With the large increases in corn acreage in the Mid-South and the increasing cost of nitrogen fertilizer, attention to fertilizer management is paramount. Major points to consider are how applications should be made within and across fields, and when and how much to apply to realize the greatest return.

Peter Scharf and colleagues at the University of Missouri published results of research conducted to assess across — and within-field variability in required nitrogen rates, and corn yield response to application timing. The reports offer guidance that will be helpful to Mid-South growers striving to apply nitrogen fertilizer to corn in the most economically efficient and environmentally sound manner.

Corn producers generally apply the same nitrogen fertilizer rate over whole fields and sometimes over whole farms.

In studies conducted in three major Missouri soil areas (Delta alluvial, deep loess, and claypan), the researchers found that economically optimal nitrogen fertilizer rates (EONR) were different both among and within fields. Their results also indicated large differences in soil nitrogen availability over short distances within a field.

Among fields, median (middle value) EONR ranged from 56 to 186 pounds per acre, indicating that different nitrogen fertilizer rates are needed for different fields. This indicates that traditional yield-goal nitrogen rate recommendations either over- or under-fertilize the majority of corn production fields.

Within-field variability in EONR was high. Uniform application of nitrogen fertilizer across a whole field would have resulted in half of each field receiving either too much or too little fertilizer. The level of variability in within-field EONR suggested that nitrogen fertilizer applied at variable rates within a field could be both economically and environmentally beneficial.

With current high corn prices, over-application probably will be more frequent since the cost of over-applying nitrogen is less than the reduced income from lost yield potential with under-application. They concluded that there is a need to develop and use technology that accurately predicts EONR for different fields and at various locations within a field.

Timing of application.

Producers typically apply a significant portion of nitrogen fertilizer before or at planting, or before corn emerges. Generally, this is done to allay the fear of irreversible yield loss resulting from early-season nitrogen deficiency.

Studies were conducted throughout Missouri in 28 experiments. When data from all of the experiments were pooled, there was little evidence of yield reductions or irreversible yield loss when the total nitrogen fertilizer application was delayed as late as growth stage V11, or about five to six weeks after emergence. This finding occurred even though nitrogen-deficiency stress was highly visible in some cases when late applications were made.

Only small yield reductions (3 percent) were measured when nitrogen application was delayed until V12 to V16, or up to about seven weeks after emergence.

Full yield was not realized in the experiments when nitrogen application was delayed until silking. However, only a 15 percent yield reduction occurred when the total amount of nitrogen fertilizer was applied at silking.

The researchers concluded that the risks of yield loss associated with mid- to late-vegetative nitrogen applications to corn are acceptable, and may be less than the risks of nitrogen loss associated with fall and/or preplant applications.

They further concluded that when unplanned delays in nitrogen application occur or when severe in-season nitrogen loss occurs, an economical response to nitrogen fertilizer is likely to occur until silking.

Nitrogen is the single most expensive input used in corn production. The above findings and resulting conclusions should be considered in planning the judicious use of this expensive input.