Corn roots

How to manage nitrogen in wet corn fields

While corn that was planted on time in Louisiana is up and most has a pretty good stand, new challenges are arising with nitrogen fertilization in the wet conditions around the state.

The start of the 2015 corn season has been challenging for the majority of Louisiana. Much of the corn that has been planted was planted later within the optimum window, while several producers are still trying to decide on whether to plant the final intended acres.

While corn that was planted on time is up and most has a pretty good stand, new challenges are arising with nitrogen fertilization in the wet conditions around the state. Much of this corn is between the V2 to V6 leaf stage and has good growth. However, only a limited number of acres have had any measureable N applied. This has increased interest in aerial application of part or all of the intended fertilizer as urea. While these applications have the potential to be very beneficial and common — especially in the Delta of Mississippi on high clay ground, according to Bobby Golden at Mississippi State University — correct decisions and appropriate management are needed to get the most benefit.

The first thing that needs to be decided is whether or not the crop needs this shot of fertilizer or can it be delayed until the ground applicator is suitable. Aerial application will be more costly per acre than ground applications. The best indicator of whether or not the crop needs this application will be the crop itself. Symptoms of N deficiency are an overall pale crop and shortened growth. However, these symptoms are also associated with cold and/or wet conditions. While poorly drained soils and N deficiencies can be highly associated with each other, field pattern can be a way to determine if the crop as a whole is stressed or just low areas (is the short, pale corn in a localized area or across the field).

Once areas have been identified as not just localized areas of poor drainage, other issues must be addressed. One such issue is whether the crop is suffering from an nitrogen or sulfur deficiency. These two are often confused with the other. Typically, location on the plant can be used to gauge whether N or S is the most likely culprit, with N traditionally being on lower, older growth and S being on the new growth. However, in young, developing plants these traditional deficiency locations are not 100 percent.

While determining if an N or S deficiency is present is critical, often these two will probably be present in most Louisiana systems. Additionally, if no S has been put out, then managing for both N and S will probably be necessary, especially if substantial time will pass before successive fertilizer applications.

The next factor that needs to be determined is how much N to apply. This decision will be based entirely on the farm operation. There are pros and cons for both urea and UAN, but both can be used to produce optimum growth and productivity. In this regard, producers can apply their entire intended N applications as urea aerially. Though expensive, this provides the luxury of not having to fit in another application when drier conditions arrive (especially as further planting of grain sorghum, soybeans and cotton are probably also intended).

If UAN purchased

On the other hand, if UAN has been purchased and is the intended fertilizer for the majority of the N management, a smaller application can be applied as urea to provide some flexibility until drier conditions exist. These applications should include at least 50 to 100 pounds of N per acre (as urea and ammonium sulfate) and anywhere from 10 to 20 pounds of S per acre (as ammonium sulfate). This ensures adequate N and S for the stressed crop.

However, in making these applications, it is important to readjust further N to account for this additional application. Splitting N into a couple of smaller applications can minimize N losses associated with the current adverse environmental conditions being experienced.

The final management decision that should be made is whether or not to treat the aerially applied urea. This decision often comes down to economics rather than agronomics. From an N management perspective, the application of urease inhibitors (NBPT) on aerial applied (without incorporation) should be the standard. This is due to the fact that up to 50 percent of applied urea can be lost through volatilization if the conditions are unfavorable.

Furthermore, the application of a nitrification inhibitor (DCD or nitrapyrin) may also be warranted, but this has little to do with the application and environmental conditions now and more to do with soils and environmental conditions that will be experienced in the upcoming season. Further details are provided in Choosing to use N inhibitors or not in Mid-South corn.

Even though this corn season has had a challenging start, overall what has been planted shows promise. Therefore, management decisions should be taken that will help maintain this potential. With current prices, inputs must be warranted prior to application to ensure return on the investment. Producers must watch their crops and weather on these applications.

Beatrix Haggard is an LSU AgCenter soil fertility specialist; Josh Lofton is an LSU AgCenter field crop agronomist.

TAGS: Management
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