INDUCTION OF OVULATION:
The most common cause of prolonged anovulation (greater than six months) in mares is Cushing’s Syndrome. A Cushing’s Screen (see BET Protocol) should be performed in these mares. If they are positive, they will not respond to the following protocols unless treated for Cushing.
Anovulatory mares have serum concentrations of progestagens that are less than 1.0 ng/mL. A progestagens assay should be performed before starting any protocol. Additional progestagen assays should be performed following completion of the protocol to detect ovulations, which may have occurred even though the mare did not show estrus. These mares will respond to prostaglandin injections and exhibit normal estrous cycle if TT4 levels are sufficient (ie greater than 12.0 ng/mL).
A Total T4 assay should be performed in all anovulatory mares or any mare not exhibiting normal estrous cycles or ovarian follicular development. Mares with TT4 concentrations below 12.0 ng/mL will not respond adequately to the anovulatory protocols. Response will be significantly improved when sufficient thyroxine is given orally to elevate TT4 concentrations between 14 and 20 ng/mL. The dosage should be titrated so that these concentrations are present 24 hours following the dosage of thyroxine. This simply means that the blood sample should be taken before the oral dose of thyroxine is administered and not afterwards. Thyroxine need only be given once per day. Thyroxine should be continued at least until 45-50 days of pregnancy. It is imperative to monitor TT4 concentrations to insure that the proper amount of thyroxine is being administered. Too much will cause weight loss and hyperexcitability, and too little will be ineffective.