University of Illinois

No. 14/June 27, 1997

Corn Thrives in Warm Weather

Only 2 weeks ago, it seemed that warm weather would never arrive. But it did, with temperatures rising into the 80s and 90s by June 20. The corn crop responded as expected, with very rapid growth. Except in those fields affected by excessive moisture, the color of the crop is excellent. Earlier this week (June 23 and 24), corn planted early ranged in size from V6 (six leaf collars visible) and about 18 inches tall at DeKalb to V1213 and more than 48 inches tall near Belleville. As reported earlier, stands are generally excellent.

During the cool weather in May, we often had to use the "low cutoff temperature"--calling the low daily temperature 50 degrees F when it was actually less than that. When the daytime high temperature is above 86 degrees F (30 degrees Celsius), we similarly use 86 degrees F as the high temperature. The point of these cutoff temperatures is that growth is already very slow at 50 degrees F, and so it doesn't slow much more at lower temperatures. On the high end, growth is very rapid at 86 degrees F, and it doesn't increase much more at even higher temperatures. Growing-degree days (GDD) are calculated as the average temperature for the day (high plus low divided by 2) minus 50.

A few other points about growing-degree days:

  1. About 65 GDD are needed to add a leaf to the corn plant. That means two to three leaves per week during very warm weather.

  2. With average high temperatures in the mid-80s in the midsummer, much of the difference in GDD accumulation from one day to another is from differences in minimum night temperature, rather than in maximum day temperature, which often is cut off at 86 degrees F.

  3. Medium-maturity hybrids for central Illinois require about 2,700 GDD from planting to maturity. Much of the crop was about 150 to 200 GDD behind average by the end of May. Although it might seem good if we could catch up on this GDD shortage, doing so would require higher-than-average night temperatures, which are often unfavorable for best growth and yield. It would be much better if the growing conditions in the last half of September allow the crop to fill grain during that period.

  4. Hybrids require slightly more than half of their seasonal GDD requirement to reach pollination. The GDD requirement to reach pollination, however, varies more with hybrid maturity than does the GDD requirement from pollination to maturity.

Given the generally favorable condition of the corn crop at this point, are there any concerns? Not many, but watch the following as summer weather unfolds:

  1. Deep green leaf color usually means that leaves are functioning at high rates of photosynthesis. Loss of green leaf color or loss of leaf area before grain is filled usually means reduced yields.

  2. Slight symptoms of drought stress have appeared in areas that haven't received much rain. Leaf curling is the most common drought symptom; and, the earlier in the day leaf curling occurs, the more serious that degree of stress.

  3. Although lack of adequate water has been considered to be most damaging at pollination time, leaf canopy deterioration during grainfill has probably been the major weather-related problem in recent years. Most stress-timing research was done years ago with older hybrids, and improved hybrids have greatly reduced the amount of barrenness under dry weather at pollination. Although this hypothesis needs testing, it may be that water shortages after pollination might cause as much damage as shortages before or during pollination.

  4. Full canopy cover, with very little sunlight striking the soil surface at noon during early grainfill, is essential for highest yields. Hybrids, plant populations, and row spacing all affect the canopy. Check the amount of sunlight getting through the canopy in early August as a way to assess management of each hybrid.

  5. Corn thrives on lots of sunlight, adequate soil moisture, and moderate--not hot--temperatures. In general, weather comfortable to people in August is also favorable for corn yield. The favorable years of 1992, 1994, and 1996 were all characterized by cooler-than-average August temperatures. The crop is off to a better start this year than in any of those years, so we should be in position for high yields if we get timely rains and good August and September weather.

Emerson Nafziger, Department of Crop Sciences, (217)333-4424