The Value of Using Optical Sensors to Determine Nitrogen Rates
Figuring out the optimum nitrogen (N) rate is no easy feat thanks to year-to-year and field-to-field variability. Applying a flat rate isn’t a good idea either because it can result in over- or underfertilization. That magic number in wheat can range from zero to more than 120 lb. of N per acre.
To determine N rates, Kansas State University researchers compared optical sensors, such as the Ag Leader OptRx, Topcon Cropspec and Trimble GreenSeeker, and a multispectral imager, such as the MicaSense RedEdge, to the standard formula based on soil test data. In most cases, using optical sensors resulted in reduced N rates without negatively impacting wheat yields.
The 2014/15 growing season was extremely dry until early May, with limited organic matter mineralization up to then. Under these conditions, N recommendations were as follows:
- In the absence of soil test information, the recommendation averaged 78 lb. N per acre.
- Adding the results from fall or winter soil tests and using the standard K-State N equation cut the recommendation by 24 lb. to 54 lb. per acre.
- Using a crop sensor-based N system shortly after green-up, reduced the N recommendations yet again by 10 lb.per acre to an average of 44 lb.
- Using the sensor twice, shortly after green-up and at second joint, cut N rates another 17 lb. per acre to 27 lb.
How did the yields turn out? The average yield was 52 bu. per acre, with a range from 27 bu. to 73 bu. per acre. There was virtually no response to N rate. In other words, reducing N rates by using sensors didn’t result in yield loss, saving input costs and increasing overall profitability.
In 2015/16, precipitation started in mid-April and grain fill conditions were cool and moist. Under these high-yielding scenarios, there was a greater response to N rates. In one location, grain yield increase continued up to 125 lb. N per acre. The N recommendation from using an optical sensor reading at Feekes 4 was lower than the recommendation based on the formula using soil test data. Grain yields were equal at the two rates, which means the optical sensor readings had greater efficiency. Using the N recommendation generated by the optical sensor at Feekes 4 and Feekes 7, coupled with two topdress applications, reduced total N rates by half compared with the soil-test-based recommendation while maintaining yield.