Grain Ear Flex Types and Management Considerations

INSIGHTS

• Ear flex is how the corn plants adjust in size and number of kernels as a response to stress or management practices.
• Understanding how management such as seeding and nitrogen rate influence ear flex characteristics is critical for management.

The ability of a corn hybrid to influence the number and depth of kernels is often referred to as its capacity to “flex”. Yield potential increases or decreases depending upon how hybrid flex characteristics interact with severity and timing of abiotic stress. The final number of kernels produced can be negatively influenced as early as the V5 growth stage while the potential number of kernels is being determined and remains vulnerable throughout early grain fill stages. The merit of a third flex trait (Figure 1), kernel size, is often underestimated. Kernel size and weight is highly influenced by favorable plant health and growing conditions throughout the latter half of grain fill stages (R3-R5).

Heritable traits like number of kernel rows, ovules produced per row and kernel size can influence how individual hybrids may respond differently to stress at specific growth stages. The term ear “flex” is commonly used to refer to how some hybrids with these phenotypic traits can take advantage of favorable environments. Flex characteristics can also provide insights to how some hybrids are able to minimize yield loss when under stress.

Figure 1. Examples of corn ear characteristics.

Characterizing Types of Hybrid Ear Flex

Ear flex characteristics almost always come up during hybrid selection and management discussions. Most often ear flex phenotypes are applied to seeding rate planning. As result of this, the seed industry has commonly placed hybrids into one of the following three categories:

1. Flex-ear hybrids that get bigger at lower populations with favorable agronomic conditions.
2. Semi-flex ear hybrids that maintain number of kernels and size at higher populations yet flex out at lower populations.
3. Fixed ear hybrids that have very small changes in ear size when planting at lower and higher populations.

Classifying hybrids into flex or fixed ear types is often done by observing ear sizes at low populations. However, most hybrids available get lumped into the semi-flex category that is not only qualified by their ability to flex at low populations, but their ability to “maintain” number and weight of kernels at higher populations or when stress occurs. Ear length is the easiest phenotype for agronomist to visually quantify, whereas the equally important but harder to quantify characteristic of kernel depth and weight is often overlooked, resulting in misclassification of hybrids. Since the ability of most hybrids to flex comes from multiple traits that influence kernel number as well as weight, ear flex scores can be more accurately determined from comparing grain yields at lower and higher densities. Planting lower densities allows plants to maximize individual plant yield potential, whereas at higher density, individual plant yield potential is limited due to neighbor competition.

Hybrid Flex Type Applies to Hybrid Management

Although the descriptions are self-explanatory, it can be challenging to understand which one provides the most advantages or disadvantages with specific growing environments and agronomic management practices. Some agronomists believe that by better classifying hybrids into one or more approaches describing how they flex may help to uniquely manage and minimize potential yield/economic loss

Understanding how and why hybrids flex differently opens the ways to take advantage of customized placement and management practices. The prevailing agronomic belief is that the following four flex traits, and their influence on physiological processes at specific crop growth stages, influences the amount of grain per plant produced.

1. Kernel row number (girth) flex hybrids could be negatively influenced by stress that coincides with when row number is being determined prior to the V6 growth stage.
2. Early kernel per row (ear length) flex hybrids could be negatively influenced by a stress occurring from V7-VT growth stages as the potential number of kernels is being formed prior to pollination.
3. Late kernel per row (tip back) flex hybrids could be negatively influenced by stress occurring during pollination and grain fill stages (R1-R3) that would result in aborted kernels and tip back. Split applying nitrogen and managing foliar disease are believed to be more important with these types of hybrids.
4. Kernel weight (size or density) flex hybrids are highly dependent on kernel weight from increased kernel depth or tightly pack starch molecules to maximize yield potential. These hybrids are more susceptible to stress such as drought or insufficient nitrogen in the last 30 days prior to black layer. Avoiding sandy soils without access to irrigation is an example of how to manage a hybrid like this.

In addition to these four traits, some hybrids are known to add additional grain per plant by producing a second ear at the node below where the primary ear forms. This is more frequently observed in the Western Corn Belt planted at ultra-low seeding rates that normally have low annual precipitation rates and no access to irrigation. Establishment of a second ear in these environments usually only occurs in years with above normal precipitation combined with low seeding rates.

The reality is that most hybrids are dependent on all four of the main flex traits to some degree. Using these principles to manage specific hybrids would be more valuable if seed company corn portfolios were more dependent on truly fixed (determinate) or flex (indeterminate) ear types of hybrids that were unable to respond to stand loss or environmental stresses.

In relation to management choices, plant density, nitrogen rate and hybrid selection are three of the easiest changes to make. Determining the best nitrogen rate is complicated by environmental interactions that can reduce or increase plant available nitrogen in various years. Previous research has predominately deemphasized the need for hybrid specific nitrogen recommendations.1,2 However, there are trials reporting differential hybrid response to nitrogen availability.3,4 Others have observed hybrids responding differently to nitrogen rates in combination with incremental seeding rates5,6 although reports of fixed ear type hybrids not needing incremental nitrogen as seeding rates increased to an optimal level have also been reported. 7 It is more likely that ear flex types have more of an influence on optimum seeding rate than on optimum nitrogen rates. Planting hybrids with flex ear characteristics can help reduce potential yield loss when the environment or pests reduce plant populations lower than the desired target.

All photos are either the property of Syngenta or are used with permission.

References:

¹ Bundy, L.G. & P.R. Carter. (1988). Corn hybrid response to nitrogen fertilization in the Northern Corn Belt. J. Prod. Agric. 1:99-104. https://doi.org/10.2134/jpa1988.0099

² Gardner, C.A.C, P. L. Bax, D. J. Bailey, A. J. Cavalieri, C. R. Clausen, G. A. Luce, J. M. Meece, P. A. Murphy, T. E. Piper, R. L. Segebart, O. S. Smith, C. W. Tiffany, M. W. Trimble, & B. N. Wilson. (1990). Response of corn hybrids to nitrogen fertilizer. J. Prod. Agric. 3:39-43. https://doi.org/10.2134/jpa1990.0039

³ Gambin, B.L., Coyos, T., DiMauro, G., Borras, L., & L. Garibaldi. (2016). Exploring genotype, management, and environmental variables influencing grain yield of late-sown maize in central Argentina. Agricultural Systems 146:11-19. https://doi.org/10.1016/j.agsy.2016.03.011

⁴ Haegele, J.W. & F.E. Below. (2013). Transgenic corn rootworm protection increases grain yield and nitrogen use of maize. Crop Sci. 53:585-594. https://doi.org/10.2135/cropsci2012.06.0348

⁵ King, K. (2021). Dissecting the interaction of nitrogen, density and genetics in maize. [Master’s thesis, Iowa State University]. Iowa State University Capstones, Thesis and Dissertations. https://doi.org/10.3127/etd-20210609-91

⁶ Mastrodomenico, A.T., J.W. Haegele, J.R. Seebauer, & F.E. Below. (2018). Yield stability differs in commercial maize hybrids in response to changes in plant density, nitrogen fertility, and environment. Crop Sci. 58:230-241. https://doi.org/10.2135/cropsci2017.06.0340

⁷ DeBruin, J., Hensley, R., Underwood, H., & Munaro, E. (2023). Yield response of maize hybrids with different ear flex to nitrogen rate and plant density. Agronomy Journal, 00, 11111. https://doi.org/10.1002/agj2.21495