Recent research on corn has tended to show variability in N response. Brown et al. (1993) reported that economically optimal N rates among 77 sites in Illinois ranged from zero to more than 200 lb N per acre. Results from other studies show similar variability in time and space. Even with such variability, results over environments have been combined and used to develop an N fertilizer rate guideline in Illinois based on anticipated corn yield (Hoeft and Peck, 2002). This guideline suggests providing 1.2 lb of N (or a different factor based on the relative prices of corn and N) for each bushel of expected yield for corn following corn, with credits given when corn follows a legume or when manure has been applied to the field.
While we know that yields, and thus actual N requirement, cannot be predicted with accuracy, the use of this guideline has proven to be satisfactory in most years and on most fields. As N costs have increased in recent years, however, it has become clear that using proven yield as a predictor of N rate tends to result in using more N than can be economically justified. This has led to the use of recent data to formulate N rate guidelines, with adjustments based on N and corn prices (Nafziger et al., 2004). Data from this project have been used extensively in developing this new approach.
The present study was designed to assess the response to N rate of corn following corn and corn following soybean, over a number of years and locations in Illinois, in order to find predictive relationships to help improve the correspondence between N rate and actual crop need for fertilizer N.
Rotations to support this study were established in 1998, and data collection on N rate response has been ongoing since 1999. The study is being conducted at the following sites and soil types (with expected corn yield), on the six University of Illinois Crop Sciences Research and Education Centers: DeKalb – Flanagan sil (175); Monmouth – Sable sicl (180); Urbana – Drummer sicl (170); Perry – Clarksdale sil (140); Brownstown – Cisne sil (115); and Dixon Springs – Belknap sil (bottomland - 140) and Grantsburg sil (upland - 120). The study at the Dixon Springs upland site began one year later than at the other sites.
A split-plot design was used, with previous crop – corn or soybean – as main plots, and N rates – 0, 45, 90, 135, 180, and 225 lb N/acre – on corn split within main plots. Corn followed corn on the same set of plots each year, with each N rate assigned to the same subplot. Soybean was planted into the third main plot each year, in preparation for corn with N rates the following year. Subplot sizes ranged from 10 x 30 to 20 x 50 ft.
Harvest for yield was done on the center two rows of each subplot. Yield data were analyzed using nonlinear regression (PROC NLIN) with the quadratic + plateau model. Where the Q+P model did not fit the data well – when yields declined at the higher N rates and/or when the model did not meet convergence criteria – the data were fit to a quadratic model. Economically optimal N rates (EONR values) were calculated from the quadratic function in each case using a cost:price ratio ($ per lb of N:$ per bushel of corn) of 0.10.
The maximum return to N (MRTN) approach uses the same data as is used to calculate EONR rates and yields, but it uses N and corn prices to convert yields and N rates into dollar return (above the yield at zero N) per acre. The net return is calculated for each of a series of N rates (the calculator currently uses a range of 0 to 240 lb N/acre in 10-lb increments) then returns are averaged over trials to produce a response curve of RTN over N rates. The maximum point of this curve is found, and the N rate at this maximum is taken as the MRTN N rate.
The 2008 season was the tenth year of data collection from this research, and may be the last year that we run the trial in its present form. Because the 2008 cropping season was so unusual, with cool, wet weather in the spring and a slow start to the crop but an extended growing season and high yields, we will also compare 2008 results with those from the first nine years of the study. We will also report here on the question about the relationship of EONR to yield among sites and years.
Yields at the optimum N rate were higher at all locations in 2008 compared to yields averaged over 1999-2007 (Table 1.) For corn following soybean, this difference ranged from 5 bushels per acre at Urbana to 78 bushels per acre at the bottomland site at Dixon Springs. For corn following corn, the range was from 12 bushels higher at Urbana to 85 bushel per acre at Monmouth, where there were some relatively low yields for corn following corn during some years before 2006. On average, yield levels were 44 bushel per acre higher than the 9-year averages for corn following soybean, and 49 bushels higher for corn following corn.
The high yields in 2008 tended to be associated with the need for more N at most sites, but the change in ratio of optimum N rate to yield at that rate (lb N per bushel of yield) did not change consistently among locations (Table 1.) For corn following soybean, the optimum N rate in 2008 was lower than the long-term optimum rate at Monmouth and DeKalb, and was higher at the other five sites. The lb N per bushel ratios were lower in 2008 than the long-term averages at those two sites and at the Dixon Springs bottomland site, where yields were also very high in 2008, but was higher in 2008 at Brownstown, where the optimum N rate was the maximum rate used. At the other three sites, yields and N rates were higher to a similar degree, and the ratios did not change by much. For corn following corn, optimum N rates in 2008 compared to the long-term averages ranged from 9 lb less in 2008 to 68 lb more at the Brownstown site, where the optimum N rate in 2008 was 225 lb N/acre, or the highest rate used.
Contrary to what we have observed over numerous site-years of regional data and for both corn following corn and corn following soybean, the relationship between EONR values and the yield at EONR among individual sites from this experiment was surprisingly strong in southern Illinois (Figure 1). In this region, each 1-bushel increase in optimum yield was associated with
0.9 and 0.8 lb higher optimum N rate for corn following corn and corn following soybean, respectively. Correlation coefficients for both lines are significant. This same phenomenon was observed in central Illinois (Perry and Urbana), where CC and SC bother required about 0.75 more N for each 1-bushel increase in yield (Figure 2). Results from northern Illinois (Monmouth and DeKalb) showed no such relationship for either CC or SC (Figure 3).
A relationship between yield and N rate has wide implications, since it would reinforce the need for more N with high yields. But we need to examine this phenomenon in more detail, including the possibility that using two sites, one with high yields and N rates and the other with lower yields and rates, might create such a relationship from the inclusion of two data clusters.
The tenth year of this trial produced rather unusual N responses, and these data will affect the overall N response function based on numerous site-years. Combining data over 10 years from two or three sites unexpectedly indicated a linear relationship between optimum N rate and the yield at that rate, with similar responses for both corn following corn and corn following soybean, in both southern and central Illinois. This relationship was not found in either rotation in northern Illinois. Because the variability over years is due mostly to weather and not to soils or other predictable factors, it will remain difficult to predict N needs even if such a relationship holds up; as we saw in 2008, yields can be much higher than normal in a given year, regardless of how the crop is managed.
Brown, H.M, R.G. Hoeft, and E.D. Nafziger. 1993. Evaluation of three N recommendation systems for corn yield and residual soil nitrate. Ill. Fert. Conf. Proc., R.G. Hoeft (ed.). pp. 43-49.
Hoeft, R.G. and T.R. Peck. 2002. Soil testing and fertility. In Illinois Agronomy Handbook, 23rd Edition. College of Agricultural, Consumer, and Environmental Sciences, Dept. Of Crop Sciences, UI Extension, University of Illinois.
Nafziger, E.D., J.E. Sawyer, and R.G. Hoeft. 2004. Corn nitrogen fertilizer response across environments and crop rotation. North Central Extension-Industry Soil Fertility Conference Proc., Des Moines, Iowa, November 17-18, 2004, pp. 5-11.