Illinois Fertilizer
Conference Proceedings
January 21-23, 2002
| Home | 2002 Index | Search |
|
|
| Home | 2002 Index | Search |
E.D. Nafziger, R.G. Hoeft, E. Adee, A.H. Anderson, R.E. Dunker,
S.A. Ebelhar, L.E. Paul, and G.A. Raines1
A report on FREC Project Number 190, 1998-2001 and 2001-2004.
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 (or a slightly different ratio based on the 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. 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.
Recent examination of data from a longterm previous crop x N rate study at Monmouth, Illinois revealed that the economically optimal N rate based on data averaged over 16 years (Bullock and Bullock, 1994) was 147 lb N/acre for corn following corn, with a yield at the optimal N rate of 144 bu/acre, and so an N requirement of slightly more than 1 lb N/bu. For corn following soybean, the optimal N rate was only 94 lb N/acre and the yield at that N rate was 170 bu/acre. This is substantially lower than the amount of N that would be recommended: 170 bu/acre times 1.2 minus 40 lb N credit for soybean is 164 lb N/acre.
The present study was designed to assess the response to N rate of corn following corn or soybean over a number of years and locations in Illinois. SPAD meter readings and post-harvest soil nitrate determinations were included to assess the usefulness of these technologies in improving economic and environmental soundness of N rate recommendations for corn.
A split-plot experimental 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. Subplot sizes ranged from 10 x 30 to 20 x 50 feet. In 2001, planting was done in a timely manner at all locations.
SPAD measurements were taken three times during the season: at about V7-V8; V13-15; and VT-R1. Harvest for yield was done on the center two rows of each subplot. Following harvest, soil samples to a depth of 3 feet in 1-foot increments were taken for nitrate analysis. Yield data were analyzed using nonlinear regression, and economically optimal N rates were calculated from quadratic-plateau functions using a cost of $0.20 per lb of N and a corn price of $2.00 per bushel, or a cost:price ratio of 0.10.
Growing conditions ranged from below average to favorable at the different locations in 2001, with yields about average or less than average at DeKalb, Orr Center, and Brownstown, and yields average to above average at Monmouth, Urbana, and Dixon Springs. Responses to N rate were fit well by the quadratic-plateau model at most of the sites, but the quadratic model fit better for corn following soybean at Monmouth and Brownstown, and for corn following corn at Perry. General responses to N were similar to those we have seen in previous years: optimal N rates for corn following corn ranged from 140 to 225 lb N per acre, and yields at optimal N rates ranged from 128 at Perry to 189 at Monmouth (Table 1). For corn following soybean, the optimal rate of 98 lb N per acre produced 175 bu per acre at Monmouth, while at Brownstown, it took 225 lb of N to produce the optimal yield of 151 bu per acre (Table 1).
Based on data averaged over all sites in 2001, the economically optimal N rate for corn following corn was 34 lb N/acre higher than for corn following soybean, and yields for corn following soybean were 17 bu/acre higher than for corn following corn (Table 1). Based on soil type, the yields of the soils in this study would be expected to average about 149 bu/acre. That would call for a recommended rate of N of 149 x 1.2 = 178 lb N/acre, very close to the optimum N rate of 180 lb N/acre that was calculated based on averages across all trials. Similarly, corn after soybean would be projected to need 178 minus 40 lb N credit, or 138 lb N/acre, close to the optimum rate of 146 lb N/acre calculated from all of the data.
As we have tended to see in the past, there is not a good correlation between yield level and optimum N rates. This illustrates the difficulty of trying to guess yield levels and corresponding N rates to use in particular fields. As we have seen most of the time, the technique of averaging data over locations in order to produce N response curves (Bullock and Bullock, 1994) may produce more usable recommendations than most alternatives. SPAD readings generally showed that leaf color as affected by N rate developed fully only after V7-V8 at most locations, but N deficiencies tended to be discernable by V7. The Urbana data from 2001 is typical of SPAD data from other sites, and it shows that correlations between SPAD and final grain yield tend to improve as plants get older (Figure 1). While N deficiency can be detected in plants around the V7-V8 stages, such plants tend to have fairly high SPAD readings, and it appears that N redistributes later in the season, after which leaf color is more indicative of likely yield level.
We now have data from three full years of this study and can start to address
our overall goal of relating N rate to yield of corn following corn or following
soybean. Figure 2 shows the overall response of yield
to N rate for the two rotations, averaged over 19 environments over the 1999-2001
seasons.
Attempts to find ways to predict N response in these trials have not been very successful. While predicted yields (and N rates) based on soil type do show some relationship to actual observed yields (Figure 3), there is a lot of variability in these responses, and there is no indication from the data of a clearly useful way to fine-tune recommendations to fit fields.
Even if we somehow knew what yield would be produced in a given field before a given season, indications are that we would not have a strong basis for adjusting N rate. As Figure 4 shows, the correlation between observed optimum N rate and the yield at the optimum N rate is not very strong among environments, even though it is somewhat better for corn following corn (r2=0.50) than for corn following soybean (r2=0.13).
To date, our results do not lend strong support either to modifying the present system of N rate recommendations or to using another predictor besides expected yield to fine-tune recommendations for individual fields. The average ratio of optimal N rate to yield over 19 environments was 1.08 lb N/bu of yield for corn following corn and 1.10 for corn following soybean, after adding the N credit for soybean. Based on the overall response functions, these ratios were 1.17 and 1.10 for corn following corn and corn following soybean, respectively. Based on yield goal according to soil type, these ratios were 1.23 and 1.15, respectively. While we know that the guideline for choosing an N rate for an individual field will usually not fall exactly on the seasonal optimum for that field, there is no obvious basis for making this recommendation system more precise.
1 E.D. Nafziger and R.G. Hoeft are Professors, E. Adee is Senior Research Specialist, A.H. Anderson is 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. 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. 1997. Soil testing and fertility. In Illinois Agronomy Handbook, 1997-98. Circular 1344, College of Agricultural, Consumer, and Environmental Sciences, Dept. Of Crop Sciences, Cooperative Extension Service, University of Illinois.
