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Weston (4) and Harper (1) Sherwood enjoy a June afternoon climbing around the combines and wondering why its not time for harvest. Their parents, Josh and Brooke Sherwood, farm south of Wilbur.
Photo by Brooke Sherwood

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FARMER'S TOOLBOX

AMMO: Putting nitrogen management in the spotlight

AMMO session focuses on inputs to influence yield, protein

May 2018
By Trista Crossley


At the last Agricultural Marketing and Management (AMMO) winter workshop of 2018, Dr. Romulo Lollato, Extension wheat and forages specialist and assistant professor from Kansas State University (KSU), tackled nitrogen management and explored current research on intensive wheat management.

Because wheat kernels are 60 to 70 percent starch and only 8 to 15 percent protein, yield is mostly a function of starch, while nitrogen availability impacts protein. Lollato explained that the more starch a kernel has, the less percent protein and vice versa. Protein starts to accumulate in the kernel about 10 days after flowering with significant starch accumulation beginning a few days later. About 20 days from flowering, the kernels have nearly half of their protein (see slide 1). Conditions late in the grain-filling period, such as drought or heat stress, can impact starch accumulation and therefore the ratio of protein to starch.

“You guys here typically have very cool grain-filling conditions, so it allows for the crop to do a lot of photosynthesis and put a lot of starch in the grain, which allows for higher yields,” Lollato said. “Go to Kansas or Oklahoma where we get hot really quickly during the grain-filling period, and we are going to shut down the starch accumulation early. That’s why our yields are lower and protein concentration goes up.”

For wheat to build starch, it needs sunlight or CO2, while protein requires nitrogen. Lollato explained that just increasing the amount of sunlight the plants get isn’t necessarily going to increase the amount of starch because it’s actually dependent on the kernels on each plant to pull in accumulated sugars—Lollato called this a sink strength. In other words, starch accumulation in wheat is limited by the number of kernels a plant has, while protein is limited to the amount of nitrogen available to the plant.

Since 2006, KSU has been doing applied research in nitrogen rates and application timing at more than 30 trial locations across the state in varying rainfall zones. Lollato said the results have shown that if growers apply more than 60 percent of their nitrogen at jointing or boot stage, the protein levels were above average.

“In other words, slightly delaying that nitrogen application so it’s available at jointing or later allows you to manage for higher protein and higher yields,” he said.

KSU research done in the last two years has shown that on average, growers needed 150 pounds of nitrogen per acre—or 2.3 pounds of nitrogen per bushel—to maximize their yield potential. For higher protein, growers needed slightly more nitrogen, about 170 pounds per acre or 2.6 pounds per bushel. Nitrogen included in the soil profile was included in those figures.

The takeaway from the research, Lollato said, was that by providing more nitrogen, up to a point, both yield and protein are increasing, albeit at different rates. At some point, nitrogen levels are sufficient for yield, but the protein can still increase. Eventually, there comes a point where both yield and protein level off and nothing is gained by applying more nitrogen (see slide 2).

Lollato said farmers have a short window in season to help manage protein by looking at the nitrogen concentration of the flag leaf.

“There is some research in the Pacific Northwest showing if there is less than 4.1 percent nitrogen in the flag leaf, you can still get a protein bump from a late season application of nitrogen. If the flag leaf has more than 4.1 percent, then you are in that e
xcessive range,” he said.
Managing nitrogen for yield and protein is a function of four factors:

• Source;
• Rate;
• Time; and
• Place.

In his presentation, Lollato focused on time and rate of nitrogen application. According to the KSU studies, there’s a steep increase in nitrogen uptake in wheat between jointing and flowering stages with nearly 80 percent of nitrogen uptake done by flowering (see slide 3). Applying nitrogen before planting helps with fall tillering, while nitrogen available during the winter/early spring is more important for head size determination. Late spring applications will help protein, but not yield (see slide 4).

“I mentioned that yield is limited by how many grains the head has; that sink strength is what we are determining here (right after winter dormancy and before jointing)—tillers and head size. It’s important that nitrogen is on the root zone at this time so it can affect the number of heads or number of spikes per head,” he explained.

To help producers figure out how much total nitrogen to apply, Lollato began his calculation for hard red winter wheat by assuming producers need 2.4 pounds of nitrogen per bushel based on 12.5 percent protein (see slide 5). That number is adjusted for a 50 percent efficiency loss, which he called “our biggest bottleneck.” He added that producers can easily adjust that number for lower protein wheats, such as soft white.

Lollato then multiplied that 2.4 pounds of nitrogen by the yield goal of the planted variety and adjusted it by the amount of organic matter mineralization in the soil and the amount of nitrogen in the soil profile (or used a standard 30 pounds of nitrogen per acre for Kansas conditions when soil profile nitrogen is unknown). Producers then need to take into account any other previous nitrogen and crop adjustments they’ve made (see slide 6) to come up with their field-specific number.

Producers were urged to do soil samples in order to avoid applying too much nitrogen.

“It is very rare to see producers collecting a soil sample. When we aren’t doing sampling, we are probably guessing how much nitrogen we are going to put out,” Lollato said.

Lollato also touched on using technology to help producers manage nitrogen, including using active crop sensors and a nitrogen-rich strip to help determine potential yield by comparing it to the rest of the field using the Normalized Difference Vegetation Index, or NDVI.

“That technology exists and is something you should be looking at as well,” he said.

To wrap up his presentation, Lollato talked about managing yields based on different varieties’ response to seeding rates and their disease susceptibility.

“I think we tend historically to overlook the value of management,” he said after his presentation. “A lot of it has been said to be dependent on variety or environment. What I wanted to share today is that there is room for us to manage the crop for whatever objective we have. Of course the first objective is profitability, but is that profitability going to come through more protein or is it going to come from less protein?”