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.