Wet spring weather leads to recommendation of nitrogen applications

From the MSU News Service

BOZEMAN — With a wetter-than-average May in most of Montana, providing additional mid-season nitrogen to wheat could boost grain protein levels and increase profit, according to a Montana State University Extension specialist.

“With the drought conditions last fall and winter, many producers likely fertilized with less nitrogen than normal due to lower than expected yields,” said Clain Jones, MSU Extension soil fertility specialist and professor in the Department of Land Resources and Environmental Sciences.

He noted that most of Montana east of the Continental Divide saw 150 to 300 percent of normal May rainfall, “But higher precipitation means that yield potentials are higher than were expected when initial fertilization decisions were made. Higher grain yield essentially dilutes nitrogen in the plant, often resulting in lower grain protein.”

Jones said that topdressing additional nitrogen may produce higher yields and protein this year in Montana.

“Of course, it could also be hot and dry until harvest, so it’s not a sure thing,” he said, adding, “Application needs to be timed correctly with precipitation and crop growth stages. The decision to apply mid- or late-season nitrogen to increase protein also depends on whether nitrogen can be applied without substantially damaging the crop and if the expected protein response and protein discount are high enough to justify the cost of fertilizer and application.”

Application timing and considerations

Nitrogen easily moves to roots once it’s on the soil, but most liquid nitrogen that is sprayed on crops, called foliar nitrogen, remains where it lands on leaf surfaces until rainfall or irrigation moves it to the soil.

“Little nitrogen is actually taken up through the leaf surface,” Jones said. “Foliar nitrogen needs to be washed off the leaf and moved into the soil by about a half-inch of water. For this reason, it’s not recommended that growers apply nitrogen to dryland fields unless there is significant rainfall predicted.”

Jones encouraged agricultural producers to make decisions about additional nitrogen applications soon. Fertilizer amount and timing decisions should be based on whether the goal is to increase yield or protein.

“Nitrogen applied before heading (emergence of the wheat head) will most likely improve yield if soil moisture is available, while nitrogen taken up during and after heading should increase protein,” Jones said. “Timing of in-season nitrogen application should be based on plant growth stage and rainfall forecast rather than a particular date. The ability to incorporate nitrogen fertilizer with rain or irrigation is more important than application at a particular growth stage.”

However, Jones said, there may be a limit to how much late-season nitrogen can be applied to boost protein due to the increased risk for lodging and leaf burn.

The chance of leaf burn increases with the amount of nitrogen applied, Jones said, so it’s suggested that producers apply no more than 30 pounds of foliar nitrogen per acre. Liquid urea tends to produce less leaf burn than urea-ammonium nitrate (UAN, or 28-0-0), and using streamer bars — which apply liquid nitrogen between the rows to minimize direct leaf contact — can substantially decrease leaf burn.

Response to nitrogen also depends on sufficient sulfur, Jones said.

“If a grower sees yellowing upper leaves, the crop may be sulfur deficient, whereas nitrogen deficiency shows up as yellowing lower leaves,” Jones said. “A foliar application of 3 to 5 pounds sulfur per acre as ammonium thiosulfate or ammonium sulfate should correct the problem, but, as with nitrogen fertilizer, foliar sulfur is dependent on sufficient rain or irrigation water to wash the sulfate into the root zone.”

Economics and testing

Nitrogen concentration at heading in the flag leaf, or the uppermost leaf of the stem, can be tested to determine whether a late-season nitrogen application will likely boost protein, according to Jones.

“Based on Montana research, grain protein is likely to increase with late-season nitrogen if the flag-leaf nitrogen concentration at heading is less than 4.2%,” he said. “The potential for yield or protein response to nitrogen is highest with lower flag-leaf nitrogen concentrations, but low flag-leaf nitrogen concentrations also mean it will take more nitrogen to make high protein.”

For example, 30 pounds of nitrogen per acre is expected to boost irrigated wheat protein by about one protein point for a 60 bushel-per-acre yield, but only half a point for a 120 bushel-per-acre yield, according to Jones’s review of research published in the MSU Extension bulletin, “Practices to Increase Wheat Grain Protein”

The ratio of fertilizer cost to protein discount may influence whether applying additional nitrogen is justified at a given flag-leaf concentration. If either the protein discount is lower or the cost of fertilizer is higher, then the critical flag-leaf nitrogen needs to be lower yet to justify additional nitrogen, Jones said. For help with this calculation, he recommends reading The Soil Scoop’s “Nitrogen Management for Grain Protein,” a free MSU Extension publication available at The Soil Scoop — MSU Extension Soil Fertility | Montana State University, online at https://landresources.montana.edu/soilfertility/soilscoop.html .

“Unfortunately, flag-leaf analysis cannot tell how much nitrogen to add, or the final protein level,” Jones said. “More late-season nitrogen is required to increase protein by a point as yield potential increases.”

For future growing seasons, growing pulse crops that fix nitrogen in the soil and minimize nitrogen leaching immediately before a wheat crop can increase the odds of having higher grain protein and possibly lower nitrogen fertilizer needs, Jones said. Soil acidification caused by nitrogen fertilizer is a growing problem in Montana, and pulse crops are a sustainable alternative to increasing nitrogen fertilizer rates.

For questions about nitrogen application decisions or soil nutrients, contact Jones at [email protected] or 406-994-6076. More MSU Extension resources are available at https://landresources.montana.edu/soilfertility .