This year, the Bureau of Land Management (BLM) plans to round up 6,000 of the wild horses and burros that roam the Western states. It's part of an expensive, ongoing effort to manage the population of these symbols of the American West. But animal rights groups say the federal agency's practice of rounding up the horses by helicopter is cruel and wasteful. Here, a brief guide by the numbers:

Number of wild horses that roam BLM lands, according to agency estimates

Number of burros running along with them

The ideal population level, according to the BLM

20 percent
Rate at which a horse herd can grow each year, according to BLM officials. The BLM also notes that a herd is capable of doubling in size every four years, and that land and wildlife are destroyed by overpopulation. Caroline Betts, a University of Southern California professor of microeconomic and international finance, says such estimates seem overblown

Number of wild horse that used to roam the North America plains, according to some estimates

Number the wild horse population had dropped to by the 1960s. As a result, Congress passed the Wild Horse and Burro Act in 1971, making the federal government responsible for protecting and managing the herds. Initially, the BLM focused on providing the horses with food and water to survive droughts, but its focus has shifted to regulating the population to ensure that the horses have enough food, water, and land. "Horses really do destroy the land sometimes," says Ben Noyes, a wild horse specialist. "They muddy up spring water and contaminate it with waste, and when they eat grass they actually completely uproot it, leaving their ranges bare."

Over 200,000
Number of horses that have been rounded up by the bureau since 1971

Number of wild horses and burros that were rounded up in the fiscal year ending in September

Number the bureau plans on rounding up this year. The horses are herded by helicopter into mobile holding pens, then taken to holding sites. "Very few horses escape the helicopter, but when they do, the activists who gather at the trap sites to protest the roundups give a hearty cheer," says Max Bearak in The New York Times. At the holding sites, horses get medical examinations and vaccinations, and the males are castrated. Healthy horses are put up for adoption.

Number of female horses the BLM hopes to give fertility control treatments, as a more humane way of limiting the population

Amount a standard horse goes for at auction; they're inexpensive because they're difficult to train. "They're not used to someone coming out with a bucket of feed," says the coordinator of the BLM's information center, Debbie Collins, "and they've never had a stall, or really any contact with humans."

Number of wild horses that died of dehydration in a July 2010 roundup in northeast Nevada, outraging animal rights activists. Another horse broke its leg and had to be shot and killed

0.25 percent
Number of horses rounded up in fiscal 2010 that suffered "direct fatalities," according to the BLM

Number of horses currently in holding facilities, both long and short term, according to an AP report

$36.9 million
Cost of holding the horses, for the 2010 fiscal year. "This mismanaged federal program is bankrupting American taxpayers and devastating our remaining wild horse herds," says Suzanne Roy of the American Wild Horse Preservation Campaign

Sources: Associated Press, LA Times, NY Times (2)