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Agronomy Day 2009

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Tour A

Managing Spring Nitrogen for Illinois Wheat

S. A. Ebelhar L. E. Paul
S. A. Ebelhar, E. D. Nafziger, E. A. Adee and L. E. Paul
Department of Crop Sciences
University of Illinois
sebelhar@illinois.edu   lylepaul@illinois.edu

Adding wheat to crop rotations of corn and soybeans has been shown to increase the yield levels of these crops by up to 15% over monocultures. Proper management of wheat, including optimum fertilization, is essential in optimizing its effects within crop rotations.

The proper rate of spring fertilizer nitrogen (N) application to wheat has taken on a new meaning with the high cost of N fertilizer and the high current price of wheat. Recent studies across the state have allowed us to better determine the optimum economic N rate to apply in the spring to wheat. But there are several complicating issues.

The rate of fall N applied to wheat does have some effect on spring application, but we will assume for now that 20-30 lb of fall applied N was applied.  The table below has the spring recommended N rate based on N price, wheat price and percent soil organic matter (OM). In general, optimum N rates decrease as soil OM increases (Figure 1).  This is due primarily to the N released from OM, along with a lower probability of N losses from the higher OM soils.

The first step in determining a spring N recommendation is to calculate the amount of N equivalent in value to one bushel of wheat.  For example, a bushel of wheat at $6.00 would “buy” 10 lbs of N at $0.60 per lb. The second step is to then find the column in Table 1 that corresponds to this value, and determine the suggested N rate based on estimated soil organic matter. This suggested N rate is based on N application at greenup (Feekes Growth Stage 3.0 [GS 3]), and there are adjustments for N timing and sources.

Within an OM level, N rates change as the ratio of N price to wheat price changes. So at the same wheat price, N rates decrease as the price of N increases.  And, at the same N price, N rates move in the same direction as the wheat price.

On low OM soils in southern Illinois, research has shown that spring N rates can be decreased 10% if one of the following applies: 1) spring application is delayed to late tillering (Feekes GS 5.0-6.0; see Figure 2 below), 2) spring N applications are split between Feekes GS 3 and Feekes GS 5-6, or 3) when either a nitrification inhibitor or a slow/ controlled release N source is used.  On high OM soils, spring application timing has had little effect. It is also recommended that no more that 150 lbs of spring N be applied to wheat grown on low OM soils, and no more than 90 lbs be applied to wheat grown on high OM soils.

Nitrogen recommendations are based on equipment delivering a uniform application of N across the spread path. Significant problems with lodging and yield loss can occur at higher N rates if uniform application does not occur. If there is uncertainty about the uniform application of N, especially with spinner-type fertilizer buggies, then N rates should be reduced by about 20 lb/acre in order to prevent overfertilizing some areas.   Be aware that this might well decrease overall yield, since N is likely to be deficient in those strips with lower rates. For some, hiring application done to assure more uniform spreading might be more cost-effective. 
No-till farmers have an increased chance for N losses and reduced N efficiency by wheat and should increase their N rate by about 10 lb/acre, but should not exceed the 150 lb N/acre maximum total spring rate.

For wheat after corn, there can be significant amounts of N tied up by the corn residue, but there can also be large amounts of residual N if the corn does not fully utilize applied N. If residual N is suspected, a soil test for N in the fall can tell you if sufficient N is present for wheat establishment.  If not, then 25-30 lbs of fall applied N is sufficient. If significant amounts of carryover N are found or suspected, it might be helpful to soil test in the spring just prior to spring application, with rates adjusted accordingly.

If the wheat is serving as a companion crop for legumes or legume-grass seedings, then N rates to the wheat crop should be reduced by 20-25% to limit vegetative growth of the small grain and thus produce less competition for the young forage seedlings.

Soil Situation OM Level  Lb of N that 1 bushel of wheat will “buy” 
> 13 9-13 5-9 < 5
Low in capacity to supply nitrogen: inherently low in organic matter (forested soils)

< 2%

150 120-150 90-120 60-90
Medium in capacity to supply nitrogen: moderately dark-colored soils 2-4% 100-120 80-100 60-80 40-60
High in capacity to supply nitrogen: deep, dark-colored soils > 4% 70-90 50-70 30-50 30
Table 1. Recommended spring nitrogen application rates for wheat. Rates assume no more than 30 lb of fall-applied N and spring application at greenup.

Effect of organic matter (OM) level on wheat response to N rates and the economic optimum N rate (based on $0.45/lb of N and $4.50/bushel of wheat).Figure 1: Effect of organic matter (OM) level on wheat response to N rates and the economic optimum N rate (based on $0.45/lb of N and $4.50/bushel of wheat).
Effects of timing of N application to wheat in southern Illinois (<2% OM) on wheat response to N rates and economic optimum N rates.Figure 2: Effects of timing of N application to wheat in southern Illinois (<2% OM) on wheat response to N rates and economic optimum N rates.
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