Research during the past decade has raised many questions concerning how farmers should estimate nitrogen (N) fertilizer needs for corn after soybean. In 2001, a group of farmers worked together to estimate the yield losses associated with reducing rates of spring-applied N to 50 lb/acre below those usually recommended.
The studies were made possible by funding from the Iowa Corn Promotion Board, the Iowa Soybean Promotion Board, and the Integrated Farm and Livestock Demonstration Program through the Iowa Department of Agriculture and Land Stewardship. Trials were conducted on fields where farmers had applied two rates of N in alternating strips. One rate was intended to be the normally recommended rate, and the other was 50 pounds N/acre lower (Table 1). All fertilizer N was applied shortly before or soon after planting.
Table 1. Summary of precision farming trials to assess the yield losses associated with reducing rates of spring-applied N to 50 pound N/acre below those normally recommended.
|Form of N
|Reduced Rate||Normal Rate|
a Any N added with phosphorus is not shown.
b N was not incorporated, but it was incorporated or injected at all other sites.
Fertilizer was applied in replicated strips going the lengths of 23 different fields in various parts of Iowa. Each strip was usually two combine swaths wide, so each trial covered approximetely 20 acres. The fields were harvested with combines having yield monitors and global positioning system (GPS) receivers.
The average for the lower rates of fertilization was 101 pounds N/acre. Yields were increased by an average of 3 bushels/acre by adding the extra N. Based on averages across all sites, the lower rate was more profitable because most farmers paid more than 3 bushels of corn for 50 pounds of N last year.
UAN solutions are often applied to soils before crops have emerged.
The higher N rate was very close to the rate that would be called for by old rules based on yield goals and credits for N supplied by legumes. Multiplying the average yield by 1.2 and subtracting normal credits for the preceding soybean crop, for example, would produce a recommendation near 150 pounds N/acre.
Yields at neither the higher nor lower rates of N fertilization showed significant relationships with the amounts of N applied. This finding gives reason to question the notion that estimates of N fertilizer should be linked directly to yield levels. More attention should be given to the possibility that N is used more efficiently by crops grown under conditions that result in higher yields.
Yield responses great enough to pay for the extra N were more common at the lower than at the higher rates of fertilization. However, the mean yield response was only 4.8 bushels/acre for the eight sites where the lower rate was less than 100 pounds N/acre. This yield increase hardly paid for the extra fertilizer at last year's prices. If the grain averaged 0.7 pounds N/bushel, only 7 percent of the extra N was accounted for in the grain harvested from these sites.
The rate of N fertilization needed to maximize profits this year may be different than last year. However, there is clear need to continue to assess the risks associated with reducing N rates below those normally considered to be optimal. Farmers who would like to help assess these risks and have combines equipped with yield monitors and GPS should contact Brad at (515) 294-9726.
This article originally appeared on page 24 of the IC-488 (3) -- March 18, 2002 issue.