Historical university, industry and presented studies predominantly found hybrids respond similarly to nitrogen (N) availability. - Trial results suggest high RTN (response to nitrogen) ratings identify hybrids that are more sensitive to N limited conditions. However, high RTN ratings are not a good indicator of response to intensive crop and N management practices, such as split applications or increased rates.
- Later relative maturity (RM) hybrids that undergo a longer grain fill period are shown to be more sensitive to N shortages and are indicated with increasing RTN scores.
- RTN ratings lack the ability to predict economic optimum N rates making it difficult to predict how hybrids would perform at different levels of N availability, which render it challenging to create an actionable N management plan.
- Analytical approaches to N management that adjust for environmental factors, such as in-season soil and plant tissue testing or predictive N modeling tools, can provide more accurate, timely in-season decisions for a more profitable N management program.
Identifying differences among corn hybrids in nitrogen use efficiency has long been investigated for improving management. Numerous studies have been conducted with the goal of understanding hybrid by N response. The following article is a brief summary of RTN trials and how to best interpret and utilize ratings when considering best management practices.
Evaluating Hybrids for Response to Nitrogen
Trials were conducted at 21 locations in 2018 to compare 13 Golden Harvest hybrids’ response to RTN for better understanding of RTN ratings as a management tool. RTN is used by some seed providers to quantify the yield loss of a hybrid under N limited environments in comparison to the yield at a non-limiting N rate. Based on trial results, a value of 0-1 is assigned to individual hybrids and used as a metric to compare to the N response of other hybrids. The RTN value signifies the % yield a hybrid lost due to limited nitrogen availability (Figure 1). The same 13 hybrids, ranging from 103 to 114-day RM, were planted at all locations to provide consistency in hybrid ratings across growing environments. The distribution of trials and the average yield penalty per location are outlined in Table 1. The significant effect of environment and soil type on nitrogen availability can be observed across trials. Individual locations ranged from as little as 0% to 69% yield loss at the most stressed locations. On average, limited nitrogen availability resulted in a 34% yield loss across locations.
Hybrid Response to Using RTN
For identification of hybrids that most consistently have high/low RTN ratings, all 21 trials were combined and summarized for response trends. RTN ratings averaged 0.32 across 13 hybrids and ranged from 0.26 to 0.36 (Table 2). Yield loss in limited N environments ranged from 61-85 bu/ac across all hybrids with a 24 bu/ac variance (Graph 1). Previous interpretations of how to best manage hybrids with higher RTN ratings have implied they will be responsive to incremental nitrogen rates and split application timings, while maintaining above average yield potential in low N environments. These data (Graph 1) suggest a lack of relationship between yield and RTN score when high nitrogen rates were applied, indicating RTN scores likely have little to do with hybrid response to incremental N rates. In the low N treatments, a trend for decreased yield as RTN scores increased suggests that hybrids with higher RTN ratings are a better indicator of hybrids more sensitive to N loss.
Hybrid RM in Relation to Nitrogen Management
Relative Maturity (RM) is a common indicator of how long a corn hybrid requires to complete its grain filling period, otherwise known as reaching physiological maturity. Due to fuller season hybrids having a longer and later grain fill period, it is reasonable to anticipate they may respond differently to nitrogen. A mobile nutrient, such as nitrogen, will decrease in availability as the season progresses due to plant uptake and soil N losses, lending to fuller season hybrids being further disadvantaged. Observations from 2018 trials indicate a linear relationship between hybrid RTN score and RM (Graph 2). As hybrid RM increased, RTN ratings also increased. This relationship supports the concept that fuller season hybrids are more sensitive to yield loss and illustrates the importance of higher intensity N management for fuller season hybrids.
Predicting Hybrid Response at Different Levels of N Availability
It is important to note, due to the trial design, it is not possible to extrapolate what may have happened in situations with less severe N loss. The following theoretical example illustrates potential yield response curves of hybrids receiving different nitrogen rates. This demonstrates how the critical amount of nitrogen needed to achieve the economic optimum rate could vary significantly among hybrids with the same RTN score (Graph 3).
Summary
Trial results did not illustrate high RTN ratings as being a good indicator of hybrids that are responsive to more intensive N management practices, such as split applications or increased rates. However, lower RTN ratings did identify hybrids that yield better under extreme N limiting conditions. Differences among hybrid RTN ratings do not appear to be large enough and consistent enough to justify hybrid specific management. The magnitude of RTN differences among hybrids would likely be less pronounced in low N stress situations representative of normal corn production scenarios. The results of RTN studies do support the observation that hybrids with a longer grain fill period are most susceptible to yield loss in low nitrogen environments and highlight the importance of intensive nitrogen management for these hybrids. RTN ratings are not able to predict economic optimum nitrogen rates or how hybrids would perform when managed to those levels, and therefore, have limited utility in creating hybrid specific N management plans.
Conclusions from this work suggest RTN ratings are of limited use in differential hybrid N management due to 2factors. First, the experimental design limits the ability to predict hybrid differences at rates in between the high and low rates utilized in testing. Second, the strong influence of environmental variability on hybrid nitrogen use efficiency requires an extensive multi-year and location evaluation of hybrids to gain confidence in differences between hybrids. Because of the relatively short life span of hybrids, characterization may not be completed until late into a hybrid lifecycle. Due to lack of actionable N management options associated with characterizing hybrids, analytical approaches that adjust for environmental factors, such as in-season soil and plant tissue testing or predictive nitrogen modeling tools, likely provide more opportunity for in-season management to correct for potential yield loss.
Contact your Golden Harvest Seed Advisor with questions or for aditional agronomic insights.
Photos are either the property of Syngenta or used under agreement.
Syngenta hereby disclaims liability for third-party websites.