Setting Nitrogen Rate Targets
By Greg Stewart, OMAFRA Corn Specialist

2001 is shaping up as a challenging year for keeping corn production input costs in line with potential returns. With rising fertilizer prices, nitrogen management will play a significant role in attempting to realize potential profits. From an economic perspective, the goal is simply stated. Apply a nitrogen rate such that higher rates won't produce enough extra corn to pay for the extra nitrogen, and lower rates will cost you more in lower corn yields than you would save from lower nitrogen rates. Of course, the reality is that nitrogen dynamics within your corn fields are quite complex and hitting the nitrogen bull's-eye becomes very difficult.

General Recommendations and Seasonal Variability

Because of the elusive nature of nitrogen targets, some question why 'general recommendations' would even be attempted. Nevertheless, general recommendations such as those found in OMAFRA Publication 296 (see Table 1) or rules of thumb such as .9 to 1.2 lb of N for every bushel of expected corn yield serve as starting points. The fine-tuning of these recommendations then requires some additional effort and information.
General recommendations are usually based on long-term, well-run rate studies, but we know that year-to-year variability in weather and in the response of corn to nitrogen is large enough that the recommendations often look wrong as we look back on a particular season. One piece of information that is helping to sort out the year-to-year puzzle is the residual soil nitrate level that can be measured in the early stages of the growing season by using a soil test. Figure 1 illustrates the soil nitrate levels measured in early June, each year, on the same plots at the Elora Research Station, under the same management system, over the past few years. Notice that in 1998 and 1999, soil nitrates were higher than average, mainly due to low precipitation and above average spring temperatures. In hindsight it appears that had we used this sort of tool to lower N rates in 1999 by 20-50%, we would have reduced N costs without affecting yields on many fields. The idea of soil nitrate testing across many acres seems unmanageable to most growers; however, a simpler, but unproven, approach might be to establish 'plots' within fields that you sample carefully for nitrates (30 cm sample depth) each year at the same time, in the same place. A record of these nitrate levels, in time, may allow you to fine-tune your overall N rates based on the year-to-year changes in residual soil nitrate in the soil.
Table 1: General recommended nitrogen rates for corn.
Region

Expected Grain Yield (bu/ac)

 

100

130

150

160

175

 

Most Profitable Nitrogen Rate (lbs N/ac)

Eastern

85

120

150

160

180

Western/Central

95

110

120

120

125

Southwestern (> 2800 CHU)

150

165

175

180

190

Note: For side-dress applications in Southwestern Ontario, reduce nitrogen recommendations by 15%.
Assumes corn following grain corn

Source: OMAFRA Publication 296












Credits to Previous Crops

This is certainly not the year to ignore nitrogen credits due to previous crops or manure applications. OMAFRA recommendations call for a significant reduction in nitrogen rates based on previous crops (Table 2.). Remember that these credits refer to N rate reductions from what you would apply for growing corn after corn. So if a producer is growing 150 bu/ac corn after red clover west of Toronto and is in a 2900 heat unit area, the N recommendation would be: 175 - 40 = 135 lbs N/acre. An interesting study out of Cornell University looked at lower input systems within various crop rotations. Where red clover stands are good this 40lb credit is conservative and many research plots have shown that 50-80lbs of N optimizes corn yields after clover. Table 3 points out that, in this New York study, corn after wheat/red clover with an N rate of 85 lbs/acre yielded quite similar to other rotations where nitrogen was applied at a rate of 145 lbs/acre.
Table 2: Reduction in general nitrogen recommendations for corn based on previous crop.
Previous Crop

Nitrogen Rate Reduction (lb/ac)

Established forage - under 1/3 legume

0

Established forage - 1/3 to 2/3 legume

50

Established forage - over - legume

100

Perennial legumes plowed in seeding year

40

Soybeans or field beans (< 2800 CHU)

30

Soybeans or field beans (> 2800 CHU)

15

Cereal - straw removed (< 2800 CHU)

10

Silage corn

10

Grain corn and other crops

0

Source: OMAFRA Publication 296.

Cost of Nitrogen versus Price for Corn
Consider the situation where you are growing corn after soybeans on land with no recent history of forages, legumes or manure. Assume general recommendations or past history suggest you apply 160 lbs N/acre. If you break that nitrogen down into 4 increments of 40 lbs each, it is usually easy to pencil out the first 2 or 3 increments as being cost-effective to apply. That last 40 lbs increment from 120 to 160 lbs, however, becomes increasingly difficult to justify in the face of rising nitrogen costs and depressed corn prices. Table 4 illustrates that at low N prices, the additional $12 in fertilizer cost is paid back with 4 bushels increased yield. Run fertilizer costs up, and that final 40 lbs of N must generate nearly 7 bushels in higher yields to warrant the higher rate. If you are already on the high side of N rates a 7 bushel increase will be hard to generate. However, cut back too far, and 7 bushels may disappear in a hurry. OMAFRA recommendations indicate that when the price ratio of $ /lb of nitrogen: $/lb of corn shifts from 5:1 to 7:1, a reduction in N rates of approximately 15 lbs/acre is warranted to optimize net returns.

Table 3: Average corn yields in different crop rotations from a 6 year study (1992-1997) Aurora Research Farm, New York.
Rotation

Conventional Management
(145 lbs N/acre)
Root worm insecticide

Low Input Management
(85 lbs N/acre)
No root worm insecticide

 

corn yield (bu/ac)

Continuous corn

124

95

Soy-corn

144

129

Soy-corn-corn

139

101

Soy-wheat-corn

147

139

(wheat underseeded to clover) Source: W. Cox and G. Bergstrom, Cornell University
Site Specific Management

When trying to sort out nitrogen rate targets, the picture gets both more complicated and more exciting because of the natural variability that exists between and within fields. Complicated, because getting a handle on this variability usually requires some additional effort in terms of mapping of yield patterns, soil sampling (i.e., organic matter), remote imaging (i.e., bare soil images), soil nitrate level eterminations, laying out variable N rate strips across the field, etc. It is also exciting, because once some of this variability is understood, you can feel much more confident in selecting those fields or parts of fields that are your best candidates for significantly lowering N rates. If specific information of this nature is unavailable, you can still make some educated guesses that the fields that have some history of manure or forages, are lower in topography, or higher in organic matter will be less likely to sacrifice yield in the face of reduced N rates.

Table 4: Assessing the impact of applying an additional 40 lbs N/acre at various nitrogen fertilizer prices.

Nitrogen Cost
$ /lb of N

Nitrogen Cost
$/40 lb N per acre

Increased corn yields required to cover additional N cost.
(corn @ 3.00/bushel)

.30

12.00

4.0

.35

14.00

4.7

.40

16.00

5.3

.45

18.00

6.0

.50

20.00

6.7

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