E.D. Nafziger, R.G. Hoeft, Eric Adee, R.E. Dunker, S.A. Ebelhar,
and L.E. Paul 1
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. As N costs have increased in recent years, however, it has become clear that using proven yield as a predictor of N rate tends to result in using more N than can be economically justified. This has led to the use of recent data to formulate N rate guidelines, with adjustments based on N and corn prices (Nafziger et al., 2004). Data from this project have been used extensively in developing this new approach.
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 need for fertilizer 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 + plateau 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 (EONR values) 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.
The maximum return to N (MRTN) approach uses the same data as is used to calculate EONR rates and yields, but it uses N and corn prices to convert yields and N rates into dollar return (above the yield at zero N) per acre. The net return is calculated for each of a series of N rates (the calculator currently uses a range of 0 to 240 lb N/acre in 10-lb increments) then returns are averaged over trials to produce a response curve of RTN over N rates. The maximum point of this curve is found, and the N rate at this maximum is taken as the MRTN N rate.
One of the questions we answer using N response data taken from the same sites over years is how much the average EONR values and the MRTN values differ. The average EONR value indicates the annual amount of N that we would apply if we had foreknowledge of the response for that year, so could adjust the N rate to the EONR. This should provide maximum profit, since the N supply would always equal the demand. The MRTN approach incorporates variability over years, and hence is closer to the more traditional approach to predicting N response based on a series of trials, differing mainly in the use of net return instead of yields averaged over years.
To assess how these two approaches differ, we calculated EONR values for each year at each location and for SC and CC, and compared the average EONR values and yields at the EONR to the N rate and yield at the MRTN, based on data averaged over the years of the study. We then calculated the increase in return from EONR (that is, having the “perfect” N rate each year) over that from MRTN, which is based on data averaged over years. Optimum N rates and returns to N were calculated using an N price of $0.40 per pound and a corn price of $4.00 per bushel.
For most locations for corn following soybean, the average EONR was 7 to 13 lb N/ac less than the MRTN, though at Perry and at the Dixon Springs upland site, this difference was 23 and 32 lb N/ac, respectively (Table 1.) Yields ranged from 0 to 5 bu/ac higher using the EONR, and this value was 3 bu/ac or less in all but one site. The added profit available if we were able to know and apply the exact EONR each year ranged from $7 to $23/ac, and was more than $15 /ac at only two locations. In one of these (Brownstown) much of this increase was due to higher yield with the EONR, which reflects the large amount of variability over years in the yield at the EONR. At this location, the CV of yield at EONR over years was 38%, compared to less than 20% at most other locations. The added “profit” from using the annual EONR was also high at the Dixon Springs upland site, where the variability in yield was also high but where there was a substantial saving in N (32 lb N/ac) compared to using the MRTN.
For corn following corn, the difference between the average EONR and the MRTN was greater at most locations than for corn following soybean, though these values were correlated among sites (Table 1.) The savings in N from using the EONR each year compared to using the MRTN rate ranged from 8 to 42 lb N/ac. The difference in yield between the two methods was only 1 to 3 bu/ac, or about the same as for SC. While the CV of yields at the EONR over years was greater for CC than for SC, this did not translate into a greater yield advantage from year-specific N rates. Instead, most of the higher profit potential from the use of EONR rates each year in CC came from saving N; the Perry, Brownstown, and Dixon Springs upland sites, which also had lower yields, required an average of 26, 33, and 42 lb N less per year than if the MRTN rate had been used each year.
It was not surprising to find that both EONR and yields at EONR varied substantially over years. Three of the seven sites in SC and four of the sites in CC had EONR values ranging up to the maximum N rate used (225 lb N/ac). All seven SC sites and three of the CC sites had EONR values less than 100 lb N/ac in at least one year. It is clear that, while knowing and using the EONR in advance for a field or part of a field (the ultimate goal of site-specific N application) has potential to reduce N use and to increase profit, it is much less clear that the high degree of variability among years in both EONR and yields at EONR will ever allow us to do this with accuracy.
Based on nine years of data, if we used the seven sites involved in this trial as a “simulated” field with equal acreages of each soil (and climate), then knowing the optimum N rate and using it each season in each part of the field would, compared to using the MRTN N rate, increase yield by 2 bu/ac and decrease N use by 15 lb/ac if the field were corn following soybean, hence raising the return to N by $15 per acre. If the field were corn following corn, yield would increase by 1 bu/ac and N use would decrease by 22 lb/ac, also providing an increase of $15 per acre in return to N. Unfortunately, because the variability that we see over years is due almost entirely to weather and not to soils or other predictable factors, it will be difficult to capture any of this saving.
1 E.D. Nafziger and R.G. Hoeft are Professors, E. Adee is Principal Research Specialist, and R.E. Dunker, S.A. Ebelhar, and L.E. Paul are Agronomists, Dep. 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.
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.
Nafziger, E.D., J.E. Sawyer, and R.G. Hoeft. 2004. Corn nitrogen fertilizer response across environments and crop rotation. North Central Extension-Industry Soil Fertility Conference Proc., Des Moines, Iowa, November 17-18, 2004, pp. 5-11.