Corn Nitrogen Application - Assessing Potential Loss

Questions have surfaced more frequently than usual this winter about the potential loss of nitrogen from various N sources and application techniques. Let's dig for answers!

Question # 1
Weed 'n Feed:
Spraying UAN and pre-emerge herbicides in the same tank.

The most obvious advantage is that N application costs are zero if you plan on spraying the pre-emerge product anyway. This system can eliminate a pre-planting operation that might delay planting and moves it into a slightly more flexible post-planting window. If the UAN is sprayed on bare soil, the N losses (ammonia volatilization) will be dependent on temperature and rainfall after application. These N losses could be quite low (0-5%) if temperatures are cool (less than 20°C) or if rainfall occurs within 48 hours. In reduced tillage situations where more of the UAN is applied to crop residue, or if temperatures are warmer and rainfall absent, the N losses may be significantly higher (15%), so the technique should be avoided. A reasonable approach on tilled ground, would be to take the recommended rate and apply 8-10% more N for surface-applied UAN applications. If the N Calculator (refer to www.gocorn.net) recommended 120 lbs N/ acre, then moving to 130 lbs per acre buffers you against N volatilization losses. This could cost $8 per acre, which offsets some of the advantages of this approach.

Some producers have kept application accuracy high and application costs low by towing a cart and injecting UAN down coulters on the planter (refer to Figure 1). Some data suggests that banding UAN will result in higher yields than broadcast UAN. In a two year study at two Ohio locations, corn following corn yielded 9 bushels per acre better with banded UAN than with broadcast UAN.

Figure 1. Dunlogon Farms of Stayner Ontario can bring 1,600 gallons of UAN (32%) in a tow-behind-cart in their attempts to keep application costs low and N losses minimized. N is banded 6 inches off the row on the corn planter.

Banded UAN produced 5 to 6 bushels per acre more corn than broadcast UAN in studies in Illinois as well.

Question # 2
Side Dress with the sprayer:
Can I apply UAN with my sprayer and 90 foot boom in early June to save time and cost? Will streamer nozzles eliminate volatilization losses? Do I use the same recommended rate as if I were sidedressing?

The most applicable research in this area has been done comparing planting time sprayed UAN versus dribbled-on UAN. Ammonia volatilization losses are lower for dribbling techniques (which should approximate streamer nozzles) compared to broadcast spraying UAN. The possibility of experiencing higher temperatures during early June increases significantly compared to early May. Therefore the risk of ammonia loss increases as well. The streamer nozzles will help but not eliminate the risk of N loss.

The best approach is to apply in late May-early June in an effort to avoid hot days (volatilization) and before the corn is beyond the 6-leaf stage (leaf burn). Since this is earlier than typical sidedress timing, and since there is potential for some N loss due to the surface application, go with the N Calculator recommended rate for planting time applications, not the sidedress recommended rate. On fine-textured soils, this will cost you more for N ($15/acre) compared to true sidedressing and offsets the benefit of the faster application.

Question # 3
Sidedress urea:
I can't source anhydrous ammonia in my area and urea would be the next cheapest source. Can I broadcast the urea in June into standing corn?

The ammonia volatilization risk is higher with urea than with UAN (UAN is only one-half urea). The potential loss from broadcast urea in June depends on a number of factors. The following conditions would all contribute to greater N losses from ammonia volatilization:

• high soil temperatures
• high soil pH (>pH 7.5)
• coarse soil texture
• low organic matter content
• high amounts of surface residue
• lack of rain for 10-14 days following application.

The research is quite variable in measuring losses from surface applied urea. It is substantially higher than surface applications of UAN and can reach 40% N loss. This means significant additional costs and the practice has few upsides. A more appropriate system would be to capture the efficiency advantages of sidedressing and the potential lower costs of urea by investing in a system to sidedress the granular product (refer to Figure 2).

Figure 2. Claussen Farms of Brucefield Ontario sidedress urea with an air cart in their efforts to capitalize on sidedress efficiencies and the cost advantages of urea over UAN. Note the applicator bar using coulters to incorporate urea in a skip row configuration.

Other Options
Other N sources that are specifically designed to reduce N losses are available that have not been discussed in this article. Relatively new products like poly coated urea (ESN) appear to be less susceptible to ammonia volatilization losses due to slower accumulation on the soil surface and less pH change around the particle. As more experience is gained with these products, they may become additional tools in maximizing N use efficiency.