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Every year farmers play a game of risk with N management. One type of agronomic or production risk occurs when a corn field becomes deficient in N fertilizer and yield is lost. Quantifying this agronomic risk for a specific field is difficult because outcomes are dependent upon many factors. Indeed, farmers across Iowa use several N management practices that differ in N forms—several commercial types in addition to animal manure, with different timing and application methods, and of course, different rates of N across several landform regions with differing rainfall patterns and at least two common cropping systems.
Our research indicates that spring rainfall is a primary risk factor for N loss and deficient corn N status, but early and mid-summer rainfall could prevent post-spring application of N and, therefore, is a secondary risk factor should a grower delay N application. The Iowa average (1893 – 2014) May-June (spring) and July-August (summer) rainfall are similar with slightly more rainfall in the spring (8.5”) than in summer (7.4”). A climate risk context is given by separating the data into the current climate normal period (1981 – current) and past period (1893 – 1980). Spring and summer rainfall are uncorrelated in Iowa (Figure 1). Using the fact that spring and summer rainfall are independent, we identify rare years with the 95th percentile. Prior to 1981, only seven years fall outside the 95th percentile but in the 35 years since 1981 this has happened in 11 years (32%).
Change in excessive rainfall has implications for N risk management. Growers should know the risk for loss of N from their fields (Figure 1). The 11 extreme years identified in 1981 - 2014 can be grouped by whether the extreme rainfall occurred in the spring, summer, or both. Years with extremely high rainfall in the spring (4 of 11 years) would imply higher risk for N loss from pre-spring application. Years with extremely high rainfall in the summer (3 of 11 years) would imply the risk of inability to get into the field to apply post-spring N. Finally, years with extremely high rainfall in both spring and summer (4 of 11 years) would imply risk to both management approaches.
The Iowa Soybean Association On-Farm Network and Environmental Programs & Services teams have been tracking effects of rainfall on corn N management using late-season aerial imagery and corn stalk nitrate surveys since 2006. When data of yield response to N are not available or difficult to collect, the annual survey based on the late-season corn stalk nitrate test guided by the aerial imagery of the corn canopy provide important post-season feedback information. Farmer participation in annual feedback surveys is as critical as going through an annual medical check-up.
Data collected by agronomists and farmers from more than 3,500 corn fields across Iowa between 2006 and 2014 were used to develop equations describing the relationship between rainfall, N rate and management This analysis enables researches to estimate the risk to a specific field of experiencing deficient status N by the end of the season (more detailed in a manuscript “Integrating Field and Climate Data for N Risk Management”).
The data collected over the last 10 years indicate that different management practices, a combination of timing and N form, will require slightly different N rates for achieving optimal corn N status (Figure 3). The distribution of N rates shown as box plots in Figure 3 indicate that the spring and sidedress applications require slightly lower rates that those in the fall. Also, the variability in N rates needed to produce the optimal N status with fall-injected swine manure applications are slightly higher than that with commercial N sources such as UAN and AA.
It can be noticed also that a range of rates for SD N (UAN or AA) and Spring UAN for both corn after corn and corn after soybean are in the same ballpark as those produced by the Iowa State University N Calculator. The value of the ISA data, however, is that inferences about other practices such as Fall AA and Spring AA can be made.
The deficient N status suggests that the supply of N from the soil and fertilizer was likely not adequate, and plants will likely respond to additional N with above break-even yield response.
The optimal N status suggests that the supply of N from the soil and fertilizer matched plant demand, and additional N will produce above break-even yield response less than 50% of the time.
The excessive N status suggests that the supply of N from the soil and fertilizer exceeded plant demand, and additional fertilizer will not increase yield response.
For the risk neutral farmers vthose who ignore the risk and do not want to gamble, the data in Figure 2 would be enough to guide their N management. These farmers will ignore the effect spring rainfall and other factors.
For risk-averse and risk-tolerant farmers the N calculator should help quantify the risk of being deficient for a range of rainfall, N rates, management and cropping system scenarios. The value of the risk calculator to the risk-averse or risk-taker farmer is shown in Table 1.
| Total rate,
lb N /acre
|May through June rainfall, inches|
|Fall Anhydrous Ammonia|
|Fall Injected Swine Manure|
|Sidedress UAN or Anhydrous Ammonia|
|Spring Anhydrous Ammonia|
Aggregated data of annual N status surveys clearly shows a greater probability of N deficiency with each additional inch of May through June rainfall, which has a much larger impact (ten to 16 times greater) on N deficiency than an additional pound of N fertilizer. Simply put, both rainfall and N rates impact the likelihood of N deficiency but these effects are in different directions, with the overwhelming impact being from early season rainfall (Table 1, representing southern Iowa). The data in Table 1 also clearly shows that, in general, starting with more N does not rapidly decrease risk values.
As shown in Table 1, when all other factors are equal and the total N rates used by farmers range from 130 to 160 lb N/acre, corn after soybean fields in the Southern Iowa with 13 inches of May-June rainfall will have a 26 to 60 % chance of late-season N deficiency; with 16 inches, a 40-70 %; and with 19 inches, a 50-80 % chance of N deficiency.
Based on historical data, fields in the southern Iowa that received Spring AA may have about 15-30% less chance to be deficient than other form and timing combinations.
The online calculator estimates clearly indicate the higher risk of deficient corn N status for fields for corn after soybean compared to corn after corn. While this survey does not allow us to draw a clear cause and effect relationship, other studies suggest that it is likely that corn after soybean fields lose more N (references) than corn after soybean. These differences are likely because soybean residue have a lower C-to-N ratio and mineralize much faster that corn residues. It is also likely that some fields with corn on corn have a longer history of manure applications, and therefore, higher yield potential and demand for this nutrient.
Similar to other N diagnostic tools, the stalk nitrate test outcomes can be variable. The data collected between 2006 and 2010 from on-farm replicated strip trials with two N fertilizer rates (Agronomy Journal, 2012, 104:1284–1294) indicate the change of receiving above break-even yield response to the application of an extra 50 lb N/acre when the stalk nitrate test is deficient is between 55 -75%. This probability is about 30-60 % when the stalk nitrate test is optimal and less than 30% when stalk nitrate test shows the excessive stalk nitrate value of larger than 6000 ppm. The false positive (when the test suggest deficient status when it is truly not) can arise when corn plants exactly use up all N at the time of physiological maturity thus maximizing yield, so that additional N would not produce economic response.
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The first map shows average historical rainfall in May and June, computed for 4km x 4km regions within the state, between 1997 and 2018. In May and June, this historical rainfall is cumulative over the period between May 1 and today, otherwise, the map shows the full cumulative rainfall during both May and June.
The second map shows the deviation from the historical average rainfall for each region. In May and June, this map will update daily, and rainfall totals should include all but the last 24-48 hours. Otherwise, the map shows the most recent May-June period.
All rainfall totals are gathered from Iowa Environmental Mesonet .
This applet uses aggregate climate information as well as GIS data to provide rainfall estimates for specific locations.
We are very thankful to all farmers, agronomists, technical service provides, partner industry professionals and university scientists who participated and contributed to the on-farm field and environmental studies used to develop this tool.