Mar. 08, 2010
By Jim Camberato, RL (Bob) Nielsen , Eric Miller, & Brad Joern Agronomy Department, 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 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 following 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 the fewest number of bushels. At some level of N, grain yield stops increasing with more N. Consequently, applying N above what 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 so-called "new" approach to N rates is not necessarily new, but simply 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 will usually be 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, climates, 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 sites.
To date, more than 100 trials have been conducted around the state. About 69 percent of them are corn following soybean and the remainder are primarily corn following 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 have used sidedress liquid UAN simply to facilitate trial logistics. Similar N results would be expected from late pre-plant or sidedress anhydrous, but not necessarily from early pre-plant anhydrous or 28 percent 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.
More Discussion on N Management
The specific AONR can vary from field to field and from year to year for a single field. This is not particularly surprising since we've always known that predicting optimum N rates for any given field in any given year is challenging, primarily due to the difficulty of predicting soil N supply and growing season weather. Soil N supply can provide as much as half of the total N available to the crop. Weather influences both soil and fertilizer N efficiency. Crop health, N uptake, and N use efficiency are weather and soil dependent.
Soil or fertilizer N lost to leaching, denitrification, or volatilization represents N that is no longer accessible to the plant. Anhydrous ammonia is the least risky of the N sources in this regard 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 anhydrous to nitrate. Urea-based forms of N should be incorporated to minimize volatilization losses. For surface-applied urea containing fertilizers, urease inhibitors 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 uptake. Failure to recognize or manage these risks of N loss will require higher N rates to attain economic 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. Certain tests like the Pre-Sidedress Nitrate Test can be used for manured fields or soils with very high organic matter content. The end-of-season stalk nitrate test can be used as a "report card" to help you evaluate whether N was over-applied or under-applied this past year.
Another set of N management tools we have been evaluating are optical reflectance sensors that offer a "snapshot" of the current N status of a crop. These tools might be useful in fine-tuning sidedress N rate decisions later in the growing season (with the understanding that mineralization and N loss rates for the remainder of the growing season are difficult to predict).
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.
Adjusting Nitrogen Rate for Soil Nitrogen Supply and Nitrogen Loss Potential
In the absence of testing methods to fine-tune N rates for a given field in a given year, one can rely on educated guesses about the extent of soil N supply and N loss based on field history and current year weather patterns. We know from our field trials that a reasonable average AONR for corn following soybean is 186 lbs/ac of applied N or a lower EONR that is based on current N cost and grain price (Table 1 or the on-line N calculator). In fields with low soil N supplying capacity or high N loss potential, consider increasing the N rate by 20 to 30 lbs/ac. In fields with high soil N supplying capacity or minimal N loss potential, consider decreasing the N rate by 20 to 30 lbs/ac.
Soil N supply and N loss potential are very much related to soil characteristics and, of course, soils vary geographically around the state. As we conduct more and more trials, we are beginning to see regional differences in AONR that make some sense as we consider the soils in those regions of the state. More trials are needed before we can confidently develop recommendations for every region or soil association in Indiana.
4-Year Summary of Corn Response to Nitrogen Fertilizer
The average Agronomic Optimum N Rate (AONR) for all of our corn/soy sites since 2006 was 186 lbs/ac total applied N (with an average trial yield of 196 bu/ac). At the five Purdue locations where we conducted paired trials of corn/soy and corn/corn in 2007-2009, the average AONR for corn/corn was 47 lbs greater than for corn/soy while average corn/corn yields were 20 bu/ac less than the corn/soy yields.
Based on $0.40/lb N and $3.50/bu corn, the average Economic Optimum N Rate (EONR) for all of our corn/soy sites was 168 lbs/ac total applied N or 18 lbs less than the average AONR. However, the average yield at the EONR was only 0.5 bushel lower than that at the AONR. The EONR values for other combinations of N cost and grain price are listed in Table 1. If you want to determine EONR for other N and grain prices, use the on-line N calculator for Indiana at this Web site.
