Thirsty crops

Pollination. It’s the week that can make or break your corn crop. A lot of corn this year reached that critical growth stage sooner than it usually does, thanks to being planted weeks earlier than usual. Is reaching pollination earlier than normal good or bad?

A study by the Climate Corp., an insurance company, indicates in Iowa and other western Corn Belt states the odds of beating a hot week for pollination are much greater if the corn pollinates before July 15-21, but in the eastern Corn Belt, the benefit is negligible.

However, this summer if weather predictions hold and temperatures are warmer than normal through July and into August, there may be little or no advantage for reaching pollination early. With so little moisture in reserve in the subsoil and hot, dry weather persisting, the corn plant has little to draw on. With more than half of Iowa short on moisture in the topsoil and subsoil in late June, regular rainfall throughout the rest of the 2012 growing season is needed.

Iowa State University Extension climatologist Elwynn Taylor said in late June that most of Iowa would need at least an inch of rain per week to make a decent yield. Corn planted May 1 will use about 1 inch of water during the last week of June. This increases to about 1.25 inches a week for early July and 1.5 inches a week for mid-July. About 1.25 inches a week is needed in late July and early August, 1 inch a week in mid-August, and 0.7 inch a week in early September, with a rapid decline in need for water through September.

What if it doesn’t rain?

If it doesn’t rain, crops have to rely on moisture in the subsoil. If the crop had 5 inches of water in the subsoil in late June to draw from without receiving rain, theoretically the crop would last another three or four weeks. But it doesn’t quite work that way. Corn under drought stress will roll its leaves to conserve water. This is good for overall survival, but leaf rolling can impact yield potential.

Every 12 hours of leaf roll probably reduces yield about 1%, maybe up to 2% as corn gets closer to tasseling. Toward tassel and through brown silk stage (just before and during pollination), yield loss can be anywhere from 5% to 10% for every 12 hours of leaf roll.

What does Taylor and the U.S. Drought Monitor say? In late June, Taylor thought dry, hot conditions would continue through most of July, but he expects a break in August with the occurrence of more normal weather. The National Drought Mitigation Center identifies areas of dryness and drought on a map at www.droughtmonitor.unl.edu/DM_midwest.htm.

What about soybeans? Drought stress for beans is a much more important factor during stages of reproductive growth than vegetative growth. “If we get rain later in the growing season, the chance of ending up with a reasonably good soybean yield still exists,” says Taylor. The August-term weather forecast looks a little more favorable for that to happen.

Especially when corn is silking and tasseling, it’s critical to get the needed rain. “But, if the weather’s hot and the wind is blowing, even if you get an inch of rain per week, it is used up pretty fast,” says Lee Woudstra, who has grown corn for many years with wife Mary near Hospers.

“This year our corn was planted early and has a good root system. But when there’s little or no moisture left in the subsoil, there’s nothing there for the roots to reach. We’ve really relied on that reserve moisture over the years here in northwest Iowa.”

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NICE EARS: Corn looked good last year in early August in this field belonging to Lee and Mary Woudstra in Sioux County. “We had rain and reserve soil moisture, too,” says Lee, shown checking how well the crop pollinated and filled out. In 2012 it may turn out to be a different story, as hot, dry conditions are cause for concern.

This article published in the July, 2012 edition of WALLACES FARMER.

All rights reserved. Copyright Farm Progress Cos. 2012.