Issue No. 8, Article 12/May 18, 2007
Evaluating Corn Stands
Thanks to less rainfall than had been forecast in many areas, Illinois producers planted nearly the entire 2007 corn crop by mid-May. Though I wrote two weeks ago that the low percentage planted by the end of April might predict yield loss, the rapid progress during the past two weeks has, we hope, made that prediction too low.
Most Illinois corn fields have what appears to be good to outstanding stands, with very high emergence percentage and good uniformity. While this is not the case in every field, I do not think that we need to talk much about replanting, since it's likely to be rare in Illinois this year. Exceptions might be areas that received a lot of rainfall or where cutworms have taken out many plants.
Most people have done a great deal of stand counting, and there's little need to go into detail about how to do this well. Fairly uniform stands do not need many counts to provide a good estimate, while highly variable stands require a considerable number of counts. It's common in this type of spring to have low stands only in parts of fields, though, such as in areas where water stands or cutworms attack. Taking counts only in the affected area is more useful than taking field average counts in such cases, especially if any replant will be confined to damaged areas only.
While emergence uniformity has generally been very good, there may be planter issues in some fields that caused non-uniform distribution of plants down the row. Because this is an issue that seems to persist, in part because of competitive efforts by planter companies, we have continued to look at stand distribution in a few studies. It has become convention to evaluate plant spacing uniformity using the standard deviation, which is calculated using individual plant-to-plant distances. A perfect stand with the same distance between all plants would have a standard deviation of 0. Standard deviations measured in actual fields range from about 2 inches, which is very uniform, to more than 6 inches, which is very "sloppy." Lower plant populations usually have larger variability (standard deviation values), because skips are longer. Missing plants and doubles both contribute about equally to raising the standard deviation.
In a study conducted at DeKalb and Urbana in 2006, we thinned a "uniform" 36,000 plants per acre to 30,000, 24,000, or 18,000 plants by removing plants either on a uniform pattern or on a deliberately "random" pattern. Table 3 shows standard deviations and yields, averaged over the two locations.
Plant spacing variability had little effect on yield at either 18,000 or 30,000 plants per acre, but the "random" stand yielded about 13 bushels less than the uniform stand at 24,000 plants per acre. We think the principle at work here is that the yield of individual plants at relatively high populations (30,000 or more) is strongly limited by closeness of their neighbors, and having those neighbors a little closer or farther apart doesn't make much difference. At low populations (18,000 or less), individual plants are not "crowded" by their neighbors (plants are about 12 inches apart at this population), so having neighbors a little closer or farther away doesn't affect yield much. At 24,000 plants per acre, where plant spacing averages about 9 inches, plants crowded by closer neighbors yield less, while those with neighbors farther away don't yield much more than at lower populations, so the average yield is less than when spacing is uniform.
In production fields, we think that these findings support some of our earlier work that showed that plant spacing variability has little effect on yield as long as populations are relatively high. In one study, we found that skips decreased yield and doubles increased yield, but only to the extent that they resulted in differences in plant population. In other words, population is the key, and variability in plant spacing means little when population is high enough. There are exceptions--for example, when planters malfunction and leave large gaps and clusters of plants. But modern planters with monitors provide good protection against such problems.
Plant size uniformity is probably more important in most fields than plant spacing variability. Because size variability is difficult to measure and difficult to cause, it has not been studied very much. From our work a number of years ago, we know that uneven plant size (caused in that case by planting at different times) reduces yield, to the point where, if a plant is planted three weeks after its two neighbors, it will often yield nothing. We concluded that all size differences have the potential to reduce yield but that the differences in size have to be substantial (three to four leaf stages early in the season) before yield loss starts to mount up.
What about size differences among plants in relatively uniform stands? I recently noted a smaller plant between two larger plants in what appeared to be a very uniformly emerged field planted at Urbana on April 2.
Varying plant sizes in a corn field planted at Urbana on April 2.
It is possible that this plant emerged later than its two neighbors, though emergence was more uniform than current plant size seems to be. The smaller plant is at stage V3 (3 leaf collars visible) while those on both sides are in late V4, so probably about 1.5 stages more advanced. This difference in stage will persist as the plants develop; in fact, because the growing degree-days (GDD) needed to produce each new leaf decrease at later stages, the stage difference might well increase. The problem is that the smaller plant, even if the cause of its being smaller isn't persistent (that is, if there isn't insect damage or some other "permanent" problem), will face increasing competition for water, light, and nutrients, and it could well end up yielding much less than its neighbors.
If you have similar unevenness in plant size, it might help (next year, at least) to see whether smaller plants have experienced something that might have caused them to emerge later or grow more slowly after emergence. Candidates for such causes include damaged seed that germinates and emerges late, old crop residue that lands on top of seed to keep it from coming up on time, planting into unevenly moist soils that required rainfall to bring some plants up, and disease or insect damage to the roots or the mesocotyl (the tissue that connects the seed and the base of the plant). Smaller size differences might be due simply to some seed landing with the germ face up and some down, with more growth needed for the latter to emerge.
Beyond noting that size differences exist and perhaps finding possible causes, we do not have a good way to estimate the yield effects of such unevenness in plant size. Part of this is because size variability is seldom "neat," with just two sizes of plants, but rather consists of a range of sizes. The effects on individual plants depend on the relative sizes of the two (or four) neighbors on either side. We have even found that size differences across rows can affect yield. It makes a good exercise to mark plants by height and stage, either on a map or with small stakes or labels, then come back at the end of the season to take individual ear weights and relate these to plant size differences. Remember that the size of such plants relative to their neighbors changes during the season, so be sure to record the time when size differences were measured. Watching these plants change over the season might also provide some useful information.--Emerson Nafziger