Hailed as a way to funnel "green" payments to growers, the Conservation Security Program has faced a difficult birth, although it was part of the 2002 farm subsidy law. Some lawmakers have tried to limit it to a one-state pilot this year rather than a nationwide program.

Iowa Sen. Tom Harkin, author of CSP, said a spending cap "threatens to strangle this program ... before it even gets started." Harkin is Democratic leader on the Senate Agriculture Committee.

Cost of the program initially was estimated at $2 billion through 2011, most coming after the 2007 expiration of the farm law. But more recent estimates put the cost at $9 billion, if CSP is considered an entitlement, like crop subsidies, and sends payment to any farmer who qualifies.

A cap would hold spending at the initially estimated level, but also could constrain the size of the program.

"The farm bill was written carefully not to place any hard cap or arbitrary limit on the CSP - just as there are none on USDA's commodity programs," Harkin said in a statement.

"The harsh limit proposed in the budget will lock many farmers out of CSP and severely threaten the conservation and environmental benefits President Bush and Secretary (Ann) Veneman have been taking credit for."

CSP backers said the program would offer three levels of payments to participants, based on how much work they did to make soil, water and wildlife preservation a part of their operations. Maximum payment would be $50,000 a year.

Veneman said the budget included a record $3.9 billion for land, water and wildlife conservation, carrying out an 80 percent increase mandated by law for conservation.

"This is implementation of the farm bill," Veneman said.

The Sustainable Agriculture Coalition said a cap on CSP would "radically restructure" the most innovative program in the 2002 farm law, signed only nine months ago.

Conservation groups said the budget would not enroll as much land in the Wetlands Reserve as allowed by law - 178,000 acres instead of the 250,000 acres possible. The National Wildlife Federation called it "a drastic cut in one of the most successful tools" to preserve and restore wetlands.