By Heidi Smith, DVM
Once mares go through a transitional heat in the spring (see article on transitional heats), if all is normal, they will get into a routine of regular heat cycles. On the average, the mare will ovulate (produce an egg) every 21 days. (If a mare cycles more quickly than every 18 days or more slowly than every 24 days, she should be checked for abnormalities.)
At some point before ovulation, the mare will come into estrus or "heat." In a regularly cycling mare, the heat period during which they will stand for breeding may range from only a day or two on up to a week or more. The most consistent part of the heat period is that it will end shortly after ovulation. The duration of standing heat prior to ovulation may range from less than a day up to about a week.
After the previous heat (or the ending of the transitional heat), the remains of the follicle (the fluid-filled structure on the ovary that bursts to release the egg or ovum) turns into a structure called a corpus luteum, which in breeding barns is generally referred to by its initials as a CL. This structure produces the hormone progesterone, which makes the uterus a habitable place for a pregnancy to form. Approximately two weeks after ovulation, the uterus is able to "sense" the presence or absence of a pregnancy. If no pregnancy is present, the uterus will produce a hormone called prostaglandin, which causes the CL to regress and to quit producing progesterone. As soon as this happens, the pituitary gland signals the ovaries to begin building follicles again. One (or sometimes two) follicles will emerge from the ovaries' stock of immature ova. The follicle produces the hormone estrogen, which causes the mare to come into heat and exhibit behavior that she is receptive to the stallion.
When the follicle reaches maturity, more hormones are released from the pituitary, causing the follicle to soften and rupture, releasing the ovum so that it can be picked up by the oviduct, the small tubule that leads from the ovary to the uterus. The ovum will travel about halfway down the oviduct. This is the site where fertilization of the ovum takes place.
When the mare is bred, the sperm travel very quickly to the midpoint of the oviduct. The majority generally reach their destination within 15 to 20 minutes of breeding. They will survive for quite a long period of time there, if the mare is healthy and the sperm are healthy. In general, we can count on the sperm to live in the oviduct for at least 48 hours, and in some cases, they have been known to survive in the oviduct for as long as 6 or 7 days. Because the ovum will only live for about 6 to 12 hours after ovulation if it is not fertilized, we ideally want to have the sperm present in the oviduct and waiting for the ovum. That means that we try to breed the mare prior to the time that she ovulates. Furthermore, since the sperm will live for so long in the oviduct, it will not increase conception rates to breed more frequently than every 48 hours during heat. In fact, because the uterus has to get rid of all of the excess fluid and contamination introduced at breeding, more frequent breeding will sometimes overwhelm the uterus's defenses and actually lower the chances of conception.
Once the ovum is successfully fertilized, it will remain at the midpoint of the oviduct for approximately 5 days. This gives us a window of opportunity to help the mare who has difficulty clearing excess uterine fluid or fighting contamination by doing uterine lavages and/or administering the hormone oxytocin to help the uterus contract and get rid of the excess fluid. It has always seemed a bit paradoxical that we could help the mare get rid of fluid by introducing more fluid into the uterus, but it has now been shown that the mild stretching of the uterus that occurs during lavage or infusion causes natural oxytocin to be secreted that make the uterus contract and expel the fluid. We now mimic that by injecting very small doses of oxytocin.
After the fertilized ovum (now an embryo) reaches the uterus, it influences the uterus not to release prostaglandin, hence preventing another heat cycle from occurring. The mare will usually make additional follicles around the time that the heat cycle would have occurred, and these will in turn form additional CL's to produce more progesterone.
 |
|