Mare care for the equine breeding/foaling season

Preparing the non-pregnant (open) mare

• If the goal is to achieve an early pregnancy, ensure light therapy has been initiated by February 1 at the latest (average start would be in April); past this date, there will be no advantage to placing your mare under lights
• Research and make stallion selections as soon as possible as most stallion stations will begin collecting in early-mid February; complete all paperwork and stallion contracts and make your payments.
• Complete a client contract with the Equine Reproduction Laboratory (ERL) prior to arrival for first exam
• If a mare is new to you, ensure all previous medical and reproductive history are available to be shared with the veterinary team
• Ensure she is current on all vaccinations, deworming and farrier work prior to arrival or arrange for the ERL team to meet these requirements
• A mare must be systemically healthy to encourage the best reproductive results!
• If your mare is in the midst of the spring transition phase, our team can attempt to hasten the first ovulation if requested

Using a recipient mare

• If a recipient mare is necessary to meet your goals, inquire early for an application through the ERL to ensure a mare is reserved for you; it is on a first come first serve basis
• If you are providing a recipient mare, we recommend:
  • She is between the ages of 4-13 years
  • She has successfully given birth and nursed/raised a foal previously
  • She is currently reproductively sound; if this is unknown schedule a breeding soundness evaluation (BSE) to ensure she is a good candidate
  • She is able to be haltered and led into a set of stocks safely
  • She is current on all vaccinations, deworming and farrier work prior to arrival or arrange for the ERL to meet these requirements

Seasonality of reproduction

Photoperiod (amount of daylight compared to darkness) is truly the regulator of seasonal reproduction. Generally, changes within the mare’s reproductive cyclicity is defined by four seasons of ovarian function: the winter anestrus, spring transition period, physiologic breeding season, and fall transition period. The winter anestrus is usually encountered November-March. This ovarian phase is identified by minimal follicular growth (follicles <20mm in diameter) and low estrogen levels. Behavior is characterized by lack of estrus signs and not actively rejecting a stallion.

The spring transition period is identified through multiple “clustered” follicles on ovaries examined via transrectal ultrasonography. This period typically lasts for 2-3 months, resulting in multiple waves of follicular development and regression. Follicular growth is responsible for the production of estrogen which may manifest itself as irregular or prolonged “in heat” behavior. Eventually, one dominant follicle during the late-transitional phase will ovulate, signaling the end of transition. The average first ovulation of the year is approximately May 1 in Colorado.

During the physiologic breeding season, the mare demonstrates a 21-day inter-ovulatory period on average and is described as seasonally polyestrous. The estrus or standing heat phase of her cycle generally lasts 5-7 days whereupon she is receptive to a stallion. The diestrus phase of her cycle, where she will actively reject the advances of a stallion, typically lasts 14-16 days. The physical act of ovulation is due to rupture of the follicular wall, whereupon the egg (oocyte) is released from the follicle and picked up by the oviduct.

The fall transition of reproduction represents a time of year which is the exact opposite of the spring transition. It typically falls between September-November. The increased darkness during this time of year results in eventual cessation of ovulations and follicular waves, and signifies the initiation of winter anestrus.

Imposed seasonality

Many equestrian breeds support an imposed seasonality as there is a preference for performance horses to be born earlier in the year. This will lead to more mature yearlings, precocious 2-year-old racehorses, and competitive futurity horses. Ultimately this leads to increased annual earnings for these horses. In the northern hemisphere, the universal birthday for Thoroughbred racehorses is January 1, to best support this endeavor.

There are several methods by which equine owners and breeding managers can encourage an earlier spring transition period through artificial lighting schemes. In the northern hemisphere, light therapy is often initiated at the end of November or early December for a February 1 ovulation. The therapy still requires 60-70 days to be effective whereupon 16 hours of light is provided and 8 hours of darkness. This typically works by tacking on light at the end of the day, whereupon lights are initiated at dusk and remain on until roughly 11 p.m. The type of light is not as important (fluorescent vs. incandescent). However, at least 10-foot candles are recommended, which loosely translates to a 200-watt light bulb in a 12×12 stall. Photometers can be borrowed to measure this, or several apps are now available for download via smartphone.

Another option for horse owners and breeding managers who do not have the option to keep mares in a stall is an individualized light therapy mask. The Equilume was originally designed similar to a racing mask with blinkers positioned on the outside of the right eye so as not to interfere with the handler located on the traditional left side. An LED low-level blue light (465 nm) provides 50 Lux of diffuse light into the horse’s eye. This individual mask has demonstrated the ability to advance seasonality in mares as well as those housed indoors maintained with white light. Based in Kildare, Ireland, the company boasts several options since its inception. The Belfield is a seasonal mask with a 5-month lifespan allowing for 7 active hours (4-11 p.m.) per day. The Curragh mask maintains a similar active light exposure but allows for the eye cup to be replaced annually. Finally, the Cashel is a rechargeable mask that will maintain the charge for up to 15 active hours a day.

Methods to drive follicular growth during transition

Deep anestrus is defined as evidence of follicles fewer than 20 mm in diameter on a mare’s ovary. The transitional mare is defined as maintaining multiple follicles greater than or equal to 20 mm in diameter. The transition phase typically lasts 60-70 days, often accompanied by irregular estrus patterns leading to owner frustration. Multiple methods have been attempted to hasten the transitional phase in an attempt at driving an earlier first ovulation. Once a mare is identified to be in the midst of the transitional phase, a treatment plan utilizing small doses of a hormone that promotes follicular growth will begin. The treatment is discontinued when one or more follicle(s) reach greater than or equal to 35 mm in diameter. Breeding management plans will commence and insemination can be timed appropriately. This treatment does not work in every mare and should be discontinued if no significant follicular development occurs within 10 days of treatment.

Preparing your pregnant mare for foaling services

• Identify the ideal location for your mare to foal based on your experience and comfort level
• A veterinarian should evaluate the late gestational mare at least once to confirm fetal location and identify early signs of problems
• If a Caslick is in place, it is recommended to open it 2-4 weeks prior to parturition
• The ideal foaling location should be quiet, comfortable, and easy to disinfect
• If foaling a mare out at home, ensure you have access to an alert system, chains, towels, and gloves, as well as umbilical clamps, umbilical dips, enemas and colostrum supplementation if necessary
• Pregnant mare pre-foaling vaccines should include:
  • 5 mo (120d): EHV 1 (#1)
  • 7 mo (210d): EHV 1 (#2)
  • 8 mo (240d): Rotavirus (#1)
  • 9 mo (270d): EHV 1 (#3), Rotavirus (#2), Clostridium (#1)
  • 10 mo (300d): Rotavirus (#3), Clostridium (#2), 5-way + WNV and Rabies

Most late gestational mares are recommended to arrive to a foaling facility 2-4 weeks prior to their due dates unless a problem is identified. Upon arrival, the veterinary team will perform a general physical examination which will also include identification of your mare’s body condition score (BCS), as the majority of fetal growth occurs in the final 3 months of pregnancy. They will also identify the presence and quality of mammary secretions and observe for evidence or a history of vulvar discharge. Notation of a Caslick in place will be made with preparations for removal within 2-4 weeks of the anticipated foaling date.

A baseline palpation and transrectal ultrasound examination will also occur upon arrival. The veterinarian performing the ultrasound will pay special attention to fetal movement, fetal positioning (through identification of the foal’s eye), fetal fluid appearance, and measurement of the combined thickness of the utero-placental unit (CTUP). 

Once complete, your mare should be placed within a comfortable setting, away from the regions of the practice or farm with excessive horse traffic. It is also recommended the mare be maintained under lights for the final 2-3 months of gestation to help prevent post-partum anestrus and decrease the duration of pregnancy by 7-10 days. The area chosen for foaling should be clean, dry, visible, and easy to disinfect. Options should include a large foaling stall with shavings or straw bedding or a large grassy field.

All pre-foaling vaccinations recommended for the specific region/population and diagnostic tools should be on hand and unexpired. An organized kit containing general supplies, pre-foaling supplies, and neonatal foal supplies should be labeled and easily accessible. Additional post-partum products should be inventoried and may include banked frozen colostrum, colostrum measuring devices (brix refractometer), colostrum alternatives, milk replacers (both powders and pellets), and even the possibility of mares to serve as nurse mares in an emergency.