Crop color is good to outstanding in most fields, though Nafziger has seen a few fields with N deficiency symptoms.

"These are likely fields where N was not yet applied, was applied so late that it was not yet being taken up, or where a lot of N loss has occurred," he said. "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, without access to the roots."

The fact that "non-rescued" fields are looking mostly well-nourished reminds us that guesses about how much N we have lost, versus how much root function has been compromised, are usually not very accurate, he added.

In any case, the dark green color means that photosynthetic rates are high, and as leaf area and light interception approach their maximum, the crop is producing sugars at maximum rates. This is exactly what the crop needs to do as it enters the pollination stages, Nafziger said.

"We do not 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," he said.

"We need to remember that the effect of good conditions now can be overcome if it turns dry and hot later in July. If that happens, we will start to run out of water and kernel numbers will drop, along with yield potential."

Nafziger said 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," he said. "These would include 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 turn brown, he said it should be easy to see which kernels are fertilized and starting to increase in size. In round numbers, maximum yield potential is expected when kernel numbers are in the range of 15 to 20 million per acre.

At 32,000 ears per acre, that means 500 to 550 kernels per ear. If conditions remain good for two weeks after brown silk, there tends to be little abortion, Nafziger said. But any of these leaf symptoms that occur during the two to three weeks after brown silk may signal some loss of developing kernels.