Illinois
Fertilizer Conference Proceedings
January 22-24, 2001
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E.D. Nafziger, R.G. Hoeft, Eric 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.
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 a N fertilizer rate recommendation system has led, in Illinois, to development of a 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. The recommended amount of N per bushel is based on the ratio of N cost to corn price; 1.2 lb per bushel would reflect a relatively favorable ratio, with low N cost relative to corn price, and an increase in this ratio due to higher N costs or lower corn prices would lower the recommended N rate. 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 long-term 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 a 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 ft. In 2000,
planting was done in a timely manner at all locations except in southern Illinois,
where excessive rainfall caused damage to the crop.
SPAD measurements were taken at 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 ft. in 1-ft. 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 $0.27 per lb of N and a corn price of $1.80 per bushel, or
a cost:price ratio of 0.15.
This was the first year in which both previous crops were available at all locations. Rainfall was lower than normal in July at Urbana and Dixon Springs and in August at Monmouth and Dixon Springs. May and June rainfall totals were above normal at all locations (Table 1). As a result of very wet conditions following planting, yields at Brownstown and at the upland site at Dixon Springs were quite low-only about 40 bushels per acre at Brownstown and about 60 bushels per acre at Dixon Springs. Variability was also high at these two sites, and as a result we decided not to include these data in the discussion. Besides low yields and variability, the results at Brownstown indicated that corn following corn produced more than three times the yield of corn following soybean. This may have been due to more soil saturation.
In 2000, responses to N rate were fit well by the quadratic-plateau model
at three of the sites, but at both Perry and at DeKalb (Figure
1), the quadratic model fit the data better, due in part to lower yields
at the highest N rate. General responses to N were similar to those we have
seen in previous years. Optimal N rates for corn following corn ranged from
123 to 185 lb N per acre, and yields at optimal N rates ranged from 111 at
Dixon Springs and Urbana to 188 at Monmouth (Table 2).
For corn following soybean, the optimal rate of 87 lb N per acre produced 199
bu per acre at Monmouth, while at Dixon Springs it took 184 lb of N to produce
the optimal yield of 133 bu per acre (Table 2).
Based on data averaged over all five sites in 2000, the economically optimal
N rate for corn following corn was 42 lb N/acre higher than for corn following
soybean, even though yields were similar (Table 2).
If yields of about 142 bu/acre were used as expected yields across these locations,
then the recommended N rate for corn following corn would be 142 times 1.2,
or 170 lb N/acre, very close to the predicted 172 lb N/acre we calculated.
Similarly, corn after soybean would be projected to need 172 minus 40 lb N
credit, or 132, again almost exactly the 130 lb N/acre optimum calculated from
the data. In general, optimal N rates were higher for corn following corn than
for corn following soybean at those locations with higher yields, while this
difference tended to be less at the lower-yielding locations. An exception
to this was at Perry, where stands and early growth were not very favorable.
After two years of collecting N response yield data at diverse locations in Illinois, it is becoming apparent that, although responses at individual sites vary considerably, the technique of averaging data over locations in order to produce N response curves (Bullock and Bullock, 1994) may produce more usable recommendations. Using the average over five locations in 2000 produced optimal N rates for corn after corn and for corn following soybean only 9 and 6 lb/acre less, respectively, than those based on 10 locations over both 1999 and 2000. While these findings will have to be combined with more data from additional years before firm conclusions can be drawn, the results from fairly disparate responses over the two years generally support the present recommendations for N use on corn in Illinois.
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 2000 clearly shows this (Figure 2), but in this location, where late-season dryness affected yield and where the difference in yield level due to previous crop was very large, SPAD readings did not correlate with yield very well across the two rotations. At this location, a SPAD reading of 55, which generally indicates that corn is adequately supplied with N, was associated with yields around 180 bushels per acre when corn followed soybean but only about 120 bushels per acre when corn followed corn.
With expected changes in N prices due to price increases in natural gas, it
is useful to note how the recommended N rate differs with changes in the N
cost:corn price ratio. If the anticipated corn price is held constant at $2.00
per bushel, then based on the N response data from 10 sites over 1999 and 2000,
the recommended N rate decreases about 1 lb of N per acre for each 1-cent rise
in the cost of a pound of N (Table 3).
Table 1. Growing season rainfall at the six trial locations, 2000.
Table 3. Recommended N rate and yield at that N rate for different N prices.
Figure1. N rate response of corn following corn or soybean at five Illinois locations in 2000.
1E.D. Nafziger and R.G. Hoeft are Professors, E. Adee is Senior Research Specialist, A.H. Anderson is Visiting Senior Research Specialist, and R.E. Dunker, S.A. Ebelhar, L.E. Paul, and G.A. Raines are Agronomists, Department 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. 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.
