At fall seeding time, Southwest winter wheat farmers have no way of knowing how much nitrogen the crop will need to meet yield goals.

“If they know what final yields will be, they can select the exact amount of fertilizer the crop will need,” said Brian Arnall, Oklahoma State University nutrient management specialist.

“But when they are planting, they have no clue what they need for fertility.”

Arnall, speaking last summer at the Precision Ag Technology Conference in Fairview, Okla., said nitrogen is a moving target.

“Nitrogen is the most dynamic of nutrients. Even with soil tests, estimating nitrogen needs can be tough because available levels in the soil can change from adequate to too short within three months. Potassium and phosphorus levels do not change that much.”

But Arnall said sensors and nitrogen-rich strips can help wheat farmers predict how much nitrogen they need to finish a wheat crop. In some years, sensing can save farmers money by allowing them to reduce nitrogen fertilizer rates. Other years, sensing allows them to increase nitrogen to match mid-winter conditions and capitalize on early season growth to achieve higher yields.

“This is not a cookbook program,” Arnall said. “But it is better than basing wheat fertility on 20-year averages, which can miss by as much as 80 pounds per acre.” He says success depends on equipment, soil, and management. It requires a little tweaking.”

He said in many cases improved profit potential will not be as good the first year as in subsequent seasons. “But farmers can allow sensors to decide when to make fertilizer applications.”

Arnall said a 2003 test plot made 93 bushels of wheat per acre “when 60 bushels was the best it had ever produced. Weather was just right.” Extra nitrogen paid off. “Some years, we need different nitrogen rates.”

In test plots, the best rate over a 30 year period came from 49 pounds of nitrogen per acre. “That’s not a lot of nitrogen, but farmers need to zero in on yield potential. We don’t know that until maturity, not until we harvest.”

Nitrogen-rich strips and a Green Seeker sensor, however, can improve the odds considerably. “We can tell if you need nitrogen or not. Some years farmers can use just residual nitrogen.”

He said a Green Seeker sensor, moved across nitrogen-rich strips (as many as 24 test strips of wheat inside a field with graduated nitrogen rates) in February indicates which strips and which nitrogen rates offered the most yield potential.

That system in 2003 and 2008 resulted in yields 40 bushels above the three-year average and prediction in February was within 3 bushels per acre of actual yield.

“Analysis in February tells us how good the season has been to that point,” Arnall said. “We can’t predict a late freeze, however.” He said 2009 was a tough year.

“Most of the time, eight out of ten years, sensor-based nitrogen application results in better profits.”

Arnall said an Oklahoma farmer has been using nitrogen-rich strips for several years and saved $2 per acre in his first year. That was up to $14 an acre in 2008. The first year he applied nitrogen fertilizer on only 21 percent of his acreage.

In 2007, he applied nitrogen to 100 percent of his acreage and in 2008 added nitrogen to 50 percent of the crop.

“He figures it takes him 8 hours to put in the strips and 8 hours to read them. Over five years, he invests 80 hours installing and analyzing the strips. Over that time, he figures he’s saved $384,000 in fertilizer. That’s $4800 an hour.”

He said of 7 recent Oklahoma trials only two produced better results than the sensors. “We will not hit every time but it’s better than the current practice. We want to get 10 out of 10 predictions right. That will be amazing and we are working on it.

“We’re taking the guesswork out of nitrogen application. We’re not always cutting back on nitrogen fertilization. Sometimes we add more if we need to.”

He said most County Extension agents have Green Seekers in their offices. And assistance may be available from the Natural Resources Conservation Service through the Environmental Quality Improvement Program (EQIP).

“Also, a pocket sensor, about as big as a calculator, soon will be available for about $200.”

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