Farmers intending to replace old planters have a multitude of recent innovative features to consider, including planters with the ability to seed more than 1 hybrid or variety within a field. These new planters, often referred to as multi-hybrid or multi-cultivar planters, are configured with the equipment needed to automatically switch between 2 or more crop cultivars on-the-go. This technology allows growers to use prescription maps to match hybrids or varieties with specific field conditions, and will likely be most beneficial in fields with variable landscapes. Initial implementation has largely focused on variable placement of corn hybrids, but the technology could potentially be used with any crop.
Precision farming pioneers have long envisioned that hybrid or cultivar would be an important input for variable management (Dudding et al., 1995). Extension agronomists consistently rate corn hybrid selection as 1 of the most important factors for maximizing yield (Coulter and Van Roekel, 2009; Elmore et al. 2006; Thomson McClure, 2014). Variable cultivar planting takes this management decision to a higher level, allowing growers to choose the best-adapted cultivar for each part of the field.
Commercial availability of multi-cultivar enabled planters makes it easier than ever to deploy a zone-based management strategy for crop cultivar selection. This Crop Insights discusses strategies to identify candidate fields and develop appropriate multi-cultivar prescriptions, as well as review some potential applications for multi-cultivar planting in corn and soybeans. Although many of the principles discussed can be applied to numerous crop species, the focus of this article will primarily be multi-hybrid strategies for corn.
Two conditions are necessary for a multi-hybrid planting strategy to provide a yield advantage. First, there must be significant within-field variation in yield due to environmental or management factors, including landscape topography and other soil variables (i.e., the more uniform a field, the less likely that multi-hybrid planting will increase yield). Secondly, there must be a difference between hybrids in yield response to the within-field environmental variation.
A statistical technique originally developed in the 1960s (Finlay and Wilkinson, 1963; Eberhart and Russell, 1966) has commonly been used to describe yield stability of a corn hybrid across a range of environments. This method involves developing a linear regression of yield for a given hybrid vs. the average yield of all hybrids tested across the same (multiple) environments. This provides a measure of relative yield stability for a given hybrid. A regression slope of 1 represents average yield stability; with more stable hybrids (commonly referred to as “defensive” or “workhorse” hybrids) having a slope 1 (Figure 1).
The average yield of all cultivars at a location is referred to as that location’s environmental index. Although originally developed to characterize yield stability across multiple locations, this same model can be applied to assessing hybrid response to variability within a field and evaluating the potential value of variable hybrid planting.
Figure 2 shows a hypothetical field in which yield performance is nearly (or “relatively”) constant across the entire field. In this scenario, yield would be maximized by planting the highest-yielding cultivar across the entire field.
This example visualizes a scenario in which the environmental factors in question have little or no influence on yield of either cultivar. In reality, all fields have some degree of spatial variation in yield due to environmental or management factors; the greater this variation, the more potential there is for differential placement of cultivars within the field to increase yield.
Figure 3 shows a hypothetical field in which yield varies due to environmental factors, but the 2 cultivars respond similarly to the environmental variation. The environmental index in this example could be reflective of any environmental factor or combination of factors that contributes to spatial variation and impacts grain yield, such as drainage, disease pressure, or soil properties; or management factors such as tillage or crop history. Although this field has substantial variation in yield across the landscape, cultivar A still out-yields cultivar B across all environments in the field; therefore, yield would be maximized by planting the entire field to cultivar A. It is important to remember that substantial variability in yield across a field does not automatically mean that variable placement of 2 cultivars will provide a yield advantage.
Figure 4 shows a scenario in which both conditions are met for multi-cultivar planting to be advantageous: variation in yield due to environmental or management factors and differential cultivar response to this variation.
In this field, yield would be maximized by planting cultivar A in the higher-yielding regions of the field and cultivar B in the lower-yielding areas, a scenario represented by the solid lines in the figure.
Pioneer split-planter trial near Harlan, Iowa in 2001. Split-planter trials have been used extensively over the past 20 years to study the value of variable hybrid placement.
Even though commercial availability of multi-hybrid planting technology is relatively recent, the potential value of within-field variable hybrid placement has been studied extensively by Pioneer and university scientists for the past 20 years. Studies have typically involved using a conventional planter to plant 2 hybrids across a field using a split-planter arrangement. This method allows paired comparisons of 2 hybrids throughout an entire field to determine if they performed differently in different environments within the field. Numerous Pioneer on-farm split planter trials were conducted beginning in 1996, when the rapid adoption of yield monitors among growers made collection of spatial yield data across entire fields feasible for the first time (Doerge and Gardner, 1998) (Figure 5 and Figure 6).
Results of university split-planter studies generally have not supported widespread implementation of multi-hybrid planting. In the majority of studies, the hybrids responded similarly to within-field variation. A 3-year split-planter study conducted in 5 fields in New York found that spatial variability in yield differences between hybrids occurred in only 4 out of 15 site-years (Katsvairo et al., 2003). A study conducted from 1997 to 1999 in dryland production in eastern Colorado also found that the 2 hybrids tested responded similarly to in-field variation (Shanahan et al., 2004). Two studies conducted at multiple locations in eastern Illinois both found that there was no significant spatial variability in yield differences between hybrids in most fields tested (Miao et al., 2006a; 2006b).
A study was conducted by Pioneer and USDA researchers in the late 1990s using split-planter experiments to evaluate the potential yield benefits of variable hybrid planting in irrigated and dryland corn production in the far western Corn Belt (Doerge, 2000). Results showed that variation in grain yield across the landscapes in test locations was associated with site characteristics that do not change over time, such as elevation, pH, organic matter, soil color and soil electrical conductivity; however, there was no evidence that hybrids responded differently to these site characteristics at either the dryland or irrigated locations in any year of the study. These results are consistent with the scenario shown in Figure 3, in which environmental variation exists but hybrids responded similarly to it, making the best management strategy to plant the higher performing hybrid across the entire field.
One noteworthy aspect of all of these studies is that the hybrids used generally were not selected based on any specific agronomic characteristics. In the study conducted by Shanahan et al., an early maturity and late-maturity hybrid were compared. The other studies simply compared hybrids that were commonly used within their respective regions at the time. Even though the results of these studies did not show that variable hybrid planting would have provided much value in most cases, they do not rule out the possibility that a multi-hybrid management strategy using hybrids selected based on specific agronomic characteristics appropriate for certain field areas could be beneficial. The accumulated body of research in this area suggests that the greatest likelihood of success with multi-hybrid planting would be to target implementation to select, highly-variable environments, using hybrids carefully selected based on yield-limiting factors in the field.
Initial attempts at developing multi-hybrid planting prescriptions have often followed in the footsteps of strategies developed for variable-rate seeding. Variable rate seeding prescriptions typically involve varying the seeding rate based on spatial variation in yield potential, where more productive areas usually receive a higher seeding rate (in the case of corn) and less productive areas a lower rate. Management zones are developed according to expected yield performance, often using past yield history as a basis, or soil characteristics as a proxy for productivity (Butzen et al., 2009). Applications of this framework to multi-hybrid planting have typically involved splitting a field into higher-yielding and lower-yielding areas and then planting an “offensive” hybrid to the high-yield areas and a “defensive” hybrid to the low-yield areas.
This method of creating multi-cultivar prescriptions offers the advantages of being widely applicable and relatively straightforward to develop and execute. However, research suggests that it is unlikely to provide yield benefits in corn on a consistent basis, for the simple reason that very few modern hybrids meet the criteria of being truly “offensive” or “defensive.” A recent review of performance data on over 2,500 corn hybrids tested in 7 or more environments found that only 6% met the definition of offensive (slope >1) and 8% met the definition of defensive (slope < 1), while the vast majority of hybrids (86%) were classified as “stable” (Lauer and Hicks, 2005). These findings are not surprising given that modern corn breeding programs have largely focused on developing hybrids that will provide consistent performance across a wide range of environmental conditions (Pierce and Nowak, 1999).
Consider source of yield variation
To realize a benefit from multi-hybrid planting, it will most likely be necessary to go beyond simply characterizing spatial yield variation — understanding the factor or factors driving that yield variation and selecting hybrids accordingly will be required. By comparison, variable-rate seeding per se is generally simpler because the only 1 management criterion is under consideration – seeding rate, which is adjusted either higher or lower based on productivity or other factors. Modern hybrids are typically characterized for numerous agronomic traits such as drought tolerance, disease resistance, root strength, etc., which provide a wide range of potential criteria for creating multi-hybrid prescriptions. Figure 4 illustrates 1 of the 2 conditions necessary for multi-hybrid planting to be advantageous is a differential hybrid response to variation in productivity. Knowing both the field conditions and hybrid characteristics for success under those conditions is critical.
One of the environmental factors most likely to provide the basis for a successful multi-hybrid management strategy is soil moisture. This factor meets both the criteria for successful multi-hybrid planting — too much or too little soil moisture causes substantial variation in yields, and crop cultivars frequently differ in their response to insufficient or excessive moisture. Just as importantly, these differing responses are usually well-characterized. An example of recent research in this area is a collaborative study between Pioneer, Raven Industries, and South Dakota State University comparing conventional and variable planting at several locations in South Dakota. This study involved placing hybrids with greater tolerance to wet conditions in low landscape positions where there was likely to be excess moisture early in the season and more drought-tolerant hybrids at upper landscape positions likely to experience drought stress later in the season. Preliminary findings from the study have shown promise for this strategy, with yield benefits in the range of 5 to 8 bu/acre at some of the study locations. (Sexton et al., 2013; 2014).
There are many other possible applications and placement criteria for multi-cultivar planting. A multi-cultivar prescription could potentially involve multiple criteria, such as planting a drought-tolerant hybrid on high ground prone to moisture stress, and a disease-tolerant hybrid on low ground prone to a foliar disease. In some cases it may prove beneficial to select hybrids or varieties based on a predetermined multi-cultivar strategy, whereas in other cases the greatest benefit may be derived by first selecting the best available genetics and then using a multi-cultivar planter to optimize their placement.
Not all potential applications would necessarily require a multi-cultivar planter to execute, but may be easier to implement with the ability to switch cultivars on the go. Some applications may have limited utility now, but could become more valuable in the future with the development of new genetics and technologies. If multi-cultivar planting is widely adopted, it is possible that new technologies could be brought to market specifically to make use of this capability.
Soil moisture. As previously discussed, soil moisture is probably the most obvious candidate to form the basis of a multi-cultivar strategy. Multi-cultivar planting could allow a more drought-tolerant cultivar to be planted on hill slopes, sandy areas, or other areas prone to drought stress. Drought-tolerant cultivars could also be planted in pivot corners in areas with central pivot irrigation. Conversely, a cultivar more tolerant to saturated soils or “wet feet” could be planted in low-lying or poorly drained areas.
Disease resistance. Hybrids or varieties with greater genetic resistance to disease could be placed on low-lying ground or other areas more prone to disease. Disease-resistant hybrids or varieties could also be placed in areas that are inaccessible for aerial applications of foliar fungicide such as along tree lines, near wind turbines or powerlines, or near populated areas.
Stress emergence. Stress emergence ratings for Pioneer® brand corn products help categorize their genetic potential to emerge under stressful environmental conditions (including cold, wet soils or short periods of severe low temperatures) relative to other products. Multi-cultivar planting could be used to place a hybrid with a high stress emergence rating in areas of a field prone to poor emergence conditions, such as productive areas that may have high levels of residue, or low-lying areas that are slower to dry out and warm up in the spring.
Insect resistance / refuge placement. Multi-cultivar planting would allow virtually limitless flexibility in placing structured insect refuges within a field. While this capability currently has limited utility in the Corn Belt due to the transition to blended refuge corn products, it could be useful in cotton-growing regions that require structured refuges, as a research tool, or possibly with a future insect protection technology in corn or other crops. Multi-cultivar planting would also allow selective placement of an insect-resistant hybrid or variety along fencerows or grass waterways to protect against insect pests that move in from field margins, or in areas of a field at higher risk of insect damage due to prior cropping history or management practices.
Herbicide resistance. With multi-cultivar planting, a hybrid or variety with additional herbicide resistant traits could be placed along field margins, field entrances, or grass waterways to allow spot-spraying for management of weed species moving in from adjacent fields or fence-rows, or from seed brought in on machinery. Planting a herbicide-resistant hybrid or variety along a field margin could also be used to protect against herbicide drift from an adjacent field.
Iron deficiency chlorosis tolerance. Soils with pH above 8.0 can result in alkalinity-induced chlorosis and reduced yield in corn and soybean. Corn hybrids and soybean varieties both vary in their tolerance to elevated soil pH. Multi-cultivar planting would allow planting a hybrid or variety tolerant to chlorotic conditions in some areas of a field, and another hybrid or variety which is more productive on lower pH soils. A Pioneer / University of Nebraska study conducted during 1998-2001 explored the possible value of multi-hybrid planting for increasing corn yield on high pH soils in Nebraska, although weather conditions during the study were generally not conducive to inducing chlorosis symptoms and results were ultimately mixed (Doerge 2002).
Standability. Multi-cultivar planting could potentially be used to help reduce the impact of lodging on yield. In the case of corn, this could involve planting a hybrid with stronger roots and/or stalks along field edge or other area prone to wind lodging. For soybeans, a shorter stature variety could be placed on highly productive soils prone to lodging due to excessive plant height.
Maturity. Generally, planting of similar maturity of hybrids or varieties would be an important component of a multi-cultivar strategy; however, in some cases it could prove advantageous to selectively place products with differing maturities. A shorter maturity hybrid or variety could be placed in low-lying areas or other parts of a field prone to slow maturity and drydown in the fall. A shorter maturity hybrid could also be placed in the end rows of a field to allow the field to be opened up earlier in the harvest season, prior to harvesting the rest of the field.
Variable placement by hybrid or variety maturity could also be used to mitigate frost risk. Cold air accumulates in low-lying areas, putting them at a greater risk of frost damage. Placement of an earlier maturity hybrid or variety in these areas could reduce the risk of frost damage prior to physiological maturity, while allowing a fuller season hybrid or variety to be placed on higher ground less susceptible to early frost.
Seed-applied technology. Multi-cultivar planting could be used as a means to selectively place products based on seed-applied technology rather than genetics, or potentially based on both. As an example, this could involve placement of seed with a specific fungicide seed treatment in a part of a field prone to disease or an insecticide seed treatment in areas of a field at higher risk of insect damage due to prior cropping history or management practices. Populations of nematode species are known to vary by soil texture, with larger and more damaging species often more prevalent in sandy soils. Seed with a nematicide seed treatment could be placed in portions of a field at greater risk for nematode damage. With the rapid growth in seed treatment and seed-applied technologies, the potential applications for selective placement using a multi-cultivar planter will likely expand in the future.
Seed production. Multi-cultivar planting technology could be useful in hybrid seed production, as it could allow greater flexibility and efficiency of planting male and female rows and border rows.
Although it’s hypothetically possible that a poorly designed multi-cultivar prescription could actually result in yield loss, this outcome is probably unlikely in most cases. The more realistic risk for most growers would be that multi-cultivar planting provides no yield advantage, or a yield advantage that is insufficient to offset the additional cost and complexity associated with multi-cultivar planting. Multi-cultivar planting would substantially increase the complexity of planting operations due to the need to create prescriptions and handle a larger number of cultivars. Multi-cultivar planting would also likely increase the frequency of planter fills, particularly if a prescription is heavily weighted toward one product, which would increase the amount of time needed for planting.
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¹Pioneer Agronomy Information Manager
²Pioneer Agronomy Research Manager