NASS's weekly Crop Progress & Condition Report is widely followed during the growing season, but University of Illinois crop sciences professor Emerson Nafziger questions the usefulness of early-season numbers toward predicting final yeild, as they represent a subjective estimate.
To test whether the early-season indicators are good predictors of final crop yield, Nafziger compared the late May "Good" to "Excellent" (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 bushels per acre), had a late May G-E rating of 78%.
But not surprisingly, he does say "as the season progresses, the correlation between crop ratings and final yield improves."
The most recent crop condition ratings from USDA showed a 5-point drop in the percentage of corn rated to G-E to 72%, but Nafziger points out that "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." He continues, "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."
A question that is often asked at this crop development stage is whether stress now will reduce final yield potential even if rainfall returns to normal levels. Nafziger says, "The short answer is no; we have no evidence that a corn plant that undergoes moderate water stress during the first half of vegetative growth — say, through V10 or so — suffers irreversible loss of potential kernel number or size."