Do You Want Your Broodmare to Foal Early?
by Dr. Frederick Harper, Extension Horse Specialist, University of Tennessee
An old saying states, “Patience is a virtue.” In a society where “instant-everything” is the order of the day, this saying is not practiced by many.
Some of those who must still practice patience are owners of pregnant broodmares. With a gestation length of 335-340 days, they just have to wait until the appointed time.
You may ask, “Is there anything that can be done to reduce the length of a mare’s pregnancy?”
The answer is yes.
Horse owners use artificial light to trick non-pregnant mares into an earlier estrous than normal each spring.
Mares are long day breeders. That means that more hours of light (artificial or sunlight) trigger their reproductive system into activity in the spring.
Researchers at Texas A & M University studied the effect of added length of daylight on a group of pregnant broodmares. They used 16 hours of daylight and 8 hours of darkness in a group of pregnant broodmares starting December 1 until they foaled in the spring. A control group of mares remained under normal daylight hours only.
Extending the number of daylight hours during the last trimester of pregnancy shortened the length of gestation by 10 days. The birth weight, height, heart girth as well as length and circumference of the cannon bone of these early foals were no different from those born later.
Reproductive factors, such as time from foaling to ovulation, start of estrus and subsequent conception rates, were not affected by the use of artificial light in pregnancy.
Another Texas A & M University study found that thin broodmares with a body condition score of 4.5 or less had longer gestations than broodmares in a moderate body condition of 6 or greater.
Thin broodmares carried their foals nine days longer than broodmares in moderate body condition.
A University of California study also showed that nutrition had an effect on length of gestation. Well-fed broodmares foaled 4 days earlier than broodmares on a maintenance ration.
From these studies, it is obvious that broodmare owners can influence the length of gestation within limits. These are factors that one may use to shorten the length of gestation in broodmares.
First, feed broodmares a balance ration that keeps them in a body condition of 6-7.5. This is a body condition score system in which 1 is emaciated and 9 is very fat.
Second, use artificial light so broodmares receive 16 hours of daylight from early December (assuming broodmares foal in March and April) until they foal. Mares that foal later in the year probably should be placed under artificial lights at the start of their third trimester.
Normally, a 200-watt bulb is used in a typical stall. It should be bright enough to read a newspaper. Broodmares can be kept in outside pens with additional light of a similar magnitude.
If you want an earlier foal next year, make sure your broodmare is in moderate body condition, fed a balanced ration in the late pregnancy and get 16 hours of daylight in the last trimester.