High fertilizer nitrogen (N) prices will add significant costs to corn production this year. Concurrently, spot shortages of some N products may occur. What steps can producers take to get the most return from added N? What are some options for managing fertilizer applications?

Decision 1: Where to apply N and at what rate?

Allocate more N to where it is needed most. If your N costs are high, or products are in short supply, then allocate more N to the situations with greatest potential response to applied N. This allocation would be to the most responsive crops and rotations. Table 1 gives suggested preplant corn N rates for various rotations. Also closely evaluate the rate required in each rotation for your soils and geographic location. If you have been applying N at rates above the ranges suggested in Table 1, then consider reducing them into the suggested range. If you are already within the range, and have not observed N deficiencies in the past, then consider decreasing rates to the lower part of the range. Research studies conducted in Iowa in the past 10 to 15 years indicate that in many years N rates in the lower part of the ranges are adequate. Also, data from diagnostic tools such as the late spring soil nitrate and fall cornstalk nitrate tests or canopy sensing (visual or sensors) may further clarify adequate rates.

Because corn is so responsive to N, if fertilizer N supply is short it is probably better to apply a lower rate of N to all corn acres than to skip fields. Exceptions are

1. fields with adequate rates of manure,
2. first-year corn after alfalfa, and
3. fields receiving adequate rates of other forms of N such as by-products.

Because corn rotated with other crops requires less N and returns higher yields than continuous corn, consider rotating corn after soybean or after alfalfa instead of planting corn after corn. An example of a long-term rotation study from the Iowa State University Northeast Research and Demonstration Farm is shown in Table 2. Decisions to change rotation should be weighed carefully. Not growing corn and instead planting soybean after soybean, for instance, will likely lead to lower soybean yield and increase the potential for disease and nematode problems.

With the increased N prices and low corn prices, it would be justified to reduce rates somewhat. However, when fields are very responsive to applied N, reductions should not be dramatic, in general perhaps not more than 10 to 20 percent less (for example, moving from 150 to 120 lb N/acre in a corn-soybean rotation would be a 20 percent reduction). Remember, greatest yield increase comes from the first units of applied N, and least from the last. Looking at the yields in Table 2 for the C-S rotation, the first 80 lb N returned 41 bushels per acre ($82.00 per acre at $2.00 per bushel corn at a cost of $16.00 per acre at $0.20 per lb N). The last 80 lb of N returned only 3 bushels per acre ($6.00 per acre at $2.00 per bushel corn at a cost of $16.00/acre at $0.20 per lb N). At a corn/nitrogen price ratio of 10:1, an economic N rate for the C-S rotation would be approximately 120 lb N/acre, with a yield of 148 bushels per acre. Reducing the N rate to 100 lb N/acre results in a predicted yield decrease of 3 bushels per acre.

Decision 2: What N is being applied?

Take into account all N being applied to cornfields. Nitrogen recommendations are for the total amount of N needed. Therefore, add up the N coming from various fertilizers such as diammonium phosphate (DAP), weed and feed urea-ammonium nitrate, and starters. These amounts should then be subtracted from recommendations such as those shown in Table 1.

Decision 3: What are alternative N sources?

Use alternative N sources such as manure, biosolids, and N-containing by-products (such as liquid ammonium sulfate). Now is a good time to closely measure the nutrient content of animal manure and carefully apply agronomic rates.

Decision 4: How can productivity be improved?

Adopt proven crop management practices: soil conservation; IPM; adapted high-yielding hybrids; crop rotations; and optimal soil pH, phosphorus, and potassium levels. These practices help to ensure optimal N use efficiency.

Decision 5: How well is N applied?

Calibrate applicators, apply fertilizer products and manure accurately, and use the correct method.

Summary

Applying these practices and management options can help increase returns from dollars spent on N. Increased fertilizer N costs will detract from overall profitability, but carefully assessing N needs and application options will help minimize expenses and increase overall return on fertilizer N investments.

Table 1. Suggested N rates for corn production based on crop rotation.

Adapted from Table 1 of Iowa State University publication PM 1714, Nitrogen Fertilizer Recommendations for Corn in Iowa.

Table 2. Effect of crop rotation on average yield and response to applied N for the period of 1979 to 1998, Northeast Research and Demonstration Farm, Nashua, IA.

C, corn; S, soybean; O, oats; A, alfalfa.

^aThe nitrogen source is spring incorporated urea.

^bOats are undersown with alfalfa, and no hay is harvested that year. Three harvests are made in the "Hay" year (year after seeding).

Adapted from Effects of crop rotation and nitrogen fertilization on crop production over a 20-year period, by Antonio Mallarino and Ken Pecinovsky. pp. 13-16. Northeast Research and Demonstration Farm report, ISRF98-13.