Illinois Fertilizer
Conference Proceedings
January 25-27, 1999
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E.D. Nafziger, R.G. Hoeft, E. Adee,
R.E. Dunker, S.A. Ebelhar, L.E. Paul, and G.A. Raines1
There has been a great deal of recent work on the response of corn yield to fertilizer N rates. Much of this work has been stimulated by the need to provide a stronger basis for determining optimal economic rates of N application, and by the need to minimize environmental consequences of corn production. Most results of such work have shown a large amount of 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. Similar results reported from other studies make it quite clear that N responses are extremely variable in time and space.
While the response to N fertilizer has been found to vary substantially among fields, the need for an N fertilizer rate recommendation system has led, in Illinois, to development of an N rate recommendation based on anticipated corn yield (Hoeft and Peck, 1997). This recommendation is for 1.2 lb of 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. An obvious problem with this recommendation is that yields, and thus actual N requirement, cannot be predicted with accuracy. Still, the use of this recommendation has proven to work quite well in most years and on most fields.
Studies conducted for 11 years at Monmouth, Illinois, showed economically optimal N rates of 173 lb N/acre for corn following corn and 142 for corn following soybean (Bullock and Bullock, 1994). Recent examination of more data from the same study indicated that optimal N rates for continuous corn during the period 1983 through 1996 ranged from 54 to 24016 N/acre, and that the N amount required per bushel of yield at the calculated optimal N rate ranged from 0.59 to 1.98, with an average slightly more than 1.0 lb N/bushel.
The present study is designed to assess the response to N rate of corn following
corn or soybean over a period of years at a number of locations in Illinois.
SPAD meter readings and post-harvest soil nitrate determinations are designed
to assess the usefulness of these technologies in improving economic and environmental
soundness of N rate recommendations for corn.
As this was the first year of this study, it was possible to assess the response of corn following only corn or soybean-not both-at all locations except at Perry, where an ongoing study including both previous crops was adapted for this work. At Dixon Springs, separate trials were established in bottomland and upland positions, and the trials included tillage as a variable. A trial established at Brownstown suffered from excessive rainfall and will not be discussed. A split-plot experimental design was used, with previous crop (corn or soybean) as main plots (after 1998), and N rates-0, 45, 90, 135, 180, and 225 lb N/acre-on corn split within main plots. Subplot sizes ranged from 10 x 30 feet to 20 x 50 feet. Table 1 gives soil types and agronomic practices for each trial in 1998, and monthly rainfall totals are given in Table 2.
SPAD measurements and harvest for yield were done on the center two rows of each subplot. Following harvest, soil samples were taken in one-foot increments to a depth of three feet for nitrate analysis. Yield data were analyzed using regression, and economically optimal N rates were calculated from quadratic functions using a cost of $0.25 per lb of N and a corn price of $2.00 per bushel.
This was the first year of what is planned to be a multiple-year study, and in most cases only one previous crop, rather than two, was available. In the three sites (DeKalb, Urbana, and Dixon Springs) that had only corn as the previous crop, responses to N rate were significant, and generally followed a curvilinear shape (Figure 1). The only exception was at Urbana, where the response within the range of N rates was essentially linear. Economically optimal N rates at these locations ranged from 124 in the upland site at Dixon Springs to the maximum N rate used-225 lb N/acre-at Urbana. The N requirement ranged from 0.87 to 1.27 lb N per bushel of yield (Table 3).
At Perry, where corn was grown following both corn and soybean, corn following corn responded more to N rate up to about 135 lb N/acre than did corn following soybean, then yield decreased at higher N rates, while corn following soybean showed a more typical response, leveling off at the highest rate (Figure 2). The economically optimal N rate for corn following corn was higher than for corn following soybean, as was the yield at the optimal N rate. These factors combined to produce a large difference in N needed per unit of output-0.63 lb N/bu for corn following corn, and 1.08 lb N/bu for corn following soybean (Table 3). These results are in contrast to other work at Monmouth that showed lower optimal N rates and higher yields-and thus lower N requirement per unit of yield-for corn following soybean (Bullock and Bullock, 1994). At the Monmouth location, where corn followed soybean, the response of yield to N rate was nearly linear (Figure 3), and the optimal N rate was taken as the highest N rate used, with strong indications that higher N rates would have substantially increased yields. The yield of 203 predicted at the highest N rate (225 16 N/acre) resulted in a ratio of 1.1 1 lb N/bu of yield, which is within the range of commonly observed values when corn follows soybean, but which is relatively high given that 40 lb N/acre is usually credited to corn following soybean.
SPAD readings were not taken at all locations, and data from them will not be presented here. In general, SPAD readings responded to N rate in much the same way as did yield. Soil nitrate measurements are not yet available.
The 1998 crop year was the first for this study, and while it was not possible to draw many conclusions with regard to previous crop effects, N responses differed substantially among locations. Yields ranged from expected levels at Dixon Springs and Urbana to higher-than-expected at other locations. All studies will continue in 1999 and include the full set of treatments.
Table 2. Monthly rainfall for the five locations of the previous crop x N rate studies.
Figure 3. Corn yield response to N rate at Monmouth, where corn followed soybean.
1 E.D. Nafziger and R.G. Hoeft are Professors, E. Adee is a Senior Research Specialist, and R.E. Dunker, S.A. Ebelhar, L.E. Paul, and G.A. Raines are Agronomists, Dept. 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. 111. Fert. Conf. Proc., R.G. Hoeft (ed.). pp. 43-49.
Bullock, D.S. and D.G. Bullock. 1994. Calculation of optimal nitrogen fertilizer rates. Agron. J. 86:921-923.
Hoeft, R.G. and T.R. Peck. 1997. Soil testing and fertility. In Illinois Agronomy Handbook, 7997-98. Circular 1344, College of Agricultural, Consumer, and Environmental Sciences, Dept. Of Crop Sciences, Cooperative Extension Service, University of Illinois.
