DENVER, April 6  Mack Louden worries that his 30,000-acre ranch sits in the cross hairs of the Army’s plans to expand its Piñon Canyon Maneuver Site at Fort Carson, and he, along with other Colorado ranchers, are increasingly upset about the idea.

“Where we live, how we live, it’s all going to die a slow death if the Army gets our land,” said Mr. Louden, a fourth-generation rancher from Las Animas County, along the southern edge of the state.

He and other ranchers are to testify on Monday before a committee of state lawmakers in support of a bill that seeks to keep the Army from acquiring nearly a half-million acres it says it needs to train soldiers in the nuances of modern warfare.

Colorado law grants the federal government permission to condemn land for some purposes, like building courthouses and post offices. And the Defense Department lifted a moratorium this year on land acquisitions to allow the Piñon Canyon expansion.