Illinois
Fertilizer Conference Proceedings
January 24-26, 2000
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E.D. Nafziger, R.G. Hoeft, E.A. Adee, A.H. Anderson, 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 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. 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 was 147 lb N/acre for corn following corn, with a yield at the optimal N rate of 144 bu/acre, and thus a N requirement of slightly more than 1 lb N/bu (Bullock and Bullock, 1994). 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 by 30 to 20 by 50 feet. In 1999, planting was done in a timely manner at all locations, but at Perry in western Illinois, poor soil conditions at planting and heavy rainfall after planting resulted in some stand loss.
SPAD measurements were taken three times during the season: at about V7-V8, VT, and R2-R3. 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 $0.18 per lb of N and a corn price of $1.80 per bushel, or a cost:price ratio of 0.1.
This was the first year in which both previous crops were available at all locations. While July and August were quite dry at all locations (Table 1), yields at the three northern locations — DeKalb, Monmouth, and Urbana — were quite high, while those in the southern three locations — Perry, Brownstown, and Dixon Springs (upland) — were lower than normal, primarily due to dryness after pollination. However, the yield advantage expected for corn following soybean, especially under dry conditions, did not occur consistently; overall, corn following corn yielded about the same as corn following soybean (Table 2).
Based on data averaged over all sites in 1999, 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, 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 Perry, where stands and early growth were not very favorable
Even though yield levels varied considerably in 1999, there was a generally positive relationship between optimal N rates and yields at those N rates (Figure 1). 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 locations in 1999 generally supported the present recommendations for N use on corn in Illinois.
SPAD readings generally showed that substantial visual differences in leaf color due to N status developed only after V7-V8 at most locations (data not shown). Differences in leaf color as measured by SPAD were generally significant by VT and developed further by the time the final readings were taken several weeks after pollination.
Table 1. Monthly rainfall totals for 1999 at the six trial locations.
1 E.D. Nafziger and R.G. Hoeft are Professors, E.A. 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, 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.
