Optical sensors guide nitrogen rates You've been growing wheat for years. You've made some impressive yields. But, you are never really sure how much nitrogen your crop needs. Even with timely tissue samples, you can't easily provide ideal nitrogen levels across an unevenly growing wheat crop.

The reality is that you choose to apply enough nitrogen to make sure the crop doesn't suffer. Fertilizer is cheap compared to the unacceptable blow of under-performance.

Economics, environmental regulations and a desire to improve wheat yields are driving farmers to find more efficient ways to supply the precise amounts of nitrogen their crops need. Randy Weisz, small grain Extension specialist, North Carolina State University, gave growers the latest recommendations and promises for the future at AgTech 2000 and the Southeast Vegetable and Fruit Expo in Greensboro, N.C.

Successful wheat growers are using a combination of tiller counting and tissue testing to fine-tune their wheat nitrogen applications. The techniques work well on uniformly growing wheat. But, time of planting, changes in soil type and other factors typically create both low and high tiller densities across the same field. Growers need some way to apply more nitrogen in areas of low tiller density and less nitrogen where plants already have what they need. A system of optical sensors that essentially count tiller density on the go, combined with a variable-rate nitrogen applicator, may be the solution.

"There should be a way to go to a field in season, while the crop is growing, and figure out how much nitrogen the wheat needs," Weisz says. "We know a lot about wheat nitrogen needs. When you plant in October and harvest in June in North Carolina, the wheat needs almost no nitrogen for the first three to five months. The best time to put the majority of the nitrogen out on a healthy crop with plenty of tillers is in March at growth stage five, when the crop is taking up a lot of nitrogen. If the stand is thin, the best time to apply nitrogen is in February, to stimulate further tillering. We recommend taking tissue samples at growth stage three and five to determine nitrogen needs."

As a guideline, Weisz says a thick stand of wheat has at least 550 tillers per square meter. Wheat with fewer than 550 tillers per square meter should receive most of it's topdress nitrogen in February. But, the small grains specialists cautions that early nitrogen applications can stimulate vigorous growth and subject tender wheat to freeze damage. To avoid supplying too much nitrogen to thin stands of wheat in late January or early February, Weisz suggests splitting the nitrogen, with half going on early and the other half in March.

"We are fairly comfortable with those recommendations," Weisz says. "We are still evaluating whether or not we should variable rate apply nitrogen on thick and thin spots within a field. How do you find the spots and how do you apply variable rates of nitrogen?"

At least two companies, according to Weisz, have developed optical sensors that attach to sprayers. These sensors look in front of the sprayer and determine tiller density. Thin stands trigger higher nitrogen rates and thicker stands trigger lower nitrogen rates. One of these units is being marketed in Kentucky. The other is still under development.

Weisz is working on another technique for determining stand thickness and varying nitrogen rates. He is comparing near infrared photographs of wheat fields with hand counts of tiller densities in those same fields.

"We hand count the tillers in each field and calibrate the photographs according to the information we get from the field," he says. "Different colors identify different tiller densities. The problem with this system at this point is that each field must be individually calibrated to be accurate. If we are going to use this technique today, we have to go to one known thick area and one known thin area in each field to calibrate the photograph. Keeping in mind our guideline of 550 or more tillers per square meter in thick stands and fewer than 550 tillers per square meter in thin stands, we feel confident we can use either sensors or aerial photographs to make nitrogen application rate decisions."

Still unknown is whether or not this technique will either increase yields or reduce excess nitrogen rates enough to make the technology economically practical. Several members of the audience pointed out that factors other than lack of nitrogen, such as low sulfur or soil hard pans, can be responsible for low tiller density.

"Our work is ongoing," Weisz says. "We have a tissue test at growth stage five that is pretty good for determining how much if any additional nitrogen should be applied. We know that we should apply some nitrogen to thin stands by growth stage three. This optical scanning and remote sensing with infrared photography and variable rate application is showing promise."