HARDIN COUNTY — The following article was written by Dr. Peter Thomison, OSU corn specialist (C.O.R.N. Newsletter, 2016-22).

Throughout northern Ohio, there are corn fields exhibiting drought and temperature stress. Leaf rolling is common and uneven, stunted corn growth is widespread. Most of these stressed corn fields are at a stage of development that is especially susceptible to drought and high temperature injury – pollination.

High temperature stress is usually associated with drought. Heat stress and drought intensify damage to corn and soybean but either may cause major crop injury alone. The recent high temperatures have generated questions about the impact of high temperatures on corn. How much heat stress can these crops withstand without incurring major yield losses even when adequate soil moisture is available? Corn originated as a tropical grass and can tolerate exposures to adverse temperatures as high as 112 degrees F for brief periods. Optimal daytime temperatures for corn typically range between 77 degrees F and 91 degrees. Growth decreases when temperatures exceed 95 degrees F.

University of Illinois agronomist Dr. Emerson Nafziger notes that “afternoon temperatures in the mid-90s are not a problem for corn….. if they have enough soil water available. …. plant temperatures have been raised to 110 or higher without doing direct damage to photosynthetic capacity. The level required to damage leaves depends on the temperature the leaf has experienced before, but it generally takes temperatures above 100 in field-grown plants”.

According to Iowa State University agronomists, high temperatures may have a double impact on corn “The first is the increase in rolling of corn leaves in response to moisture deficiency. By rule-of-thumb, the yield is diminished by 1 percent for every 12 hours of leaf rolling - except during the week of silking when the yield is cut 1 percent per 4 hours of leaf rolling. ….The second impact is less obvious initially. When soil moisture is sufficient, as it is for the most part this July, the crop does not have a measurable yield response to one day of temperatures between 93F to 98 F. However, the fourth consecutive day with a maximum temperature of 93F or above results in a 1 percent yield loss in addition to that computed from the leaf rolling. The fifth day there is an additional 2 percent loss; the sixth day an additional 4 percent loss. Data are not sufficient to make generalizations for a heat wave of more than six days, however firing of leaves then becomes likely and very large yield losses are incurred. Generally a six-day heat wave at silking time is sufficient to assure a yield not to exceed trend.” Some corn agronomists believe newer corn hybrids may have more tolerance to heat and drought than the hybrids did when this rule of thumb was originated.

Are corn hybrids that show greater leaf rolling more susceptible to drought injury than other less affected hybrids? Not necessarily. In a recent OSU evaluation comparing two drought tolerant and two drought sensitive hybrids, differences in leaf rolling were not readily evident. However, they were observed at Hoytville, in 2012, a site-year that experienced severe drought stress. Leaf rolling ratings for the drought tolerant hybrids were lower or about the same as the drought sensitive hybrids. However, other types of drought tolerant hybrids may express different leaf rolling responses.

A Putnam County Ag Tour is going to be hosted by the Putnam County Soil & Water Conservation District on Saturday, August 13 starting at 7:30 a.m. with breakfast at the OSU Extension office. At 8:30 a.m., the tour will start at the Soil Health Agricultural Research Plot (SHARP) site across from the landfill with Jim Hoorman talking about the Soil Health site and John Bormuth from Legacy talking about the Nutrient Toolbar. At 10 a.m., participants will be taken to see a Two-Stage Ditch at Bill Oedy’s farm and Albert Maag, PSWCD and Kayla Miller, OSU will discuss water monitoring results. At 11 a.m., Vennekotter’s Swine operation will be visited with presentations by Curt Tobe, PSWCD on Senate Bill 1, Brian Schroeder, Cooper Farms, and Glen Arnold, OSU on Manure Management equipment. The final stop will be at the Ellerbrock and Welch Hops site to discuss the Ohio HOPS Union. Speakers will be Scott Welch and Brad Ellerbrock, owners and operators. The Hops will be harvested in late July and early August. The tour will return to the Ag Center at 12:30. Participants will also see the I-9 bridge and flood mitigation and Blanchard River logjam removal. To reserve your seat, register online at https://www.surveymonkey.com/PCAgTour2016 .