Issue No. 9, Article 5/June 1, 2012
Early Season Crop Ratings and Yield
The National Agricultural Statistics Service's weekly Crop Progress and Condition provides a subjective estimate of the condition of crops each week of the growing season, reported by percentage in the categories of very poor, poor, fair, good, and excellent. For convenience, many people combine the good and excellent categories to indicate how much of the crop is in good shape.
As a subjective measure, a crop rating tends to reflect mostly how the crop looks on a given day. But because the rating is one of the few actual numbers available starting early in the season, many like to use it to make guesses about yield potential. It's no surprise that as the season progresses, the correlation between rating and final yield improves.
To see how well this works early in the season, I plotted the late May "good + excellent" (G–E) percentage for each of the past 12 years against final corn yield for that year (Figure 1). The 28% rating in 2002 was one of the lowest on record, and that year turned out to be a very poor one, with a yield of only 136. That single year means that there is some correlation over years between the G–E rating in late May and yield (R2 = 0.21). But when 2002 was removed, there was no relationship at all between the two factors (R2 = 0.00). Going back to the worst corn year in the past 30 (1988), the late May G–E rating was 78%, and the crop yielded 73 bushels per acre. So it's clear that the crop rating early in the season has almost no accuracy in predicting yield.
Figure 1. Relationship between percentage of the corn crop rated good or excellent (G–E) in late May and final Illinois corn yield, 2000–2011. Source: NASS.
This month the G–E rating was 88% on May 6, which is very high. It dropped to 74% on May 13, rose to 79% on May 20, and dropped sharply to only 66% on May 27; the current rating is at about the average for the past 12 years. It's also about the same as it was in 2011, even though this year's crop is much advanced compared with a year ago.
How can a week with no major weather events trigger such a drop? In this case, the 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. When you add to that the increased unevenness of plant size 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.
This leads to the question often asked at this stage of crop development: is stress reducing final yield potential, even if rainfall returns to normal levels? The short answer is no; we have no evidence that a corn plant that undergoes moderate water stress during the first half or so of its vegetative growth--say through V10 or so--suffers irreversible loss of potential kernel number or size.
Leaf rolling brought on by lack of adequate water is never a good thing, because rolled-up leaves do little or no photosynthesis. So plants under stress accumulate little dry matter, and after weeks of stress this reduction stunts growth and starts to diminish the potential for recovery. Leaves and stems that develop under such conditions tend to remain small, and this reduces their ability to photosynthesize fully even if water becomes available later.
If yields are lower than desired at the end of the season, though, many people will recall having seen stress symptoms during early vegetative growth and point to stress as an explanation. This often includes the observation that there were fewer kernel rows on the ear than expected or than that hybrid should have. It's likely that stress can reduce kernel row number, though it is very difficult to show early stress as a cause. Seasons that produce low yields almost always have stress during the second half of the season, and separating the effects of earlier and later stress is not possible. Ears that show "zippering"--loss of kernels rows due to abortion--lose rows to stress after pollination, not during early vegetative growth.
In contrast, corn that undergoes moderate stress during early vegetative growth often yields very well in years when such stress is confined to the early season. In part this is because growth of the ear and tassel up through mid-vegetative stages requires very small amounts of the plant's resources, so modest reductions in plant sugar have little effect. Also, these plant parts develop in the interior of the wrapped leaves and so are well protected from the effects of inadequate water.
The 2012 crop continues to develop at a rapid pace: growing degree-day accumulations in Illinois from May 1 to 27 exceeded normal by 115 GDD, or more than 30 percent. Average plant height on May 27 was 11 inches, which is likely a record for late May; in 2004 the average was only 7 inches, and that was on May 30. So the earliest-planted fields--those planted in mid-March--accumulated more than 1,000 GDD before the end of May. These fields need to accumulate only about 300 or so GDD to reach pollination. With normal temperatures this will happen by mid-June.
Along with being warmer than normal, May has been drier than normal in all parts of Illinois; rainfall across regions has ranged from less than an inch to some three inches. While the lack of normal amounts of rainfall continues to be a concern, fields where the roots are tapping well into the soil water generally have reasonably uniform growth and good crop color.
Other benefits to dry May weather are the near-total absence of drowned-out field areas and of the excessive losses of N that have followed wet spring weather in recent years. Soil conditions remain conducive to deeper rooting, and this could provide real benefits if dry conditions occur later in the season. Some plant diseases that require wet weather to develop are also minimized.
So on balance, the warm, dry weather has been favorable, and we do not believe that there has been any substantial loss of yield potential in most areas up to now. But of course the point will come where current soil water supplies can no longer provide water at rates high enough to sustain maximum growth rates, and the need for rainfall will become more urgent.--Emerson Nafziger