Emerson D. Nafziger, Howard M. Brown, and Fabián G. Fernández 1
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. Our own work, funded by FREC (Project Number 190) has been quantifying this variability, and these results have stimulated us to propose a new approach to using N rate data to formulate N use guidelines (Nafziger et al., 2005).
Working with agronomists in other Corn Belt states, we have developed a new approach to making N rate guidelines (Sawyer and Nafziger, 2005). This approach uses data from most of the recent N response work that has been conducted in Illinois, including both small-plot and on-farm results. This method involves calculating return to N based on data from each trial, then averaging these returns (at each N rate increment) over appropriate trials.
One of the main concerns in taking such an approach is whether or not the database is sufficiently large to support the outcome adequately. We undertook this work to enlarge the database on which N rate guidelines are based, using replicated, field-scale, N rate trials at numerous on-farm locations throughout Illinois, representing the diversity of soils and weather in the state.
Cooperators throughout Illinois were solicited to participate in this study through various publications, as well as in person. Criteria set out for participation were flexible, and included only that corn follow either corn or soybean, that N rates applied the previous fall or at the time of planting (for example, as starter or herbicide carrier) not exceed 40 or so lb N per acre, and that producers have the ability to apply N rates accurately. Partly because the request was made late (after funding was received), nearly all of the participants applied rates in the spring, mostly as sidedress after crop emergence. Some 30 cooperators participated in 2007 to conduct 36 trials. Sites were fairly uniformly distributed within Illinois, but the concentration in central Illinois was higher than in northern or southern Illinois.
Nitrogen rates of 0, 50, 100, 150, and 200 lb N per acre (in addition to basal N rates applied uniformly across the field) were applied in a randomized complete-block design with three reps. Strip length and width were chosen by cooperators, with the general suggestion that harvest be done in only part of the width of strips in order to reduce border effects, and that plot length be no more than one-quarter mile long, in order to minimize total yield loss due to low N rates.
Data were taken either with a weigh wagon or a combine-mounted yield monitor. Yields were calculated for each strip on the basis of 15% moisture. Yields were averaged across reps, and curves were fit to the data using PROC NLIN of SAS, with the quadratic + plateau function chosen as the default. The quadratic portion of the curve was used to calculate the economically optimum N rate (EONR) and yield at that N rate. In order to simplify the presentation, we used the N cost:corn price ratio of 0.1 to calculate EONR for each site.
Every site provided usable data in 2007. For reporting and consolidation, individual sites were separated into Northern, Central, and Southern locations, and in Central and Northern locations into corn following corn (CC) and corn following soybean (SC) sites. There were no CC sites in Southern Illinois in 2007.
The SC trials generally showed responses similar to those we have seen previously, but consistency among sites was higher than in 2006. In Northern Illinois, the three SC sites all showed response to N, and N increased yields by an average of about 65 bushels per acre (Fig. 1). In Central Illinois, yield levels and the shape of the N response were almost identical among 10 of the 12 sites; all but one trial had an optimum yield of 200 bu per acre or more (Fig. 2). The average projected yield without N (most trials had some basal N, typically 20 to 30 lb) was 118 bu per acre, while the yield at the optimum N rate averaged 220; in this set of trials the use of N increased yield by 103 bu per acre. In Southern Illinois, N more than doubled average yield, from 73 bu per acre without N to 180 bu per acre at the optimum N rate (Fig. 3). All of the trials responded well to N, but the shapes of the responses were not as consistent as in central Illinois.
Responses to N of corn following corn (CC) were similar to the responses in the SC trials. Unlike the results in 2006, when N increased yield of six northern Illinois CC sites by an average of less than 40 bu per acre, the increase in 2007 was about 83 bu per acre (Fig. 4). Still, the response to N was only modestly larger than in the northern Illinois SC trials. In Central Illinois, N increased yield by about 90 bu per acre (Fig. 5), less than the yield increase from N in SC trials. One of the CC trials in Central Illinois showed no response to N at a yield level of 215 bu pr acre, for reasons that are not at all clear. Two additional sites showed a linear response to N.
In order to see how results from these on-farm sites in 2007 differed from N response data (gathered through 2006) used to formulate current guidelines, the N rate required to produce the maximum return to N (MRTN) for these trials was compared to the MRTN values returned by the N rate calculator at the website http://extension.agron.iastate.edu/soilfertility/nrate.aspx. In Northern Illinois, the 2007 SC trials produced MRTN values similar to those from the calculator, while the value based on four CC trials, while larger than in 2006, again were less (by 39 lb N per acre) than the value returned by the calculator, and were almost the same as the value from the SC trials in 2007 (Table 1). In contrast, the CC trials in 2007 in Central Illinois produced an MRTN rate of 192 lb, 43 lb higher than the MRTN for the SC trials. This value is increased by the linear responses at two locations (the optimum N rate at such locations is the highest rate used, as long as the slope of the line exceeds the ratio of N price to corn price, which here is 0.1.) There was, however, also a non-responsive site, which deflates the MRTN.
In Southern Illinois, the MRTN based on the six SC sites was 185 lb N per acre, or about 20 lb more than the MRTN returned by the calculator based on 64 previous studies (Table 1). The effect of the 2007 data on the MRTN will be relatively modest for all three regions, however. Values from 2007 were not greatly different in most cases from values calculated using data through 2006. Also, as the number of trials represented in the data set increases, this effectively dilutes the effect of the added data.
On-farm trials went very well in 2007, due mostly to excellent cooperators and those who worked with them. Credible data were received from all locations. Trials where corn followed soybean (SC) produced more or less “typical” response data, with optimum N rates generally similar to those calculated from earlier trials. The N response was unusually low in most corn following corn (CC) trials, however, with average optimum N rates substantially less than those calculated from earlier studies, and also less than optimum N rates calculated for the 2006 SC trials. We think this was due mostly to larger than normal amounts of carryover N from the dry weather in 2005. Amounts of N needed to optimize yield tended to decrease as the ISNT values in the 0-12” soil depth increased. This reflected to some extent the high yields and high N rates needed in Southern Illinois, where ISNT values are low, and the lower than normal response to N, especially in the CC trials, where ISNT values tended to be higher.
Figure 4. N response for the on-farm, corn following corn (CC) trials in Northern Illinois in 2007.
Figure 5. N response for the on-farm, corn following corn (CC) trials in Central Illinois in 2007.
1 Emerson Nafziger, Howard Brown, and Fabián G. Fernández are Professor, Growmark Agronomy Manager and Adjunct Associate Professor, and Assistant Professor, Dep. of Crop Sciences, University of Illinois.
Nafziger, E.D., R.G. Hoeft, Eric Adee, R.E. Dunker, S.A. Ebelhar, and L.E. Paul. 2005. Assessing variability in corn response to N rate. pp. 12-18, Proc. Illinois Fertilizer Conference, Peoria, Illinois, January 24-26, 2005.
Sawyer, John E. and Emerson D. Nafziger. 2005. Regional approach to making nitrogen fertilizer rate decisions for corn. Proc. North Central Extension-Industry Soil Fertility Conference, November 16-17, 2005, Des Moines, IA.