Nitrogen fertilizer is probably the biggest concern for most corn growers. Carrie Laboski, Department of Soil Science, University of Wisconsin-Madison said, depending on when N fertilizer was purchased and if/when grain was contracted, the N:corn price ratio for any particular farm may range from 0.05 to 0.15. The maximum return to N (MRTN) fertilizer rate is influenced by price ratios not prices of fertilizer or grain. For example, the MRTN for corn following corn on a high yield potential soil is 135 lb N/a at a price ratio of 0.10 regardless of whether that price ratio was obtained with $0.33/lb N and $3.30/bu corn or $0.55/lb N and $5.50/bu corn (Figure 2); but when the price ratio is 0.05 (for example $0.30/lb N and $6.00/bu corn) the MRTN is 165 lb N/a. To select the most appropriate N rate use the MRTN rates outlined in UWEX A2809.
Effect of price level on return to N fertilizer at the 0.10 N:corn price ratio for corn following corn on a high yield potential soil. Black diamonds indicate the MRTN of 135 lb N/a. White diamonds indicate the range of N rates that have profitability within $1/acre of the MRTN.
High prices have increased the economic risk of over and under application of N fertilizer. Figure 2 shows the effect of the price level on return to N fertilizer at a N:corn price ratio of 0.10 for corn following corn on a high yield potential soil. Higher grain price levels result in more economic risk for under fertilization as shown by the steepness of the left hand side of the curves in Figure 2. At current record high N fertilizer prices, the economic penalty for over application of N fertilizer is greater than at fertilizer prices more typical of five to 10 years ago as shown by how quickly the right-hand side of the curve drops. The profitable range of N fertilizer rates become narrower at higher price levels. At current higher price levels, there is still over lap in the MRTN profitable range for different price ratios, but the overlap is not as great as when price levels are lower.
Because of high N fertilizer prices, many farmers are asking if there is an ideal timing of N fertilizer that would result in greater efficiency for crop uptake. For sandy soils, sidedress N applications are a must to reduce the probability of N leaching before the crop has a chance to use it. On medium- and fine-textured soils that are moderately well- to well-drained, there is probably minimal benefit to sidedressing N compared to preplant applications because there is usually not much opportunity for N loss via leaching or denitrification. However, some preliminary research results from Lancaster Ag Research Station in 2007 suggest that there is likely a benefit to having some N applied preplant or in a starter fertilizer if the majority of the N will be applied at sidedress, if growing silage corn. On poorly- and somewhat poorly-drained soils, there may be a benefit to sidedressing N to minimize the opportunity for denitrification. If conditions for denitrification exist (warm, wet soils), then use of a nitrification inhibitor would be economically beneficial. Remember that urea containing fertilizer must be incorporated into the soil by tillage or rainfall. Ammonia volatilization will usually be prevented if 0.1 to 0.2 inches of rain falls within 24 hours of application. If 0.1 to 0.2 inches of rain falls within two to four days after application, then some ammonia volatilization will occur.
Significant volatilization losses occur if no rainfall occurs within five days of surface urea application. If physical incorporation is not possible and rainfall is not predicted, consider using a urease inhibitor to prevent urea volatilization for 10 to 14 days. Use of both nitrification and urease inhibitors is economically viable only if conditions for N loss are likely.
Fertilizer N use efficiency is maximized when weeds are controlled as shown in research conducted at Arlington Ag Research Station in 2006 and 2007. When weeds were controlled preplant or when 4 inches tall, the economic optimum N rate was generally similar and less than the MRTN rate at the 0.05 N:corn price ratio. However, when weed control did not take place until the weeds were 12 inches tall, corn required about 100 lb N/a more to maintain yield compared to when weeds were controlled earlier. Failure to control weeds resulted in yield losses that could not be over come by applying more N. Thus, the moral to the story is to control weeds early to maximize N use efficiency.