Introduction to Equine Breeding – Part II

Introduction to Equine Breeding – Part II – Dr Sarah Seitz (BSc, DVM)

Last time, I gave a brief overview about seasonality and the estrus cycle. This article will cover the basics of pregnancy, lactation and foaling. The average length of pregnancy in a mare is between 335 and 342 days (an easier way to remember this is 11 months, 11 days).

After the mare ovulates, the ovum is short lived and must be fertilized within 12 to 18 hours. Fertilization of an aged ovum (over 12 hours old) is possible, but this increases the incidence of early embryonic death. This is important from a management aspect because the mare must be bred as close to ovulation as possible. Only fertilized eggs are capable of moving from the oviduct into the uterus, and this usually occurs 6 days after ovulation.

A key factor in embryo survival is transuterine embryonic migration. This occurs from day 10 until day 16 after ovulation, and refers to the movement of the embryonic vesicle from the tip of one uterine horn to the tip of the other horn. This movement inhibits the release of PGF2alpha from the endometrium which serves to maintain the presence of the corpus luteum which is necessary for pregnancy maintenance. Knowing this is important because if your mare has anything in her uterus such as cysts or scar tissue that can interfere with embryonic movement, the embryo may not survive past 16 days. After 17 days post ovulation, embryo fixation occurs at the base of one uterine horn, and the embryo continues to grow. A fetal heartbeat can be seen by ultrasound at day 25. Implantation starts at about day 35 and endometrial cups form. Endometrial cups are important because they secrete equine Chorionic Gonadotropin (eCG) which causes small follicles on the ovary to ovulate and form small corpora lutea which secrete progesterone for pregnancy maintenance. If a mare loses her pregnancy after day 35, the endometrial cups remain functional in her uterus, so she will not come into estrus anytime soon (she still “thinks” she is pregnant).

About 4 to 6 weeks before your mare is due to foal, oestrogen levels begin to rise and progesterone levels decline. This will trigger the development of her mammary glands, which dramatically increase in the last 2 weeks of pregnancy, and she may begin “waxing” (dried colostrum on her teats) 1 to 2 days before foaling. A relatively inexpensive and accurate determination of impending foaling is to measure the calcium levels in her milk. There are commercial tests kits available for this purpose, and a sharp rise in calcium levels are indicative that parturition is imminent.

Parturition, or foaling, is divided into 3 stages. The first stage lasts 1 to 4 hours, and is the preparation for birth. This stage is characterized by increased myometrial activity. Your mare may seem uncomfortable and colicy due to increased oxytocin levels, and the cervix relaxes and dilates. The foetus changes position from being “upside-down” to being “right-way-up” with front legs extended. Finally, the cervical star of the chorioallantois ruptures to allow for passage of the foetus. The second stage is the expulsion of the foetus and lasts 15 to 20 minutes. If your foal has not been delivered within 20 to 30 minutes, there may be a problem and intervention is necessary. The foal should come out with front feet and nose first. If your foal is delivered within the intact amnion (white bag), the amnion must be torn open so the foal can breathe.

The umbilical cord will break when the mare stands up – do not break or cut the cord yourself. The third stage of parturition is expulsion of the placenta. The placenta is usually expelled within 45 minutes. If it has not been expelled within 3 hours, it is considered to be retained and requires intervention. Unlike cows, horses are sensitive to effects of a retained placenta. I plan to write a whole article dedicated to retained placenta and its treatment. Once the placenta has been expelled, it should always be examined and weighed. Make sure it is complete, has normal appearance and weighs 10-11% of the foal’s weight.

Once the foal has been delivered, you must make sure that colostrum intake is adequate. Colostrum contains antibodies (IgG’s) that protect the foal from pathogens. Failure to ingest adequate colostrum can lead to foals becoming weak and often, these foals do not survive. If you suspect that the mare has poor quality colostrum, the foal must receive colostrum for a colostrum bank. There are commercial snap test kits (I like this one) that are available that test the IgG levels in the foal’s blood. If IgG levels are low, the foal will likely benefit from a plasma transfusion and supportive care. Ensuring adequate consumption of colostrum after birth is one of the most important aspects of foal management.

For the first 3 months of its life, your foal receives almost all of its nutrition from its mother’s milk. It is important that your mare be fed properly during this time. The amino acids methionine and lysine are of particular importance because they are critically needed by the foal. If your mare is losing weight, you can supplement her with fat (vegetable oil). Horses digest fat easily and it is a good energy source. Check your mare’s udder daily to ensure that she is producing milk for her foal.