According to the 2015 Final EA for the Water Canyon Wild Horse Growth Suppression Pilot Program, mentioned in the July 5 roundup announcement,
When injected, the GonaCon vaccine targets the reproductive hormone gonadotropin releasing hormone (GnRH) secreted by the hypothalamus of the brain. Antibodies produced in response to the vaccine inactivate endogenous GnRH, which in turn eliminates stimulation of the pituitary gland and gonads in males or females. The resulting “immunocastration” renders animals unable to produce reproductive steroids or gametes.
The EA was copied to the project folder in ePlanning with other related documents.
The Proposed Action, Alternative A, called for the use of PZP-22, but the Decision Record authorized Alternative B, similar to Alternative A except the mares would be treated with GonaCon-B instead of PZP-22.
A keyword search of the EPA web site yielded no results for GonaCon-B.
The description of Alternative B (bottom of page 34 in the pdf) refers to the registration of GonaCon-Equine, suggesting a rebranding or reformulation of the product.
A 2017 labeling amendment dropped the restricted-use designation and increased the interval between primer and booster from 30 days to 90 days.
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Western Horse Watchers Association 