The USDA analysis projected the impact on net farm income would be less than a 1pc fall in the short-term and that in the "short run, agricultural offset markets may cover these costs."

(The "offset markets" referred to income some farmers could make by the sale of carbon credits.)

Over the medium - and long-term the USDA study predicted "costs to agriculture to rise but remain modest (3.5pc and 7.2pc falls in net farm income, respectively)."

But it said "benefits to agriculture from an offsets market rise over time and will likely overtake costs in the medium and long term."

The report's executive summary concluded that under the cap and trade scenario in the House bill "the agricultural sector will have modest costs in the short-term and net benefits – perhaps significant net benefits – over the long-term."

But in his testimony, American Farm Bureau President Bob Stallman pointed out, "even with a robust agricultural offset program, the bill still does not make economic sense for producers because a number of sectors will be not able to participate."

The ability to benefit from an offset program would "depend to a great degree on where the producer is located, what he or she grows and if his or her business can take advantage of the program," Stallman noted.

The USDA's analysis, which was based in part on modelling conducted earlier by the Environmental Protection Agency (EPA), included benefits landowners would gain from planting trees for carbon sequestration on pasturelands and farmland.

Nebraska Senator Mike Johanns demanded to know from Agriculture Secretary Tom Vilsack and EPA Administrator Lisa Jackson how many crop or pasture land acres would go out of production as a result a cap and trade program aimed at reducing greenhouse gas emissions.

"The whole purpose of this hearing is to be honest," Johanns, a former Agriculture Secretary, pointed out.

But neither Obama Administration official could answer Johanns question. "We do not have a number," Jackson said.

Johanns concluded, "Unless you can quantify this, you can't sell this plan. It becomes a hope and prayer plan for agriculture."