I’m not going to go into all the behaviors exhibited by a stallion in a feral or pasture breeding situation today, as most horse breeding operations today use halter handling of the stallion for live cover breeding or semen collection.

What’s that lip curling thing?

When a stallion teases a mare, he may vocalize to the mare, invesigate her tactilely (usually with his lips and muzzle, particularly over her haunches, vulva, and rear legs if permitted by the handler), and investigate her olfactorily using the Flehmen response. The Flehmen response is a particular curling of the lips displayed by equids, felids, ungulates, and other mammals to facilitate the transfer of scent molecules from the air  into the vomeronasal organ (also called the Jacobson’s organ). It is named for its proximity to the vomer and nasal bones, and is located on the roof of the mouth. The vomeronasal organ contains chemoreceptors that allow the stallion to detect pheromones. This behavior allows the stallion to detect odorant chemicals, such as those from the mare’s urine, to determine the presence of estrus. The behavior may also be displayed when a horse suffers from minor abdominal pain.

What’s the normal precopulatory interaction between a stallion and mare?

Most stallions will interact with the mare as soon as allowed, normally acheiving erection within two minutes, and are ready to mount within 5 to 10 seconds of achieving a full erection. Now you can see why it’s so important to keep a teaser stallion separated from a mare in estrus, especially if he is not the intended breeding stallion.

What’s the normal copulatory behavior of a stallion?

Many stallions, particularly those that have been pasture-bred, will feign a mount from the side to be sure the mare is in standing heat before mounting from the rear. Mounting is followed by insertion of the penis into the vagina, a series of six to eight pelvic thrusts, ejaculation, and dismount. In a pasture breeding situation, the mare will often walk out from under the stallion, while in a hand breeding situation, the stallion may be backed off the mare or mounting dummy if he was being collected an artificial vagina (AV) for artificial insemination. On an organized breeding farm, the entire process, including washing, occurs within one to two minutes of the stallion entering the breeding shed.

For more information on stallion behavior during breeding, check out this chapter exerpt from Sue McDonnel at the University of Pennsylvania, New Bolton Center.