LANDER, Wyo. (AP) — The Bureau of Land Management has been rounding up hundreds of horses in Fremont County this week.

Helicopters are being used to gather the horses, who are then loaded into trailers and taken to corrals. The Associated Press reports that the mares are being given birth control shots known as PZP.

The BLM is trying to bring the wild horse reproduction rate down to 10 percent, down from an estimated 15 percent now.

Last month, a citizen advisory panel recommended that the BLM should consider sterilizing some mares to control booming mustang populations and protect rangelands.

The agency is also preparing to round up some wild horses in Sweetwater and Carbon counties next summer. It’s taking public comments on the plan.