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5 questions about corn

1. How serious is the threat of herbicide-resistant weeds?

Dead-drop serious. “Two years ago, I said we were on the edge of the precipice (regarding resistance),” says Mike Owen, Iowa State University (ISU) Extension weeds specialist. “Now, we have pretty much stepped off the edge.”

Three years ago, one third of Iowa soybean fields surveyed had glyphosate-resistant waterhemp populations, Owen says. He believes it's worse in 2012. By now, those fields have been rotated with corn, making it a corn problem, too.

More disconcerting is multiple-mode resistance. In Illinois, scientists have identified a waterhemp biotype that resists triazines, ALS and PPO inhibitors, and glyphosate. It's expected that scientists will find a waterhemp biotype this year or in 2013 that resists these modes of action and HPPD inhibitors, says Pat Tranel, University of Illinois (U of I) weed scientist.

What to do? Rotating modes of action is a good step. Applying them at different times, such as a preemergence application followed by a postemergence one, is better. Ditto for planting a competitive crop stand. You may also want to consider cultivation on thickly infested areas, says Owen.

“Each tool may be individually weak, but many tools working together can have a strong cumulative effect,” adds Matt Liebman, an ISU agronomist.

2. How much can a hot day hurt yields?

By itself, very little. “A 100° day doesn't have an adverse effect, assuming there is ample moisture in the soil,” says Elwynn Taylor, ISU Extension climatologist.

Strung together, though, hot days can clip yields. The fourth day of 94° temperatures and above accompanied by 50% humidity chops yield potential.

“Even if you have good moisture in the soil, you lose 1% of yield,” says Taylor. “The fifth day, you lose 2% more yield; the sixth day, you lose an additional 4%.”

Problems worsen if this occurs during silking. Temps at 94° or above – combined with 50% humidity on the fourth day – clip yields 3% rather than 1%.

3. What's happening when upper leaves stay green and lower leaves die?

Disease, insects, or nutrient deficiencies may first cross your mind. In many fields last year, this was the telltale sign of drought damage.

“Last year, people thought it looked like potassium or nitrogen deficiency, but it was water deficiency,” says Emerson Nafziger, U of I Extension agronomist.

4. Does test weight influence yield?

Very little. In fact, it's not uncommon for drought-stressed corn to have a high test weight per bushel, says Bob Nielsen, Purdue University Extension agronomist

Kernel weight is a much bigger yield-determining factor. It's one reason USDA yield forecasts increased over the course of the 2011 growing season.

“There were better measures on kernel weights as the season progressed,” says Roger Elmore, ISU Extension agronomist. “Test weight doesn't correlate to yield, but kernel weight does.”

Yield is determined when kernel weight is multiplied by kernel number, says Nafziger. “If test weight alone was important, popcorn with a test weight of 62 pounds would yield 400 bushels per acre,” he says.

5. Can corn yield well with a thin stand?

It's tough, particularly if it hasn't happen by early August.

“When you see all that sunlight hitting the ground, that sunlight is not doing any good,” says Nafziger. By then, a spotty stand translates into scrawny yields.

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