Sizing Up the Wheat Crop

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Sizing Up the Wheat Crop

04 May 2012

US - In at least some places in Illinois, the wheat crop has been through heat, drought, floods, frost, and hail, all within the past six weeks. While the crop-condition ratings have been high all spring - 80 per cent of the crop was rated as good or excellent at the end of April - many people wonder if the crop can produce high yields following such unusual spring weather, including University of Illinois crop sciences professor Emerson Nafziger.

On the positive side, the crop got off to a good start last fall, and with the mild and relatively dry winter, it came into the spring in great shape. In March, record-high temperatures drove very rapid development and, by far, the earliest onset of heading ever seen.

"Eight per cent of the crop was headed by April 8. By April 22, the date when we typically see the start of heading, 55 per cent of the crop was headed. By the end of April, 80 per cent of the crop was headed, compared to an average of only 6 per cent headed by this date over the past five years," said Professor Nafziger.

Is such rapid development, including early heading, a good thing?

"We don't have much to go on because we have never seen this before," he said, "but it is likely that the unusually high temperatures and rapid progress through vegetative stages in March may have reduced plant weight and leaf thickness by a little. This may explain why leaf colour has stayed paler than we're used to, and why there was some leaf damage, or at least loss of colour, from the frost events of the second week of April."

Of perhaps greater concern was the effect of the low temperatures on heads that were either emerging at the time of the frost or that were "in boot" - that is, inside the sheath of the uppermost leaf, soon to emerge.

It helps that heads are held upright and thus radiate little to the sky on cold nights. However, cold can damage the flowers of developing heads. In serious cases, the heads can be sterile with no grain forming. Professor Nafziger notes that this is very rare in Illinois.

While most reports so far are that grain is developing normally, it would be a good idea to look closely at heads approximately two weeks after head emergence to make sure that grains are forming.

"If there is any head sterility, we would expect it in areas where the temperatures dropped into the mid-20s when the heads were in boot," Professor Nafziger said.

It is possible that the warm temperatures early in the year and rapid plant development may also have limited head size. Professor Nafziger notes that the head numbers appear to be good in most fields, "but there have been a few reports that the number of grains per head may not be as high as we'd like."

"By a week to 10 days after flowering - that is, after we see anthers outside the heads - we can count the kernels per head. This, along with head counts, can give us a preliminary yield estimate," explained Professor Nafziger. At 14,500 kernels per pound of grain (a conservative kernel size), one head per square foot with 20 kernels will translate into about one bushel of yield per acre.

With early heading, favorable (cooler) temperatures during grain fill might increase kernel size. The main thing to watch in the coming weeks is how well leaf colour and leaf area hold up.

With grain fill now half-completed in the earliest-headed fields, leaves need to remain green for only a few more weeks to complete grain fill. In cases of leaf damage from hail or insects, the potential to fill grain will depend on how much green tissue, including heads and stems, remains active until maturity.

The early heading and recent cool weather may extend the "six weeks from heading to harvest" rule of thumb by a week or so, especially for fields that headed by mid-April. "We expect fields that headed that early to be at, or close to, combine-ripe by the end of May. This should provide a boost for double-crop soybeans this year," said Professor Nafziger.

He also expects that early harvest may expand the double-crop area into central and even northern Illinois this year.

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