Also, the testicles of a draft stallion are small in relation to their body size compared to light horses. Typically, the draft stallion tends to have a low sperm concentration, high semen volume, and a large amount of gel in the ejaculate. Because of these factors, the semen often must be centrifuged to maintain proper concentration and sperm quantity for optimum reproductive efficiency in artificial insemination. These factors make successfully transporting cooled and frozen semen from heavy horses more difficult and time consuming. Twinning is most common in Thoroughbreds and draft horses.
Twins result when a mare ovulates and conceives on two follicles during a heat cycle.
Drafts have even been known to ovulate with three follicles and conceive triplets. Due to their large uteruses, they are more likely to carry twins to late gestation or even foaling. This makes it very important to have draft mares sonogrammed by a veterinarian 14-15 days after ovulation to detect unwanted twins. As with light mares, one embryo may be crushed during this early phase of pregnancy, allowing the other to mature. Unfortunately, the higher tendency for the development of multiple follicles does make predicting ovulation more difficult in draft mares.