The Bureau of Land Management (BLM) estimates that 40,605 wild horses and burros (approximately 33,780 horses and 6,825 burros) are roaming on BLM managed rangelands in 10 western states based on the latest available data, compiled as of February 28, 2013. Wild horses and burros have virtually no natural predators and their herd sizes can double every four years. As a result, the agency must remove excess animals from the range each year to control herd sizes. The estimated current free-roaming populations exceeds by nearly 14,000 the number that the BLM has determined can exist in balance with other public rangeland resources and uses. The maximum appropriate management level (AML) is approximately 26,677 (DOI BLM website).
National Academy of Sciences Committee Review: Using Science to Improve the BLM Wild Horse and Burro Program, A Way Forward. A 2013 review by the National Academy of Sciences of the Wild Horse and Burro Program detailing the effects these animals have on the landscape and constructive changes that could be implemented into the program.