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Reproduction - Estrous Cycle and Teasing

estrus behavior | teasing methods | group teasing | individual teasing

Introduction: The reproductive cycle in mares is classified as seasonally polyestrus. This means that they are reproductively active at a defined portion of the year, during which they will cycle repeatedly. Each of these estrous cycles are driven by a coordinated system of interdependent hormonal responses that stimulate appropriate behavioral and anatomical changes. These changes combine to provide favorable conditions to establish and support pregnancy.

Estrus Behavior: The estrous cycle in mares is divided into two distinct periods, diestrus and estrus. During diestrus, progesterone produced by the corpus luteum is dominant. This period is characterized by behavioral non-receptivity and even aggression towards a stallion that approaches too closely. During diestrus, mares are commonly described as "out of heat." For most mares, diestrus begins at 1 to 2 days after ovulation and continues for 14-16 days.

Mares that are in estrus or "in heat," are primarily influenced by the hormone estrogen. Estrogen causes them to exhibit a receptive behavior when a stallion approaches. Mares that are in estrus will typically show one or more of the following behaviors:

• Attentive, passive approach (in contrast to aggressive approach often seen in diestrus)
• Leaning into the stallion with either the shoulder or hindquarters
• Raising the tail
• Periodic inversion of the vulva, which is often called winking or blinking
• Posturing (hindquarters rotated forward) and urinating

Estrus typically lasts 4-7 days, beginning 4-6 days before ovulation and continuing 1-2 days after ovulation has occurred. However, the length of time that mares are in estrus is much more variable in late winter/early spring. During this period it is not uncommon for mares to remain in heat for up to 2 weeks.

Estrus behavior also varies between mares. Some mares consistently show strongly prior to ovulation, others may never show more than passively. The intensity that a mare shows estrus is usually not related to fertility. However, mares that consistently fail to show estrus can be difficult to breed and require more frequent palpation exams to continually determine where they are in their cycle.

Estrus behavior can also change in the same mare when exposed to the teaser stallion under different circumstances. Following are some examples of these circumstances and how teasing behavior may deviate from normal:

• Maiden - Mares that have never foaled often show estrus at a distance but reject the stallion out of fear on contact.
• Foaling mares - Mares with newborn foals will often aggressively reject the stallion as he approaches. Care should be taken if the teaser is brought into a barn where mares with foals are stalled. For example, make sure that all foals are standing before the stallion enters the barn. This practice will help prevent injury caused by mares inadvertently stepping on a foal as they charge at the stallion.
• Stress - Mares will often fail to tease if under stress. Some sources of stress include long trailer rides, recent introduction into a group, and any source of excitement.
• Pregnancy - Pregnant mares will often show estrus-like behavior when teased, especially at around the second month. This behavior is usually less evident than that which was shown during estrus.

Introduction: The development of reliable methods to assess and record teasing behavior of mares is certainly one of the most important tasks undertaken at breeding facilities. Accurate and up to date teasing records are absolutely essential in the success of equine breeding programs.

Breeders typically use a scoring system to record daily changes in the teasing behavior of the mares in their care. One popular scoring system uses the numerals 0-4 to describe a mare’s behavior toward a stallion. Each teasing score corresponds to a type of reaction generally exhibited by mares and signals their readiness to breed (Table #1).

Estrus behavior usually becomes more pronounced as the follicle progressively develops. For many mares, estrus begins with passive behavior 4-6 days before ovulation, then teasing behavior will intensify as ovulation approaches. Teasing scores will usually be detectably reduced by 24 hours after ovulation; however, many mares will usually "tease in" for 1-2 days post ovulation.

A good teasing program uses a teaser stallion that is manageable and exhibits strong libido. Virtually any method is acceptable that can consistently and safely permit a stallion to have relatively free access to nuzzle a mare and provide an observer the opportunity to assess her receptivity to the stallion. Most breeders find the ability to effectively perform both individual and group teasing methods important since each method has distinct advantages and disadvantages. The importance of a sound and consistent teasing program to a successful breeding operation cannot be overstated. For the majority of mares, it is a very reliable and cost efficient method of evaluating estrus.

Group teasing: This method provides breeders an excellent means of following the cyclic activity of individuals within groups of mares. Mares in group teasing situations are given the opportunity to freely associate with the teaser for a period of at least 15 to 20 minutes. The advantages of group teasing include efficiency and effectiveness. Often an astute observer is given the opportunity to distinguish subtle changes in the mare’s behavior that may not be as noticeable in an individual teasing situation where the mare is restrained.

A major drawback of this method is that it often fails to detect estrus in mares that are either fearful of approaching the stallion or fearful of mares already near the stallion. This is especially true early in the estrus period and in maiden mares. For these reasons, the manager should keep in mind that as the number of mares teased in a group increases, it is more likely that observation of estrus in some mares will be missed. Generally, it is safe to begin the season teasing 10 to 15 mares in a group. Because some of the mares in each group become pregnant, this number may be increased as the season progresses. Another problem associated with group teasing is that mares that have been newly added to the group will often tend to stand quietly away from the others mares or keep close to a newfound friend. In these cases, the observer must make a special effort to take notice of any degree of attentiveness that the mare may express toward the stallion. In these situations, mares will occasionally need to be removed from the group and teased individually.

Individual teasing: These teasing methods are especially useful in breeding operations that employ hand breeding. For safety reasons it is essential that all mares be individually teased before the stallion is allowed to approach the mare.

Careful observation and recording of teasing characteristics are key to making appropriate breeding management decisions. Following are some of the most important factors to consider when assessing teasing behavior in mares:

• Normal Diestrus: Mares will typically tease "out of heat" (score of 0) for 14-16 days before returning to heat. Many will violently reject the stallion during this period.
• Prolonged Diestrus: If a mare is out of heat for a period longer than 18 days, it may indicate either pregnancy or false pregnancy. If it is during winter, it may mean that she has returned to an anestrus-like state, or if Ovuplant was used to induce ovulation during her previous cycle, she may be experiencing a normal side-effect of the treatment.
• Short diestrus: If the diestrus period is shorter than 14 days, an endometrial infection or other uterine inflammatory condition should be suspected.
• Split estrus: Often noticed during spring transition, mares can show heat for several days, go out for 1 to 4 days, then again exhibit estrus behavior. This occurs when follicles develop but fail to ovulate and regress, permitting a second follicle to develop.
• Pregnancy: While mares will usually reject the stallion for most of their pregnancy, they will often show some estrus behavior at around day 30-60 of pregnancy. These mares usually do not exhibit the same intensity of estrus behavior as they did when they were in estrus and will rarely allow the stallion to mount. Care should be taken if using artificial insemination (AI) to avoid inseminating mares that have a closed cervix and a thick, sticky feel to the inner lining of the vaginal vault. These mares could be pregnant.
• Wet Mares: Mares with foals at their side will not typically show the same level of estrus intensity as they would when they are alone.
• Maiden: Mares that have never been bred before vary in their interest in teasing. Maiden mares with strong estrus behavior are often resistant to live cover.
• New arrivals: While these mares may be ready to be bred, they will sometimes fail to show estrus until they become acclimated to their new surroundings. Usually, these mares can be successfully teased within an hour or two of being placed in a stall and provided hay. Also, these mares will often be too distracted to tease in a group setting and will need to be caught and teased individually until they become accustomed to the influence of their new herd.

Teasing should be performed daily until a mare is determined to be safely in foal. Afterwards, mares should be teased periodically through the rest of the breeding season. It is often an advantage to pasture pregnant mares near a stallion’s exercise pen. The benefit of this is two fold: it allows the stallion to be housed in a more natural setting near mares, while at the same time, allowing the manager to keep track of their teasing behavior. Mares that return to estrus can be removed and placed back into a more intense management scheme.