"COMPLETE RIDER" YOUR #1 HORSE DESTINATION

Republished with permission from Equine Research Center Getting Your Mare To Cycle   Lighting Programs: Lighting programs are used to artificially alter the mares' natural breeding season. It is known that in nature it is the increasing day length that is primarily responsible for initiating mares to cycle each spring. Lighting programs aim to artificially imitate the increasing day length by exposing mares to stall lighting. By exposing mares to 16 hours of light starting December 1st, we can induce them to be cycling in February as opposed to their natural start up time of May. Who should go on lights? All barren and maiden mares that you want to breed before May 1st. I also recommend early foaling mares go under lights as sometimes these mares foal, go through the ten-day foal heat period and shut down into an anestral state. A regular box stall should be lit with a 200-watt incandescent bulb (or equivalent). The intensity of the light is important. A general rule of thumb to check for adequate light intensity is that you should be able to read newspaper print in all corners of the stall with ease. The duration of light is calculated to expose the mare for 16 hours per day. For example, if your mare is exposed to natural daylight from 8 a.m. to 3 p.m. and comes into her stall each afternoon at 3 p.m., the stall lights should be kept on from 3 p.m. until midnight for a total exposure of 16 hours of light to the mare. I normally recommend evening lights as opposed to morning lights, as this mimics nature's changes more closely. It takes approximately 60 days of increased photoperiod to induce ovarian follicular activity with estrogen production. Initially, the heat periods seen are of the transitional type, but will be followed by fertile estrus cycles. Starting your lighting program by December 1st usually ensures that the mare is having fertile estrus cycles by the start of the breeding season.   Hormone Regimes: Various hormone regimes have been used to either help predict the time of ovulation and/or to help initiate the mare to ovulate when she is in the transitional period. Progesterone & Estrogen: Progesterone (150 mg) and estradiol (10 mg) (P&E) are given by intramuscular injection once daily for 10 days. These hormones inhibit the production of FSH (follicle stimulating hormone) and LH (luteinizing hormone). When the injections stop, you get a FSH and LH surge, producing follicular development with ovulation usually occurring 9, 10 or 11 days after the last injection. For P&E therapy to be effective, the mare must be in either a transitional or cycling state. Progesterone: Progesterone can be given as daily progesterone (150 mg) injections or orally as altrenogest (Regumate [R]) 0.444 mg/kg daily. Progesterone inhibits ovulation but not follicular development. When treatment stops, cycling and *transitional mares can be expected to ovulate approximately 10 days after the last dose. Mares with inactive ovaries or with follicles less than 20 mm in diameter usually fail to respond to this treatment. Therefore, it is important to ensure the mare is transitional or cycling with at least a 20 mm follicle present before using a progesterone based therapy. GnRH: Administration of GnRH over a 14 to 21 day period has been shown to produce ovulation in *anestrus mares, however, these mares failed to produce a CL (corpus leuteum). If the mares were transitional (with follicles greater than 20 mm), a CL was present post-ovulation. Currently, successful administration of GnRH requires intensive administration varying from twice daily injections to continuous administration. Where GnRH may become more useful is with the possible development of a controlled release implant releasing GnRH constantly over a 2 to 3 week period. ***Anestrus begins in late fall when the mare stops cycling and her ovaries are inactive. It is followed by the transitional period before regular ovulation begins.   Page [1] [2] [3] [4] Adapted from an Equine Research Centre article by Moira F. Gunn, B.V.M.S., Armstrong Bros. Inglewood, Ontario

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