transitional mares
The transitional period between anestrus and estrus is officially complete once a mare has had her first heat of the year. Getting some mares to this point, however, and deciding when to breed them can be tricky, veterinarians recently concurred.

They discussed the topic at the 2018 American Association of Equine Practitioners Convention, held Dec. 1-5, in San Francisco, California. Charlie Scoggin, DVM, Dipl. ACT, from Rood & Riddle Equine Hospital, in Lexington, Kentucky, and Dirk Vanderwall, DVM, Dipl. ACT, professor of equine reproduction at Utah State University, in Logan, co-mediated a Table Talk session about transitioning mares. The format allowed attendees to share their practical experience, as well as related research.

Transitional Mares 101

Most, but not all, nonpregnant mares enter a state of reproductive “winter quiescence” when the days become shorter. This annual event is called anestrus and means the mare isn’t fertile, which (in nature) prevents her from foaling during months of inhospitable weather and limited food supply.

These broodmares subsequently transition back into their regular cycles as day length increases. However, if owners are hoping for an early season foal, this transition might require human intervention in the form of artificial lights or medical treatment.

Scoggin and Vanderwall reminded attendees that follicle-stimulating hormone (FSH) is one of the first hormones to rise during transition. This triggers the ovary to produce egg-filled follicles that ovulate after stimulation by luteinizing hormone (LH). Sometimes during transition, LH lags behind FSH, meaning the mare has large follicles that might never ovulate on their own. Such mares might need medical intervention at this point to force ovulation.

This prelude prompted the first of many discussions: How fertile is that first ovulation following anestrus?

The moderators agreed that unfortunately, data are limited to answer this question with a high degree of certainty. One conference attendee suggested the first ovulation of the season isn’t the most fertile; yet he would never pass up an opportunity to breed a mare. He did note, however, that he’d only recommend breeding if the rest of the urogenital tract appeared healthy on physical and reproductive examinations. Other attendees agreed, stating they short-cycle transitional mares (make them ovulate early to begin the next, hopefully more fertile, cycle), whereas some said they believe the first ovulation after anestrus is just as fertile as any other.

Under the Lights

The remainder of the discussion focused on lighting protocols—a key component of managing most transitional mares—specifically using commercially available blue-light masks and fine-tuning artificial (i.e., barn) lighting. Approximately one-third of attendees indicated they use light masks; agreeing that they liked how mares wearing the masks aren’t restricted to their stalls for supplemental lighting from overhead lights.

Attendees and moderators also discussed tips and information about blue-light mask use:

  • The masks can be cost-prohibitive for some operations, forcing farm managers to pick and choose which mares wear them;
  • When they first became available several years ago, the masks only seemed to last one season. However, attendees said some masks now appear more durable, and veterinarians try to get two seasons out of a single mask. Managers can stretch mask longevity by waiting until later in the season before starting its use, mentioned several attendees.
  • Typically, breeding operations build up to the recommended 16 hours of light (by gradually adding supplement light from 4 p.m. to midnight) followed by eight hours of dark in a stepwise manner, confirmed attendees. Other data have shown, however, that there’s apparently a “light-sensitive” portion of the day that interrupts the dark phase. Using artificial lighting only during this shorter, light-sensitive phase appears as effective as the 16 hours. This alternate artificial light technique is referred to as pulse lighting.Attendees said increasing overall light doesn’t necessarily help mares transition. Instead, it is decreasing amount of darkness that spurs the transition. Those practitioners believe that having lights come on for one hour in the middle of the night could be just as effective as having them on for eight hours at the end of the day. In addition, this “pulse lighting” minimizes stall time and could reduce utility costs associated with lighting large operations.
  • While 16 hours of light starting Dec. 1-15 in the Northern Hemisphere is the “magic number,” geography and other factors still make a big difference in how each operation manages its transitional mares.

With the technology currently available, farm managers sometimes forget basic facts about lightbulbs. For example, Scoggin and Vanderwall reminded attendees that you should be able to read a book from anywhere in the stall, or else it’s too dark. Blue lights, LED lights, incandescent … it doesn’t matter; they all need to be turned on and off appropriately. Farms that turn the lights on at 6 p.m. and off at 6 a.m. because no one is available to turn them off at midnight have problems with transitional mares. Finally, they encouraged managers to check bulbs often to make sure they don’t need changing.

Medicinal Options

Attendees briefly discussed several additional techniques to kick-start transitional mares, including vaginal progesterone devices; administration of deslorelin, estradiol cypionate, or GnRH-agonists such as buserelin, and, in the future, possibly recombinant equine FSH; constant exposure to teaser stallions; warm saline lavages (which people agreed are controversial); among others.

Take-Home Message

Ultimately, the moderators agreed, all medical interventions must be used in conjunction with appropriate lighting and geographical considerations. As one participant noted, “What works in Florida does not necessarily work in Wyoming.”