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Issue No. 14, Article 6/June 26, 2009

Too Hot for Corn?

The corn crop in Illinois is finally mostly planted, though it's likely that some of the acres yet to be planted a week or two ago will not be planted to corn this year.

There have been several inquiries this week about hybrid maturity if corn must be planted this late for reasons such as having corn herbicide applied. The growing degree-days we can expect if summer temperatures are about average from July 1 to 50% chance of first frost are about 1,900, 2,100, and 2,450 for northern, central, and southern Illinois. Above-normal temperatures will increase these numbers by about 150 GDD. So even if we "credit" late-planted corn 300 GDD less than rated maturity due to reduced requirements, it will be very risky to plant hybrids that need more than 2,400 GDD in central Illinois. This would suggest that hybrids for planting now be rated at about 100 days RM or less in central Illinois, while those in southern Illinois could still be in the 105- to 108-day range. To be clear: it is very risky to plant this late regardless of hybrid or its maturity, and it's not realistic to expect yields above 100 bushels per acre for corn planted in Illinois in the last week of June.

One of the more common questions this week has been the effect of the high temperatures on corn. Daytime high temperatures have exceeded 90 °F for most of Illinois over most of the last 4 or 5 days. Low temperatures have also been high, reflecting the high dewpoints. Dewpoint, the temperature at which the air is saturated with water vapor (that is, at 100% relative humidity), is a measure of the water content of the air. Night temperature at this time of the year is typically close to the dewpoint, and a dewpoint in the low to mid-70s is quite high. In fact, the water content of air with a dewpoint of 75 degrees is some 60% higher than the water content of air with a dewpoint of 60 degrees.

We have often said in the past that high nighttime temperatures increase respiration rates, thereby reducing the amount of sugarproduced by photosynthesis during the dayavailable for growth processes in the plant. That is true regardless of the developmental stage of the crop, but the consequences of this differ depending on whether the crop is in a sensitive growth stage. Fortunately, corn plants during vegetative growth are less sensitive to temporary reductions in the sugar supply than are plants during pollination. The earliest-planted corn in Illinois is getting close to tasseling, however, and as pollination approaches, shortages of sugars usually have considerable negative effects on yield potential. For most of the Illinois crop, though, high night temperatures this week will not have much negative effect on yield potential. If warm nights persist, or if they return by the time of the peak pollination period (likely to be about the third week of July this year), yield potentials could drop.

What about daytime highs in the 90s? As long as the crop has enough water to keep the stomata open to allow photosynthesis to proceed normally, temperatures in the lower to mid-90s present no real challenge to corn plants. Corn is a plant of tropical origins, and its photosynthetic rate reaches a maximum at or even slightly above 90 degrees. As temperatures rise above 90, the rate will usually hold steady, only starting to drop when temperatures approach 100, providing there is adequate water. We use a cutoff of 86 degrees to calculate modified GDD, but that is because high temperatures are often associated with moisture stress, and moisture shortage lowers the temperature at which the photosynthetic rate is maximized.

So if we must have a week with high temperatures in the 90s, the last half of June is a relatively favorable time. It helps some that winds have been relatively calm, which helps reduce water loss through the leaves and preserve soil moisture. With the cutoff of 86 degrees and average high temperatures this time of year only a few degrees less than that, high daytime temperatures have only a modest effect on the rate of crop development. High nighttime temperatures, in contrast, directly increase the daily GDD accumulation. For example, typical day/night temperatures of 83/63 produce 73 ­ 50 = 23 GDD, while a 92/74 temperature combination produces 80 ­ 50 = 30 GDD (the high temperature is taken as the cutoff temperature of 86 degrees, and the average of 74 and 86 is 80.) The difference of 7 GDD may not seem like much, but over a week's time it's a difference of more than one leaf stage during mid-vegetative growth.

Leaf appearance and increase in plant height have been very rapid with the warm temperatures of the past week. The corn we planted here at Urbana on April 9 is now about 6 feet tall and at stage V13­V14. The last three or four leaf stages usually appear more quickly than the 50 GDD per leaf we normally assign, due to very rapid stem growth during this period. Thus we expect to see tassels on this crop by about July 1, and pollen shed could come as early as July 4. The last planting, in late May, is only about knee-high and so has at least a month to reach the VT, or tassel-emerged, stage. As is normal with high temperatures during vegetative growth, plant height is increased relative to the leaf stage. This suggests that late-planted plants will end up tall, and likely with smaller stalk diameters.

With the rapid plant development resulting from above-normal temperatures this year, GDD reductions from late planting will be considerably higher than they were in 2008. This means that development is partly catching up after a late start. But the downside is that we will likely end up with lower plant dry weight, leaves with less weight per unit area and perhaps less area, and perhaps one or even two fewer leaves. This could translate into incomplete canopy cover during and after pollination. This almost always means lower yields, even if water supplies remain adequate.

On the positive side, much of the crop I have seen in the last week that is past V6 or so has good color, at least in areas that have not had excess water problems. There are still concerns about the loss of N from some fields, and some producers have decided to apply some supplemental N. But high temperatures mean rapid leaf appearance and growth, which usually means that a relatively high percentage of the leaf area that we see is young leaf tissue.

Young tissue has lower (but increasing) chlorophyll content and so paler color. But when good canopy color can develop under high temperatures, this indicates that the N supply is adequate to meet the needs of the crop and that the root system is functioning well to take up the N, at least up to its present stage. If temperatures drop to normal and we continue to get high amounts of sunlight, then the crop color will improve even more. A consistent dark-green color during mid- to late vegetative stage reflects a good N supply, but more importantly indicates a high likelihood of good leaf function going into the pollination period. If we continue to get rain, we could be set up for some good pollination conditions. What happens after that remains unknown, but at least good pollination is a start.--Emerson Nafziger

Emerson Nafziger

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