The primary methods we suggest to improve corn nitrogen utilization are:
1) Use split application timing to limit nitrogen fertilizer exposure and losses when crop demand is low.
2) Take measures to minimize volatility potential of urea-based nitrogen sources.
- Inject liquid UAN-solution in the soil.
- Apply urea during the “first split,” rather than later when temperatures are higher.
- Limit the amount of urea-based fertilizers you broadcast of your total N program.
- Apply urease inhibitors to urea-based sources when you broadcast them on corn.
Nitrogen sources and application - Using the right nitrogen source and application method may be more important to corn grain yield, than how much you apply. It is becoming more popular to aerially apply urea to corn fields, but while this method is quick and convenient compared to injecting liquid nitrogen, there is risk associated with its use on corn that requires more management. Urea (46-0-0 or 41-0-0-5) or urea-containing nitrogen sources, including UAN-solution (N-sol, 32%, or 28-0-0-5), are subject to volatilization loss when applied to the soil surface (either broadcast or dribbled in a band). This risk may offset convenience or other savings. In fact, no-tillage research studies in Missouri and Tennessee show UAN-solution and urea broadcast on the soil surface reduced corn yield potential 9 to 23 percent compared to ammonium nitrate broadcast, UAN-solution injected, or anhydrous ammonia injected. Thus, we strongly suggest using methods to minimize urea volatility potential. Of course, injecting UAN-solution in the soil solves this issue, and is why this practice is standard. Broadcasting urea-containing sources during the “first” split application period should reduce risk, because temperatures are generally cooler and likelihood of rainfall capable of incorporating nitrogen is greater. These environmental conditions reduce the potential for volatility losses. On the other hand, hotter temperatures and more infrequent showers later in the summer, promote high potential nitrogen loss. You can reduce volatility by adding proven urease inhibitors, such as Agrotain, to granular urea or UAN-solution. Urease inhibitors temporarily slow the activity of the urease enzyme. But you’ll still need timely rainfall or overhead irrigation to get urea-based N into the soil to reduce loss and make it available so the plants can use it. Tillage can also incorporate urea, but may not be practical, and should be assessed relative to savings. Sole reliance on granular urea to supply nitrogen for corn also exposes plants to leaf burn from broadcast application, if you use the preferred split application strategy. Thus, this strategy requires strict management, multiple applications, and a little help from Mother Nature in order to be successful for corn production.
Nitrogen application timing – Abundant spring rainfall which saturates soils for extended periods greatly increases potential nitrogen loss through denitrification and leaching, limiting the amount of nitrogen available for crop use later in the season. Denitrification happens when microorganisms in saturated soils turn nitrate nitrogen into nitrogen gases that escape into the air. Warm soil temperatures speed up this process. Research indicates denitrification rates range from 2 to 3 percent per day at soil temperatures from 55 to 65 ˚F. Denitrification rates increase to about 5 percent per day when soil temperatures are warmer. Nitrogen application timing can have considerable effect on potential nitrogen loss. Since corn nitrogen requirement is high, and it is the first crop planted in the spring (when losses can be high), application timing can have tremendous impact on corn productivity and your profitability. Therefore, we strongly encourage you apply nitrogen fertilizer at specific times according to corn need — in other words “spoon feed” your crop using a split application strategy. This split application method reduces the likelihood of considerable nitrogen loss due to wet weather before crop uptake, and generally increases crop response to nitrogen fertilizer. Corn uses less than 10 percent of its nitrogen before rapid vegetative growth begins. This growth spurt usually begins in late April through mid-May in Mississippi, depending on seasonal temperatures and planting date. Thus, there is no good reason to apply considerable nitrogen in March, when it is not needed until May. You can use nitrogen more efficiently if you apply only a small portion of nitrogen just after plants emerge. This reduces the amount of nitrogen exposed to potential early season loss. Apply the bulk of your nitrogen fertilizer just before the growth spurt, when the plants need it most. Our standard nitrogen recommendation is to apply no more than one-third of the total nitrogen near planting/crop emergence. Apply the remaining nitrogen about 30 days later. Corn should be taller than 12 inches or at V6 growth stage for the second application.
Nitrogen placement – Close nitrogen sidedress placement in relationship to the crop row is not particularly necessary for corn production, particularly if some nitrogen was previously broadcast or starter fertilizer is used. This is largely because nitrogen is relatively mobile in the soil-water solution, compared to some nutrients. Furthermore, more than 90% of nitrogen uptake will occur after corn is more than knee-high. Corn also has a fibrous root system, which develops substantially more lateral growth than tap-rooted crops, such as cotton or soybeans. In fact, corn roots will likely extend to the row middles before plants are knee-high. If you side-dress the second split-application when corn is about 12 inches tall or at the V6 growth stage, as is preferred to optimize fertilizer efficiency, close knife placement may prune and/or burn lateral roots. For these reasons, we generally suggest corn growers place side-dress knives near the edge of raised beds. If you have twin-row or narrow row patterns, nitrogen placement in the row middle may be more practical. If knives are positioned in row middles, the outside knife on each side of the applicator should be modified to apply one-half of the intended nitrogen rate, since it will run between the same rows twice.
Nitrogen rate recommendations – The MSU Extension Service generally recommends using 1.3 pounds of actual nitrogen for each bushel of corn yield goal. For example, the nitrogen recommendation for a goal of 160 bushels per acre is: (1.3 lb N × 160 bu/A) = 208 lb N/A. However, although corn is responsive to nitrogen, this yield response increases to an optimal level, but then becomes level when other factors become more limiting. Thus, it is not really practical to push corn yield and profitability by further increasing nitrogen rate, if you are already using an optimal rate and best management practices, combined with good growing conditions. Yield improvement can only be gained by addressing any and all limiting factors. Our research shows you can use 10 to 15 percent less nitrogen than the standard recommendation if you are growing corn on lighter, sandier soil. These recommendations were developed and based upon using split application strategy. Therefore, applying nitrogen in a single application, especially an early preplant timing, may require a higher rate. How much more would be needed depends primarily upon how long soils remain saturated, which determine potential nitrogen loss. The split application strategy is preferred to minimize this guesswork, considerable risk, and expense. Nitrogen recommendations for corn in the south are based on corn yield goal because our warm, wet winters keep nitrogen from carrying over from year to year. This is different from the Midwest, where consistently cold, dry conditions effectively stop nitrogen loss during the winter, allowing them to forgo yield goals in their rate determinations.