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 2008 to conduct about 35 trials. As has been the case in previous years, the number of trials 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 2008. 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 2008.
The SC trials generally showed responses similar to those we have seen previously, but consistency among sites was higher than in either 2006 or 2007. In Northern Illinois, the two SC sites and the one CC site showed similar responses to N (Figure 1). In Central Illinois, yield levels and the shape of the N response were similar among most of the SC sites, and, with some exceptions, yields were very high (Figure 2). Yields without N (most trials had some basal N, typically 20 to 30 lb) ranged from less than 50 to about 180 bushels per acre in central Illinois. This range is somewhat larger than we have seen in previous years; yields without N in some trials in eastern Illinois were very low, due in part to excessive wetness and cool soil temperatures early in the season in this area.
One cooperator in Macon County southeast of Decatur conducted a trial with the same N rates, but with fall-applied anhydrous ammonia compared to 28% UAN solution applied after planting, with N timing/form split within N rate. Results (Figure 3) show that spring-applied N produced higher yields than fall-applied N. The N rate calculated to produce the optimum yield (at the N:corn price ratio of 0.1) was 169 lb N/acre for fall-applied N, and 174 lb N/acre for spring-applied N, and that the yield from spring applied N at the optimum N rate was 222 bu/acre, 28 bu higher than the optimum yield for fall-applied N. These results suggest that fall-applied N was less effective than spring-applied N for reasons in addition to the fact that more fall-applied N was likely lost.
Responses to N of corn following corn (CC) in central Illinois trials showed generally greater responses to N rate than has been the case in recent years (Figure 4). The average yield at the highest N rate was approximately 100 bu per acre higher than the average at the lowest N rate, and a number of the response curves were linear in shape, suggesting limited availability of N. This is likely the result of extensive loss of soil N under wet conditions, but as the results in the fall-versus spring N application trial indicate, effects of the weather on roots might also have been involved.
Results in southern Illinois were quite similar to those in recent years, but without some of the low yields we have observed in previous years (Figure 5). A considerable amount of corn in southern Illinois was planted late or replanted in 2008, and the high yields reflect the unusually favorable conditions, including high rainfall and a season that extended into October.
On-farm trials went very well again in 2008, due mostly to excellent cooperators and those who worked with them. Credible data were received from all locations. Trials in 2008 were characterized by high yields in nearly all locations, and with higher than normal responses to N rate. This included having several sites with linear responses to N rate, and numerous sites with high optimum N rates. This is much unlike the responses in 2006, where N responses were very flat in many cases due, we think, to large amounts of carryover N from the dry weather in 2005. This illustrates the value in generating and using as much N response data as possible to support current N rate guidelines.
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.