With the wet spring in parts of the state, there are still some wheat fields that haven't been topdressed with nitrogen. Some of those fields are starting to turn yellow and appear to be nitrogen deficient, said Dave Mengel, K-State Research and Extension soil fertility specialist.
That may prompt the question, Mengel said, "How late can nitrogen be applied on wheat and still pay, particularly with the price of nitrogen so high?"
"Research from K-State and other universities has shown that nitrogen-deficient wheat will almost always respond to additional nitrogen fertilizer by increasing yield and protein content - as long as the nitrogen is applied before flag leaf emergence," Mengel said. "But once the wheat heads, producers are unlikely to receive an economic yield benefit from applying nitrogen, even if the wheat is nitrogen deficient."
If a field of wheat has not yet received any nitrogen fertilizer, there is still time to apply the nitrogen, but it needs to be done as soon as possible, the agronomist said.
Exactly how much nitrogen should be applied is a function of stand, growth stage, and the amount of nitrogen applied last fall.
"In many cases, wheat is still prior to or just beginning to joint, so the application rates should be 50 to 70 pounds nitrogen per acre. As it gets later in the spring, responses will decrease, and the rate of nitrogen applied should be reduced," he said.
Topdressing on late-season wheat can cause physical damage from the applicator, or foliar burn from liquid fertilizers, but these factors can be managed.
"Narrow tires and wide swath widths will minimize physical damage to the wheat," Mengel said. "Using granular materials such as urea will minimize leaf burn and work well, if we get some rain to move the dry fertilizer into the soil. Volatilization loss from urea may be a concern this year where there is not enough plant canopy to reduce evaporation of water from the soil surface. Growers who will be applying granular urea to small wheat, especially if the soil is a little wet and the weather is warm, may want to consider the use of Agrotain, a urease inhibitor, to reduce the potential for volatilization losses."
If liquid nitrogen is used on late-season wheat, it can be applied with solid stream nozzles, such as "streamer bars," to reduce the potential for leaf burn, he added. Leaf burn is a serious concern when it occurs on the upper leaves, since the wheat relies on these leaves for grain fill.
"No fertilizer, liquid or dry, should be applied once the head begins to emerge," the K-State agronomist said.