Recent rainfall across much of the Corn Belt has some growers wondering if they have enough nitrogen left in the soil for their corn crop.
Agronomy experts with Pioneer Hi-Bred say growers need to evaluate fields and, in some cases, develop a nitrogen rescue strategy.
"Each year presents new environmental challenges," says John Shanahan, Pioneer agronomy research manager. "Nitrogen is the single most expensive input, which means there are numerous reasons to get it right - the right amount, the right timing on the right acres. Growers who don't apply enough risk reduced yields." Early-season nitrogen stress creates irreversible yield loss. According to the University of Kentucky, for each day of moisture saturation, 3 to 4 percent of nitrates in the soil are lost.
"Corn requires nearly half of its total nitrogen supply between V8 and tasseling," Shanahan says. "Pioneer recommends side-dressing nitrogen between V4 and V8, allowing a safety margin for weather and soil conditions that delay nitrogen application or the movement of nitrogen to the roots."
Soils tests are a useful tool for evaluating nitrogen in fields. Testing soils at least 12 inches deep can help indicate the amount of nitrogen currently available to the corn plant. Another option is to use optical sensors mounted on nitrogen application equipment.
Optical sensors emit modulated light of the appropriate wavelength onto plants and measure how much is reflected back to the sensor. This measure of "crop greenness" correlates with the plant chlorophyll content. Estimating chlorophyll content also estimates crop nitrogen status.
"A grower also can estimate nitrogen by answering some key questions," Shanahan says. "When was the nitrogen applied, what form was used, how much was applied and what have field conditions been following the application."
If a grower determines his or her corn crop is nitrogen deficient, an additional side-dress or rescue nitrogen application is an option.
"Because nitrogen is so expensive, side-dressing is a great option," Shanahan says. "Spreading applications throughout the growing season reduces risk. If wet spring conditions result in nitrogen losses, growers can increase side-dress rates. If warm temperatures and moderate rainfall result in high nitrogen mineralization and a nitrogen-sufficient crop, growers can reduce the side-dress rates."
Growers who need a rescue application can apply nitrogen with a high-clearance sprayer using ammonium nitrate. Broadcasting urea is another nitrogen application option.
"Growers need to evaluate what type of equipment and nitrogen sources are available," Shanahan says. "If a grower is going to make in-season applications, he or she needs to have a plan in place and work with their local applicator. Once the crop is about knee-high, ensuring sufficient nitrogen is a priority."
Pioneer is developing nitrogen-efficient hybrids. Research focuses around improving the yield of corn using native and transgenic genes. In addition to improving the utilization of nitrogen, researchers are working to lower the level of nitrogen inputs while maintaining yield.