In the lower two-thirds of Iowa this week, heat and dry weather stress caused corn leaves to curl. This “leaf rolling” is a survival mechanism for corn plants, notes Virgil Schmitt, Iowa State University Extension agronomist in southeast Iowa. Corn plants with curling or rolling leaves have been a common sight in many fields in southeast Iowa south of highway 92 for about the past two weeks. Farmers a little further north of highway 92 have also seen some leaf rolling.

CURLING CORN: Hot, dry weather caused corn leaves to curl in some Iowa fields the past week or so. It’s especially evident in southeast Iowa where it’s been driest, but you can see corn curling in central Iowa too. This corn was curling near Boone on June 23.

As a general rule, corn yield potential is reduced 1% for every 12 hours of leaf rolling except during pollination when it’s 1% every four hours. Schmitt says rainfall amounts in his region so far in June have varied from a half inch to 1.5 inches in the northern half, to more than 3 inches south of Interstate 80 to more than 5 inches in the far southwest corner of Muscatine County and the far northwest corner of Louisa County. Amounts diminished to less than a quarter inch in most of Lee County.

Hopefully, the dry areas will get timely rains for pollination

For most of southeast Iowa, he says another 1.5 to 2 inches is needed yet in June to fill the soil profile to help crops get through July and August, if normal rainfall occurs. However, the far southern counties will need more. “What we received this week helped,” says Schmitt. “Hopefully we will start picking up some timely rains as we approach tasseling and pollination.”

Almost the entire southeast quarter of Iowa is considered abnormally dry, according to the latest U.S. Drought Monitor map. All or portions of 35 counties are included, up from 22 the previous week. The northern third of Iowa has the most topsoil moisture with more than 90% rated adequate to surplus.

In central Iowa, most of the corn is well past V6 growth stage and has greened up nicely as of June 24. But the hot weather this past week or so has caused corn leaves to curl in some fields, in areas lacking rainfall.

What effect does heat wave having on corn yield potential?

The answer depends on the growth stage of the corn at the time of the heat, says Mark Johnson, ISU Extension agronomist in central Iowa. The number of kernel rows you eventually see around the ear is determined during V6 to V8 or V9 (leaf collar) stage. Number of kernels per row depends on pollination, how many ovules get pollinated.

For about 2 weeks after pollination, kernels can be aborted from the tip of the ear on back, says Johnson. Size and weight of kernels is determined from that point on to when the corn plant reaches physiological maturity, which is when the black-layer forms in the corn kernels, late in the growing season.

“Leaves curl during the heat of the day as a defensive mechanism, which minimizes loss of water through evapo-transporation,” says Johnson. “If leaves wrap severely and/or stay wrapped through the evening and are wrapped right away in the morning, there is mostly likely some yield loss occurring. From what I’ve seen, areas of fields with compacted soil fit the second scenario, while the bulk of the field, in many cases, fits the first scenario.”

Can your corn’s roots reach deep for plant-available water?

How much of any given field is stressed by the heat is largely dependent on the corn roots’ ability to reach plant-available water in soil. This in turn depends on soil texture, structure, percentage of soil organic matter, etc. “If there is no compaction in the root zone and there is good aggregation of soil particles, the plant will not reach the wilting point as early as in fields or parts of fields that have soil compaction and therefore less plant-available water,” says Johnson.

Keep in mind heat stress is one thing and moisture stress is another. “The combination is what really hurts once the ear has begun development, and especially in the reproductive stages, the R stages of plant growth,” he says.

A couple of rules of thumb for corn are:

1) Corn yield potential is reduced 1% for every 12 hours of leaf rolling except during pollination when it is 1% for every 4 hours of leaf rolling.

2) Four days at or above 93 degree temperatures equal 1% yield loss. The fifth day it becomes 2% and the sixth day it’s 4%. Since it is hard to separate moisture stress from heat stress, Johnson says he’s sure there is a lot of variation in these rules of thumb.

How does hot weather affect soybean yield potential?

For soybeans, the R1 growth stage has begun in many fields in central Iowa and most of the remaining fields will reach that stage by the end of this week, by June 24.

“Soybean plants begin their reproductive stages or R stages of growth while still in the vegetative or V stages,” Johnson notes. “Because soybean plants are so adaptable and put on more blossoms than will ever develop into pods, and they continue to put on more blossoms for a long time, soybean plants really aren’t affected much by high temperatures at this time in the growing season.”