Under natural circumstances, mares tend to give birth to foals in the spring and summer, after 11 months of gestation. This is when grass is most plentiful, so the new mother has plenty to eat, promoting milk production. Nature's normal cycle isn't in sync with mare owners who want racing foals, so humans have devised methods of bringing mares into heat earlier than normal so her foal is born earlier in the year.
Mares normally come into season, or heat, from April to October. Given the 11-month gestation period, that means foals start hitting the ground in May and the latest babies of the year in September. If you breed a mare near the end of the annual seasonal cycle, she'll be carrying baby weight in the heat of the following summer. A mare's heat cycle corresponds to daylight lengthening as winter turns to spring. According to Colorado State University's Equine Reproduction Laboratory, changes in the brain's level of melatonin affected by additional daylight stimulates hormone production and the mare comes into heat.
Thoroughbred and standardbred racehorses are officially 1 year old the first January 1 after they're born. Some horse show registries also use the Jan. 1 date. That means a foal born in late January or early February really is close to a year old on his official birthday, while a foal born December 31 is legally a year old the next day. Those born in May or June, under nature's timetable, are at a size and competitive disadvantage compared with foals born the same year but a few months earlier.
Fooling Mother Nature
Many racehorses have actual birth dates in February or March. That's because breeders fool Mother Nature by putting mares under lights. Their biological calendars tell them it's spring and start estrus. To effect this, you must stable your mare so you can turn on the lights at the appropriate time. To fool her body into thinking spring is in the air even when snow is on the ground -- or falling -- you must turn on the lights at night at least two months before early ovulation occurs. According to CSU, most breeding farms turn the lights on around December 1, with the idea of starting breeding around the week of February 15. Waiting until then reduces the likelihood of that most unwanted of potential racehorses, the December foal. The Omaha Ministry of Agriculture recommends a minimum of two 10 foot-candles of light exposure at mare eye level. However, much depends on the darkness of your barn walls. The ministry states that a 200-watt incandescent bulb usually adequately illuminates a box stall. You can either keep a constant level of light, approximating the hours of daylight for spring in your region, or add 30 minutes per week until you achieve that level of spring light. Putting your lights on timers avoids the need to turn on lights in the early morning hours.
An 11-month gestation is about 340 days, but that's an estimate. A nearly six-week leeway exists, three weeks on either side of the average. If your mare's been bred before, it's likely the gestation period for this foal will be about the same. About a month before foaling, your mare's udder begins distending. Her nipples fill up four to six days before foaling; colostrum begins to drip a day or two prior to the event. If you are foaling out your mare at home, invest in a mare cam so you can see what's happening without hovering. Some mares will wait until no humans are around before delivering.
- Ontario Ministry of Agriculture: Foaling and Predicting Foaling Time
- Equine Medical Services: So, You Want to Breed That Mare
- Equine Reproduction: When Should You Breed Your Mare?
- Colorado State University Equine Reproduction Laboratory: See The Light -- Advancing the Breeding Season for Early Foals
- Ontario Ministry of Agriculture: Artificial Lighting for Mares
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