No. 5 Article 4/May 4, 2012

Sizing Up the Wheat Crop

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

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, with by far the earliest onset of heading we have seen: 8 percent of the crop was headed by April 8, and by April 22, the date when we typically see the start of heading, 55 percent was headed. By the end of April, 80 percent was headed, compared with an average of 6 percent by this date over the past five years.

Is such rapid development, including early heading, a good thing? We don't have much to go on since we have never seen this before. But it is likely that the unusually high temperatures and rapid progress through vegetative stages in March meant having leaves a little thinner than usual. This may explain why leaf color has stayed a little paler than we're used to, and why there was some leaf damage, or at least loss of color, from the frost events the second week of April.

Of perhaps greater concern was the effect of the low temperatures on heads, either those emerging at the time of the frost or those "in boot" (inside the sheath of the uppermost leaf, soon to emerge). It helps that heads are held upright and radiate little to the sky on cold nights. But the flowers of developing heads can be damaged by cold, and in serious cases the heads can be sterile, with no grain forming. This is very rare in Illinois, and while so far most reports indicate normal grain development, it would be a good idea to look closely at heads two weeks or so after head emergence to make sure grains are forming. If there is any head sterility, it would be expected in areas where the temperatures dropped into the mid-20s when the heads were in boot.

It is possible that the early warm temperatures and rapid plant development may also have limited head size to some extent. Head numbers appear to be good in most fields, but there have been a few reports that grains per head may fall below what may be needed for high yields. By a week or 10 days after flowering (after we see anthers exerted from heads), kernels per head can be counted. This, along with head counts, can give a preliminary estimate of yield. 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 per acre.

With early heading we would normally expect favorable (cooler) temperatures during grain fill, which might increase kernel size. But the main factor to watch over the coming weeks is how well leaf color and leaf area hold up. While there has been some rain during flowering in some of the crop over the past week, the dry weather during much of April might have limited inoculum development, and we're optimistic that Fusarium head blight (scab) may not be a serious issue in most areas.

With grain fill now half-completed in the earliest-headed fields, leaves need remain green for only a few more weeks to complete grain fill. In the case of damage from hail, 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. But 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; early harvest may expand the double-crop area into central and even northern Illinois.--Emerson Nafziger

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