No. 7 Article 7/May 21, 2010

When Will Corn Start to Grow Again?

With 96% of the corn crop planted and 78% emerged by mid-May this year, there should be a lot of joy going around about the 2010 Illinois corn crop. Instead, local news this week talked about the "need to replant" and other woes. Most people have observed that the crop is not growing well and that it has poor color. So is corn off to the great start we were talking about several weeks ago or not?

The first point we need to remember is that the start to the Illinois corn crop is not highly correlated with the end: final yield. The great starts in 2004 and 2005 ended with high and low yields, respectively, and the poor starts in 2002 and 2009 ended with low and high yields, respectively. It's a long season, and weather effects have a complicated (and mostly unpredictable) interaction with how we manage the corn crop. That means that, while we can feel good about early planting and good stands at this point, trying to use current (May) crop condition to predict yield is simply a guess. We know that clear and visible negatives--poor stands, uncorrected nutrient deficiency, heavy weed competition--will usually not result in high yields, but the effects of clear and visible positives at this point in the season are much less certain.

The corn we planted here at Urbana on April 5 is now (on May 19) in stage V6 and about 8 inches tall; plants are fairly short for the stage they're in. Corn planted on April 21 is at V2-V3. Growing degree-day accumulation from April 5 to May 18 has been 475, while 296 GDD accumulated between April 21 and May 18. Table 2.1 in the 24th edition of the Illinois Agronomy Handbook indicates that it takes 315 and 555 GDD to reach stages V3 and V6, respectively, so the crop is developing about as expected for corn planted in late April but ahead of predictions for the earlier-planted corn.

As we can note from the slowdown in GDD accumulation, temperatures were warmer in the first three weeks of April than they've been since; only about 70 GDD accumulated here at Urbana over the past week, compared to more than 100 for the same week in April. We've also had more rain in the first three weeks of May than in all of April.

With low temperatures in the 30s on May 8 and cool, cloudy, wet weather in most places since then, the crop still appears pale, with slow growth. In fact, "growth" (seen as increases in height and in plant "spread") is usually fairly slow in early vegetative stages, and we tend to project weather we don't like as also being poor for corn. Warmer, drier weather, expected by the end of this week, should rapidly improve crop appearance. Whether cool weather in May affects final yield is still speculative, but it's certain that weather patterns through the rest of the season have more effect on yield than temperature during May.

So if corn is up with a good stand, water is not standing in the field, and all other management factors are in place, we've done what we can to set the crop up to reach its yield potential. Once growth resumes in earnest with warmer temperatures and sunshine, the earliest-planted crop will quickly reach its rapid-growth stage, when stem elongation begins. This happens at or just after V6 and then accelerates, with most rapid stem elongation after V9 or V10, when the internodes in the central part of the stalk start to elongate.

After V6, rapid leaf appearance and expansion add quickly to leaf area; this in turn increases the photosynthetic capacity, which further increases the rate of growth. With luck, the crop will look better and be growing rapidly by early June in most fields. If the weather into June is good, then we'll forget the slow growth we're seeing now and start to look forward to having a good crop canopy in place and a good start to pollination by early July.--Emerson Nafziger

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