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Issue No. 1, Article 11/March 23, 2007

Spring Nitrogen Rates for Corn

Last fall the price of corn was rising during harvest and fall nitrogen application, and the price of nitrogen was lower than it had been six months earlier. Since then, the corn price has stayed high, and N price has risen. This, in addition to our having new N response data from 2006, makes spring N rates a topic of great interest.

Our new approach to N guidelines involves using all of the N response data we have from Illinois trials and calculating the "net return to N." Based on all of the N responses from a region and previous crop (corn or soybean), we find the N rate at which the return to N is maximized. We call this N rate the point of maximum return to N (MRTN). This approach also involves finding a range, centered at about the MRTN, over which the return to N is within a dollar per acre of the return at the MRTN. We consider this approach to provide a guideline to the best N rate, designed to help producers choose a rate based on economics (cost of N and price of corn), rather than a single "recommended" N rate.

Before the recent N price increase, one producer asked whether he should consider applying more N in the spring, because the guideline rate had increased when calculated using the new price of N. The answer in this case was "probably not," both because the rate that had been applied was still within the (recalculated) guideline range and because the cost of applying 20 or so more pounds of N would make the N more expensive. The rise in N price in the past two months means that the ratio of N price to corn price is not much different than it was last fall. Because it's the ratio of N price to corn price that determines the "best" N rate, the N rate choice made last fall is still valid.

We conducted some 60 N rate trials in 2006, adding to the 300 or so trials conducted through 2005. Unless such new N response data show very different responses than previous data, adding new trials doesn't change the overall response very much. To our surprise, 2006 was a rather unusual year for N response in Illinois, especially for corn following corn in central and northern Illinois. In fact, corn following corn over much of the northern half of Illinois showed less response to N than corn after soybean. We think that this was due to dry weather in 2005 and through the winter of 2005-2006, resulting in a considerable amount of carryover N available to the 2006 crop.

As a result of this lower response to N for corn after corn in 2006, adding the data into the guideline N rates for corn following corn reduced the guideline N rate. Corn following soybean responded more typically to N rate, so those rates changed little. We did not have many corn-following-corn N rate trials in southern Illinois, so we have no basis to change those guidelines. Because of unusually good rainfall and growing conditions in southern Illinois, however, corn responded to N rate more than usual in 2006, thus raising the N rate for corn following soybean in southern Illinois.

To show the changes brought about by including the 2006 data, I calculated guideline N rates both before and after including that data. I used an N price of $550 per ton of anhydrous ammonia ($34 cents per lb N) and a corn price of $4 per bushel. In northern Illinois, adding the 2006 data reduced the N rate for corn following corn, from 211 to 195 lb, and increased the N rate for corn following soybean slightly, from 143 to 147 lb N per acre. In central Illinois, the N rate for corn following corn dropped from 184 to 178, and the rate for corn following soybean was almost unchanged, dropping by 1 lb to 177 lb per acre. In southern Illinois, the larger-than-normal response to N in 2006 meant that adding the new data raised the guideline N rate from 167 to 174 lb N per acre. The rate for corn following corn in southern Illinois, based on previous data, is 179 lb N per acre.

How is it possible that corn in central and southern Illinois, whether it follows corn or soybean, seems to need N rate in the range of 176 to 179 lb, plus or minus the range of about 20 lb per acre? The simple answer is that this is what the data, from a total of some 300 trials through 2006, tell us is the "best" N rate. If we knew nothing else about a given field, this is probably a rate that can be used with little fear of having too much or not enough N. For corn following corn, we have some evidence that high corn yields need a little more N, but there's only a small chance that any field will have inadequate N at an application rate of 200 lb of N per acre, which is the upper end of the guideline range. Corn following soybean is less likely to respond to higher N rate, unless soil organic matter content is relatively low and growing conditions are favorable.

Along with N rate questions, the high price of corn means that most producers are interested in making sure the crop suffers no deficiency of any kind, which leads some to use any input whose shortage might possibly limit yield. One such input is starter fertilizers, including those with micronutrients such as zinc, sulfur, and copper. Response to starter fertilizer has been fairly elusive in Illinois trials, with the chance of response decreasing from north to south, and from no-till to tilled, partly because warmer soils encourage root growth and nutrient uptake from the bulk soil. We would expect strip-till to react more like tilled soil than like no-till with respect to starter, simply because the seedbed is tilled and roots should be able to grow unrestricted.

There is little evidence to suggest that new hybrids are more likely to respond to starter, whether micronutrients are added or not. It's possible that rapid early growth of many hybrids may enable roots to reach nutrients more quickly, even if those nutrients are not banded close to the seed. This is not to say that starter should not be used, but it's doubtful that it will pay to put starter on now if there has been little reason to do so before now. Instead, keep aware of the need to place seed into soil conditions that are conducive to root growth, to allow the plant to develop a root system early and without restriction. Water limits yields much more often than nutrients do, and the only way to help the plant to take up water when it needs it most is to encourage good rooting. --Emerson Nafziger

Author:
Emerson Nafziger

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