Source: Purdue University

Nitrogen fertilizer costs remain volatile but continue to be one of the most expensive variable costs for corn. Applying "more than enough N" is no longer cheap "insurance" as it once was many years ago. Applying "more than enough N" is also not environmentally friendly. High N fertilizer costs and environmental impacts should encourage growers to critically evaluate their N fertility program, including application rate, fertilizer material, and timing.

Nitrogen rate recommendations for a given field were traditionally linked to its historical yield levels3. For corn/soybean, the traditional rule of thumb was an N rate equal to about 1 lb of N per bushel of expected yield. For corn following either corn or wheat, the recommendation was equal to about 1.2 lbs. of N per bushel.

These rules of thumb infer that the more N you apply, the more grain you harvest. Actually, yield response to N is usually not a straight-line relationship. In reality, the first pounds of applied N typically return the greatest number of bushels and the last pounds of applied N typically return fewest number of bushels (figure to right). At some level of N, grain yield stops increasing with more N. Consequently, applying more N than the crop can use is dollar wasteful and environmentally distasteful.

Throughout the Midwest, most land-grant universities have moved away from yield-based N rate recommendations toward data-driven recommendations that are sensitive to N and grain prices4. This "new" approach to N rates links documented yield responses to N with the relative economics of grain price and N cost.

A couple of new terms or acronyms have developed from this approach. The term "Agronomic Optimum N Rate" or AONR defines the N rate that will produce maximum grain yield, regardless of cost. The term "Economic Optimum N Rate" or EONR defines the N rate that will result in the maximum dollar return to N. The EONR is usually less than the AONR, will usually decrease as N prices increase, will usually increase as grain prices increase, or may remain the same if the ratio between nitrogen cost and grain price (N:G) remains the same.

The "new" approach requires yield data from numerous field trials documenting corn yield responses to N fertilizer rates across different soil types, growing seasons, crop rotations, hybrids, tillage systems, etc. Until recently, such yield response data available for Indiana were quite old and few in numbers. We began our current N rate trials in 2006 at seven of Purdue’s research centers plus a number of on-farm sites5.

To date, nearly 150 trials have been conducted around the state. About 67% of them are corn/soy and the rest are primarily corn/corn. The N rate treatments have ranged from nothing but starter N to as much as 286 lbs/ac applied N. Most of the trials used sidedress liquid 28% UAN simply to facilitate trial logistics. Similar results would be expected from late pre-plant or sidedress anhydrous, but not necessarily from early pre-plant anhydrous or 28% or fall anhydrous. Most of the trials were conducted on fine-textured soils: silt loams, silty clay loams, and the like. All of the trials have been field-scale; meaning that the individual N rate "plots" are usually field length by some multiple of the combine header width. Most of the trials have been harvested with the aid of GPS-enabled yield monitors.

Regional and Soil Differences in Optimum N Rate
Nitrogen used by the corn crop originates from soil organic matter and crop residues as well as from applied fertilizer. Some soils provide as little as 25% of the crop N requirement, the remainder coming from fertilizer N. Other soils provide in excess of 50% of the crop N demand, with a lesser amount needed from applied N. The N supply and N loss potential of a soil are related to soil properties and, of course, soils vary geographically around the state. In general higher organic matter and better drained soils provide more N to the crop and retain more fertilizer N than lower organic matter, more poorly drained soils. As we conduct more and more trials, we are identifying regional differences in AONR (Fig. 1) that make sense as we consider the soils in those regions. Our current research suggests that poorly-drained fine textured soils in northeast, eastcentral, and central Indiana require about 221 lb N/ac to optimize yield of corn/soy, whereas the AONR for better-drained fine textured soils in westcentral and northwest Indiana is about 173 lbs N/ac. These results were obtained with efficient fertilizer application methods and timings. The EONR is less than the AONR, based on the relative cost of N and value of grain. Economically optimum N rates can be found in Tables 1-3.

More field research is needed in several areas of Indiana and on coarse textured soils to determine whether other regional and soil specific recommendations are warranted. Please consider collaborating with us in conducting on-farm research N rate trials (see pg. 5 for more information).

More Discussion on N Management
Although we report a single AONR for a region the specific AONR can vary from field to field and from year to year for a single field. For example, the optimum N rate for our research site near West Lafayette was 165, 130, 186, 182, and 186 from 2006-2010, respectively. This variation in optimum N rate is not particularly surprising since we’ve always known the difficulty of predicting soil N supply, fertilizer N loss, and growing season weather. Weather influences both soil N supply and fertilizer N efficiency. Crop health, N uptake, and N use efficiency are also weather- and soil-dependent.

Soil or fertilizer N lost to leaching, denitrification, or volatilization is no longer accessible to the plant. Anhydrous ammonia is the least risky of the N sources because it is the slowest to convert to the nitrate form that is susceptible to leaching or denitrification losses. Nitrification inhibitors can be used to further delay the conversion of ammonium to nitrate. Urea-containing fertilizers should be incorporated to eliminate volatilization losses or a urease inhibitor can be used to delay the initial conversion of urea to ammonia (reducing the risk of volatilization loss). Finally, sidedressing N will minimize the "window of opportunity" for N loss prior to plant uptake6. Failure to recognize or manage these risks of N loss will require higher N rates to attain optimum yield.

Even if you take steps to minimize the risk of N loss, predicting the optimum N rate for a particular field in a particular year remains a challenge. Several tools exist that may improve N management. These include: the Pre-Sidedress Nitrate Test7 which can be used to estimate soil N supply in manured fields or soils with very high organic matter content, a chlorophyll meter8 or active sensor in conjunction with a high-N reference strip which can be utilized during the growing season to evaluate crop N status, and the end-of-season stalk nitrate test9 which can serve as a "report card" to determine whetheN was over- or under-applied

The bottom line on N use in corn is that we’re dealing with a biological system that interacts with everything under the sun, including the sun. We cannot accurately predict the weather. We cannot accurately predict soil N supply throughout the year. Yet, we cannot afford (financially or environmentally) to simply apply "more than enough". We can minimize the risk of fertilizer N loss by understanding the processes and matching N source with placement and timing. We can develop average N rate recommendations that will work in "average" years. We can attempt to fine-tune those recommendations with tests, models, optical sensors, or simply educated guesses.

We Are Looking for On-Farm Trial Cooperators
Our long-term objective is to develop more region- or soil-specific N rate guidelines. Conducting N rate trials on farmer’s fields is the best way for us to expand our efforts and increase the database for making regional recommendations. The general protocol for such trials is to apply strips of six N rates (for example, 70-110-150-190-230 lbs/ac N), repeated no fewer than 2 times across a field. Size of individual plots (a single N rate strip) can be length of field by some multiple of combine header width. Use of combine yield monitors is strongly encouraged primarily because they greatly reduce the harvesting logistics of such a trial. The general protocol for such a trial can be downloaded from the Web at http://www.agry.purdue.edu/ext/ofr/protocols.html.

If you are interested in conducting on-farm N rate trials, contact Jim Camberato (765-496-9338 or jcambera@purdue.edu) or Bob Nielsen (765-494-4802 or rnielsen@purdue.edu). We will work with you to come up with the best compromise between our desires for statistical soundness and your desire for logistical simplicity.

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