The heat dome causing sweltering temps across most of America this week is also doing a number on your corn crop, says Iowa State meteorologist Elwynn Taylor.

Taylor, speaking at Purdue’s Top Farmer workshop yesterday, says corn loses 1% of yield after four consecutive days of temperatures over 90 degrees F – even if you have good soil moisture.

What’s worse, the heat is cooking pollen. “If it’s the week of silking, you’ll lose 3% each day,” he adds. 

One week of consecutive 90-plus days would add up to a 4% yield loss not counting pollinating time. After a week, leaves start to fire on corn, and bean plants start to abort pods.

Taylor (left) says Iowa State economist Bob Wisner is expecting a 153 bushel per average corn crop this year, with prices around $6.15 per bushel at harvest for December contract. His range is 148.4 bpa to 159 bpa.

“In a normal year, the yield would be just over 160 bpa, but because this year is influenced by La Nina, even though it has gone to neutral, Wisner is looking at a conservative 153 bushel per acre,” he says. “It will be hard for the nation to come in with a 160 bushel yield, even if everything turned out perfect from now forward.”

In Indiana, 62% of the acres were rated good to excellent last week. Those numbers are expected to fall across the corn belt due to heat.

If more than 50% of acres are in good or excellent condition in early July, you are on track for above trend line yields, says Taylor. But a lot can change - quickly - because it's a crucial time for corn plants. This is a late crop; nationwide, corn is usually half silked out by early July but this year it was only 10%.

La Nina impacts yields

With a guy like Taylor in the room, you just have to ask: is this crazy weather a sign of global warming?

“La Nina brought the extremes in the winter, and those tornadoes in odd places like Alabama,” he says. “La Nina causes higher and lower temps, all across the corn belt, compared to normal. It’s a concern. La Nina tends to give us a dry winter in plains and south, thus the fires in Arizona and wet conditions in Montana. It’s following the same exact pattern as what happened in 1955 and 1974.”

Taylor is worried that a recent 30-day SOI (Southern Oscillation Index) reading shows La Nina may make a reappearance. If that’s the case, there’s a chance for early frost this fall – not good considering how much of the crop went in late.

Another concern is that summer temps in 2010 were warmer than usual, divided right down the continental divide. “In history, every time this happens we have a big wheat crop and below trend yields in corn,” says Taylor.