E.D. Nafziger, R.G. Hoeft, Eric Adee, R.E. Dunker, S.A. Ebelhar,
and L.E. Paul 1
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 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 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.
To assess the effects of year-to-year variability on calculated optimum N rates, we calculated for each location the N rate that maximized return the N (the optimum N rate) starting with the first year of data (1999) and adding additional data for each year, through 2006. This was done for corn following corn (CC) and for corn following soybean (SC). In all cases discussed here, optimum N rates and returns to N were calculated using an N price of $0.30 per pound and a corn price of $3.30 per bushel.
The economically optimum N rate (EONR) – that rate at which the net return to N (RTN) was at its maximum – differed considerably among years at each location, and did not always correlate well with the yield difference between SC and CC rotations at the EONR. At DeKalb, the EONR averaged 50 lb of N less for SC compared to CC, and SC averaged 191 bu per acre, 17 bu more than CC (Table 1). Except for the first year of the study at this location, the EONR for CC was always higher than for SC, and the yield of CC was always lower (Fig. 1). This was also the case at Monmouth, where the average EONR was 61 lb N greater for CC than for SC, and where the average yield penalty for CC was 37 bu per acre (Table 1). This location was characterized by some very large yield differences, exceeding 50 bu per acre in three of the eight years (Fig. 2). Differences were inconsistent at Urbana, where CC on average required 17 more lb N to produce 15 less bu per acre (Table 1), but where, except for one year when CC yielded more than 60 bu less than SC, yield penalties for CC tended to be modest. Perry was an unusual location, in that CC had a slightly higher yield (4 bu per acre) than SC, but required an average of 37 lb more N per acre to achieve that yield (Table 1). Not only was the average yield difference small, it also varied less over years than at most locations (Fig. 4).
The Brownstown site frequently suffers from lack of water, and the average SC yield there is only 118 bu per acre (Table 1). Poor conditions in two of the eight years rendered results unusable. While SC averaged 13 more bushels per acre than CC at this site, the amount of N required did not differ on average, though differences among years were quite variable (Fig. 5). It is difficult to make projections based on such variable data. The two sites at Dixon Springs showed considerable differences, with the yield level in the bottomland site 50 bu higher than at the upland site, but with the same amount of additional N (29 lb per acre) needed to optimize yield at both sites (Table 1). Yield differences between SC and CC averaged 3 bu for the upland site, where differences were within a range of 20 bu on either side of zero (Fig. 6), and averaged 13 bu at the bottomland site, where differences ranged from about zero to 25 bu or so (Fig. 7).
With the expected increase in continuous corn acreage, there is considerable demand for data that indicates what the yield “penalty” (compared to SC) is expected to be, and also how much more N is needed when corn follows corn. This study to date has generated 54 site-years of data with which to make such predictions. The problem remains, however, about how to make sense of the large amount of variability that occurs among locations and among years. If we average all of the data from Table 1, we can say that corn following corn yields 13 bu, or about 8 percent less, than corn following soybean, and it requires 32 lb more N. We could further state that the yield penalty tends to be higher in Northern Illinois, where yields of SC are higher, and that the additional N needed to produce CC yields is also greater in this area. While correct, the variability is so great that predictions based on such averages will in almost every case appear to be “wrong” by many. The challenge remains to educate on the fact that high variability always means a low chance that a prediction will seldom turn out to be “exact”.
1 E.D. Nafziger and R.G. Hoeft are Professors, E. Adee is Principal Research Specialist, and R.E. Dunker, S.A. Ebelhar, and L.E. Paul are Agronomists, Dep. of Crop Sciences, Univ. of Illinois, Urbana, IL.
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.