Following general worries about dry weather in March and April, many parts of the state have over the past week received a month's worth of rainfall in only one or two storms. The Illinois corn crop was 68% planted as of May 4, and for the crop that was emerging, moderate amounts of rainfall were welcome. Storms with inches of rainfall, hail, and high winds are never welcome. And in other places, much corn planting remains to be done, and continuing wet weather is starting to shorten the season for fields that have yet to be planted.
Statistics indicate that only the west, southeast, and southwest crop reporting districts still have more than 50% of their corn crop yet to plant. We normally think that planting delay penalties are a little milder in southern Illinois than in the northern part of the state because of the larger number of warmer days, but this varies so much with the rainfall pattern that it's hard to put numbers on it. If we can get the rest of the crop planted before late May, statewide yield potential should not be greatly reduced by late planting. On average, we expect corn yield to drop about 1/2 bushel per day of delay during the first third of May, about 1 bushel per day during the second third, and about 1 1/2 bushels per day during the last third of the month. These are, as I noted previously, variable from year to year, depending on weather patterns; only last year (2002), corn planted in mid-May yielded more in most areas than corn planted at the "ideal" time (late April).
While we wait for fields to dry out so planting can resume, we are in some local areas dealing with ponding in parts of fields and hail damage to emerging crops in other fields. Standing water damage is probably easier to deal with; by the time the water recedes, we will be able to tell whether plants survived and whether the drowned-out areas are large enough to replant. When plants are just emerging, or when seeds have sprouted and are about to emerge, they are at a vulnerable stage for injury or death because of lack of oxygen caused by saturated soils and/or standing water. Small seedlings are "air breathing" (well, they don't really breathe, but they need oxygen), and like other organisms that oxidize accumulated food materials for energy and growth, lack of oxygen leads to rapid death. Soils hold some stored oxygen, and rainwater carries dissolved oxygen, especially if its temperature is low; but it's probably unrealistic to expect seedlings underwater to survive for more than three or four days. Older plants can last longer, both because they produce their own food materials and because their roots and leaves are larger and so escape oxygen deprivation a little better.
Hail is another concern in some areas. Corn plants just emerging are vulnerable to direct hits by large hailstones, but the number of large hailstones falling on an acre is usually not enough to wipe out a large percentage of plants. Smaller hailstones that shred leaves are not much danger because there isn't much leaf area to shred. Protection of the growing point, which is just forming about 3/4 inch deep in emerging plants, is provided by the layer of soil, which absorbs the shock. As a result, it is hard to kill germinating corn seedlings with hail, though soft soils may provide enough "give" to allow some physical injury. Emerging soybean plants are more easily damaged, but so far Illinois doesn't have many emerging soybean fields.
Regardless of the cause of damage, the key to survival and regrowth of small corn and soybean plants is the health of the growing point--the tip of the stem from which new leaves are generated. Though it takes some patience, the surest way to monitor this is to see whether plants start to regrow soon after conditions improve. As happened in 2002, it can be cool enough in May that regrowth is slow and we waste time by waiting. With average soil and air temperatures this year, if regrowth is not visible within 3 or 4 days of improvement of soil conditions, then healthy plant counts should be made and replanting decisions considered.--Emerson Nafziger