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July 24, 2002

Indiana crops stressed out by infrequent, inadequate rain

WEST LAFAYETTE, Ind. – Showers and thunderstorms that swept across Indiana late Monday (7/22) and early Tuesday (7/23) gave crops a much-needed boost, but corn and soybeans will need more of the same to stand a fighting chance at respectable yields this fall, said Purdue University agronomists.

Crops are exhibiting varying degrees of stress connected to weather patterns that dumped too much rain on the Hoosier state during spring planting, then barely a drop of moisture combined with excessive heat this summer.

Indiana corn is all over the board in appearance, height and health, said Bob Nielsen, Purdue Extension corn specialist.

"The most certain assessment we can make about the crop this year is that it's extremely variable," Nielsen said. "You can find areas of the state where corn looks awfully good – the color is good, and the leaves are not rolling up very much at all on some of these hot days. Then you can drive not very many miles away and find an area that's not received the rains, or maybe some other factors are playing a role, and you've got crops that look real tough."

Corn condition declined statewide this past week, according to the Indiana Agricultural Statistics Service (IASS), based at Purdue. In the week ending Sunday (7/21) 36 percent of the corn crop was rated "good" to "excellent," down from 48 percent the previous week and 72 percent during the same week in 2001, the IASS reported. Corn rated "poor" to "very poor" stood at 27 percent.

More than two-thirds of the state corn crop had not silked by the beginning of this week, the IASS reported. A year ago at this time, 81 percent of the crop had silked.

"Because so much of the crop was planted late this year, we're obviously not looking at record yields," Nielsen said. "I think the best we were hoping for would have been average yields, but that would have required perfect conditions after that late planting, which we really haven't had. The average, or trend, corn yield for 2002, based on historical yields, would be 141 bushels per acre.

"The one determining factor at the moment is that a lot of the state's corn crop is just now coming into pollination. So the weather over this next three weeks will be very important in determining how successfully that pollination occurs, and what kind of kernel set we get on those developing ears."

Cooler temperatures that moved in with this week's rain also should provide relief to crops, Nielsen said.

Farmers in Indiana and other Eastern Corn Belt states planted late because soils were too soggy to support equipment during the traditional late April/early May planting period. Under pressure to get the crop in, many farmers tilled fields prior to planting that were wetter than desired, creating high levels of soil compaction. Planting followed in fields still excessively moist, leading to further compaction.

As a result, crop development is stunted in many fields because layers of compacted soil have limited root growth, Nielsen said.

"Those are the kinds of situations where drought stress is typically the worst," he said. "There's a lot of compacted soil out there, from tillage operations, from tire tracks and from the planting operation itself. In the worst of these fields the net result has been a root system that's been very limited on how deep it can grow through those compacted layers."

Typically, corn is most vulnerable to moisture deficiency in July, said Tony Vyn, a Purdue cropping systems specialist. The crop needs about an inch of rain per week.

"This is the most critical time for corn," Vyn said. "Generally speaking, we define it as a period from 10 days prior to silking to the two- to three-week period after silking. That's also the time that corn requires its maximum rate of water uptake. At the moment, corn is utilizing about 0.2 to 0.3 inches of water per day." The critical period is 1-2 weeks later this year for late-planted, underdeveloped corn.

Where rainfall has been spotty, corn plants have only one place to go to draw moisture, Vyn said. "They're getting it entirely from subsoil reserves, but only to the extent that roots are active in those zones with plant-available water, and only to the extent that subsoil reserves are sufficient."

Drought-related stress is hurting corn in another way. Plant-destroying insects are feeding both below and above ground, Vyn said.

"This year there were problems with corn rootworm feeding on the roots themselves, and now there are rather significant numbers of rootworm beetles feeding on the silks," he said. "That raises some concern over the effectiveness of pollination."

Bottom line: Lower yields.

Corn stress manifests itself in many ways. Vyn and Nielsen listed the following telltale signs:

• Leaf rolling – May begin as minor curling during the heat of the day and progress to more severe rolling at all hours.

• Unusual color – Leaves display a grayish cast. A dead leaf will appear as a bleached-out straw color. Lower leaves can take on a yellowish hue if experiencing nutrient deficiency. Yellowing along leaf edges usually is symptomatic of potassium deficiency; yellowing in the center of a lower leaf often indicates nitrogen deficiency. "Lower leaves may show these symptoms even when adequate levels of fertilizer have been applied, because availability to corn roots has been compromised by dry soil conditions," Vyn said.

• Abnormal growth pattern – Tasseling and silk emergence out of synchrony.

• Shallow roots.

Soybeans also are suffering but are more forgiving of July drought.

"Beans have a much better ability to tolerate stress at this point in July," Vyn said. "The bulk of yield performance for soybeans is typically determined by moisture availability and environmental conditions in August. That's the time when we have pod fill. What makes or breaks soybeans is not the weather in July, it's the weather that occurs in August."

Soybeans are shorter than usual for this time of year, but Vyn said farmers should not be overly concerned.

The IASS rated Indiana's soybean crop 39 percent "good" to "excellent," down from 51 percent the week of July 7-14. Twenty-four percent of the crop was rated "poor" to "very poor." Only 37 percent of soybean acreage is blooming, compared to 77 percent a year ago. Nine percent of the crop is setting pods, well off the 20 percent average for mid-July.

More information about crop stress and related topics is available at Nielsen's Chat 'n Chew Café; Web site; the Purdue Agronomic Drought Stress Information Web site; and the July 19 issue of Purdue's Pest & Crop Newsletter

Writer: Steve Leer, (765) 494-8415, sleer@purdue.edu

Sources: Bob Nielsen, (765) 494-4802, rnielsen@purdue.edu

Tony Vyn, (765) 496-3757, tvyn@purdue.edu

Ag Communications: (765) 494-2722; Beth Forbes, bforbes@aes.purdue.edu; http://www.agriculture.purdue.edu/AgComm/public/agnews/

Related Web sites:
Purdue University Department of Agronomy Extension page

Purdue News Service: (765) 494-2096; purduenews@purdue.edu


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