"The federal management program for wild horses has been something of a financial and animal welfare disaster for quite some time," Pacelle wrote in blog, A Humane Nation.
"In recent years, the Bureau of Land Management has rounded up tens of thousands of horses, causing distress and fear and some occasional deaths, without any reasonable expectation to adopt out these animals.
"That has resulted in a swelling captive population of wild horses - now more than 45,000. Almost half of the agency's entire budget goes toward captive horse management," he said.
"If there is pressure or reason to reduce the population, then the primary management tool from this point forward should be fertility control, rather than costly and sometimes dangerous round-up and removal regimes."
Pacelle said PZP can be used to maintain sustainable wild horse populations, since the American public wants wild horses roaming the West.
"By using more fertility control to humanely reduce wild horse populations on the range, and having fewer horses in long-term federal holding pens on the government dole, US taxpayers can save tens of millions over the next decade."
The Humane Society sponsored the registration of the vaccine.