Don’t let a mare’s transitional period confuse your horse-breeding plans.
Courtesy of Americas Horse Daily
Transitions can be hard wherever they appear: between paragraphs in a term paper, going from a reining large fast circle to a small slow or getting anopen (nonpregnant) mare’s reproductive system to gear up for breeding season.
Scientifically speaking, a mare’s “transitional period” is the two- to eight-week span prior to the point where a mare ovulates for the first time in the breeding season. In those weeks, the mare’s reproductive system – from hormones to organs – transforms from the anestrus of winter to the estrus of spring. And it can be confusing for those waiting to get her bred.
During winter, a mare’s reproductive system goes into anestrus: The system shuts down its hormonal activity and ovulation cycles in response to the shorter day length perceived by the eye and interpreted by the pineal and pituitary glands. As the days in spring lengthen, the mare’s reproductive system gears back into action.
In the northern hemisphere, by late April or early May, mares return naturally to estrus, where the ovaries regularly produce follicles that ovulate eggs for fertilization. To bring mares into season as early as February, breeding farms regularly use artificial photoperiod – lengthening the day artificially by putting mares “under lights.” Whether it happens in February or May, for two to eight weeks before that first ovulation of the season, the mare’s reproductive system is in “transition.”
“In deep anestrus in winter, the follicles on the mare’s ovaries are typically less than 10 to 20 millimeters in diameter for several weeks or months in a row,” says Dr. Patrick McCue, a veterinarian and reproductive specialist at Colorado State University’s Equine Reproductive Laboratory; he also writes the Journal’s “Breeding Shed” column. “Once follicles grow to 20 or 25 millimeters, the mare is in transition. Some mares will have only one follicular wave in the transition period: The mare might start with small follicles and then grow one large follicle that goes on to ovulate. “A majority of mares will have a series of follicular waves where different follicles will increase in diameter and then regress repeatedly. Finally, a dominant follicle will grow and eventually ovulate.”
Once the mare ovulates for the first time, the transition period ends, and she’ll typically continue to cycle regularly for the season. Mares follow a 21-day heat cycle: five to seven days in heat, where a follicle ovulates on day 3, 4 or 5; and 14 to 16 days out of heat as the egg is either fertilized and a pregnancy established, or another follicle developed for ovulation.
What makes the transitional period tricky is that its duration is hard to predict from mare to mare on the same farm and from year to year in individual mares. It can also fool breeding managers because transitional mares often exhibit sporadic or continuous behavioral heat even through they are not ovulating. “Every time a mare grows a follicle, estrogen is produced, which brings a mare into heat,” Dr. McCue explains. “In the transitional mare, the first follicular wave will usually regress, estrogen levels decrease and the mare will go out of heat. Another follicle will develop with a rise in estrogen again, and the mare comes back into heat. “The transition period is really identified by a time when the mare is in and out of heat – or some mares could tease in heat for six to eight weeks in a row – without ever ovulating.” Without an ovulation it’s not a fertile cycle.
Managing the Transition
In any breeding program, the ultrasound is a valuable tool for managing the transitional period. “Every spring, we have owners call with questions about a mare being in heat for a long time,” Dr. McCue says. “We ask them to bring the horse in for an ultrasound. Usually, we can tell based on one ultrasound exam approximately where the mare is in her transition period. “On ultrasound, not only can you see the size of a follicle but you can also look for evidence of a previous ovulation by the presence of a corpus luteum, the structure that forms on the ovary after the ovulation of a follicle,” he explains. “That will tell you that the mare is no longer transitional but has ovulated and then you can expect the normal, rhythmic 21-day cycle.”
If the mare has small follicles, 15 to 20 millimeters in diameter, it will usually be a week or two before she could have a large, dominant follicle, and Dr. McCue might recommend another ultrasound in a couple of weeks. If she has larger follicles, closer to 35 millimeters, she could be approaching the end of transition and ovulation. “In that case, we could administer an ovulation-inducing agent like human chorionic gonadotropin (hCG) or deslorelin acetate,” he says. “We commonly induce ovulation in the late transition period so we know that the follicle is going to ovulate, otherwise, that follicle could hang on for many more days.” The first ovulation is fertile, and a late transitional mare could be bred in that case. Once the mare ovulates – either naturally or induced artificially – she typically cycles regularly after that.
Ultrasounding during transition can especially help in a hand-breeding program that relies on teasing. “When a mare is cycling, the rule of thumb is tocover the mare on the second or third day of heat, and then cover her every other day until she goes out of heat,” Dr. McCue says. “You typically end up covering her 2.5 times per cycle, on average. “But if the mare is in transition, you could cover her every other day for a month! Or for what you think is several cycles, when she’s really not ovulating. That’s a lot of wear and tear on a stallion with no chance for pregnancy.” In a pasture-breeding program, judicious ultrasound can also help you better time when to put your stallion into a mare herd. Turning him out when open mares are showing behavioral heat but are still in transition probably won’t produce pregnancies.
While most farms put mares under lights to bring them into season earlier, it’s also an effective way to manage the transitional period. Once you begin to lengthen a mare’s days artificially, she usually starts to ovulate 60 days later. Farms typically start putting mares under lights in early December so that they will begin cycling in early February. Of course, the most common reason breeder do that is to have a foal born earlier in the spring; depending on the foal’s intended use, an earlier birthdate might or might not be an advantage. “Another advantage for breeders,” Dr. McCue adds,” is by getting your mare cycling by mid-February, you gain more potential breeding opportunities during the season. “We all take for granted that if the mare doesn’t get pregnant, we’ll have another cycle, but that’s not always true. Sometimes a stallion isn’t available or the mare has problems on a cycle. It’s not a given that you’ll always have multiple cycles to breed on in a breeding season. Advancing the first ovulation of the year gives you more opportunities.” Putting mares under lights also allows you to time when the transitional period happens, rather than waiting on it to happen.
A lot of research has gone into different hormone therapies and their effectiveness at bringing mares into estrus earlier in the year and in shortening the transitional period. Researchers have looked at just about everything from gonadotropin-releasing hormone to progesterones and prostaglandins. Generally speaking, none are as effective as artificial photoperiod at bringing mares from deep anestrus into the transitional period and estrus. Several therapies can be effective in shortening the transitional period.
Typically, they work best with mares in deep anestrus. And their effectiveness depends greatly on administration and dosage. “One of the pitfalls of hormone therapy in deep anestrous mares,” Dr. McCue says, “is if the mare fails to get pregnant. You can stimulate follicular development and cause a mare to grow a follicle and ovulate, and it will be a fertile ovulation – she can get pregnant and the pregnancy will proceed as normal.
But if she doesn’t get pregnant, she could very likely fall back into anestrus.” That is more likely with a mare that is treated in deep anestrus. However, used with an artificial photoperiod program, hormone therapy can usher mares through transition faster without the risk of returning her to anestrous. Hormone therapies are best used on a customized, case-by-case basis. “For example, deslorelin – a GnRH agonist – can stimulate follicular development,” Dr. McCue says, “and we’ll sometimes use low-dose deslorelin treatment in transitional mares or a mare that shuts down inexplicably during the physiological breeding season.”
In addition, progesterone (or synthetic prosestins such as Regumate) as proven effective at timeline a mare’s first ovulation of the year when used on mares that have been under lights. “Unfortunately, right now, there is no commercially available, FDA-approved drug to stimulation of follicular development in the transition period,” Dr. McCue says. There are ways to manage a mare’s transitional period in timing and duration, but as with most transitions, it’s most important to understand what’s going on and just get through it.
Getting the Lights Right
“To use artificial photoperiod effectively, the main factors are timing, intensity and duration of light,” says Dr. McCue. “For the timing, mares need approximately 16 hours of light and eight hours of darkness – studies have shown that horses need a dark period. “Mares can be outside under natural sunlight during the day, but whether they are in a paddock or in a barn at night, the lights need to come on at dusk to extend the photoperiod exposure to 16 hours. Studies have also shown that adding lights at dusk works better than adding lights before dawn.
“For example,” he says, “in Colorado in December, the sun comes up around 7:30 a.m. and goes down around 4:30 p.m. So we set out light timers to come on at 4:30 p.m. and to shut off between 10:30 and 11 p.m.” Intensity of light is also important – it must be 10 foot candles or greater – and Dr. McCue recommends buying a simple light meter at a local hardware or electrical shop to measure it exactly. “If you are in a stall or paddock, use that meter at the height of a mare’s eye, and the gauge will tell you the amount of foot candles that your lights are providing. Too little intensity will not stimulate the mare’s endocrine system to do what you want.” He adds that “almost any kind of light” will work, from incandescent to fluorescent.
A rule of thumb is enough light to read a newspaper by anywhere the mare could be, whether it’s in a 12×12 stall or outdoor paddock. For the duration, “it takes at least 60 days of stimulatory artificial photoperiod to take a mare from deep anestrus to the point of first ovulation,” he says. “In quite a few mares, it will be more than that, 75 to 80 days, but some are faster.” Once a mare is through her transitional period and is cycling, you will need to continue housing her under lights to keep her cycling until the natural daylight has reached 16 hours. If you get her cycling by mid-February and then stop putting her under lights, she will revert back to anestrus without continued light stimulation. In most situations in the northern hemisphere, an owner can stop artificial lights by April 1 since spring is well under way and there is a sufficient duration of natural sunlight to keep the mares cycling.