4. Achieve Uniform Plant Spacing
Growers instinctively prefer corn stands with uniform plant-to-plant spacing. A "picket-fence" stand is both aesthetically pleasing and presumably higher yielding.
How is plant spacing uniformity measured?
Seeding specialists and agronomists have long used 2-related statistics, coefficient of variation (CoV) and standard deviation (SD), as the preferred metrics to quantify meter performance and plant spacing uniformity. A SD value of 2.0 inches or the corresponding CoV value of 0.33 are widely cited as the thresholds above which, corn yield loss would be expected (Nielsen, 2001). The CoV is easily calculated by dividing the SD by the average plant spacing. For example, the SD corresponding to a CoV of 0.33 at an average spacing of 6.0 inches is 2.0 inches. More recently, engineers have also devised a "singulation" metric as an indicator of seed spacing uniformity, although there is no industry standard as to how it is calculated.
Spacing metrics poorly correlated to yield
Agronomists have long known that the various planting outcomes that result in increasing CoV and SD, and declining singulation values can have widely different impacts on resulting individual-plant grain yield (Nafziger, 1996; Doerge et al., 2002 and Nafziger, 2006). Thus, the use of easy-to-measure plant spacing metrics that are poorly correlated with individual plant yields has unfortunately created a tradeoff between convenience and accuracy. This has no doubt contributed to inconsistent results in past research seeking to explain the impact of within-row plant spacing on corn grain yield (Krall, 1977; Nafziger, 1996; Nielsen, 2001; Doerge and Hall, 2000; Doerge et al., 2002; Lauer and Rankin, 2004; Liu et al., 2004a, 2004b, 2004c; Nielsen, 2006).
Individual plant yield determinations provide new insights
A 2002 Pioneer study (Doerge et al.) uniquely allows for quantifying the impacts of common and realistic non-ideal planting outcomes on grain yield. This study was conducted in 4 different environments (2 in Iowa, 1 in Missouri and 1 in Minnesota), across a wide yield range of 109 to 206 bu/acre, and using hybrids with 3 very different genetic pedigrees. In this study, within-row spacing measurements and grain yields were determined on >6,000 individual plants.
- As expected, differences in grain yields resulting from common non-ideal planting outcomes were indeed observed and are listed in Table 1.
- These non-ideal planting outcomes typically, but not always, resulted in lower grain yield. The notable exception is that a double slightly increased yield. But yield losses for all other planting outcomes varied over a rather wide range, from zero to -0.26 lbs. grain for the 2- or 3-plant groupings depicted in Table 1.
- By far, a skip is the planting outcome that contributes the most to yield loss. Plants adjacent to a skip only partially compensate for the missing plant.
- In general, yield loss due to misplaced plants is negligible if plants are displaced from their preferred location by no more than ½ of the normal plant-to-plant distance.
Table 1. Corn grain yields resulting from various planting outcomes. Yield impacts are averaged across 4 study locations.
- The CoV and singulation readings on the planter monitor are valuable real-time indicators of meter performance but poor predictors of the agronomic consequences of common, realistic non-ideal planting outcomes.
- The planting outcome causing the greatest yield loss is % skips. A skip is defined as an in-trench distance between seeds of ≥1.75 times the desired plant-to-plant distance (for example, at 34,848 plants/acre in 30-inch rows, the average distance between seeds or plants would be 6.0 inches, and a skip would therefore be a plant-to-plant distance of ≥1.75 x 6.0, or 10.5 inches).
- Not all skips are caused by the planter. Missing plants resulting from unsuccessful germination or emergence will reduce grain yield just as much as planter skips and are to be equally avoided. Emerged plant spacing, along with population, gives the best prediction of yield performance.
- Occasional doubles do not negatively impact yield. A true planter double, or 2 seeds held in 1 cell of the planter meter, is not necessarily a bad thing. First of all, there is at least enough increase in yield from a double to offset the cost of the extra seed that is being planted. And if having an occasional double (i.e. 1-2%) helps ensure fewer or no skips, then such an outcome would be preferred. This would be the case if adjusting the planter vacuum setting upward could reduce the occurrence of empty cells, even if the higher vacuum setting increases doubles slightly.
Past inconsistent research results explained
There are several explanations for the lack of agreement in the results from past plant spacing studies, which were all conducted by highly-qualified researchers. First of all, different planting outcomes that contribute to SD, CoV or singulation can have completely different effects on individual plant yield (Table 2). Skips are highly detrimental to yield, doubles can be slightly positive, and misplaced plants have no effect on yield until plants are displaced from their preferred location by more than 1/2 the normal plant-to-plant distance. Second, no 2 fields can be expected to have the same amount or combination of non-ideal planting outcomes. Thus, it is no wonder that comparisons from aggregated, plot- or field-wide plant spacing studies are contradictory if the sources of plant spacing non-uniformity are not considered. Unfortunately, this lack of consideration has been true of most plant spacing studies.
Table 2. The contribution of 5 common non-ideal planting outcomes to 2 statistics used to describe seed and plant spacing uniformity, including Coefficient of Variation and Standard Deviation.
*Compared to perfect plant spacing.
Plant Population assumed - 34,848/acre or mean spacing - 6.0 in.
Other sources of confounding include the manner in which some field experiments have been conducted. For example, some studies have used highly artificial groupings of plants to achieve predetermined levels of plant spacing variability. In addition to being unrepresentative of "real-world" conditions, they often employ only different levels of misplaced plants (no skips or doubles) to achieve the desired spacing treatments. Table 1 and Table 2 indicate that these types of plant arrangements will have little to no impact on yield. Other plant spacing studies may have been compromised by the use of overplanting and thinning to achieve the desired plant spacing arrangements and populations. These practices are potentially confounding because corn plants can sense the presence of neighboring plants beginning very shortly after emergence due to subtle differences in the ratios of red: far red light they receive (Liu et al., 2009). These light quality differences can act as an early signal of pending competition that can initiate a shade avoidance response in the remaining plants. Thus overplanting and thinning can unintentionally result in plants that have been preconditioned to exhibit less favorable crop architecture and lower grain yield potential.
In contrast, when individual plant yields arising from different planting outcomes are considered, research results have been amazingly consistent. For example, Nafziger (1996) found that 10% skips in 4 Illinois experiments resulted in an average 8.1% decrease in yield (at 30,000 plants/acre) while the findings from the Pioneer study (Doerge et al., 2002) measured a corresponding 8.9% yield decrease (at a similar plant population). Likewise, the Illinois studies measured a yield increase of 4.2% for 10% doubles while the Pioneer data revealed a 4.7% yield increase. These similarities are notable since the genetics used in these two sets of experiments were released at least a decade apart.
Clearly, the key messages on within-row plant spacing uniformity are:
- it does impact grain yield and can be explained
- whole-field impacts on grain yield are usually relatively small, averaging about 1 to 2%
- growers should work to minimize or eliminate skips and not worry about occasional doubles or slightly misplaced plants