"The most common misconception or the biggest mistake is for veterinarians to think that light is not very important," says Patrick Burns, PhD, president of Burns BioSolutions, Lexington, Ky.
All of the hormonal treatments and drugs work best or most effectively, either after the mare has had some exposure to photoperiod
(artificially), or at the beginning of the transitional season in the spring (April-May), when the mare has started the process
on her own due to the increasing daylight.
"When you try to superimpose these hormonal therapies on mares that are in debt, deep in anestrous, the results of the therapies
are consistently terrible," Burns says.
In order to fool Mother Nature slightly, a treatment of as short as three weeks to six weeks of light exposure (16 hours of
light per day, typically until 10 p.m.) has beneficial effects. Natural light and lengthening days alone will work, but it
typically takes about 60 days. Accelerated light regimens speed the process a bit.
"No one's come up with anything better yet," Burns observes. "Typically we would like to have them on a lighting regime for
a minimum of 30-40 days before any of these hormone therapies."
With some mares, the results are quite good in as little as 21 days. There is a huge difference between no light and 21 days.
The 21-day regimen was shown when using exogenous equine follicle stimulating hormone (FSH). Even with exogenous FSH, which
will grow the follicle to ovulation, the results are far better when the mare has had at least three weeks of light. Light
initiates an entire neural-endocrine cascade into effect. We know that light has some effects on GnRH secretion, a lot of
input on the pituitary and stimulation of prolactin.
"What I preach and teach is that photoperiod is number one," says Dr. Jim Brendemuehl, Dipl. ACT, University of Illinois Veterinary
Teaching Hospital. "When one has the time to plan ahead, it is important to get mares under a lighting regime by the first
of December. It is recommended to put mares on lights 45-60 days prior to when you want them to cycle. They should also be
in a positive nutrient balance so they are gaining weight, rather than losing, and in good body condition."
Sufficient pituitary LH content
Mares must have sufficient LH in the pituitary for the hormonal drug therapies to work. Oftentimes with hormone therapy in
the field, the pituitary just does not contain a sufficient amount of LH to promote ovulation in early spring, regardless
of the stimulation of the system. The follicle is stimulated, but ovulation doesn't occur due to the insufficiency of LH.
Naturally, when the mare goes into seasonal winter anestrous, her LH synthesis decreases. There is not adequate LH stores
until far enough along into the transitional period. LH ultimately gets there after photoperiod stimulation. As she starts
cycling in the spring, FSH levels increase prior to LH. Once the mare is progressing with light stimulation, her pituitary
begins to build sufficient LH content. The last contributing factor that leads up to ovulation is the pituitary gland building
up enough LH in the gland to be able to trigger an ovulatory surge.
"In my perspective, in the transitional phase, whether mares are under artificial lights or under natural daylight conditions,
when they haven't started to cycle yet, progesterone or the progesterone/ estradiol combination is very effective to give
mares a kick start into their cyclic activity," says Dirk Vanderwall, DVM, PhD, a researcher with Northwest Equine Reproductive
Lab at the University of Idaho. "I certainly find that very helpful and beneficial clinically in mare management in that spring
Progesterone seems to help the pituitary synchronize itself and to accumulate a sufficient amount of luteinizing hormone (LH),
so the mare will go ahead and ovulate. Progestin has a negative feedback, which inhibits FSH and LH and essentially allows
a buildup of LH at the same time it is inhibiting follicular development.