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Plant Spacing Uniformity

May 15, 2003

In many areas in Illinois, this is a good time to look at corn stands, including both plant spacing uniformity and uniformity of emergence. Uniformity of emergence for those fields planted so far is generally very good; when soils remain relatively dry as the crop emerges, usually few barriers to emergence and so few problems with emergence time uniformity exist. We planted one study here on two dates (March 24 and April 23), and for both planting dates, emergence was complete within 2 to 3 days of when it started. The March planting emerged in 22 to 24 days, and the April planting in 12 to 13 days. Most stands I have seen in fields look very uniform in plant size, though standing water or soil washed off or onto emerging plants will cause problems in some areas.

The question of plant spacing uniformity continues to come up, though the replacement of older planters continues to gradually improve the uniformity of seed drop spacing down the row. I would sum up the results of recent research on plant spacing uniformity as follows: Within reason, if a planter is dropping the desired number of seeds per acre with good depth control and adequate seed covering, plant spacing variability is costing little or no yield loss in corn. When I say "within reason," I mean that the planter is not doing things like plugging up and accumulating seed to drop in heaps, that the vacuum or finger pickup units are properly set and maintained, and that the planter monitor shows reasonably uniform drop among units.

I realize that the preceding statement is not accepted by everyone, and I am aware of some field studies that show that improving "sloppy" stands can increase yields. Our own studies, plus data from Wisconsin, Ontario, and other places, though, show little or no yield response to changes in plant spacing variability. Plant spacing uniformity (or variability) is measured most often by "standard deviation" (SD), a calculation of how much plant-to-plant spacings differ from their average. A perfect "picket fence" stand, with every plant exactly the same distance from its neighbors, has a standard deviation of zero. In field measurements I have made, SD values have ranged from about 1.5 to 5 inches or so. To give you an idea, perfectly spaced seeds 7 inches apart but with 1 seed in 10 failing to emerge gives an SD value of 2.3 inches. One double rather than a skip in 10 seed drops gives an SD of 2.1. One skip and one double per 10 drops, which would mean a full stand, produces an SD value of 3.3 inches.

One problem with using standard deviation to calculate uniformity of stand is that it is so laborious; most people measure it by the "ruler-on-a-stick" method, where each distance between adjoining plants is recorded for consecutive plants down a length of row, usually for 20 to 50 plants or so. That's fine for that section, but because of the nature of skips, doubles, and other things that affect plant spacing, a section of row 15 to 30 feet long usually does not represent a field very well. I have used a "Space Cadet" that was developed some years ago, which can accumulate 100 plant spacings, and I usually take such counts 6 to 10 times within a field. Even that seems inadequate sometimes, especially when a skip 2 or 3 feet long in one segment doubles the SD. Simply, no fast and easy way exists to get a good measurement of plant spacing variability in the field.

We know from research that doubles, which have about the same effect on SD as skips, do not have the same effect on yield; many doubles will produce two ears if conditions are good and can even increase yield, while skips almost always decrease yield. It may be that SD should be replaced by a method that measures number of skips, say, longer than two or three times the average seed drop distance, per 100 feet or per 200 or 300 plants. We don't have the data to fine-tune this concept, but skips and loss of population, not seeds dropped an inch or two away from "dead center," are the real problems when plant spacing uniformity is less than we'd like, and they're the ones we need to pay attention to.

Some people have wondered if uniformity of plant spacing is also a concern in soybean. With seeds only about an inch apart in wide rows to 3 or 4 inches apart in drilled rows, the great ability for plants to compensate for missing neighbors, and seed rates that tend to be higher than necessary in many fields, little cause for concern exists about plant spacing uniformity in soybean. Improvements in metering soybean seed, however, and improvements in metering mechanisms to reduce seed injury have been welcome changes. We expect to pay increasing attention to improving efficiency of soybean seed use, including eventual sale of seed by number and more precise dropping by seed number rather than pounds per acre. Improved planting mechanisms will make such changes possible.--Emerson Nafziger

Author: Emerson Nafziger

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