Lights and ovulation drugs play a role in cycling, conception
by Hope Ellis-Ashburn
There are few more satisfying moments than when a mare checks in foal. Generally, it comes about as a result of manipulating a mare’s cycle through the use of artificial lights, ovulation drugs or a combination of the two.
Left to her own devices, with no manipulation to her cycles, the average day that a mare in the Northern Hemisphere will ovulate is April 12. Shorter periods of darkness trigger the mare to begin the ovulation cycle.
“Light goes into the eye, causing a decrease in the release of melatonin from the pineal gland, which in turn stimulates the release of GnRH from the hypothalamus,” explained Etta Bradecamp, DVM, Dipl. ACT, Dipl. ABVP, a veterinarian and equine reproduction specialist with Rood and Riddle Equine Hospital in Lexington, Ky. “This, in turn, causes the release of FSH from the pituitary gland which activates the ovaries to stimulate follicular development.
“It takes about 60 days to have enough stimulation from the shortening periods of darkness for them to reach their first ovulation.”
In an industry where the ideal birthday is much closer to the first of the year than the first weeks of spring, it becomes necessary to encourage mares to begin ovulating at an earlier date.
“What we’ve learned through research is that artificially exposing them to longer periods of daylight is the same as what happens in nature,” said Dr. Bradecamp. “We need about 60 days of those short nights to get them cycling in February. We recommend starting between Thanksgiving and Dec. 1. And 60 days from Dec. 1 is the first of February, ready to have the transitional period.”
While artificial lights have long been recognized for their role in manipulating periods of daylight and darkness, an exciting new development within the last 10 years is equine blue light therapy. Blue light is delivered directly into one of the mare’s eyes via a programmable mask that mimics the prolonged exposure to daylight/shortened periods of darkness that was once exclusively the domain of artificial lights.
One advantage of the masks is that mares do not have to come in for light therapy. However, the added warmth experienced by bringing mares into the barn during the coldest part of the season does seem to augment traditional light therapy.
“Cold weather will make them slower to start cycling,” said Dr. Bradecamp. “I always tell breeders (using these lights) that they may need to put a blanket on them when it’s cold, wet or rainy. When the mares seek shelter and are getting plenty to eat, it doesn’t affect them as much.”
As simple as it seems, there are ways to run into trouble. For optimum results, the extended lighting should be uninterrupted. In other words, don’t wait for natural darkness to occur and then bring the mare in under lights. The lighting process should begin before natural daylight ends.
Finally, regardless of how you choose to deliver artificial lighting, periods of darkness remain an important factor in your mare’s overall well-being such that the lights do not constantly remain on and she is getting enough sleep.
Often, artificial lights are all that is needed to get a mare cycling early, but sometimes pharmaceutical intervention is also necessary. Ovulation drugs can be used as a stand-alone option or in conjunction with artificial lights.
Dr. Bradecamp explained a common scenario.
“If I check a mare (for pregnancy) for a breeder at the beginning of February and she’s not cycling, I may say let’s wait another week. If I check her again in another week and another week and she’s still kind of dragging along and hasn’t started to have that first cycle of the season or even close, the breeder is likely starting to get anxious. At that time, I recommend starting [the mares] on a GnRH analog drug in a small dose twice a day. One of the most common GnRH analogs (we use) is called buserelin to try to stimulate them to release FSH if they’re not quite doing it enough on their own already.”
Administering a GnRH analog is not a guarantee.
“It works in over 50 percent of the mares you try it on. If this still doesn’t work, there are other drugs that we’ll try,” Dr. Bradecamp said. “These include estradiol, domperidone and sulpiride. Another option is to put the mares on altrenogest.”
In addition to human chorionic gonadotropin (hCG) and Sucromate, there are other drugs that can be used to manage a mare’s reproductive cycle alone or in combination with one another. These include prostaglandins and progesterone. A prostaglandin is a hormone that works to bring a mare into estrus. The most commonly prescribed are lutalyse and Estrumate. These drugs are used for mares that have already cycled and had an ovulation. They work by “short cycling” the mare to bring her back into heat.
Progesterones are naturally produced in the mare once she has an ovulation. They are what make the mare go out of heat such that she is not receptive to a stallion; they also maintain the pregnancy if the mare is bred and becomes pregnant. The hormone can also be used to synchronize mares to accelerate the first ovulation of the season.
Progesterone is administered in two forms—an injectable product in oil, and orally as the synthetic progestogen altrenogest.
Finally, there are (hCG) and Sucromate, hormones to induce ovulation. Another GnRH analog, Sucromate acts at the level of the pituitary gland, causing it to release LH, or luteinizing hormone, which acts on receptors on the follicle. hCG has LH-like activity on the follicle, inducing ovulation.
Combining Lights, Ovulation Drugs
According to Dr. Bradecamp, if a mare is placed under lights and is cycling at the desired time, the only drug that may be used on her is some type of ovulating assisting agent.
“Some people prefer to use hCG early in the year,” she said. “Some people will use Sucromate in combination with hCG. Sometimes getting the mares to have that first ovulation is a little tougher once they’ve started cycling.”
If a mare isn’t cycling, however, it becomes necessary to resort to pharmacologically stimulating her.
“If they don’t have a corpus luteum, we don’t use a prostaglandin on them until they have started cycling,” said Dr. Bradecamp. “Some people will put them on 10 days of altrenogest. When we pull them off of this and we are trying to get them to cycle, we’ll give them a dose of prostaglandins just in case they have had an ovulation that we didn’t recognize during those ten days.”
Similar Success Rates
Artificial lights, whether used alone or in conjunction with pharmacological intervention, are highly effective. Normal mares, meaning those mares without underlying health conditions, will get in foal at the same success rate seen in mares allowed to cycle more naturally.
“The only negatives are time and cost,” Dr. Bradecamp said.
Regardless of how the resulting foal comes about, lights and ovulation drugs are viable options to achieve that fulfilling moment when the ultrasound reveals the early result of your breeding goals. HB
Hope Ellis-Ashburn is a freelance writer living in Tennessee. To comment on this story, email us at firstname.lastname@example.org.