Late one cold spring night in Maryland, we had just finished foaling out a young first-foal Thoroughbred mare when a group of graduate students stopped by the farm on their way home from a party. In the spirit of things, they continued their partying outside the foaling stall, laughing, joking and carrying on. The young mare, experiencing some abdominal cramps, became disoriented and frightened by the noise. A cramp hit her and she collapsed onto the ground. After a moment , she got up and started to spin round and round getting increasing excited by the noise.

Immediately it became apparent that the life of her newborn foal lying helplessly in the middle of the stall was in jeopardy. Rushing into try and save the foal from being crushed, the mare nearly trampled both her foal and its human protector. Quickly the partying college students realizing that their college professor was in a life-threatening situation decided they' ought to find a better place to continue their party.

Within minutes the mare quieted down and Jim placed the foal back in the stall. Fortunately the mare sniffed her foal and began licking it. Nature took over and the maternal business of nurturing her newborn began again.

From research done at the University of Maryland, we found that age of the mare at foaling, sex of the foal, and time of year bred contributed to the length of the pregnancy.

Statistical analysis showed that the calculation of length of gestation related to these factors was a much more accurate estimate of the normally used 340 day average.

Later research showed that the best estimate of a specific mare’s length of gestation was determined from prior pregnancies. Without this kind of individual mare data, this chart is usually a better estimate for the length of gestation than the 340 day average.

How to Use the Chart: Locate the age of the mare at time of foaling along the top of the chart, then locate the date she was last breed along the side. Follow the column down and the row across till they intersect. This is the predicted foaling date.

There is a 6 day window around this date. On the average colts are 3 days early and fillies are 3 days late.

If the conception rate of dry mares to your stud was not as good as you would have liked this past year, improved feeding techniques might be a partial answer.

Breeders who settle a large percentage of mares each year pay close attention to the “art of feeding” horses. Many successful breeders begin a feeding program for their open mares several months before the onset of breeding season in order to manipulate the nutritional state of the mare into mimicking its evolutionary natural rhythm.

In the wild, mares are dependent on the nutritional sources associated with each season. During the late fall and winter, food becomes increasing scarce. As the herd wanders in search of food, many mares began to tap into their stores of body fat for energy.

The "average" gestation length for broodmares is 340 days. The range to produce a live, normal foal starts around 305 days and extends to over 390 days. That is roughly an eighty-five day spread - which translates into pregnancies ranging between ten and thirteen months!

For centuries, this large range has frustrated horsemen as they await the birth of the next genetically engineered generation of horses. In an effort to reduce the risk of an abnormal birth, most herdsmen have determined that they have a greater peace of mind by attending the "blessed event".

This is the crux of the matter. Mares are notoriously secretive about foaling. They want to be alone at this very private moment and are not willing to signal that the long awaited event is eminent.