What is the transition period of a mare?

The period between the anestrus season and the breeding season is the transition period. During this period, the sexual behavior of the mare tends to be erratic. The mare will come into heat, and the ovaries will develop many small follicles. The mare may remain in heat for several weeks. Oftentimes the developing follicles regress and the mare does not ovulate. It does no good to breed the transitional mare unless she happens to develop a follicle that will ovulate. This can only be determined by palpation or ultrasonic evaluation of the mare’s ovaries. The transitional mare in heat will stand to be bred, but unless she has a follicle develop to ovulate, you are wasting time and exposing the mare to a greater chance of infection and injury by breeding during the transitional period. As mentioned, the mare can be monitored during this period by palpation and/or ultrasound, but usually it is best to let the mare go out of heat or take her out of heat by giving her a synthetic progesterone for 14 to 15 days. This will usually shorten the time period to ovulation as compared to letting her go out of heat naturally. The cost of synthetic progesterone is $4.00 to $5.00 per day. The imposition of January 1 birth dates for most breeds of horses has caused us to breed the mare during a less than optimum time for fertility. An open or maiden mare that has not been on lights beginning in late November will often go through a transition period during February and March. Consequently, the first heat period in February may be long and characterized by no ovulation. No ovulation means no conception, which means no January and possibly no February foal next year.