A new document from the FDA’s Center for Veterinary Medicine provides a summary of the bovine reproductive cycle and the role of reproductive hormones, along with descriptions of how various drugs used in estrous synchronization programs affect that cycle.

The online article, titled “The Cattle Estrous Cycle and FDA-Approved Animal Drugs to Control and Synchronize Estrus—A Guide for Producers,” begins with some definitions of terms to aid in understanding the subject, such as the difference between “estrus” and “estrous.” Estrus is a noun, referring to the time in which a female animal is “in heat” or receptive to breeding and conception. Estrous is an adjective used to describe something related to estrus, such as “estrous cycle.”

The authors then describe the phases of the cattle estrous cycle, including the follicular phase, estrus and ovulation, the luteal phase and development of the corpus luteum, pregnancy versus non-pregnancy and cyclic versus acyclic responses.

The article goes on to explain the activity and applications of drugs used in estrus synchronization programs. These include:

·         Drugs in the gonadorelin class, which act similar to gonadotropin releasing hormone (GnRH). Five gonadorelin products are FDA-approved to treat ovarian cysts, which is a therapeutic use. Three of the five gonadorelin products are also FDA-approved to control or synchronize estrous cycles in cattle when given sequentially with another drug.

·         Drugs in the progestin class act similar to the hormone progesterone naturally secreted by the corpus luteum. One progestin, an over-the-counter intravaginal insert, is FDA-approved for synchronizing estrous cycles or advancing estrus in cattle either when given alone or sequentially with another drug. Melengestrol acetate (MGA), is a progestin with FDA approved for use in feed to suppress estrus in heifers but is not approved for synchronizing estrous cycles or advancing estrus in cows or heifers.

·         Drugs in the prostaglandin class act similar to the hormone prostaglandin F2-alpha naturally released from the uterus when there is no pregnancy. Five prostaglandin products are FDA-approved for several therapeutic uses and for estrous synchronization y in a single- or double-injection prostaglandin-based regimen. Four of these products are also FDA-approved for synchronizing estrous cycles or advancing estrus in cattle either when given alone or sequentially with another drug. All five prostaglandin products are prescription only.

The FDA guide makes note that extra-label use of drugs for estrous control and synchrony in cattle is not allowed. FDA regulations allow extra-label use of drugs in animals only when an animal’s health is threatened or where the animal may suffer or die without treatment, and a veterinarian must carefully diagnose a medical condition before prescribing an extra-label treatment for therapeutic purposes. Using reproductive drugs in an extra-label manner does not meet any of those requirements.

View the full document online from FDA.