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Can We Relax Yet?

June 26, 2003

With the performance of most of the corn and soybean crops now solely at the whims of the weather, many peo-ple will have anxious eyes on the sky, especially over the next month. Corn continues to develop at a rapid pace, though average height for the state is a bit below average, mostly because of the late planting in southern Illinois. With 75% of the crop rated "good" or "excellent," the crop is certainly off to a good start in most areas.

The most recent report indicates that 1% of the state's crop was tasseling by June 22. This is more than the zero percent average for this date and is coming from the crop that was planted in early April or even late March. Our March 24 planting here is about head-high to a tall person and will likely show some tassels by July 1, though the cooler weather forecast for this weekend may delay that by a day or two. On a trip to DeKalb on June 24, most of the corn looked outstanding, with probably the best uniformity of plant size and stand that I have seen in my 22 seasons in Illinois. In my (slightly biased) opinion, the uniform dark green of a solid canopy of a corn crop is truly one of the best sights there is.

Leaf color is excellent in most fields. Although we expected that soils would mineralize N (convert it to a form available to the plant) slowly due to low temperatures and May rainfall, one N study here, where continuous corn has not had N applied for 6 years now, shows much less N deficiency than it has most years. The N needed to green the crop there had to come from soil organic matter, including some that was probably mineralized last fall and that stayed in place due to the dry conditions.

Concerns still remain about the water supply for the corn crop this season, although we have had enough rainfall to recharge the surface soil. Although there's not much we can do about this, we can tell by observing the plants when they are undergoing a shortage of water. Corn leaves curl when the loss of water vapor through the leaves even slightly exceeds the rate of uptake of water from the soil. On hot, windy afternoons, water loss is so rapid that plants can show curling even if soil moisture supplies are fairly good. Soil compaction contributes to less root growth and earlier leaf curling as well.

As long as leaves curl only for an hour or two on a hot afternoon but are relaxed the rest of the day (and night, when even severely stressed plants relax their leaves), then the loss of photosynthesis is probably minor. The longer it goes without rain to replenish soil water in the rooting zone, though, the earlier in the day the leaves will roll and the more severe the effect of increasing lack of photosynthesis. This is particularly true as the plants approach the tasseling stage; if leaf curling occurs at all after tassels appear, it's a signal that water supply in the soil may be inadequate to allow silks to grow to receive pollen. Also, any lack of photosynthate (sugars) during this critical period will probably mean fewer kernels formed and filled. Cooler temperatures help, mostly by reducing the rate of water loss from leaves.

Soybeans will likely start to flower about the second week of July, but the most critical stage for moisture supply is in late July and early August, as pods start to fill. Soybean growth has finally taken off, and if water supplies stay adequate, soybean canopies should fill reasonably well in most fields. Soybeans planted in mid-May are mostly in the V4 or V5 stage now, but I saw soybeans in northern Illinois that were planted in late May and are still only in stage V1 or so. These may have their flowering delayed by plant size more than photoperiod, and it will be difficult for these to form a full canopy if they are in wide rows.

Good news on wheat: Most wheat growers are pleased to find yields coming in higher than had been expected. Reports from southern Illinois indicate that yields in the 50s and 60s are common, with some fields producing yields in the 80-bushel range. Quality concerns from Fusarium scab remain, but yields may well approach the 61 that we harvested as a state average in 2001. I expect to hear about how "intensive management" helped bring about such high yields this year. Although some of the "intensive" practices are no doubt sound, favorable conditions this year will mean that the sound management that most producers use, while not "intensive" in the sense that the word is usually used, will produce very good yields. As an example, I heard in northern Illinois that some wheat fields were sprayed with foliar fungicides several weeks ago, even though virtually no fungal disease has developed under the favorable weather conditions there. Such practices may be "intensive," but they certainly reduce profits. Sound management and spending more money to produce a crop do not necessarily go together.--Emerson D. Nafziger

Author: Emerson Nafziger

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