Issue No. 4, Article 9/April 17, 2009
Are Your Corn Stands "Precise" Enough?
Many producers have heard about the dangers of "imprecise" corn seed placement and of ways to fix this with various after-market products designed to produce that "picket fence" stand that we all want. Even those who have decided not to spend the money to "upgrade" relatively new planters with such equipment wonder if this decision will cost them yield and money.
The picket-fence stand is probably not often quite as uniform in terms of plant size and spacing down the row as we imagine it to be. But we all admire a good, even stand of corn, and "precision" certainly has visual appeal. Of course, by the time plants reach their mature height, we can see "drive-by" precision mostly as the uniformity of spacing of the plants in outside rows. Plants in outside rows tend to be more uniform in size than those inside the field, because they get more water and sunlight. They also tend to be planted at slower speeds and may benefit from that as well.
Even the best-planted fields can sometimes develop uneven plant size and plant spacing as the result of conditions after planting that lead to stand loss or plant damage. Such effects can only be seen by getting out into the field, and in many cases there might be nothing that could have been done differently to avoid such problems. We have found that plants respond to loss of competition from their neighbors (that is, by loss of stand) by increasing their ear size. Of course, the increase in ear size of remaining plants can't compensate fully for the loss of plants, so stand loss almost always means lower yields per acre.
Plant spacing variability has usually been measured as the standard deviation (SD) of plant-to-plant spacing. Taking individual plant-to-plant spacings is time-consuming, and so many of these measurements are made on fairly small numbers of plants (and small sections of row), often without enough samples in a field to get very good estimates. It is very easy to find one section of row where uniformity looks good and another section, often very close by, where uniformity doesn't look very good, especially when one or two missing plants or doubles is all that it takes to make such a difference. All this means that "precision" of spacing is not very easily measured, even though we often think it's easy to gauge by eye.
Here is a summary of findings and thoughts on issues having to do with corn plant stand establishment and uniformity:
1. Uniformity of plant spacing in the row takes on less meaning as populations increase, and it is nearly meaningless at plant populations in common use todaythat is, with average plant spacing often about 6 inches or even less. Plant spacing variability (SD) of 6 to 8 inches that some have reported is virtually impossible to produce at plant populations in the 30,000 range or higher, unless the planter malfunctions or we deliberately set out to produce such an effect.
2. The term "precise" is relative--the least precise stands today are probably better than the best stands we could produce with shoe openers and plate-type planters in the 1960s. When you can see a shifting pattern of aligned plants as you drive by a field, you know that the drop spacing was relatively uniform. One extensive measurement of spacing uniformity in a farm field produced a standard deviation of 2.4 inches, while that in a "sloppy" field nearby was 3.4 inches. This is probably about the range we can expect in fields with full stands.
3. Skips (due to missed drop or failure to germinate) and doubles contribute almost equally to nonuniformity of stand as measured by the SD. But skips and doubles have opposite effects on yield at lower populations and very little effect at high populations; they affect yield to the extent that they affect stand, and effects of stand on yield depend on where along the plant population curve the stand is.
4. If we are reaching our target population, there is almost no chance that uniformity of interplant spacing has a measurable effect on grain yield. If there is an effect, it is likely too small to spend money to correct. As a practical issue, we can never get to complete precision (SD = 0) in a field, so whether or not yield is lost from having an SD of 3 inches instead of 2 inches is not possible to answer.
5. Some data that have been used to show that uniformity of plant spacing pays are not presented correctly. One study we did with stands thinned to be very "sloppy" and with individual ears harvested showed that, at 30,000 plants per acre, there was a lot of ear size variability regardless of how much space plants had in the row (Figure 3). When we converted the data by dividing the yield per plant by the space that each plant occupied, there seemed to be a huge advantage to plants spaced closer in the row (Figure 4). Which is the accurate way to show such data? It is clear that we can show them in a way that makes precision seem to "pay" or, conversely, in a way that makes it look rather meaningless.
Figure 3. Grain yield per plant in a corn stand thinner to create high plant spacing variability. The population was 30,000 plants per acre, and the standard deviation of plant spacing was 52 inches.
Figure 4. Data from Figure 3, calculated as “bushels per acre” by dividing the grain yield per plant by the amount of space each plant had in the row.
6. Uniformity of seed placement is important, to the extent that it affects uniformity of emergence. Uneven plant size early translates into yield loss. This means that efforts to place seed at uniform depth and into uniform soil conditions are likely to pay off. Whether this requires some added equipment or modification is not clear. Those who plant carefully into uniform soil conditions and at speeds within the range recommended by the manufacturer might well see little difference from adding seed formers or other devices to improve uniformity, while those who want to plant at high speeds and into soils with root balls and other types of interference may find some advantage in add-ons to help keep seed in uniform conditions.
7. Some who have had problems with shallower- or deeper-planted corn might advocate changing the planting depth, but reacting to problems in a previous year is often not very helpful in the current year. Planting 1.75 inches deep is a good target. Though uniformity of depth is not easy to measure, as a practical guide you shouldn't be able to find seeds at depths more than about a half inch above the bottom of the seed furrow. --Emerson Nafziger