GonaCon™ is an EPA-registered, immuno-contraceptive pesticide. Its classification is “restricted-use” due to “non-target injection hazard.” EPA warns that “pregnant women should not be involved in handling or injecting GonaCon and that all women should be aware that accidental self-injection may cause infertility.” Children are not allowed in areas where the product is used. [4, 6] Please keep in mind that the GonaCon™ dose-in-question is meant for a horse.
GonaCon™ “works” by causing an auto-immune disorder. Behaving like a perverted vaccine, GonaCon™ tricks the immune system into producing antibodies that destroy a female’s gonadotropin-releasing hormone (GnRH). Without GnRH, a female does not produce sex hormones, does not come into estrus, and is thus infertile. Behaviorally, courtship-rituals cease. [1, 3-6]
For those wild-horse-and-burro advocates who oppose the other immuno-contraceptive — PZP — you will be disturbed to learn the following from the USDA-APHIS “Questions and Answers” sheet regarding GonaCon™:
“After evaluating GonaCon™, the Food and Drug Administration (FDA) … approved the slaughter of pigs vaccinated with GonaCon™. Similar injectable hormone-altering products are used routinely in livestock applications.” 
Good grief. So, the slaughterhouse pig ovaries used to manufacture PZP may very well come from animals who were previously injected with GonaCon™ to destroy their GnRH hormone — without which the ovaries cannot produce estrogen — and those poor pigs may also have been “routinely” injected with other similar “hormone-altering products.” Then our wild horses and burros are injected with PZP, which itself causes a marked drop in estrogen after just three treatments.  Surely, these hormonal atrocities constitute animal abuse.
GonaCon™ is long-acting. The treatment-protocol, consisting of two injections administered 30 to 60 days apart, can cause infertility for as long as four-to-five years without the need for booster shots. [3, 5] However, mares would still need to be rounded up and held captive for those 30 to 60 days to administer the injections properly. If all females in a small herd were treated per the multi-year plan, it could result in an unintended consequence — a huge gap in the herd’s age-structure, because very few if any foals would have been born during that period. Indeed, although the pesticide’s effectiveness was expected to diminish over time, a 3-year study of GonaCon-treated elk revealed that the percentage of infertile females actually increased each year, finally reaching 100%. It was also noted that every one of the treated elk suffered an abscess at the injection-site. 
Because GonaCon™ stimulates the immune-system, it will elicit the greatest reaction — the greatest output of destructive antibodies — if a mare is blessed with healthy immune-function. Such a mare will react strongly and be contracepted quickly. But she could just as easily be sterilized. In fact, GonaCon’s™ “application instructions” warn of the chance of sterilization. 
On the other hand, GonaCon™ may not work at all if a mare suffers from weak immune-function. That mare’s immune system will fail to react to GonaCon™, and she will get pregnant in spite of it. Thus, over time, there is the risk of another unintended consequence — selection for the immuno-compromised. As Jenny Powers, a National Park Service wildlife veterinarian and one of three lead scientists who participated in the elk research commented: “Any time we’re manipulative with wild animals, we’re messing with natural selection.” 
Report prepared by Marybeth Devlin on December 18, 2015 for Protect Mustangs
1. Keller, Larry. (2011, May 17) To shoot, or not to shoot, at Rocky Mountain NP. High Country News. Retrieved from http://www.hcn.org/blogs/range/to-shoot-or-not-to-shoot-at-rocky-mountain-np
2. Kirkpatrick, J. F., I. K. M. Liu, J. W. Turner, Jr., R. Naugle, and R. Keiper. 1992a. Long-term effects of porcine zonae pellucidae immunocontraception on ovarian function of feral horses (Equus caballus). J. Reprod. Fert. 94:437-444. Retrieved from http://www.ncbi.nlm.nih.gov/pubmed/1317449
3. McGrath, Matt. (2011, September 1) “Deer ‘pill’ curbs aggressive mating.” BBC News. Retrieved from http://www.bbc.com/news/science-environment-14744811
4. United States Environmental Protection Agency. (2009) Pesticide Fact Sheet. Mammalian Gonadotropin Releasing Hormone (GnRH). Retrieved from http://www3.epa.gov/pesticides/chem_search/reg_actions/registration/fs_PC-116800_01-Sep-09.pdf
5. USDA-APHIS. (2006, May 1) “GonaCon™—Birth Control for Deer: Questions and Answers.” Digital Commons@University of Nebraska – Lincoln. Retrieved from http://digitalcommons.unl.edu/usdaaphisfactsheets/7/
6. U.S. Department of the Interior. (2015) Review of Ungulate Fertility Control in the National Park Service. Retrieved from https://www.nature.nps.gov/biology/wildlifehealth/Documents/Ungulate%20Fertility%20Report_09242015.pdf
Protect Mustangs is an organization who protects and preserves native and wild horses.