<P> Despite the success of the adoption program, the BLM struggled to maintain acceptable herd levels, as without natural predators, herd sizes can double every four years . As of 2013, there were over 40,000 horses and burros on BLM - managed land, exceeding the BLM's estimated "appropriate management level" (AML) by almost 14,000 . In addition to these on - range horses, there are 49,000 additional wild horses, also protected under the Act, living in off - range corrals and pastures . </P> <P> The BLM uses limited amounts of contraceptives to control herd numbers, in the form of PZP vaccinations; advocates say that additional use of these vaccines would help to diminish the excess number of horses currently under BLM management . As of 2013, the BLM is also researching the possibility of spaying some mares to permanently prevent pregnancies, and a new vaccine, the "first single - shot, multiyear wildlife contraceptive for use in mammals", has been approved for use by the Environmental Protection Agency . </P> <P> From 1988 to 2004, Congress prohibited BLM from using any funds to destroy excess animals . In 2008, the BLM announced the possibility of euthanizing excess horses, a move which was quickly condemned by horse advocates . </P> <P> The constitutionality of the new law was disputed . Up until then, feral horses and burros were considered to be under the jurisdiction of State estray laws, and managed as unclaimed livestock that the Federal government had no right to interfere with . To test this assertion, in 1974 the New Mexico Livestock Board seized 19 free - roaming feral burros which were preventing cattle from using a watering hole on federal land . The United States District Court for the District of New Mexico held that, under the Property Clause of the U.S. Constitution, Congress could regulate "wild" animals only to protect public land from damage . The case went to the Supreme Court of the United States . In Kleppe v. New Mexico, 426 U.S. 529 (1976), the Supreme Court ruled that Congress's power to manage public land "necessarily includes the power to regulate and protect the wildlife living there ." and that, unclaimed free - roaming horses could be considered "wildlife" for purposes of determining whether Congress has the power to protect them . In United States v. Johnson, 685 F. 2d 337 (9th Cir. 1982) the act was challenged in court for being unconstitutionally vague and unconstitutionally overbroad in its definition of "unbranded and unclaimed horses". The United States Court of Appeals for the Ninth Circuit upheld the wording of the act. - </P>

The wild free-roaming horses and burros act of 1971 (public law 92-195)