Corn planted the first week of May is in about the 8-leaf stage at Urbana, and color and stand uniformity are very good in most fields. The return of cooler weather this week will slow down the rapid rate of growth, but the crop is still ahead of average in its development. With the warm temperatures that held through the first 2 weeks of June, we were accumulating the 65 or so growing degree-days required to put a new leaf on corn plants in less than 3 days.|
Although the corn crop appears to be in good shape in most fields, there are places in Illinois where thunderstorms last week caused flooding and hail injury, sometimes in the same field. Plants with six leaves or more are better able to withstand flooding than are smaller plants, but saturation of the soil results in loss of oxygen and will quickly affect root function. If temperatures are warm and soils are saturated for more than 3 to 4 days, plants will likely not survive. Cooler temperatures mean that water dissolves more oxygen, so plants usually survive longer. Even short periods of standing water may, however, affect the ability of the roots (and hence the plants) to recover and grow normally. Furthermore, nitrogen can be lost when soils are flooded, and this can affect the yield potential of the crop. The fungus that causes crazy top in corn also is favored by flooded conditions and may affect whole areas that have been under water.
While there isn't much we can do to prevent losses in flooded parts of a field, any steps that can be taken to drain water away from ponded areas will help. Where the crop has been killed by standing water, corn can be planted back, particularly if there is a way to use those areas for forage. Soybeans were also planted successfully in many flooded-out areas in 1998, though herbicide labels would not in most cases permit this.
Hail injury on young corn plants usually looks much worse than it is. In cases of very heavy hail injury, though, especially when plants are in the 6-leaf stage or older, the growing point can be killed outright by hail or may be bruised and infected by diseases that eventually kill the plant. The key to assessing hail damage is to see if the growing point (the tip of the stem, visible by splitting the leaf tissue down through the stem) has a healthy, whitish color. If so, then the plant likely will survive and grow back. If the growing point is brown or mushy, then the plant probably will not survive. A more certain way to assess hail injury is to note whether plants start to show new leaf growth. To assist in seeing such growth, you may wish to cut some plants off "clean" above the growing point where the hail has injured the plant, and then see whether new leaf growth appears within 1 to 2 days. If so, then the signs are good that the crop will grow back. Research has shown that, if the crop is capable of growing back, there is little benefit to mowing damaged leaf area off to try to "help" the crop grow back.
Previous research has given us a way to estimate yield loss to be expected from loss of leaf area. Most of the corn that lost leaf area to hail last week was probably in the 6- to 8-leaf stage at the most, with some smaller than that. Corn with fewer than six leaves suffers minimal loss from loss of leaf area, providing the growing point stays healthy. Corn in the 7-leaf stage is expected to lose only 2 percent yield when it loses half its leaf area, and 9 percent from losing all its leaf area. Moderate leaf loss25 percentreduces yield by only 1 percent, even at the 10-leaf stage. Corn that loses all its leaf area at the 9-leaf stage is expected to lose about 13 percent of its yield. These numbers can vary considerably with the weather but should be close on average. The other effect of leaf loss is a delay in crop development. This delay, which could total one to two leaf stages, will delay pollination, in some cases to worse weather conditions, but sometimes to better conditions. This delay is responsible for part of the effect on yield.--Emerson D. Nafziger