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Issue No. 10, Article 7/May 27, 2005

Let the Sun Shine

One of the hallmarks of a good growing season in Illinois is more sunny days than normal. Sunshine is the fuel for plants to grow, and more sunshine is almost always better unless plants don't have enough water or nutrients to take advantage of the sunlight that falls on a field. As you watch crops develop their canopy of leaves over the next weeks, notice how much of the sunlight the plants are intercepting, as indicated by the percentage of the soil surface that is shaded at midday. It's clear that most of the crop in Illinois won't develop its canopy as rapidly as the corn crop did in 2004 (many soybean crops were planted late in 2004, so they had to catch up), but as the corn crop passes the six-leaf stage, its growth in height and leaf area will accelerate and will stay at a high rate through tasseling.

There is a large difference in corn plant size from northern to southern Illinois, with most of the corn north of I-80 only 4 to 6 inches tall, while some early-planted fields in southwestern Illinois are knee-high. Stands are good in most fields, though some replant was necessary due to frost injury or soil crusting. Canopy color is finally getting back to a healthy green, though night temperatures in the 40s continue to slow that process in the northern half of the state. According to Bob Nielsen at Purdue, we should add a new leaf (collar) to the corn crop for every 85 growing degree-days or so, which at present accumulation rates means a new leaf every 5 to 7 days, longer if highs stay in the 70s and lows in the 50s.

Stem growth is slow until about the six-collar stage, so new leaves appear more slowly. After about the 10-leaf stage, a new leaf will appear for each 50 or so GDDs, with the last few leaves requiring perhaps even fewer GDDs than that. Thus by mid-June, leaves should be adding at the rate of about two per week. If four-leaf corn needs 4 x 85 = 340 GDDs to reach V8, it will reach that stage by mid-June if temperatures remain normal. If corn is now at V6, it could be at V10 by mid-June. Differences in stage now will likely become greater over the next few weeks, until the later corn catches up in leaf stage and height. At this point, though, we do not expect the very early tasseling and silking that we saw in 2004.

Soybean planting is 89% complete as of the beginning of this week, and by now over half of the crop has emerged. In parts of southern Illinois where I traveled on May 24, lack of surface moisture is limiting growth, or even emergence, in some fields. Check such fields to see if the seeds that haven't yet emerged are still dry, if they are swelled because of having taken on water, or if they germinated but then the small root dried out. Water loss rates haven't been excessive because temperatures have not been high, so we hope that most of the seeds that haven't emerged are either still dry or are continuing to germinate and grow roots, even if dry soils are slowing this process. Seeds that have taken on water and then dried out again or that have produced a root that then dried out are probably not going to produce plants.

If stand counting is necessary in soybean, there are several methods to choose from. I prefer counting plants in a defined length of row over the hula hoop method, because the hula hoop is such a small area that it often gives variable counts; by definition, spotty stands that need counting are highly variable from one spot to the next, so the disbelief factor ("this count can't be right") is often high when we count plants in a few square feet. I suggest using a stick or plastic pipe cut to a length of 52 inches and laying it next to the row to take a count. For 30-inch rows, multiply the count in one row by four to give thousands of plants per acre. Total the counts in two adjoining rows for 15-inch rows and in four adjoining rows for 7.5-inch rows, and again multiply by four to give stands in thousands per acre. This calculator will help assess stands using data from different methods.

Our data on replanting in soybean are not as definitive as we'd like, but expected yields because of late planting will have declined by perhaps 10% to 15% in northern Illinois and by 5% to 10% in southern Illinois by the end of May. Actual yield decreases from late planting are expected to vary widely over seasons, but if stands are less than 80,000 to 90,000 healthy plants per acre, replanting might be a good option. Of course, replanting in a field with dry soils will probably not produce a good stand until it rains again, so waiting for intact, planted seeds to emerge in such fields might be a better option than replanting.

We've had some reports of hail injury to emerging soybean plants. While it's possible that such damaged plants will regrow from buds where the cotyledons or cotyledonary leaves are attached, small soybean plants are fragile, and heavy hail will often break plants below the cotyledons or will break off the cotyledons, making regrowth difficult or impossible. When in doubt whether enough plants will regrow, replanting might be the best option, especially if it can be done by the end of May.

The wheat crop looks good to very good in most southern Illinois fields, while in northern Illinois there have been some reports of yellowing, perhaps because of viral diseases or damage from cold. Heading is complete, except in the most northern tiers of counties, and the cool, dry weather has clearly been good for the crop. The only fungal disease of note that we have encountered is strip rust, which is affecting some varieties in variety trials, especially in southwestern Illinois and possibly in areas south of Mt. Vernon. We saw very little of the disease in farm fields west of Centralia on May 24. There are reports of this disease causing considerable damage in Arkansas and in the southeastern United States, but our conditions may help us escape it for this year.

Based on the stage of the crop now, I anticipate harvest at about the normal time unless temperatures depart greatly from average over the next 3 weeks. That would mean a start to harvest around June 15 in extreme southern Illinois, late June in central Illinois, and the second week of July in the most northern part of the state. Double-cropped soybean prospects will be improved slightly if planting can be earlier than that, but wheat yields would probably be compromised by the warm weather needed to ripen it faster. As the last 2 years have shown, double-cropped soybean planting date affects yields considerably less than does the amount of soil moisture at planting and the rainfall later in the growing season.--Emerson Nafziger

Emerson Nafziger

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