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