4 July 2011

5 minute read

As we pass the longest day of the year in the northern hemisphere (June 21), with its maximum amount of sunlight, it's a good time to think about sunlight and its effect on the crop. If we think of the crop as a yield-producing factory, sunlight is the fuel that powers it. But this is a factory that has to build itself.

As leaves emerge from the whorls and green up as they open to the sunlight, they start to photosynthesize, producing the sugars that fuel all crop growth and yield. This in turn helps leaves and the rest of the plant to grow (add dry matter), helping the factory pick up speed. The factory needs to be at full speed, with a full light-gathering "roof" (crop canopy) by pollination time in order to produce maximum grain yields.

So the development of leaf area, collectively called the crop canopy, is critical to the growth process. We often say that the canopy "closes" at the time when we can't see soil anymore as we look down the rows from the end; the canopy seems to form a solid surface that covers the field. In 30-inch rows, this happens when the crop is about waist-high, generally at about stage V10. Fields planted in April or early May are at or nearing this stage now.

Though canopy closure would seem to be the point at which the crop begins to intercept all of the sunlight, that is not the case: a single leaf lets some of the light pass through, and leaves are not evenly distributed, so some amount sunlight is still reaching the ground. Plants need to develop a "leaf area index"--square feet of leaf per square foot of ground area--of 3 or more before the crop intercepts more than 95% of sunlight. In corn, this happens only when the crop is tasseled, at which time its exposed leaf area is close to the maximum.

Because the crop is growing so rapidly by stage V8 to V9, corn in narrow or twin rows does not have a very large or lasting advantage over 30-inch rows in the rate at which they increase leaf area and sunlight interception. While any such advantage is often positive, the difference in total sunlight intercepted by wide and narrow by the time plants reach full canopy is relatively minor--less than 10 percent. Pollination success and yield are determined after full canopy, and by then there is often no difference in light interception between wide-row and narrow-row corn.

Because sunlight is the driving force for all crop growth, there is considerable concern about the amount of sunlight and whether it may limit yields. The Water and Atmospheric Monitoring (WARM) program of the Illinois State Water Survey publishes monthly data on sunlight at a number of Illinois locations. The sunlight data are in a column headed "Total Solar Rad" with units of megajoules per square meter.

A megajoule (MJ) is a unit of energy equal to about 240 kilocalories or 0.28 kilowatt-hours. A square meter is about 10.7 square feet. The maximum sunlight received during a summer is about 32 MJ/square meter, and daily averages are typically about 3/4 of the maximum. In more familiar units, 30 MJ/square meter received on a sunny day is about 670 kcal/square foot or about 3/4 of a kilowatt-hour per square foot. That amount of sunlight energy is the equivalent of 33 megawatt-hours of electrical power per acre; in terms of chemical energy, it is the equivalent of some 14 tons of sugar per acre.

Only about half the energy in sunlight is in the visible wavelengths; most of the rest is infrared (heat) or ultraviolet and is "invisible" to both our eye and to plants. Plants intercept more red and blue light than green light; they reflect or transmit much of the green light and so appear green.

For a variety of reasons, the plant cannot convert sunlight to sugars with high efficiency. In fact, on a good day the plant will typically convert only about 2 per cent of the sunlight energy into dry matter. But with so much energy falling on an acre in a season, very high yields are still possible.

Of the three most recent crop seasons, 2008 and 2010 had high sunlight and 2009 had low (Table 1), with the total over three months in 2009 about 10 per cent less than the totals in the two high-sunlight years. In these three years, yields were highest in 2008 and lowest in 2010. In 2004, the highest average yield on record in Illinois came with only 1,987 MJ/square meters of sunlight over these three months, about the same as in 2009. So it's clear that while sunlight has an effect on productivity, it appears to be less important than temperature and rainfall. These factors are all correlated to some extent, making it difficult to single out the most important factor in determining yield.

Corn planted this year at the end of March at Urbana is at stage V12 and about chin-high now. The late-May planting is at V4 and less than a foot tall. Clearly, the amounts of sunlight that these two canopies will intercept during these longest days of the year differ vastly. But if the season lasts long enough and there is little water stress, the late-planted crop can still intercept enough sunlight to yield 80% or more of the early-planted crop. Both need careful attention to limit stresses and to keep the canopy in top shape in order to reach their potential.

June 2011