Corn Hybrid Response to Nitrogen Fertilizer

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By Mark Jeschke, Ph.D.¹ and Jason DeBruin, Ph.D.²

Summary

Efforts to improve nitrogen (N) management efficiency have driven interest in tailoring management practices to individual corn hybrids.
Successful implementation would depend on differences in hybrid response to N being large enough and consistent enough to justify hybrid-specific management.
University studies have commonly found either no hybrid by N rate interactions or that interactions were inconsistent across locations and years.
Recent Pioneer research found that yield response to N was affected greatly by location and year and that hybrid responses were generally similar when averaged across multiple environments.
The accumulated body of research in hybrid by N management suggests little opportunity for meaningful improvements to date in efficiency and profitability relative to other aspects of N management.
Response to nitrogen (RTN) values for corn hybrids are descriptive of the data collected but do not provide an actionable N management plan for growers to follow.
Analytical approaches that dynamically adjust for environmental factors, such as those employed by Encirca^® services, may provide a better opportunity to account for differences in hybrid genetics than static hybrid ratings based on a limited number of field trials.

Corn showing nitrogen deficiency symptoms.

Introduction

Nitrogen management is among the most uncertain and costly aspects of modern corn production. Economic pressures, along with public concern regarding offsite movement of N, have driven interest in improving the efficiency of N management in corn. Efforts to improve efficiency have often focused on application timing and varying application rates based on crop needs.

Growers and researchers have both explored opportunities for improving N efficiency through managing N differently for each hybrid. Some seed and fertilizer suppliers have also promoted the concept of unique N needs and application timings for specific hybrids.

Genetic differences in N uptake and utilization undoubtedly exist. Other characteristics, such as plant density tolerance and drought tolerance, are known to differ among hybrids, and grower management practices are often tailored accordingly. Some seed companies and dealers currently provide nitrogen response ratings and management recommendations for individual hybrids. However, the most pertinent question for growers is whether differences described by these ratings are actually large and consistent enough to justify hybrid-specific N management. A related and equally important consideration is the capability of industry and university research methods to reliably characterize hybrid differences in a way that provides useful management guidance to growers.

Hybrid by Nitrogen Rate Interactions

In order to evaluate the potential value of hybrid-specific N management, it is first necessary to define what constitutes a hybrid by N rate interaction, the different types of interactions that may exist, and the management implications of each. A hybrid by N rate interaction means that the grain yield of two hybrids respond differently across a range of N application rates. Figure 1 depicts N rate responses for two hypothetical hybrids in which no hybrid by nitrogen rate interaction results. Hybrid A is higher yielding across the entire N rate range; however, the agronomic optimum N rate would be the same for these two hybrids.

Response of two hypothetical corn hybrids across a range of N application rates showing no hybrid by N rate interaction. — **Figure 1.** Response of two hypothetical corn hybrids showing no hybrid by N rate interaction.

Figure 2, Figure 3 and Figure 4 depict different types of hypothetical hybrid by N rate interactions. In Figure 2, the two hybrids have similar yields when supplied with sufficient N, but Hybrid A has greater yield than Hybrid B at low N levels. Hybrid A, in this example, could be described as having greater yield stability under low N and would be better suited for placement in fields at greater risk for N stress.

Response of two hypothetical corn hybrids showing a significant hybrid x N rate interaction — **Figure 2.** Response of two hypothetical corn hybrids across a range of N application rates showing a significant hybrid x N rate interaction in which Hybrid A maintains greater yield stability under low N than Hybrid B.

In Figure 3 the two hybrids have similar yields under low and full N but have different responses; Hybrid A approaches maximum yield at a lower N level than Hybrid B and therefore, would have a lower economic optimum N rate. In this scenario, a grower may be able to reduce the N rate for Hybrid A without a yield penalty. Hybrid A would also be the preferred choice for placement in fields where there is a risk of N stress. This type of hybrid by N rate interaction is the most difficult to characterize experimentally as it would require testing the two hybrids at several rates in order to determine the optimum N rate for each.

Response of two hypothetical corn hybrids which have similar yields at low N and full N but have differing responses and optimum N rates. — **Figure 3.** Response of two hypothetical corn hybrids across a range of N application rates in which Hybrid A and B have similar yields at low N and full N but have differing responses and optimum N rates.

In Figure 4 the two hybrids have similar yields at low N levels, but Hybrid A has higher top-end yield with full N. Hybrid A has a greater response to N, but the optimum N rates are similar between the two hybrids. In this scenario, Hybrid A would be the preferred option in high-yielding as well as N stress risk environments. It is important to note that a greater response to N does not imply that a higher rate is needed to maximize yield, as illustrated in Figure 2 and Figure 4.

Response of two hypothetical corn hybrids which both hybrids have similar yields under low N — **Figure 4.** Response of two hypothetical corn hybrids across a range of N application rates in which both hybrids have similar yields under low N, but Hybrid A has greater yield potential with adequate N.

Experimental Methods

The first step to characterize a hybrid by N rate interaction is a close examination of the yield response of different hybrids across a wide range of N rates. Pioneer research studies comparing hybrid responses to N have typically included four or five rates (Gardner et al., 1990; Iragavarapu, 1998; Luce and Mathesius, 2009).

The second step is to only compare similar maturity hybrids that are adapted for the location where grown. For example, two hybrids of greatly differing maturity grown across a wide range of N rates could give the appearance of a hybrid by N rate interaction. In reality, these hybrids have very different grain and stover production potentials and therefore, N requirements.

The third step to demonstrating whether a true hybrid by N rate interaction is occurring is to make the comparisons across numerous environments and several growing seasons. With few exceptions in past testing, hybrid by N rate interactions that were apparent after a single year of testing at one location disappeared when the tests were conducted over several locations and years.

Response to Nitrogen (RTN)

The significant investment of resources required to test hybrids at multiple N application rates across numerous sites-years in order to fully characterize the N rate response greatly limits the practicality of routinely testing commercial hybrids. Response to N (RTN) is a method for N response characterization that has come into practice due to its lower resource requirement, as it often involves testing hybrids at as few as two application rates – a high, maximum yield rate and a low or zero N rate.

Response to N is a unitless index ranging from 0 to 1 that expresses the proportion of yield gained due to N application relative to the yield attained at the high, maximum yield rate. Hybrids with a greater response to applied N relative to their maximum yield will have a larger RTN value on the 0 to 1 scale (Krienke, 2015).xxx

The hypothetical hybrid by N rate interactions illustrated in Figure 2 and Figure 4 could both be detected using the RTN method, as they both involve hybrids that differ in their overall response to N. However, the RTN values alone do not provide a sufficient basis for driving N management decisions. For the scenario shown in Figure 2, the hybrid with the lower RTN value would be the preferred option; it has equal yield potential with full N and lower risk for yield loss under N stress. For the scenario shown in Figure 4, the hybrid with the higher RTN value would be the preferred option, as it is higher yielding under both low N and full N. With yield performance at only two N rates comprising the index, the RTN method does not provide any insight into differences in optimum N application rates among hybrids (the scenario shown in Figure 3), which limits the practical value of this method compared to testing with a greater number of N rates.

Nitrogen Rate x Hybrid Research

University Research

Over the years, numerous university research studies have compared corn hybrids for response to N. Research conducted in the 1980s at Purdue University indicated that some hybrid differences may exist (Tsai et al., 1984). However, this work compared hybrids that varied greatly in comparative relative maturity. Hybrids that differ significantly in maturity tend to have large yield differences, and lower yielding hybrids tend to plateau with somewhat lower rates of N. Subsequent multi-year studies conducted by scientists at the University of Wisconsin (Bundy and Carter, 1988) and the University of Illinois found no consistent evidence for hybrid by N rate interactions in numerous adapted hybrids.

Recent university studies have explored possible hybrid by N rate interactions among current commercial hybrids. A two-year study conducted at multiple locations in Nebraska evaluated yield of eight commercial corn hybrids at four different N application rates (Krienke, 2015). Hybrids selected for the study were intended to represent a range of N responsiveness based on input from the seed provider. Hybrids were characterized using the RTN metric. Results of the study showed a non-significant hybrid effect on RTN but a significant hybrid by year interaction, indicating that differences in hybrid response to N were not consistent across locations and years. Results did not support a priori categorization of hybrids in the study as either more responsive or less responsive to N.

A two-year study conducted at multiple locations in Illinois evaluated yield of four Pioneer® brand hybrids at four different N application rates and three plant densities (Clark, 2013). Hybrids evaluated in the study included two anecdotally characterized as more responsive and two less responsive to N. Results showed no significant hybrid by N rate interaction or hybrid by N rate by density interaction. As with the Nebraska study, results did not support the anecdotal characterization of hybrid N responsiveness; hybrids responded similarly to N rate, despite the fact that they were selected based on their supposed differences in N responsiveness.

Pioneer Research

Pioneer researchers have conducted numerous studies on Pioneer® brand corn products over the past 30 years to evaluate potential hybrid by N rate interactions. Research was conducted continually from 1987 to 1994, involving field studies at 19 different locations in Iowa, Illinois, Indiana, and Nebraska. Each experiment compared performance of 4 to 6 hybrids at 0, 80, 160, and 240 lbs/acre of applied N. Results from the final two years of the study are shown in Figure 5.

When yields were averaged across the environments and hybrids, there was a statistically significant yield response to N. The effect of environment was highly significant due to variation in growing conditions, as was the environment by N rate interaction. This suggests that the response to N varied depending on the environment. However, the N rate by hybrid interaction was not significant. This indicates that the hybrids responded to N in a similar way when averaged across environments.

Figure 5. Average grain yield response of four Pioneer brand hybrids to N application rate across six environments (1993-1994).

A Pioneer study conducted across seven locations in Illinois, Iowa, Indiana, and Nebraska in 2004 found similar results. Nitrogen rate and hybrid significantly affected corn grain yield; however, there was not a significant interaction between hybrids and applied N rates. This lack of an N rate by hybrid interaction indicates that there was no difference in the way these three hybrids responded to these N rates.

Figure 6. Average grain yield response of three Pioneer brand hybrids to N application rate across eight environments (2004).

Recent Pioneer Research

A recent Pioneer study conducted in 2012 and 2013 evaluated yield performance of several Pioneer brand hybrids under normal and low N application rates to determine if hybrids differ in yield stability under N stress. Nineteen hybrids, ranging from 106 to 115 CRM, were evaluated across four locations in 2012 and five locations in 2013 in Illinois, Iowa, Nebraska, and California.

Results of the study showed that differences in N response among hybrids were relatively small. Average hybrid RTN values were similar, ranging from 0.41 to 0.48 (Table 1). Differences in average yield among hybrids were greater at full N, ranging from 217 to 245 bu/acre, than at low N, where yields ranged from 117 to 134 bu/acre. The majority of hybrids yielded between 130 and 135 bu/acre under low N. Hybrids yielding below this range under low N also tended not to be the highest performers with full N.

Table 1. Average yield of 19 Pioneer brand hybrids with a low rate and full rate of applied N and calculated RTN across 4 locations in 2012 and 5 locations in 2013.

Differences in hybrid yield with full N appeared to be partially attributable to hybrid CRM – yield increased with fuller-season hybrids to a greater extent with full N than under limited-N conditions (Figure 7).

Average grain yield by hybrid CRM with a low rate and full rate of applied N (2014). — **Figure 7.** Average grain yield by hybrid CRM with a low rate and full rate of applied N (2012-2013).

A similar study was conducted in 2014 using a different set of 19 Pioneer® brand corn products. Corn products included in this study covered a similar range in CRM – from 105 to 114. Trials were conducted at five locations in Illinois, Iowa, Nebraska, and California.

Table 2. Average yield of 19 Pioneer brand corn products with a low rate and full rate of applied N and calculated RTN across 5 locations in 2014.

Results of the 2014 study were very similar to those of the 2012 to 2013 study. Average RTN values for hybrids ranged from 0.32 to 0.45 (Table 2). Average yield varied by 14 bu/acre among hybrids at low N, compared to 59 bu/acre at full N, meaning that differences in RTN among products tended to be driven more by differences in top-end yield rather than differences in yield stability under low N. For example, Pioneer® hybrid P0636HR and Pioneer P1339AM1™ brand corn had the same average yield under low N, but P1339AM1™ yielded 21 bu/acre more with full N, making the difference in RTN between the 2 products entirely attributable to a difference in top-end yield.

As with the 2012 to 2013 study, yields under low N fell within a relatively narrow range for the majority of hybrids, from 145 to 149 bu/acre. Hybrids yielding below this range under low N also tended not to be the highest performers with full N. Hybrids that were top yielders with full N also tended to be at the upper end of the yield range under low N. Differences in hybrid yield with full N again appeared to be partially attributable to hybrid CRM, showing an even stronger yield response than in the 2012 to 2013 study (Figure 8).

Of the corn products evaluated in the 2012 to 2014 Pioneer research studies, five were tested over all three years. Comparing performance of these hybrids among siteyears illustrates the effects of location and year on hybrid response to N. Results show that hybrid response to N varied by location and year (Figure 9), which aligns with similar findings from university studies. Differences among hybrids in RTN observed at a single location were not repeated across other locations and years, and very little consistency was observed in the relative ranking of RTN among hybrids from site-year to site-year. Much more variability in RTN was associated with site-year than with hybrid.

Recent Pioneer research comparing hybrid RTN by evaluating hybrid yield under full and low N application rates suggest that this method likely has little to no value for improving grower N management efficiency and profitability. To the extent that differences in hybrid response to N were observed in Pioneer research, they were primarily attributable to differences in yield performance under full N. Since testing at only two N rates does not allow any determination of the optimum N rate for a given hybrid, the most noteworthy management implication to be taken from this research appears to be that the yield advantage of fuller-season hybrids tends to be greater when supplied with adequate N.

Figure 9. Average RTN of five Pioneer® brand hybrids at four locations in 2012, 2013, and 2014.

Conclusions

Numerous research studies have been conducted over the past few decades to evaluate possible corn hybrid by N rate interactions. Most of these studies have either found no interactions or the interactions were inconsistent across locations and years due to variability in growing conditions. Several Pioneer studies conducted over this time period produced similar findings. The accumulated body of research has not provided a sufficient basis to drive hybrid-specific N management into common practice for most growers. The frequent variability in results, with N response of hybrids differing greatly depending on the conditions experienced in a particular location and year, calls into question the general practicality of using field experiments as a basis for hybrid-specific N management recommendations.

Future Directions

Differences in N requirements among hybrids appear to be relatively small and are likely masked by other, larger sources of in-field variability. In order to be successful, future efforts to optimize N management based on hybrid genetics will likely require a more sophisticated and prescriptive approach, rather than simply assigning a static hybrid rating based on a limited number of field trials. Dynamically accounting for other factors unique to a given field and year may provide an opportunity to better zero in on the hybrid signal by filtering out some of the environmental noise. The Encirca^® Yield Nitrogen Management Service incorporates N analytics that simulate the major processes that affect soil N, including crop growth, N mineralization, leaching, denitrification, and volatilization. Pioneer researchers are currently developing expanded analytical capabilities that will also incorporate genetic coefficients into N management recommendations.

Nitrogen management continues to be one of the most complex and challenging aspects of modern corn production. With an extensive history and ongoing engagement in N management research, as well as the advanced analytics of the Encirca Yield Nitrogen Management Service, Pioneer is better-equipped than anyone in the industry to help growers better manage their N.

References

Bundy, L.G., and P.R. Carter. 1988. Corn hybrid response to nitrogen fertilization in the northern Corn Belt. J. Prod. Agric. 1:99-104.

Clark, R.A. 2013. Hybrid and plant density effects on nitrogen response in corn. M.S. Thesis. Univ. of Illinois at Urbana-Champaign.

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, and B.N. Wilson. 1990. Response of corn hybrids to nitrogen fertilizer. J. Prod. Agric. 3:39-43.

Iragavarapu, R. 1998. Corn hybrid response to nitrogen rate and timing. Crop Insights Vol. 8, No. 11. Pioneer. Johnston, IA.

Krienke, B.T. 2015. Assessing factors influencing maize yield response to nitrogen using remote sensing technologies. Ph.D. Dissertation. Univ. of Nebraska- Lincoln.

Luce, G. and J. Mathesius. 2009. Hybrid response to nitrogen fertilizer: Are there differences? Crop Insights Vol. 19, No. 6. Pioneer. Johnston, IA.

Tsai, C.Y., D.M. Huber, D.V. Glover, and H.L. Warren. 1984. Relationship of N deposition to grain yield and N response of three maize hybrids. Crop Sci. 24:277-281.