DES MOINES, Iowa — More than 1 million acres of land have been protected from development each year under a temporary federal tax deduction, and land trusts nationwide are hoping Congress will now make permanent the credit, which includes a generous benefit for farmers and ranchers.

Sens. Charles Grassley, R-Iowa and Max Baucus, D-Mont., introduced a bill this week to do that. The Rural Heritage Conservation Extension Act of 2011 would allow landowners to deduct up to half of their income for 16 years in exchange for conservation easements, while farmers and ranchers could deduct all of theirs. Before the enhanced credit went into effect in 2006, landowners received only a 30 percent deduction for six years.

Since 2006, the number of acres protected each year has increased from about 750,000 to more than 1 million.

“We think the potential is much greater if we can make this permanent,” said Russ Shay, director of public policy for the alliance.

Farmers and other landowners won’t get the enhanced deduction for 2010 because Congress didn’t extend it until December.

Farmers and landowners who donate land for conservation retain ownership of the property but use of the land is restricted under the terms of the easement.

To qualify for the federal tax deduction, easements must meet guidelines set by the Internal Revenue Service and their state. Rules vary from state to state. In some states, farmers can keep farming land placed in conservation, while in others the land may be protected as a wetland or wildlife habitat. All conservation easements, however, bar future construction.

Doug Caulkins, 70, of Grinnell, donated 240 acres with prairie, oak savannah and meadow for conservation and, with 10 other families, he placed another property under a conservation easement.

“This is the sort of thing we need to preserve for Iowa’s heritage in the future,” Caulkins said. “Less than 1 percent of Iowa land is in the prairie or woods that were previously here. When you get that close to the margin, you need to provide an incentive to people to save those parts of the ecosystem.”