F.E. Below is a Professor, L.E. Paul is an Agronomist, and M.
Ruffo is a Research Associate, Dept. of Crop Sciences, University of Illinois,
Urbana, IL 1
Fall–applied N continues to be a popular fertilization practice because it saves the grower time in the spring, and spreads out the work for the fertilizer dealership in the fall. Despite these advantages, fall N must be viewed with scrutiny as this N is very susceptible to loss during the long time between application and plant uptake. With N loss in mind the best management of fall-applied N typically involves: 1) waiting until the soil temperature reaches a certain minimum threshold; 2) including a nitrification inhibitor; and/or 3) limiting applications to fertilizer N sources that do not contain nitrate-N or urea.
Ammonium sulfate is a potentially safe fall-applied N source over a wide range of production conditions. This advantage is largely because of their lower risk of volatilization losses compared to N fertilizers containing urea, and to their lower risk of leaching or denitrification losses compared to fertilizers containing nitrate-N. An added utility of ammonium sulfate is its ability to be applied in the fall and winter as long as the soil temperature is lower than 50°F and the field slope is less than 5% (according to the University of Illinois Agronomy Handbook). Urea-containing fertilizers, on the other hand, are generally not recommended for fall or winter applications as large potential losses of N can occur (University of Illinois Agronomy Handbook). The rapidly rising cost of fertilizer N, however, may cause growers to ignore these guidelines, and there are increasing reports of fall-applied urea being used.
New urea sources have been developed lately as a means to minimize N losses. Some sources, such as SuperU, incorporate urease and nitrification inhibitors in the formulation. The urease inhibitor reduces the conversion from urea to ammonium-N and the nitrification inhibitor reduces the conversion from ammonium-N to nitrate-N, both reducing the potential losses of mineral N. Other sources include the controlled release urea sources, such as ESN, where the urea granules are coated with different materials that reduce the release of N.
Considerable research has been conducted on the yield response between fall and spring-applied N with the general conclusion being that more N is required in the fall. Much less research, however, has been done on optimizing the source of fall-fertilizer N, which could be one way to fine-tune the practice. Others are advocating inclusion of urease and/or nitrification inhibitors (i.e. Super-U), or polymer coating of urea granules (i.e. ESN Smart N) as ways of improving urea's efficacy for fall applications. Alternatively, others would favor abolishing the practice of fall-applied N entirely, arguing that the risk of environmental damage from N losses outweighs the convenience of early N applications. Thus, fall-applied N is a management that clearly warrants continued scientific verification, and this need forms the basis of our research.
Our objective was to evaluate the role of N source in the success of fall-applied N for corn production and compare it with the spring application of these sources. The overall goal is to use this information to justify and/or improve fall applications of N. We believe that certain sources and/or formulations of N may have added utility for fall applications, especially in the cooler soil conditions of Northern Illinois.
The project was conducted at the Northern Illinois Agronomy Research Center in DeKalb, IL in 2006 on a Flanagan silt loam soil. This soil had 6.2 pH, 5% OM, 70 lb P/ac, and 344 lb K/ac. The factors under study were N application time (fall and spring), N sources (ammonium sulfate, urea, Super-U (urea plus a nitrification inhibitor and a urease inhibitor), and ESN (controlled release urea)), and N rate ((0, 50, 100, 150, and 200 lb N/ac). The fall treatment was applied on Dec. 17th 2005 and the spring one on April 21st 2006. Treatments were arranged in a randomized complete block design with four replications. An individual experimental unit consisted of four 21 foot long rows, spaced 30 inches apart, with all rows receiving the respective N treatment. The entire field was disked on April 27th and planted the following day with DeKalb hybrid DKC-60 18 (RR2/YG+) at 35,000 seeds/ac. The middle two rows were harvested on October 30th 2006 with a plot combine. A grain sample was collected at harvest and analyzed for grain protein concentration using a Foss 1229 NIT grain analyzer.
The data was analyzed using the Mixed procedure in SAS. All factors except blocks were considered fixed. Main effects and interactions were included in the model. The grain yield response to N fertilizer was modeled with a quadratic-plateau function whereas the protein responses were modeled with a logistic function with intercept and estimated with the NLIN procedure in SAS. The source x application time interaction was analyzed by contrasts.
A summary of the weather conditions between December 2005 and October 2006 is presented in Table 1. Total precipitation between these months was 28.3 inches, 6.3 inches lower than the 30-year average, whereas average temperature was 0.6ºF higher than average. Precipitation between February and August was below average except in April. The period January to April was warmer than average (averaged 36.5ºF for 2006 compared to 31.3ºF for the 30-year average), but this trend reversed between June and October.
There was a significant effect of N application time on grain yield (p<0.07). Grain yield for spring (239 bu/ac) N application was 5 bu/ac higher than fall application (234 bu/ac). Nitrogen application also had a significant effect on grain yield (p<0.0001). Unfertilized corn yield was 176 bu/ac and increased to 248 bu/ac with 164 lb N/ac resulting in a 72 bu/ac yield response (Fig. 1).
There was a significant (p=0.09) interaction between application time and N source, indicating that the N sources behaved differently when applied in the fall compared to the spring. There was no difference among N sources when applied in the fall, but they differed when they were applied in the spring. Spring applied SuperU resulted in significantly higher yield than ammonium sulfate (244 bu/ac and 234 bu/ac, respectively). There was no other significant difference between spring applied N sources. ESN and SuperU produced significantly higher yields as spring sources than as fall ones. There was no difference between application times for ammonium sulfate and urea and other interactions were not significant (p>0.1).
Unfertilized corn yield in previous years in this location was close to 115 bu/ac. The higher unfertilized corn yield in 2006 suggests a large availability of soil mineral N during this growing season probably due to the above-average temperatures between March and May that promoted soil N mineralization. Similarly, maximum grain yield was higher in 2006 than in 2005 and 2004, when it reached 200 and 160 bu/ac, respectively. Although the response to N was not different for fall and spring applied N, the significantly higher yield with spring application suggests a more efficient use of N fertilizer when applied closer to planting. However, the 5 bu/ac difference is relatively small to be of practical value for management purposes.
Grain protein concentration was also affected by N application timing (p<0.001). Mean protein concentration for spring applied N was 7.26%, significantly higher than for fall applied N, which was 7.07%. There was a positive effect of N fertilizer application rate on corn grain protein concentration (p<0.0001). Grain protein concentration of unfertilized corn was 6.21% and increased to 7.65% with 200 lb N/ac. Moreover, protein concentration response to N fertilizer differed between application times (i.e. significant time x N interaction; p<0.05). Fall and spring applied N did not differ in protein concentration with N rates lower than 100 lb N/ac, but spring applied N resulted in higher protein concentration than fall applied N with 150 and 200 lb N/ac (Fig. 2). The N timings differed in the magnitude of protein response to N fertilizer. Fall applied N resulted in an increment of 1.32 units whereas spring application increased protein 1.66 units. A fertilizer rate of 175 lb N/ac is required to reach near maximum (99% of maximum) protein concentration for both application times.
As occurred for grain yield, there was a significant source x time interaction on grain protein concentration (p<0.05). Mean grain protein concentration for the spring and fall applied N sources evaluated in the study are presented in Table 3. Nitrogen sources did not differ when applied in the fall, but they differed when applied in the spring. In the spring, SuperU had the highest protein concentration (7.42%), significantly higher than ESN and ammonium sulfate, followed by urea (7.31%) which was higher than ammonium sulfate. Ammonium sulfate and ESN did not differ in grain protein concentration when applied in the spring. Spring N application resulted in higher protein concentration than fall applied N for SuperU and urea, but there was no timing effect for ammonium sulfate and ESN.
The lack of significant differences in grain yield and protein among sources when applied in the fall can be attributed to the relatively dry conditions during the winter and early fall periods (December to March) that determined small losses of N fertilizer. Conversely, the average precipitation in April might have promoted some N-fertilizer losses and could explain the slight difference found among N sources when applied in the spring. Considering that even in an average to dry spring some slight differences were detected, larger differences among N sources are expected in years of above average precipitation in the spring. In addition, it must be considered that fall fertilization was done in mid-December, with average temperature of 32ºF.
Both yield and protein indicate that N availability was largest with SuperU and smallest with ammonium sulfate, whereas urea and ESN had an intermediate behavior. These environmental conditions also caused the small grain yield differences between fall and spring N application. Urea did not differ from the enhanced efficiency urea-sources (SuperU and ESN) either for grain yield or protein concentration.
Spring application increased grain yield by 5 bu/ac and protein by 0.3 units compared to fall fertilization, supporting the concept that nitrogen use and uptake are increased when fertilizer is applied closer to crop uptake. Spring application of SuperU resulted in the highest grain yield (244 bu/ac) and protein (7.4%), whereas ammonium sulfate in the lowest (234 bu/ac and 7.1%), when spring applied. There is no evidence that would suggest that urea should not be applied in the late fall. The small yield differences due to the timing of N application (i.e. spring vs fall) and N sources are probably because the major N loss event occurred in April, thereby affecting fall and spring timings and N sources almost equally. We propose to repeat this study in additional locations in Northern Illinois to evaluate these sources under a wider range of environmental conditions and soils, and to expand inference space for our research.
1 F.E. Below is a Professor, L.E. Paul is an Agronomist, and M. Ruffo is a Research Associate, Dept. of Crop Sciences, University of Illinois, Urbana, IL