Without having more history about your mare, it is impossible to know exactly what is occurring. Since you reside in California, your mare should be having adequate light stimulus to induce cycling. What is concerning is that you state that her ovaries are the size of small grapes. When both ovaries are this small, one concern is that she is not genetically normal (XX) but may be an XO mare. Has she ever cycled normally? Has she ever foaled? If the answer is no to both questions, you should have your veterinarian send off blood for genetic testing. If she had one small ovary and one bigger ovary, there is a possibility that she may have an ovarian tumor, such as a granulosa cell tumor. A good history (mare is often more aggressive) and a blood test for inhibin, plus ultrasound of both ovaries, are important to determine if this is a possibility. Additional help can be obtained by contacting an equine reproductive specialist (theriogenologist) at the veterinary school at the University of California-Davis.

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