Illinois
Fertilizer Conference Proceedings
January 26-28, 2004
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E.D. Nafziger, R.G. Hoeft, Eric Adee, R.E. Dunker, S.A. Ebelhar,
and L.E. Paul1
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.
Data from a long-term previous crop x N rate study at Monmouth, Illinois revealed that the economically optimal N rate was 143 lb N/acre for corn following corn, with a yield at the optimal N rate of 146 bu/acre, and so an N requirement of slightly less than 1 lb N/bu (Bullock and Bullock, 1994). For corn following soybean, the optimal N rate was 99 lb N/acre and the yield at that N rate was 174 bu/acre. This is substantially less than the amount of N that would be recommended: 174 bu/acre times 1.2 minus 40 lb N credit for soybean is 169 lb N/acre.
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
Of 68 N response curves (34 environments with two rotations—corn and soybean as previous crop—in each environment) generated from 1999 through 2003, the quadratic-plateau (Q-P) model fit the data in 60 cases, and the quadratic model fit the data in five cases, three of which were at the Perry location. There was no significant response to N rate in three environments at Brownstown, all with low yields.
As expected, there was a considerable amount of variation in the shape of the response curves over the five years and two rotations at each location. At Urbana (Figure 1) the optimum N rate for corn following corn (CC) ranged from 130 to 225 lb N/acre, and for corn following soybean (SC) the optimum N rate ranged from 107 to 192 lb N/acre (Table 1). In two of the five years, the optimum N rate was higher for SC than for CC. In all five years, the yield at the optimum N rate for corn following soybean was higher than that for corn following corn. This difference ranged from 2 to 63 bushels per acre, and averaged 18 bushels per acre.
While it is instructive to look at N responses over years, N rate recommendations are best made by averaging data across environments (years and locations), then analyzing the N response defined by these means (Bullock and Bullock, 1994). The average of the N rate optima was less than the optimum based on averages, by 13 lb/ac for CC and by 6 lb/ac for SC (Table 1).
The response to N rate for the two rotations at Urbana averaged over the four years (Figure 2) fit the Q-P model well. This was generally the case at the other locations as well, with the possible exception of Perry, where there was a tendency in some years for yields to decrease under high N rates in the CC rotation. This location is of average soil productivity, and it is not clear why yields tended to decrease at high N rates.
One of the questions we are also attempting to answer from this study is whether or not locations (soils and climate) should be grouped to examine overall N response, or whether sites should be examined separately. Table 2 gives the optimum N rates and yields at those rates for each location, based on data averaged over all years at each site. At most locations, yields over these five years agreed reasonably well with expected yields based on soil type and climate. Yields were relatively high and stable at the three Northern Illinois locations, and at optimal N rates, corn following soybean yielded about 18 bu/acre (11 percent) more than corn following corn. At Perry, corn following corn yielded about 2 bu/acre more than corn following soybean; this may have been due to random field effects, with corn following corn (which remains in the same plots) originally assigned to more favorable portions of the field. In Southern Illinois, corn following soybean yielded only 5 bu/acre (4 percent) more than corn following corn, but this difference increased with yield level among the three locations (Table 2).
While there are differences in N response among locations, the similarity in the general shape of the N response among locations does not clearly suggest location-specific modifications in the way N is recommended. At Brownstown, even though yields were low and unresponsive to N two of four years, corn required relatively high N rates to produce relatively low yields. On the other hand, the highest average yield—corn following soybean at Monmouth—required relatively less N to produce these yields. The resulting weak but negative correlation between optimum N rate and optimum yield (Figure 3) might suggest that the N rate recommendation factor does not remain constant with yield level. This needs further investigation, but these data indicate that a “flat rate” N recommendation for corn following soybean might better accommodate the variation in actual yield level than does the yield-based recommendation.
Based on the response to N rate based on all of the data (Figure 4), the optimum N rate for corn following corn was 171 lb N/acre, which produced a yield of 137 bu/acre; the N rate to yield ratio was 1.2 lb N/bu. For corn following soybean, 132 lb N produced 150 bu/acre; when the 40-lb N credit is added to this N rate, the ratio is calculated as 1.1. While individual years and sites show considerable divergence from the average (Table 2), our results tend to confirm that using 1.2 lb. N per bushel of expected yield, with appropriate adjustments, is a reasonable N use guideline. At the same time, expecting this guideline to result in exactly meeting the N requirement of the crop in a particular field in a given year is clearly not realistic; when we look back on the data from any one site-year, the optimal N rates were almost always “too high” or “too low” based on measured N response. Thus, while the yield-goal-based guideline is thus not a good ex post descriptor of N need, it is the best predictor that our applied research has been able to generate.
1 E.D. Nafziger and R.G. Hoeft are professors, E. Adee is a principal research specialist, and R.E. Dunker, S.A. Ebelhar, and L.E. Paul are agronomists, Dept. of Crop Sciences, University 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.
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. 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.
