geldings, and stallions. For the stallions, the researchers also used chemical preparations—specifically, fatty acids or cresol (a type of organic compound) from the feces or urine of females in heat, dissolved in water, which has been known to induce erections in uncastrated rats and to stimulate stallions’ libido.

The team found that stallions reacted with sniffing and flehmen responses to urine smells, but not significantly to the chemical smells, Guillaume said. The geldings, however, had low reaction levels to all the smells.

“The flehmen response seems to be a precursor to libido stimulation and is reflective of testosterone activity in the stallion,” said Guillaume. “As the stallions in our study were mostly unresponsive to the chemical solutions, we cannot recommend these substitutes for libido enhancement.”

In another experiment, the scientists studied six Welsh breeding stallions’ reactions to three different odors placed directly in their nostrils before semen collection using a dummy. They used a gauze swab to rub either in-heat mare urine, off-season non-ovulatory mare urine, or water into the stallion’s nostril three minutes before leading him to the breeding shed. Each stallion received each treatment once, on different days.

They found that the stallions had similar flehmen responses in smelling the urine of both in-heat and out-of-heat mares, Guillaume said. However, time to erection was much lower when the stallions had in-heat mare urine in their nostrils compared to out-of-heat mare urine (which had the same effect as water). These results were confirmed with 30 other stallions at three different studs.

“The flehmen behavior constitutes a preliminary analysis of the odor, and this behavior is indispensable to stimulate the stallion libido before mating the dummy,” he said.

About the Author

Christa Lesté-Lasserre, MA

Christa Lesté-Lasserre is a freelance writer based in France. A native of Dallas, Texas, Lesté-Lasserre grew up riding Quarter Horses, Appaloosas, and Shetland Ponies. She holds a master’s degree in English, specializing in creative writing, from the University of Mississippi in Oxford and earned a bachelor's in journalism and creative writing with a minor in sciences from Baylor University in Waco, Texas. She currently keeps her two Trakehners at home near Paris. Follow Lesté-Lasserre on Twitter @christalestelas.

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