Illinois Fertilizer
Conference Proceedings
January 26-28, 2006
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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.
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 use of 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 2005. 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 $2.10 per bushel.
The optimum N rate - that rate at which the net return to N (RTN) was at its maximum - differed considerably among years at each location. At the Urbana location (used as an example), the optimum N rate (the high point of the RTN curves in Fig. 1a) for corn following corn ranged from 125 to 225 lb N per acre, and averaged 164 lb N/acre. Yields at the optimum N rates ranged from 112 to 213 bu per acre, and averaged 167 bu/acre. At the same location, the N response of corn following soybean showed similar variability (data not shown), with the optimum N rate ranging from 79 to 182 with an average of 132 lb N/acre, and the yields at the optimum N rate ranging from 129 to 221 with an average of 182 bu/acre.
Corn following corn at Urbana responded to N up to the maximum N rate used, or 225 lb N/acre, in 1999, the first year of the study (Fig. 1b). This large response affected the return to N for several years; the optimum N rate was 195 lb/acre calculated using the first three years of data (1999-01), but fell to 168 lb/acre when calculated using data from the first five years (1999-03). It moved up slightly, to 174 lb N/acre using data from all seven years, but the return to N curves based on data from five or seven years were virtually identical (Fig. 1b).
At Monmouth (chosen as an example site), the optimum N rate for corn following soybean ranged over years from 61 to 137 lb N/acre (Fig. 2a), with an average of 103 lb N/acre. In two of the seven years (2003 and 2004), the return to N was close to zero at the highest N rate used in the trial (225 lb/acre). Yields at the optimum N rate ranged from 185 to 225, and average yield was 202 bu/acre. Corn following corn at this site had optimum N rates ranging over years from 114 to 191 (data not shown), with an average of 158 lb N/acre. Yield at optimum N rate ranged from 94 to 214, averaging 162 bu/acre. The range of optimum N rates over years for corn following soybean at Monmouth was smaller than at other locations. This limited range meant less change in optimum N rate as the number of years of data increased; optimum N rates calculated using 1, 3, 5, or 7 years of data were 115, 116, 113, and 106 lb N/acre (Fig. 2b).
Similar changes in calculated optimum N rates for corn following corn show considerably different patterns at the different locations (Fig. 3). Among locations, optimum yield calculated based on two years of data ranged from less than 50 to more that 200 lb N/acre. This difference decreased considerably as the number of years of data increased. The range from highest to lowest optimum N rate was about 80 lb/acre using three years of data and only about 50 lb/acre when data from five years were used. Using more than five years of data resulted in few changes in calculated optimum N rate. Using seven years of data (there were only five years of data usable at Brownstown), the optimum N rate ranged from 144 at the Dixon Springs upland site to 206 lb/acre at DeKalb.
As expected, the variability in and the size of the optimum N rate for corn following soybean was less than for corn following corn. The Dixon Springs upland and the Brownstown sites, both of which are subject to drought and low yields, showed very different responses to N, with low optimum N rates at Dixon Springs and relatively high optimum N rates at Brownstown. With the exception of the slow "recovery" of optimum N rate with additional years of data at the Dixon Springs upland site, optimum N rates at all sites were within the range of 100 to 150 lb N/acre when four or more years of response data were used.
While we anticipate that this research project will continue to run for two or three more years, evidence is developing that optimum N rates calculated based on maximizing net return to N stabilizes for a given site after about five years of response data have been accumulated and used in the calculation. For corn following corn, the optimum N rate based on seven years of data varies from 144 to 206 lb N/acre, but the four sites with optima in the middle of this range have calculated optima within 9 lb N/acre of one another. For corn following soybean, this range is even smaller, with all optima between 106 and 144 lb N/acre. While lower-yielding sites tend to need less N when corn follows corn, there is no such correspondence between yield and optimum N rate when corn follows soybean; the highest-yielding site (Monmouth) is the site with the lowest optimum N rate, 106 lb N/acre. These finding generally support the decision to disassociate N rate guidelines from expected yield, and also to apply N rate guidelines across large areas of Illinois.
Figure 1. Return to N curves calculated for corn following corn at Urbana, Illinois.
Figure 2. Return to N curves calculated for corn following soybean at Monmouth, Illinois.
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.
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.
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.
