No. 1 Article 8/March 24, 2006

N Rates for Corn in Illinois, Spring 2006

As part of the process of using N-rate response data to suggest N rates in Illinois, we will be adding new data when they become available and, in some cases, discarding older data. We have done that since we talked about N rates here last fall and so can give current guidelines for N use on corn this spring based on the best information we have.

The Iowa State website calculator continues to function with the same data as last fall. We hope to modify that data to match what we are giving here. That will include having separate recommendations for northern, central, and southern Illinois. If you have the option to choose one of those parts of Illinois when you go to the calculator, you'll know that the data have also been updated.

Dividing Illinois into three sections like this is more or less arbitrary, but for our purposes we'll consider northern Illinois to include the counties that touch I-80 and the counties north of them. We'll consider southern Illinois to be those counties that touch I-70 and the area south of that, though much of the land that lies north of I-70 but south of the terminal moraine that roughly follows Illinois Route 16 can be considered as southern Illinois from a soil standpoint. Central Illinois is the area in between. We all recognize that these designations are crude, especially in terms of soil type, topography, and the like. But they should work until we have data to define regions better.

The following tables (Tables 3 and 4) give suggested N-rate ranges for corn following corn and for corn following soybean for different regions in Illinois, at two N prices and six corn prices. The nitrogen price of $0.30 is equivalent to anhydrous ammonia at $492 per ton, and to UAN 28% at $168 per ton, while $0.40 per pound of N is $656 per ton for ammonia and $224 per ton for UAN. Ranges for prices between those can be estimated, as can ranges for corn prices between those provided. The number in parentheses after the name of the region is the number of site-years of N-response data that went into the calculations for that region. The N rate expected to provide the greatest return to N is in about the center of each range.

The central and southern Illinois numbers are similar to those generated using all of the Illinois data through 2004, which have been on the Web site up to now. The numbers for northern Illinois, however, are somewhat different. There, N rates for corn following corn are higher and those for corn following soybean are somewhat lower than for central and southern Illinois, or for the whole state taken together. These guidelines are based on the best available data we have, but we hope to add to our confidence in them as we add to the numbers of locations that we have to use in these calculations. Until then, we suggest setting N rates within these ranges based on corn and N prices.

The high price of N has raised a lot of questions about adjusting rates based on N form used and the time of application. Much of the data in these two tables are based on sidedressed N, mostly because sidedressing works so well for such trials. Hence, we would not consider the rates given to have much cushion against possible loss or unavailability. Sidedressed N is generally considered the application method that provides the most efficient uptake of fertilizer N. As recently as 2005, however, UAN placed on the surface after planting or sidedresssed was often not very effective where soils remained dry for weeks after planting. Some N was lost from surface-applied urea (part of UAN) due to volatilization, roots didn t grow into dry surface soils, and N didn't move to roots due to lack of water. This all combined to make the efficiency of N uptake poor in many fields last year.

The example from 2005 does not mean that sidedressed N isn't still an efficient way to apply N, but it illustrates the difficulty in predicting the effect of changing rates or time and form of application. What data we have on form and timing reinforce this--when done properly, most forms and times work reasonably well. I would not suggest reducing rates from those given in the tables if a change is made from preplant to sidedress, but certainly moving rates within the range given is acceptable and, in some cases, appropriate.--Emerson Nafziger

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