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Ohio State University Extension


C.O.R.N. Newsletter 2005-25

Dates Covered: 
August 8, 2005 - August 16, 2005
Harold Watters

Impact of Warm Night Temperatures on Corn Grain Yields

Authors: Peter Thomison

When we experience a hot, dry growing season like that of 2005, we often focus our attention on the appearance of stressed plants (leaf rolling, leaf firing, poor canopy closure), and relate stress symptoms or injury to lower yield potential. However warm night temperatures, which are often associated with drought, also adversely affect yield potential. High night temperatures (in the 70s or 80s) result in wasteful respiration and a lower amount of dry matter accumulation in plants. With high night temperatures more of the sugars produced by photosynthesis during the day are lost; less is available to fill developing kernels grain, thereby lowering potential grain yield. Past research indicates that corn grown at night temperatures in the mid 60s outyields corn grown at temperatures in the mid 80s. Corn yields are often higher with irrigation in western states, which have low humidity and limited rainfall. While these areas are characterized by hot sunny days, night temperatures are often cooler than in the Eastern Corn Belt. Low night temperatures account in part for our record high corn yields in 2004. During most of the 2004 growing season, temperatures were below normal. From late June through most of August, a period of time that included most of the grain fill period, weekly temperatures were cooler than normal - as much as 4 to 7 degrees below normal in August. Cool night temperatures in 2004 reduced respiration losses during grain fill. The absence of moisture stress was especially important during grain filling. In parts of the Ohio where rainfall was below average during grain fill in July and August, cooler than average temperatures minimized moisture stress. The high corn yields of 2003 were also associated with cooler than normal night temperatures during the grain fill period.

Impact of drought on grain composition
Drier and warmer than normal conditions during late vegetative development and grain fill also influence grain composition. When drought stress conditions severely depress corn yields, protein deposited early in the kernels is less diluted by starch deposited later during grain fill; consequently grain protein concentration increases. Conversely, optimum soil moisture, whether from rain or irrigation, promotes filling of kernels with starch and reduces protein content.

Assessing Potential For Nitrate Problems in Corn

Authors: Peter Thomison

Hot dry weather across Ohio has raised questions concerning the potential for toxic levels of nitrates in corn harvested for silage. Nitrates absorbed from the soil by plant roots are normally incorporated into plant tissue as amino acids, proteins and other nitrogenous compounds. Thus, the concentration of nitrate in the plant is usually low. The primary site for converting nitrates to these products is in growing green leaves. Under unfavorable growing conditions, especially drought, this conversion process is retarded, causing nitrate to accumulate in the stalks, stems and other conductive tissue. The highest concentration of nitrates is in the lower part of the stalk or stem. For example, the bulk of the nitrate in a drought-stricken corn plant can be found in the bottom third of the stalk. If moisture conditions improve, the conversion process accelerates and within a few days nitrate levels in the plant returns too normal.

The highest levels of nitrate accumulate when drought occurs during a period of heavy nitrate uptake by the corn plant. A drought during or immediately after pollination is often associated with the highest accumulations of nitrates. Extended drought prior to pollination is not necessarily a prelude to high accumulations of nitrate. The resumption of normal plant growth from a heavy rainfall will reduce nitrate accumulation in corn plants, and harvest should be delayed for at least 1 to 2 weeks after the rainfall. Not all drought conditions cause high nitrate levels in plant. If the supply of soil nitrates is in the dry soil surface, plant roots will not absorb nitrates. Some soil moisture is necessary for absorption and accumulation of the nitrates.

If growers want to salvage part of their drought damaged corn crop as silage, it's best to delay harvesting to maximize grain filling, if ears have formed. Even though leaves may be dying, the stalk and ear often have enough extra water for good fill. Kernels will continue to fill and the increases in dry matter will more than compensate for leaf loss unless plants are actually dying or dead. Moreover if nitrate levels are high or questionable, they will decrease as plant get older and nitrates are converted to proteins in the ear.

For information on testing and feeding corn with varying nitrate-nitrogen levels, check out the following

Drought-Stressed Corn For Silage - Bill Weiss available on-line at

Nitrates in Dairy Rations - Maurice Eastridge and Bill Weiss, Ohio State University Fact Sheet AS-0003-99 available on-line at

Continue Scouting for Soybean Aphids

Authors: Ron Hammond, Bruce Eisley

A number of questions have come up on how long growers should continue monitoring for the soybean aphid. We believe that soybeans should be protected at least through the early R6 growth stage which is a full seed. Past studies from other states and Canada suggest that a spray even at that stage can return some yield to the grower. However, threshold considerations do change once you get to the late R5, beginning seed fill stage, and early R6 stage. First, you should definitely have an increasing aphid population. At this stage and time of the summer, aphid populations are often holding steady, or perhaps going down. You need to make sure that the population in your field is still going up before a treatment would be required. Also, a treatment is usually suggested at these late stages only if the plants are under other stresses such as drought stress. At the R6 stage, if your soybeans are growing well and environmental conditions are not putting them under stress, a treatment might not be necessary. As the plants start entering the R7, or plant maturity stage, treatment is not recommended. Thus, because we are only in the second week of August, weekly sampling is still advised.

Retreatment of Fields for Soybean Aphids

Authors: Ron Hammond, Bruce Eisley

Because of having to spray in July and extremely heavy populations of aphids, many growers are concerned with the need to retreat for aphids. Because of the aphid’s potential to rebound to large numbers following an initial treatment, this is happening in numerous fields across northern OH. There are many reasons for this increase in aphids, including not having killed all the aphids to begin with (which few insecticides can do), poor insecticide application techniques (too low of pressure or volume, too large droplet size, etc.) that did not penetrate the plant canopy well enough, the aphid’s tremendous reproductive ability (remembering that they give live birth of all females, and can double in population size every few days), and the killing off of predators that help to keep their populations down.

What growers should be aware of is that the aphids will not always build back to high numbers. The only way to determine if aphid populations are again on the increase is to continue weekly sampling. As long as the soybeans are in a susceptible growth stage, at least through the early R6 stage (see other soybean aphid article), you would still want to use the 250 aphid per plant threshold and an INCREASING population. And the last point is the critical one; the aphid population should be on the increase. Our experience in 2003 suggests that while some of the insecticides allowed many aphids to remain alive, the population did not increase following spraying. The only way to determine whether you have an increasing population is to count the aphid numbers for at least 2 or 3 weeks. Coming back into your field once after spraying will not allow you to make this determination. You need to take at least two counts to determine if the aphids are coming back.

Ohio No-Till Council Field Day – August 18

Authors: Harold Watters

The Ohio No-Till Council will hold a field day event August 18th near Minster Ohio. Registration and exhibits will open from 1 PM with educational programs including no-till equipment operation and controlled traffic/ auto steer demonstrations starting at 2PM. There will be an evening meal, demonstrations and open exhibits until dusk.

There is a $10 early registration fee or pay $15 on site. The site is on the Bill Lehmkuhl farm at 6630 SR 119, located 4 miles east of Minster or 7 miles west of I-75 on SR 119. You may contact Bill at 419 628-4167 for more information.

Central Ohio Forage and Grassland Expo August 25

Authors: Howard Siegrist

An extensive aspect of forage production, from management to nutrition to profit, will be the focus of the Ohio State University Extension-sponsored Central Ohio Forage and Grassland Expo. The event will be held August 25 from 1:3 p.m. to 7 p.m. at Sorg Dairy Farm in Brownsville, Ohio. On-site registration is $10, which covers field day expenses and/or meal cost. Those who make advance reservations will receive a copy of Ohio State Extension's Forage Management Pocket Guide.

The Equipment demonstrations including: mowers, rakes, tedders, and balers will begin at 1:30 p.m. Other program items include: High return alfalfa production, nutritional advantages of forages, mycotoxins and mold concerns, stockpiling forages, making rotation of pastures work and grass identification, and utilizing feeding pads. OSU experts, Purdue University specialists, and industry representatives will be resourcing the event. Certified Crop Advisor CEUs of 4.0 units of CM and 0.5 unit of S&W will be offered.

The Central Ohio Forage and Grassland Expo is sponsored by Ohio State Extension's Licking and Perry County offices, in cooperation with the Licking and Perry Counties Soil and Water Conservations Districts. For more information or to register, contact the Perry County Extension Office at (740) 743-1602 or e-mail at For additional information, contact the Licking County Extension Office at (740) 670-5315.

Archive Issue Contributors: 

Anne Dorrance, Pat Lipps, Dennis Mills and Pierce Paul (Plant Pathology), Ron Hammond and Bruce Eisley (Entomology), Peter Thomison (Corn Production) and Jeff Stachler (Weed Science). Extension Agents: Dusty Sonnenburg (Henry), Keith Diedrick (Wayne), Howard Seigrist (Licking), Bruce Clevenger (Defiance), Jim Skeeles (Lorain), Glenn Arnold (Putman), Mark Koenig (Sandusky), Greg LaBarge (Fulton), Steve Bartels (Butler), Gary Wilson (Hancock), Roger Bender (Shelby) and Harold Watters (Champaign).

Crop Observation and Recommendation Network

C.O.R.N. Newsletter is a summary of crop observations, related information, and appropriate recommendations for Ohio crop producers and industry. C.O.R.N. Newsletter is produced by the Ohio State University Extension Agronomy Team, state specialists at The Ohio State University and the Ohio Agricultural Research and Development Center (OARDC). C.O.R.N. Newsletter questions are directed to Extension and OARDC state specialists and associates at Ohio State.