Sexual Maturity in Stallions and Mares - What You Need To Know

Could this be beginning of courtship between horses?

Could this be beginning of courtship between horses?

Dramatic physical and mental changes occur as horses transition from colt or filly stage to sexual maturity and understanding this maturation process will help you make important management decisions. New window.

Although the breeding season for equines is several months away, it is important that horse owners with young horses understand how horses mature sexually and recognize the basics of equine reproductive behavior for management purposes, both to prevent unwanted foals, and also to make wise decisions related to breeding young fillies and stallions.

The first months of spring mark the beginning of an important time in your horse's life—the start of the breeding season. This is the time of year when pheromones are on the wind and horses are in the mood for finding a mate.

Horses are seasonal breeders, and engage in their mating activities during the longer days of the year so that their foals born 340 days later can take advantage of the season's milder temperatures and lush forages.

Horses of both sexes are affected by changes on their hormone levels, which tell them it is time to mate. Longer days stimulate the development of ovarian follicles in the maiden mare that secrete estrogen as they grow, bringing the mare into periods of "heat" behavior known as estrus.

In the young stallion, increasing day length brings about higher blood levels of testosterone, the primary sex hormone responsible for initiating sperm production and stimulating the increase in sexually driven behavior known as libido.

It is helpful to observe the mating behavior of free-living horses to gain an understanding of the courtship and breeding rituals of horses. Herd stallions are seldom interested in mating with their own female offspring and seldom tolerate those females mating with other stallions, so it is often necessary for both females and subordinate males to leave the herd once they reach sexual maturity.

Young males usually roam about together in loosely knit "bachelor bands," looking for stray mares, mares to steal from other herd stallions or, if the young stallion is very bold, herd stallions to challenge for possession of an existing band of mares.

Did you know?

Sperm production is also influenced by season, and production increases as daylight increases beginning in March and usually peaking in May. Production then begins declining and by August. has declined drastically.

Compatible stallions and mares finally find each other and form long-term bonds, with sexual activity as part of the foundation. Because horses are social creatures that also find greater safety in numbers, additional compatible "stray" mares work their way into the band, and the same bonds are formed between each of them and the stallion.

Mares that are not compatible with the stallion typically leave the herd. The mares that do stay form social attachments with each other and with the stallion, providing a stable and secure existence for all.

Horse sexual behavior develops differently in colts and fillies. In the first few weeks of life, both colts and fillies can be seen playfully mounting their dams, initially all over the mare's body but progressively moving toward the haunches.

Fillies tend to move past this stage of sexual play after a few weeks and usually do not display further sexually oriented behavior until puberty.

Colts continue to engage in playful mounting behavior, mounting their peers as well as their mothers. Colts also start to have occasional penile erections prior to puberty, but their erections are not usually associated with mounting behavior.

It is not until horses hit puberty, sometime between their first and second birthdays, that true sexual behavior is initiated.

Courtship and mating

As the days get longer, don't be surprised if your young filly or colt acts a bit peculiar. It's the time of the year when equine reproductive hormones are calling, and your horse is just answering the call of the wild.

Horses are referred to as “long-day breeders” because they come into heat as the days increase in length in the spring. Mares are also “seasonally polyestrous,” meaning they have multiple estrous cycling throughout the spring and summer. The natural breeding season for horses in the Northern Hemisphere is the spring or summer.

Light is the controlling factor in causing mares to come into heat in early spring. Most studies have indicated a tendency toward anestrus (not cycling) in the winter months; however, some mares may cycle during this time as well.

Mares will cycle several times during the breeding season if they do not conceive and become pregnant. The most intense estrus behavior occurs when the mare is most sexually receptive to the stallion. Intense estrus behavior lasts about three days.

Puberty in equines

Most yearlings and 2-year-olds can and will breed under good circumstances. There's probably not a horse practice without a first-hand story of the pregnant filly that was only with a colt until a year of age.

Many fairly young colts can be fertile. Some young colts are quite mature behaviorally, looking like an old pro before the age of two if given access to fillies or mares. Others might be awkward and slow, but given ample opportunity, particularly with young mares of their size, will have no problem mounting, inserting, and ejaculating normally.

Researchers that have studied colt development in the semi-feral pony herd at New Bolton Center at Penn State note that in that herd and in other horses that have been studied under natural social conditions, the yearling and 2-year-old males do most of the breeding of the young fillies. The young fillies are often still living for the most part in their original band with their sires and dams at the time of their first estrus.

The Mare

The mature mare

The mature mare

Mare owners can attest to the influence of female hormones as the filly reaches maturity. New window.

Fillies usually attain sexual maturity at 12 to 15 months of age, but some reach puberty as early as 9 to 10 months and others as late as 18 months. Estrus, or heat, which is the period of the reproductive cycle when the mare ovulates and can conceive, is governed by rather complex hormonal effects.

When the mare reaches puberty, the pituitary gland releases the hormone FSH (follicle-stimulating hormone) which causes egg follicles within the ovary to grow and produce increasing amounts of estrogen. This estrogen prepares the reproductive tract for mating and fertilization and is also responsible for behavioral changes.

When the egg follicle approaches maturity, the pituitary releases a second hormone, LH (lutinizing hormone) that causes the follicle to ovulate. This usually occurs about 24 hours before the end of heat.

Beginning with their very first estrus, free-living fillies can be seen assuming the classic breeding posture in front of other horses, particularly stallions. If mature stallions show interest in these young fillies, the fillies often begin to show fear, which overrides the desire to stand for breeding.

Pubescent colts, however, may try to mate with fillies in estrus, but are often not yet capable of inseminating them. Sexual behavior is expressed by these post pubertal juveniles, but it is uncommon for breeding to be successful until 2 or 3 years of age.

During the long-daylight period of the year, a typical mature mare will have a five- or six-day period of estrus once every three weeks wherein she may raise her tail, squat, urinate and "wink" her vulva in the presence of other horses, particularly stallions.

If she succeeds in finding a mate, the process of courtship begins, which includes all the rituals of smelling, nuzzling, nipping and nickering that mares and stallions engage in prior to breeding. Mares will typically assume a rigid, wide-based stance and will urinate and wink the vulva as the stallion nips and nuzzles the mare, beginning with nose-to-nose contact and working toward the hindquarters.

The stallion will usually exhibit the flehmen response, in which he raises his head and curls his upper lip as he sniffs the mare's urine. By doing this, the stallion is believed to be testing the scents of the mare for readiness to breed.

Stallion exhibiting the flehmen response

Stallion exhibiting the flehmen response

The stallion will usually exhibit the flehmen response, in which he raises his head and curls his upper lip as he sniffs the mare's urine. New window.

There are several management tools used to bring about the onset of the breeding season or to alter the estrous cycle itself in both young and mature mares. The most reliable method used to induce the ovulatory season is the use of artificial lights to alter the mare’s perception of day length. The easiest light management program is to use lights at the end of the day to extend the perceived day length to 16 hours.

This practice entails turning on an artificial light source 30 minutes before sundown. The light source should be turned on to supply enough additional light to produce a total 16 hours of daylight (natural and artificial). There is a lag period of 60 to 90 days between the onset of daylight extension and first ovulation. Therefore, if February-March ovulations are desired, mares should be started on the lighting program before December 15.

The young stallion

Although most stallions begin to produce sperm as early as 12 to 14 months, most are at least 15 months or older before they can successfully breed. Few stallions are used at stud before two years of age and most stallions acquire full reproductive capacity at around three years of age.

This is not to say that a younger stallion is incapable of breeding earlier than an age of 12 months, and care must be taken to separate young stallions from fillies or mares before they reach sexual maturity to prevent unwanted pregnancies

A young stallion

A young stallion

A maturing stallion's increase in testosterone produces an animal with the potential to breed and also to be dangerous to handle. New window.

The stallion has two testicles enclosed by the scrotum and located in the prepubic area. The testicles produce the sperm and also the male hormone testosterone.  The scrotum protects and regulates the temperature of the testes which are usually several degrees cooler than the stallion's body temperature because higher temperatures decrease spermatozoa production.

In a two to three year old stallion, the scrotal width is smaller than in a full grown stallion and the daily sperm output is smaller with fewer reserves. For this reason most stallions under three years of age are considered sexually immature.

The maturation of sperm production and behavior often don't coincide. That means a young colt might be very willing to breed and have nearly perfect form as much as a year before his testicles and sperm production have developed. Similarly, a colt might have apparently maturing testicles, but be immature behaviorally.

Consider this

By the time a foal is four years old, you will have invested a total of $9014 for feed, farrier and vet bills, and other necessities according to a horse owner survey in 2013.

Daily spermatozoa production is also influenced by season. Lowest production, a decline of about 50% from peak values, is expected from September through February. Production of spermatozoa increases as daylight grows longer in March, peaks in May and June, and then declines significantly in July and August.

Artificially extending the length of the day by using lighting programs will enhance a stallion’s semen production.

Lighting programs should start around the first of December to advance the breeding season for February and March. The perceived daylight should be lengthened to 16 hours of light per day. Lighting programs must be routinely applied every day. Avoid housing situations that supply 24 hours of light since stallions may respond by decreasing production of semen to levels characteristic of short days.

Some horse owners worry that early breeding will influence a colt's manners and studdishness in non-breeding situations. One episode is probably not going to mean much, but if a young colt is allowed to breed frequently, research suggests that his hormonal and behavior maturation might be accelerated. That means he might be more "full of himself" at an earlier age.

Also, many horse owners are alarmed when a colt mounts his mom during foal heat. This normal behavior is seen in almost every colt at the time of foal heat. In fact, almost all of the normal sexual behaviors---teasing, marking, flehmen response, erections, mounting, sometimes weak thrusting - are seen within the first week of a colt's life.

These actions are often out of the "adult" sequence engaged in by mature horses, and they might be subtle or interspersed with action play sequences. Colts do a lot of sexual play with filly and colt playmates. They achieve erections, but rarely ejaculate in the play form of sexual behavior.

As the days get longer, don't be surprised if your young filly or colt acts a bit peculiar. They may seem distracted and edgy, but this is normal. Try to remember this behavior is to a certain extent beyond your horse's control. It's the time of the year when equine reproductive hormones are calling, and your horse is just answering the call of the wild.

If you have a young horse that hasn't been bred before or if you are a first time breeder, it is important to understand the full parameters of bringing a new foal into the world. No one can tell you not to breed your horse, but consider the costs in time, labor, money before taking on the responsibilities of a new foal. Learn more at

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