(Bonnie Berkowitz)

The historic drought of 2012 is intensifying in the most parched areas of the American heartland, roasting much of the corn and soybean crop, scorching the grasslands and pastures essential for cattle grazing, and threatening to send food prices surging in the United States and abroad.

More than a fifth of the contiguous United States rates as being in an “extreme” or “exceptional” drought, according to official statistics released Thursday. And it’s hot out there: Government ­meteorologists say this has been the hottest year on record in the lower 48 states, and there are still four more weeks of August. In the driest areas of the Great Plains and the Midwest, there are dismayingly few dark clouds on the horizon.

“The timing of this year’s heat and the real drying out that took place in June and July have been horrific for the corn and soybeans,” said Brad Rippey, a meteorologist at the U.S. Department of Agriculture.

This year was supposed to have a bumper crop of corn, with 96.4 million acres planted. But the new numbers from the National Drought Mitigation Center, based at the University of Nebraska at Lincoln, show the drought intensifying in the Corn Belt. Nearly 40 million acres of corn are baking in drought conditions rated extreme or worse.

The big drought of 2012 has been tough on farmers but even more brutal on ranchers. Most crop farmers have insurance to cushion the blow of a bad harvest, and the rise in prices is a windfall for those with something to harvest and sell.

The ranchers, however, have hungry cows and a serious quandary. The grass on rangelands is dying across the country, and the price of corn and hay for animal feed is soaring. House lawmakers scrambled Thursday to pass a $383 million drought-assistance bill that would give relief to ranchers and other livestock producers. The vote was 223 to 197.

A map of the drought shows sun-baked land stretching across the lower half of the country like a surgical mask. The northern tier of the country is fine, but the productive heartland of the Midwest, including corn states Indiana and Illinois, has seen its row crops shrivel. Among the places hit hardest and getting even drier are Nebraska, Kansas, Missouri, Oklahoma and Arkansas.

The geographical expanse of the drought has shrunk very slightly in the past week, thanks to rainfall in the East, but it remains a remarkably broad event, with more than three-fifths of the contiguous United States experiencing at least moderate drought, including the District and portions of Virginia and Maryland.

That’s the biggest, if not most severe, drought since 1956, and one that comes close to matching the sprawl of the 1934 drought at the desiccated worst of the Dust Bowl era.

Just three weeks ago, the portion of the lower 48 states receiving the two most serious drought designations stood at 11.6 percent. That area has now doubled to 22.3 percent. The jump in the past week from 20.6 percent represents an increase of about 32 million acres.

Rippey, the USDA meteorologist, did not choose to attribute this year’s drought to the long-term climate change that scientists blame in part on human activity — the link between specific extreme weather events and climate change remains the subject of research and debate — but he did say it’s shaping up as the worst drought since at least 1988. “It’s highly unusual, because obviously you have to go back 24 years to find even a ballpark precedent,” he said.

The Mid-Atlantic has been spared the worst, though the nation’s capital and its suburbs could use some rain.

For the first time in more than 140 years, the District has received below-average precipitation each month from January to July.

The drought-aid legislation passed by the House on Thursday would provide up to $100,000 for each of the hardest-hit livestock producers. But the Senate, which has already passed a five-year reauthorization of the broader farm bill, declined to take up the narrower House legislation. Both chambers are now on a five-week August recess.

At the beginning of the spring planting season, the USDA estimated a corn harvest of 166 bushels per acre. Economists estimate that is down to about 130 bushels per acre. The United States is the world’s largest exporter of corn, soybeans and wheat, and price spikes in the crop sector will ripple through markets globally.

The livestock industry has also faced stiff competition for corn from the ethanol industry because of the Environmental Protection Agency’s renewable fuel standard in the Energy Policy Act of 2005. The legislation requires that 15 billion gallons of domestic corn ethanol be blended into the nation’s fuel supply by 2022.

The target for this year is 13.2 billion gallons. On Monday, a group of 19 livestock producers filed a petition with the EPA requesting that the requirement be reduced to damp increasing corn prices.

Ranchers contend that they don’t have the safety nets that their colleagues in the crop sector depend on during hard times. The USDA had a “livestock indemnity program” that provided compensation for livestock deaths because of disaster, but that program expired in September.

Economist James Robb of the Denver-based Livestock Marketing Information Center said a rancher can normally turn a profit of about $180 per cow. This year, that’s going to be more like $80 a cow.

“With 30 million head of beef cattle in the U.S., a loss of over $100 per animal is huge,” Robb said.

One rancher feeling the heat is Bill Bunce, a superintendent of ranching operations at True Ranches, a Wyoming conglomeration of seven ranches, two farms and two feedlots that tend nearly 10,000 head of cattle. After enduring a winter with little snowfall and a dry spring, his land was hit by July wildfires that had spread from Colorado.

Bunce said the price of corn has gone up 50 percent since last spring; the price of hay in some areas has doubled. The grass and alfalfa hay mix that he buys for his range cattle costs $70 to $100 a ton a few years ago; now it costs about $200.

“Whether we lease new ground for cattle or buy enough feed to keep the herd at home, ranchers have to expect that next year’s sales are going to be a wash,” he said from his office in Casper, Wyo.

He will wean this year’s calves early and send them to the feedlot so their mothers won’t need to eat as much. He may slaughter or sell an additional 10 percent to 25 percent of his herd.

The consequences of leaving cows hungry are long-lived for ranchers. In stifling heat, bulls don’t have as much energy to mate, and hungry cows are less likely to ovulate.

When rangeland at home starts to dwindle, ranchers’ first option is to relocate the herd to greener pastures. Last year, ranchers in drought-blasted Texas could truck their cattle to grazing lands in different parts of the country.

But this year, Bunce said, there’s nothing but drought for 500 miles. There’s hardly anywhere for his cattle to go.