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Issue No. 13, Article 7/June 18, 2004

When the Rain Stops

From May 1 through June 13, 39 of 40 Illinois weather stations recorded rainfall totals above normal, with many of the largest amounts in the northern third of the state. While we often see rainfall unevenly distributed, even during extended wet periods, few areas have had much dry surface soil during the past month, and of course many fields have had low areas flooded, often more than once. This wet period will likely end at some point, and we need to think about what comes after that.

Corn continues to develop at a rapid pace, with the average height of the Illinois crop twice normal by June 13, at 34 inches. Virtually all of the crop is in the rapid growth phase now, meaning that height will increase at 2 to 3 inches per day as long as it stays warm. Unless it turns cool in June, the crop in at least half of Illinois fields is on pace to be pollinating by early July. If soil water content stays high from rainfall during the next two weeks, pollination under good moisture conditions is almost a certainty. That's the good news.

Loss of nitrogen and lack of root aeration are the largest negative consequences of extended wet weather, though cloudy days and warm nights are not ideal conditions for crop growth. We think that warm temperatures also tend to favor top growth over root growth, both by affecting physiological "balance" in the plant and also by increasing root respiration rates when soils are warm. Diseases could also be a factor. While these things are difficult to predict, and good top growth reflects relatively good overall growth rates at this time, we are concerned how this weather will affect root growth and health.

As long as rain continues to be abundant, root system problems may not affect growth and yield. But if it turns dry in July and August, root system limitations result in low rates of grain fill and an early end to building yield in corn. Any foliar or stalk diseases or any insects that decrease leaf area or the ability of the plant to move sugars will have much larger negative effects in plants with compromised root systems. Root systems stop growing actively soon after pollination, so problems now will have less chance for correction through root regrowth. A report of early formation of brace roots this past week is not very favorable; brace roots are more likely to start early and grow when sugars build up in the lower stalk, and sugars build in the lower stalk when the root system is unable to utilize them as quickly as it should. In general, the plant benefits more from deeper root systems than from a lot of brace roots, which only penetrate the upper few inches of soil.

Soybean is struggling more with wet soils than is corn, though the warm weather also has soybean growing rapidly, and so canopies are starting to develop. Soybean planted in mid-May is now at about stage V4 (four trifoliolate leaves expanded) and about 8 inches tall. Of course, warm temperatures have also shortened the life span of plants in flooded areas of fields, and some parts of many fields may still benefit from replanting if this can be done within the next couple of weeks. Expect the crop to continue to look somewhat pale and unhealthy as long as it stays wet and cloudy. Once soils dry some and nodules develop to provide nitrogen, color will improve. Although it doesn't seem so, the warm temperatures this year are probably more positive for the soybean crop than the cool temperatures were at this time in 2003. As long as plants are alive and stands are adequate, though, what happens to the crop and how it looks now will have much less bearing on yield than will rainfall during August.

If soybean is planted or replanted this late, use narrow rows and increase seeding rate some, though 200,000 seeds should still be adequate. Midseason, adapted varieties should still be used if the crop can be planted by June 25. After that, slightly earlier varieties will have a little better chance of maturing before frost in the northern half of the state.

Wheat has continued the remarkably rapid development that we noted 2 weeks ago, and harvest started last week, with 8% harvested by June 13. Early reports indicate yields that are average to slightly better than average in the southern half of the state. Wheat producers are very pleased to be able to double-crop soybean so early in June, and it is likely that double-cropping will move farther north than usual. In general, it is probably worth attempting to double-crop if there is good soil moisture to get the crop up and if planting can take place by July 1. Wheat harvest has moved into central Illinois by now and so should reach all but the northernmost areas by July 1, especially if we get some drying weather. Yields are likely to be compromised by the early loss of leaf area, disease, and filling conditions that are warmer and more humid (with warm nights) than are ideal for wheat.

Field pea is generally maturing early like wheat but not as early as some people had hoped. I saw photos this week from a field in southwestern Illinois where most plants had died without the aid of Gramoxone. It appeared that pod filling had ended early, and yield prospects in that field are not very good. In general, the warm, wet weather the crop has experienced in recent weeks has been hard on field pea, and yields are like to reflect that fact.

There have been some inquiries about following pea harvest with grain sorghum, even in the northern part of Illinois. Grain sorghum will get off to a good start if it's planted under the current warm conditions, but it is still a relatively slow starter as crops go. I would expect grain sorghum planted in early July in northern Illinois to begin heading only in late August and to struggle to fill grain as the weather cools in late September. Soybean would probably have a better chance, but it will also be risky planted that late in the northern third of the state. Very early corn hybrids might be a better choice but only because of the expected low yields of the alternatives. Forage production is a safer enterprise for such late planting if that market is available.--Emerson Nafziger

Author:
Emerson Nafziger

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