When wet weather occurs after N fertilizer has been applied, N can be lost. It can be difficult to assess how much N was lost and whether additional N should be applied. Often by the time the soil is dry enough for ground equipment, high-clearance equipment is needed and adds to the cost of potential rescue N applications. Aerial photography can provide the capability to assess N loss and assist in making good business decisions about rescue N applications.

Research comparing yield maps to aerial photos in fields with some N stress has shown that areas that are lighter green in the aerial photos are also lower-yielding. Understanding the relationship between color and yield allows prediction of yield loss potential due to N stress from aerial photos. Knowing how much yield may be lost due to N stress can help to determine whether the expense of rescue N applications is justified.

In three survey areas in the Missouri River bottom of Holt County, Missouri in 2005, aerial photographs suggested that an average of 60% of the land was in corn and that an average of 25 bu/acre would be lost from those corn fields due to N stress if no additional N was applied. Projected over the whole Missouri River bottom in Holt County, this would amount to 1.8 million bushels in lost corn yield.

Strip trials with rescue N in Miami County, Kansas in 2005 showed that large yield responses to rescue N applied in late June could be achieved, and that the size of the yield response was closely linked to corn color in aerial photos.