By Heidi Smith, DVM
- Springtime can be a frustrating time for
horse breeders as mares undergo a phase called "transitional
heat." During winter, the majority of mares go through a
stage called "anestrus" or absence of heat cycles.
As the hours of daylight begin to lengthen in the spring, the
mare's reproductive tract is signaled to begin activity again.
As the ovaries once again become active, instead of starting off with regular heat cycles, they build multiple tiny follicles (the structures on the ovaries that contain the oocytes or eggs). The follicles produce the hormone estrogen, which causes the mare to show signs of estrus or heat. However, breeding at this time will not produce pregnancy, as none of the follicles are large enough to rupture and release an egg.
This transitional period may be fairly short (a few days) or quite long (more than a month). The mare may show heat constantly during this transition, or may show very erratic signs of heat. Toward the end of this transitional heat, the mare usually will develop a larger follicle and finally ovulate an egg. Once she does this, she will usually begin a regular pattern of heat cycles. Until the mare develops regular cycles, the only way to differentiate a transitional heat from a normal heat with a normal follicle is to have the mare palpated or ultrasounded by a veterinarian familiar with equine reproduction.
One of the most frequent questions we are asked prior to the onset of transitional heat is whether we can give the mare an injection to bring her into heat. The injections used to initiate heat belong to a class of drugs called prostaglandins. They work by causing the degeneration of a structure in the ovary called the corpus luteum (or "CL"), which grows from the remains of the follicle from the previous heat. Until the mare has been through a heat and a subsequent ovulation, there is no CL present on which the drug can work. Hence, there is no way for us to bring the mare into heat with drugs before increased daylight causes her reproductive system to "switch on" for spring. Breeders who want early foals often achieve success at "fooling" Mother Nature by putting the mares under artificial lights for 16 hours a day beginning in the late fall.
 |
|