Plant Spacing Signature
Any analysis of plant spacing uniformity effects must consider the proximity of both of a plant's two neighbors. This is one of the shortcomings of using the standard deviation of inter-plant spacings as the single index of plant spacing uniformity. A simple two-dimensional spatial analysis technique was developed to overcome this limitation (Figure 2 below).
Figure 2. Plant spacing analysis tool that
depicts the position of individual plants versus
the distance to that plant's two within-row neighbors.
In a perfectly planted field with 30-inch rows and a population of 29,840 plants/acre, the within-row distance between all plants would be exactly 7.0 inches. If each plant were plotted as a dot, all of the dots would fall on the same [7,7] point on the graph. However, skips, doubles and triples inevitably occur in all commercial corn fields. Figure 2 graphically shows the different plant spacing outcomes that occur in commercial corn fields.
Finally, the within-row distances to the two neighbors of hundreds or even thousands of plants can be plotted on these axes to give a visual representation of plant spacing uniformity within a whole field. This can be thought of as the "plant spacing signature" of that field.
Figure 3. Plant spacing signature
for the Iowa 2 locations.
Visual inspection of a plant spacing signature gives an immediate indication of the quality of overall plant spacing uniformity for a particular field. This is depicted in Figure 3 for the Iowa 2 location.
Yield Maps Based on Plant Spacing
This plant spacing analysis tool can also be used to create yield contour maps that graphically show the effect of within-row plant area and plant spacing uniformity on individual plant yield. Geographical Information System (GIS) software is used to create these maps. Plant spacing yield maps are similar to regular yield maps developed from yield monitor data. The difference is that in plant spacing analysis, the coordinate system is based on the distances to a plant's two neighbors, instead of latitude and longitude. Figure 4 shows the plant spacing yield maps for the four locations studied in 2001.
The Effect of Within-Row Plant Area on Yield
These maps provide two key findings concerning the effect of within-row plant area on grain yield. One of these results was expected, the other was not.
As expected, individual plant yield on a per/acre basis decreases continuously the farther it is positioned from the origin [0,0]. This confirms previous research findings that increases in plant spacing standard deviation due to "skips" or missing plants always result in reduced overall yields (Nafziger, 1996).
The remarkable finding at all four locations is that plants at very close spacing (nearest the origin) are consistently the highest yielding per acre. For these occasional closely-spaced plants (comprising less than one percent of total plants in the four fields studied), there was no evidence of increased barrenness. These findings indicate that increases in plant spacing standard deviation due to occasional closely-spaced plants may actually improve overall grain yields.
The Effect of Plant Spacing Uniformity on Grain Yield
These plant spacing yield maps also provide new information on the effects of plant spacing uniformity at a constant plant population. In Figure 4, plants falling on the solid diagonal line connecting the [0,14] and the [14,0] points for the Iowa 1 location would all have the same within-row plant distance (7 inches), but would have quite different plant spacing uniformity.
This type of yield map can answer questions like this one: "Do plants at perfect 7 X 7 inch spacing yield more than plants at 14 X 0 inch spacing?". At perfectly uniform spacing, the expected yield is 143.4 bu/acre. At the [14,0] point, spacing is at its worst, and the expected yield per plant is 108.8 bu/acre.
Figure 4. Plant spacing yield maps for the 2001 locations:
Missouri | Iowa 1 | Iowa 2 | Minnesota
Individual plant yield increased with improving plant spacing uniformity, but only to a point. In general, individual plant yield reached a maximum level when plants were within 2-3 inches of perfect equidistant spacing. This same pattern was observed for all within-row plant distances and across all locations.
These findings help explain why the relationship between grain yield and whole-field plant spacing standard deviation is not necessarily consistent from field to field. Plant spacing standard deviation arising from missing, misplaced and extra plants all have different effects on yield. Poorly spaced and missing plants decrease yield while occasional extra plants, if anything, increase yields slightly.
Benefits of Improved Plant Spacing Uniformity
Plant spacing yield maps can also be used to estimate the potential benefits of improved plant spacing uniformity in commercial corn fields. This is done by comparing the actual yield measured at a location to the predicted yield if all of the plants were at perfectly uniform spacing. These estimations are summarized in Table 2 for the four locations used in this study.
Improving the plant spacing standard deviation from its original level down to zero improved yields by 7 to 19 bu/acre. For these four sites, the change in yield per inch of improvement in plant spacing standard deviation ranged from 1.1 to 6.1 with an average of 3.4 bu/acre/inch. This is very similar to the findings of Nielsen's (2001) extensive on-farm survey in Indiana and Ohio.
Table 2. Estimated yield improvement due to improved plant spacing at four on-farm locations in 2001.
| Missouri |
10.8 |
6.9 |
102 |
0 |
109 |
1.1 |
| Iowa 1 |
6.2 |
3.2 |
139 |
0 |
158 |
6.1 |
| Iowa 2 |
7.1 |
3.6 |
163 |
0 |
174 |
2.9 |
| Minnesota |
7.1 |
3.8 |
193 |
0 |
206 |
3.3 |
| Average |
--- |
4.4 |
149 |
0 |
162 |
3.4 |
These results suggest that even if the aggregate plant spacing standard deviation in a commercial field is improved by only one inch, the resulting yield benefit would be about 3.4 bu/acre. The yield increase needed to just offset the cost of planter meter calibration for a 600-acre corn grower using a 12-row planter is only 0.5 bu/acre (Doerge and Hall, 2000).
Non-uniformity in corn stands places a grower's sizable investment in their planter, seed and other inputs at risk of lower returns. Calibration of planter meters every 1-2 years is an inexpensive way to help manage that risk.
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