The National Ag Statistics Service's weekly Crop Progress report gives a subjective estimate of crop conditions each week during the growing season. University of Illinois Crop Sciences Professor Emerson Nafziger doubts that these early season numbers are useful predictors of final yield.

"Crop ratings tend to reflect how the crop looks on a given day," he says, "but because it's one of the few numbers available early in the season, many people like to use it to make guesses about yield potential. It's no surprise that, as the season progresses, the correlation between crop ratings and final yield improves."

The ratings give the percentage rated Very Poor, Poor, Fair, Good and Excellent for each crop category. Many people use the sum of the Good and Excellent percentages, or G-E rating, as an indicator of how much of the crop is in good shape.

To test whether the early season indicators are good predictors of final crop yield, Nafziger compared the late May G-E rating with the final corn yield for Illinois each of the past 12 years. With one exception, he found crop rating to be of no value in predicting final yield. In fact, the late-May G-E rating for 1988, the worst corn year in the past 30 years (73 bu./acre), had a late May G-E rating of 78%.

Crop ratings have been good so far in 2012, but dropped from 79% to 66% G-E between May 20 and May 27. This drop in ratings reflects the continuing dry weather.

"Because it's a subjective measure, even a crop with a good stand and good uniformity is not likely to be rated as excellent when its leaves are rolling up in the afternoon due to lack of water," explains Nafziger. "Add to that the increase in unevenness of plant size that is resulting from differences in root growth and water availability to individual plants and some loss of uniform, green color as water and nutrients become more limiting. The crop starts to look less promising."

Leaf rolling brought on by lack of adequate water is never good. Rolled-up leaves do little or no photosynthesis and accumulate little dry matter. Leaves and stems that develop under such conditions tend to remain small, which reduces their ability to photosynthesize fully even if water becomes available later. If the conditions remain unchanged over several weeks, growth is stunted, and the plant is less likely to recover fully.