The heat wave that lit up a vast swathe of the mid-west could not have come at a worse time for this year's corn.

Or delivered more of a shock to farmers. All signs had been pointing to a bumper crop this year.

A mild winter, a balmy spring – most farmers elected to plant some of their fields a few weeks earlier than usual. Mike Buis, who farms in west-central Indiana, put some of his corn in on 9 April.

Some farmers didn't even hold on that long, planting as early as March.

By 1 June, the early corn was shoulder high.

But corn, it turns out, is a sensitive creatures. June's combination of extreme heat and long dry spell proved a deadly combination.

For the late planting, the extreme heat made it too hot to pollinate. "They got zapped big time," Buis said.

Corn goes sterile if the temperature stays above 95 degrees for several days running, he said.

But even the corn planted earlier never had a chance, because of the prolonged drought. Buis whips out his smart phone to show pictures of cracked, dry earth. On some rows of corn, there are traces of fertiliser still on the ground; there has not been enough water for its absorbtion.

Some stalks never produced an ear of corn. But even those stalks that were pollinated before the heat wave are in poor shape. The hot temperatures sent feeding systems into shutdown. Some stalks produced tiny nubbins that stopped growing. Others produced full-sized ears of corn, but with scattered kernels that are failing to develop.

"It it had been dry and stayed below 95 it would have been half as bad," he said. "But we got it coming and going."