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eturn to table of contents Grain Yield (bu/acre) 250 200 150 100 50 Low N Hybrid A Hybrid B Significant Hybrid x N Rate Interaction: Hybrids have comparable yield at low N and full N but have differing responses and optimum rates. Full Nitrogen Yield Low Nitrogen Yield Response to N (RTN) Nitrogen Rate Hybrid A 200 100 0.5 Full N Hybrid B 200 100 0.5 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. Grain Yield (bu/acre) 250 200 150 100 50 Low N Hybrid A Hybrid B Significant Hybrid x N Rate Interaction: Hybrids have comparable yield with low N; Hybrid A has higher top-end yield with full N. Full Nitrogen Yield Low Nitrogen Yield Response to N (RTN) Nitrogen Rate Hybrid A 200 100 0.5 Full N Hybrid B 150 100 0.33 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. 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 zero to one 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). RTN = High N Yield — Low N Yield High N Yield 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. 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. DuPont 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 64

eturn to table of contents 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 the 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. Grain Yield (bu/acre) 200 180 160 140 120 100 80 3489 3225 3346 3394 CRM 108 0 80 160 240 Nitrogen Rate (lbs/acre) Figure 5. Average grain yield response of four Pioneer brand hybrids to N application rate across six environments (1993-1994). 115 113 110 DuPont Pioneer Research DuPont 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. Grain Yield (bu/acre) 210 200 190 180 170 160 150 140 130 34B20 (YGCB, RR2) 34H32 (YGCB) 34N44 (YGCB) * = Optimum Applied N for Maximum Yield (bu/acre) 0 80 160 240 Nitrogen Rate (lbs/acre) Figure 6. Average grain yield response of three Pioneer brand hybrids to N application rate across eight environments (2004). A DuPont 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. Recent DuPont 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. 65

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