In the early 80's we granted researchers at the University of California at Davis the right to conduct an experiment with PZP, a contraceptive vaccine. The results were very successful, especially because it did prevent pregnancy in the horses with no side effects, it could be used on pregnant mares without damage, and it proved reversible.
"It appears that this experimental contraceptive vaccine is also effective on large populations of horses and that it may serve as a humane alternative to the control of overpopulation of other feral animals including white-tailed deer, tule elk, elephants and possibly black bears"---Irwin Liu, DVM, PhD, University of California, Davis. "It far outweighs the disadvantages of culling by translocation and/or slaughter."
Liu continues, "As you can see, the original studies at the Wild Horse Sanctuary have had an enormous long term effect on the humane treatment of feral animals all over the world. ...I am truly grateful to the WHS for allowing the project to happen."