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Issue No. 14, Article 4/July 8, 2011

The Illinois Corn Crop in Midseason

While the large acreage reported at the end of June shocked the corn market, it goes almost without saying that good yields are still needed to end up with a large crop. While it is a little dangerous to guess at prospects before we see many tassels, we now have some idea about the physiological state of the crop as it enters the most critical yield-determining part of the season.

The latest report from the National Agricultural Statistics Service indicates that 8% of the Illinois corn crop was silking as of July 3--behind the 21% average of the past five years, but set to increase substantially over the next week as the crop planted in early May begins to silk. In our planting date study, the 109-day RM hybrid planted on March 31 silked on July 3, and the 113-day hybrid on July 5. That's after about 1,400 growing-degree days for the early one and 1,450 for the later one. The May 5 planting will probably silk later this week, after some 1,300 or so GDD.

From travels through much of the northern part of Illinois over the past week, it's clear that most of the corn crop there is in good shape. Exceptions are places where heavy rainfall in the last half of June left standing water in low places and flooded bottomland; little crop will recover to produce much there. These areas are much less extensive than in 2010, but parts of central and southern Illinois have more of them. The amount of loss in affected fields will depend on how much stand remains and how quickly root function can be restored.

The rest of the crop has benefited greatly from the dry weather and sunshine over the past two weeks. Root systems have clearly grown well and are tapping soil water beneath the top few inches. The state of the root system is signaled better through observing the crop than digging up roots. The primary indication of healthy roots is that the crop shows no signs of water or (in most cases) nutrient stress. Leaves in low-stress crops stay open (unrolled) through even the warmest part of warm days, and the rate of height increase is normal. Now that the surface soils are drying out, roots must be actively taking up water from deeper in the soil to keep the plant functioning well.

Crop color is good to outstanding in most fields, though I've seen a few fields with symptoms of N deficiency. These are likely fields where N was not yet applied or was applied so late that it was not yet being taken up or where a lot of N loss has occurred. In fact, with the dry weather of the past two weeks, some of the "rescue" applications that have been made, based on fear of N loss or on poor crop color when soils were very wet, may still be sitting in the dry surface soil, with no way to get to the roots. It's perhaps too early to say that such applications were a mistake, but some were likely premature, especially in fields where adequate N was applied earlier and there was no real indication that much of it had been lost. The fact that "nonrescued" fields are mostly looking well nourished now does remind us, though, that guesses about how much N has been lost (vs. how much root function has been compromised) are usually not very accurate.

In any case, the dark green color means that photosynthetic rates are high, and as the canopy closes "for real"--as leaf area and light interception approach their maximum--the crop is producing sugars at maximum rates. This is exactly what it needs to do as it enters the pollination stages. We don't have a good test to see how sugar levels are holding up in the plant, but with so much sunshine, moderate temperatures, and a healthy crop canopy, they should be as high as they can be at this point. Remember, though, that the effect of good conditions now can be overcome if it turns dry and hot later in July, in which case water will start to run out and kernel numbers will drop, along with yield potential.

The main things to watch for now are signs that photosynthetic rates might be dropping and, by the time pollination ends and silks start to turn brown, how many kernels have been fertilized. Threats to photosynthetic rates are almost always visible as leaf symptoms, including loss of dark green color, curling up due to lack of adequate water, disease damage to leaf area, or loss of leaf area from hail or insects.

After silks start to brown, it should be easy to see which kernels are fertilized and starting to increase in size. In round figures, we expect maximum yield potential when kernels number in the range of 15 to 20 million per acre. At 32,000 ears per acre, that would be 500 to 550 kernels per ear. If conditions remain good for 2 weeks after brown silk, there tends to be little abortion. But any of the leaf symptoms just mentioned occurring in the 2 to 3 weeks after brown silk may signal some loss of developing kernels.

Coming through northeastern Indiana early this week, I saw corn planted in June that was undergoing significant drought stress. There is likely some corn in similar condition in Illinois, including some that was replanted. Unless it rains soon on such corn, the root system, which never had much chance to grow before soils dried out, will remain small, and the crop will be unable to grow well as a result. As we move through July, prospects for such corn are dropping quickly.--Emerson Nafziger

Author:
Emerson Nafziger

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