Illinois
Fertilizer Conference Proceedings
January 26-28, 2006
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L.E. Paul, E.A. Adee, and S.A. Ebelhar1
With increasing nitrogen costs and concerns about efficient nitrogen use, the recommendations for nitrogen fertilizer applications rates for corn are being reviewed. Fertility specialists from the University of Illinois and other Universities are working to establish new nitrogen fertilizer ap-plication guidelines for corn. Although the nitrogen application rates for wheat are lower than for corn, economically effective application levels for wheat should also be evaluated. The amount of nitrogen to apply to produce the best economic yield in any single year is influenced by several factors. These factors include, among others, the previous crop, the organic matter content of the soil, the amount of nitrogen released from the soil, and the amount of nitrogen lost from the soil.
Recommendations for the amount of nitrogen to apply for a wheat crop have changed slightly in the past 20 years. The recommendations listed in the Illinois Agronomy Handbook (Hoeft and Peck 2002) as compared to the Illinois Agronomy Handbook (1983-84 Circular 1208) were in-creased by 10 to 20 pounds of nitrogen per acre for "stiff-strawed" varieties and by 30 to 40 pounds for other "adapted" varieties in the past 20 years. Wheat varieties have changed since the 1980's with improved straw strength and the term "stiff-strawed" varieties has been dropped from the current Handbook. Current recommendations are a range of rates depending on soil organic matter. Additional work is needed to determine the effects of the fall and spring nitrogen application timing and the best rate for each application timing to establish nitrogen rate levels. With the increasing cost of nitrogen fertilizer, the amount of nitrogen to apply for maximum re-turn will be affected.
These studies were established in the fall of the 2004 in wheat seeded into soybean fields. They were established as split-plot designs with the fall n rates of 0, 20, and 40 pounds per acre as the main plots and the spring n rates of 0, 25, 50, 75, 100, 125, and 150 pounds per acre split in the main plots. These studies are established on four University of Illinois Crop Science Research and Education Centers: DeKalb - Flanagan silt loam; Monmouth - Sable silty clay loam; Brownstown - Cisne silt loam; and Dixon Springs - Grantsburg sil.
This is a summary of one year results from the 2004-2005 growing season. Some additional data from the harvest of 2004 are included, but not all nitrogen rates were used in both years.
In general, there was a significant positive yield response to fall applied nitrogen. Although not significant at Brownstown, the trend at all locations was for a positive yield response to some fall applied nitrogen. The yield response at Dixon Springs and DeKalb was linear to the fall nitro-gen. The Monmouth site had a larger yield response to the 20# rate of nitrogen application than to the 40# application rate. (Fig. 1)
The yield response to spring nitrogen was linear at Dixon Springs (Fig. 2) also at Brownstown (Fig. 3) except when no nitrogen was applied in the fall. At Dixon Springs using the regression model, the optimum spring nitrogen rate was 150 pounds per acre. At Brownstown using the regression model, the optimum nitrogen rate with 0 pounds of fall nitrogen was 116 pounds per acre. With the 20 and 40 pounds of fall applied nitrogen, the optimum application rate for spring nitrogen was 150 pounds per acre.
At Monmouth (Fig. 4) and DeKalb (Fig. 5), the yield responses were mostly quadratic with no significant increase in wheat yields above the application levels of 75 to 100 pounds of nitrogen per acre. At Monmouth, using the regression model, the optimum nitrogen rate with no fall ni-trogen was 67 pounds of nitrogen per acre. When 20 or 40 pounds of nitrogen was applied in the fall, the optimum spring rate was 0 pounds. At DeKalb, using the regression model, the opti-mum rate with no fall nitrogen was 3 pounds. When 20 to 40 pounds was applied in the fall, the optimum level is 0 pounds in the spring.
Changing prices of nitrogen and wheat can have an effect on the optimum nitrogen fertilizer rates For example, if the ratio of the price of a pound of nitrogen to the price of a bushel of wheat moves from 8 to 1 to 16 to 1, the optimum amount of nitrogen at DeKalb changes from 3 pounds to 45 pounds per acre.
In considering the amount of nitrogen that is needed, conditions in 2005 must be taken into account. The dry soil conditions of 2005 probably meant that there was less nitrogen loss than normal and may have indicated that less nitrogen was necessary than would normally be expected. Conversely, the dry soil may have made it more difficult for the roots to absorb the nitrogen that was present in the dry areas of the topsoil.
Over the two year period of 2004 and 2005 at Dixon Springs and Brownstown, the wheat yield response to increasing spring nitrogen rates was nearly linear (Fig. 6). For the two year period, optimum levels of spring nitrogen were 89 pounds at Brownstown and 136 pounds at Dixon Springs. These amounts are the total nitrogen applications rates with no nitrogen applied in the fall.
Over the same two years at DeKalb, the wheat yield response to increasing spring nitrogen rates was quadratic (Fig. 7) after 40 pounds per acre in the fall. Although there is a positive response to added nitrogen in the spring, with the present price of nitrogen, the optimum levels of additional nitrogen in the spring in these two years was none.
When the total amount of applied nitrogen and resulting wheat yield are compared, there are varying results across the trial locations around the state (Fig. 8). The optimum total nitrogen rate varies from 3 pounds to 150 pounds per acre. When all sites are averaged, the optimum total nitrogen rate is 124 pounds per acre (Fig. 9). However, using that amount would not have resulted in the best results at any of the sites as the rate is too low for the southern Illinois sites and too high for northern and central Illinois.
Based on the limited data from this study, the results seem to indicate that using the nitrogen recommendations in the Illinois Agronomy Handbook that are based on the organic matter of the soil and the location in the state would do a fairly good job of optimizing the nitrogen rate. However, in good production years, the application rate recommendations may not be high enough in southern Illinois. The application of some of the nitrogen in the fall seems to be beneficial to yields and will replace an equal or greater amount of nitrogen applied in the spring.
Figure 1. Wheat yield response to fall 2004 nitrogen applications, all locations.
Figure 2: Wheat yield response to nitrogen rates at Dixon Springs in 2005 growing season.
Figure 3: Wheat yield response to nitrogen rates at Brownstown in 2005 growing season.
Figure 4: Wheat Yield Response to Nitrogen Rates at Monmouth in 2005 growing season.
Figure 5: Wheat Yield Response to Nitrogen Rates at DeKalb in 2005 growing season.
Figure 8: Wheat yield response to total nitrogen application for the 2005 growing season.
1 L.E. Paul and S.A. Ebelhar are agronomists and E.A. Adee is a principal research specialist, Dept. of Crop Sciences, University of Illinois, Urbana, IL.
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.
Illinois Agronomy Handbook,1983-84 Edition. Circular 1208. College of Agriculture. Dept. of Agronomy, University of Illinois Cooperative Extension Service.
