August 22, 2013

In contrast to most Nebraska summers, the last few weeks have been quite cool and comfortable, leading many to ask about potential impact on the corn crop.

Concerns for Late Planted Corn. One of the largest concerns is the impact on some of the later planted corn. Research has indicated that the number of growing degree days (GDD) needed for corn hybrids to reach various growth stages, including maturity, is fairly uniform across environments. Most corn hybrids grown in Nebraska mature between 2400 and 2700 GDDs. Later planted corn seems to “make up time,” most likely because of warmer temperatures (meaning faster growth rates) during early vegetative growth stages. The same hybrid, planted two weeks later, tends to tassel and silk about one week later. Work in Indiana and Ohio has shown that for each day after May 1 that corn was planted, it required about 6.5 GDD less to reach maturity. This means that a “2700 GDD hybrid” planted May 20 would require 2700 - (20 x 6.5) = 2570 GDD. That’s the good news. However, in this situation, there will be relatively fewer days for grain fill before average frost date, and grain filling will occur during shorter days (fewer hours of sunlight/photosynthesis). This is part of the reason for expecting lower yields from later planting dates.

Cool Conditions Could Boost Yield if Frost is Late. Cooler weather has other impacts on the crop. Weather during pollination was significantly more favorable than in 2011 and 2012. Seed sets are near optimum in fields where water stress was not severe. We have lots of kernels per ear and per acre available for grain filling. Cool weather during grain filling slows down development, but respiration slows more than photosynthesis. While somewhat less sugar is made per day in cool environments, less is lost to respiration, and the grain filling period is extended. This can allow the plants to store relatively more sugar, resulting in heavier kernels and higher yields, especially compared to seasons with high day and night temperatures. Areas of the world consistently having the highest yields tend to be high elevation (providing high light intensity). They have warm (not hot) days and cool nights (which reduces respiration). Daytime temperatures above 86-87°F do not benefit yield and increase transpiration of water. A few years ago (2009) we had a very cool late July and August which caused lots of concern about potential frost damage to the slowly developing crop. These conditions actually resulted in record yields (freezing temperatures came later than normal).

One can compute the GDDs from actual and theoretical temperatures. If a typical “hot” August day is 95°F, with a low of 72°F, we accumulate 29 GDD. [(86+72)/2 - 50 = 29 GDD. (In this equation, the high and low temperatures are added and divided by two to get an average temperature. The minimum base temperature used for corn growth (50°F) is then subtracted to get the total GDD for the day. Temperatures above 86 are usually ignored, as they don’t materially accelerate development.) On a cool day, for example with a high of 84°F and a low of 60°F, we accumulate [(84 + 60)/2]-50 = 22 GDD. If we have had 20 such days, we’d expect that we lost an average of (20 x 7) =140 GDD. This would be equal to five or six days of crop development. (Or, providing five or six days of extra grain filling period.)

Most years, the hybrids that producers select will reach physiological maturity 10 days or more ahead of average frost dates. Unless we have a frost significantly earlier than average, the cooler weather will have little impact on corn development, other than increasing yields for corn with “normal” planting dates. For later planted corn, the cooler weather may carry some increased frost/freeze risk.

Other Impacts

Potential Disease Threats. Another impact of the cooler weather is the tendency to have more dew, and longer periods of free moisture on the corn leaves in the morning. This increases the ability of some leaf disease pathogens to survive and infect corn leaves. The cooler temperatures and higher humidity favor development of some pathogens, particularly common rust and gray leaf spot. Southern rust also likes free moisture and humidity, but prefers warmer weather. Leaf disease leads to a progression of damage: death of the leaf tissue, resulting in less photosynthetic area and reduced sugar production, leading to less grain fill, which in turn leads to more stalk cannibalization, and then more stalk rots and lodging. (See the August 15 CropWatch article on scouting and management.)

Grain Fill. The National Weather Service forecast for the rest of August is for normal to above normal temperatures and below normal rainfall. Currently, significant areas of Nebraska have less than adequate soil moisture so continued warm, dry conditions will stress dryland corn during grain fill. Heat stress during grain fill tends to increase water use, transpiration and evaporation. Regardless of available moisture, heat also will result in accelerated development. The later planted fields may “hurry” through grain fill, and may be more impacted from heat stress than fields planted at “normal” planting dates.

Harvest Moisture. Both irrigated and dryland corn are fairly variable in development stages, although most of it is between R3 (milk stage) to early R5 (early dent). The range of planting dates this spring, noted in previous CropWatch articles, led to variation in development stages, resulting in a range of weather impacts on corn. Heat units received after physiological maturity tend to dry the corn crop in the field, and this (along with lower relative humidity, compared to big corn growing states to the east) is often an advantage for Nebraska producers. It is not improbable that the later planted fields will tend to have higher harvest moistures.

Yogi Berra said, “It ain’t over ‘til it’s over.” It is too soon to project yields, but models seem to indicate we are in a reasonably good position for this date, especially on the irrigated corn crop. It is too soon to worry about early frosts, and, if forecast trends hold, we may be in position for excellent yields.

Tom Hoegemeyer
Professor of Practice, Department of Agronomy and Horticulture