As we approach the end of July, with continued moderate temperatures and no heat wave in sight, it's clear that the main determining factor for yield in most Illinois cornfields will be the water supply for the rest of the season. Although there are few signs that dryness has hurt the crop, there are areas of the state where rainfall during July has been much below normal.
Make no mistake, the crop is in excellent condition in the majority of fields in Illinois. The cool, dry weather during the past weeks has slowed the development of the crop but has also helped keep the crop healthy. Leaf color, which is an important factor in yield potential at this time of year, is outstanding in most fields.
In other parts of the state, July rainfall has been heavy, and as a result the crop has continued to suffer from lack of root growth and nutrient uptake. The worst of this damage is confined to the lower parts of fields, but root development has probably been less than ideal in many fields in the wetter areas. The crop in these fields is probably not as well tapped into the soil water supply due to relatively shallow rooting.
Even where there has been less than average rainfall during July, the crop is still able to extract water from the soil in amounts adequate to keep the crop working well. Although the entire profile may not have been fully recharged at the beginning of the month, low temperatures have reduced the amount of water loss from the crop. Water loss on a particular day increases with wind speed and temperature, and is measured simply by seeing how much water depth evaporates from an "open pan"; the measurement is called "open pan evaporation," or OPE. We don't often hear of actual OPE values; but, on Monday of this week (a day with little wind and high temperature about 80°), the OPE was only 0.17 inch at Urbana. That is probably half what it might have been with windy, hot conditions.
The "crop coefficient" is the percentage of OPE that the crop uses. It varies with the development stage of the corn crop but is at a maximum (near 1) for the few weeks after tasseling. The health of the leaves and the activity and depth of the root system affect the crop coefficient. The crop coefficient is very difficult to measure, and so it is more often simply modeled than measured. It is likely, though, that the crop coefficient has been quite high for the crop in July. This would normally result in rapid water loss, but the OPE has been low enough to keep water loss relatively low.
Does this mean that the crop is "made?" Although the low temperatures have kept water loss relatively low, they have also slowed development, probably to about the same degree. So some of the advantage of early pollination has been decreased, and we will need water to keep the crop functioning well for 5 or 6 more weeks, even where it pollinated early. In parts of southern Illinois, where wet fields delayed planting to the last half of May, and in northern Illinois, where cool temperatures have persisted, the crop is just now pollinating in many fields. These fields will need 7 or 8 weeks of favorable conditions if they are to yield well.
The crop is well positioned for "typical" August weather. High temperatures and humidity are common in August, and good leaf health will help get the crop through at least the early part of such weather. Soil moisture has also been depleted in some places, and the need for rainfall will increase in these areas. Even where dryness is a problem, how well leaves maintain their color will be the main thing to watch over the next weeks.
Any loss of leaf area at this point in the season will result in reduced yields. This is a prime time to look at how well the canopy is maintaining the ability to absorb sunlight and, thereby, to produce what the plant needs for high yields.
To assess the canopy, go into fields in the late morning or early afternoon when the sun is high and bright. Look between the rows and see how much of the light is reaching the soil. A good canopy now should be intercepting more than 95% of the sunlight, meaning that the areas of sunlight that you see hitting the soil should be small and few in number. If there aren't enough plants, if the rows are too far apart, or if the leaves have been damaged due to nutrient deficiency or insects, then there will be patches or streaks of sunlight on the soil surface. Leaf diseases can reduce leaf function without reducing canopy light interception, so this technique is less effective for leaf diseases. But even if you can't fix the causes of low light interception in the crop this year, you want to keep in mind the following fact: Every bit of light that hits the soil surface during grainfill is lost to the crop and will result in loss of yield. It's worth watching.--Emerson Nafziger