Seeking the Green

Optical sensors that measure the reflectance from the corn canopy and then attempt to use that information to manage the crop have been available for more than a decade now. GreenSeeker (TM Trimble) is one of the better known sensors. The main ambition behind GreenSeeker use is to improve nitrogen rate management. There is still significant interest in the country side on how to investigate and deploy optical sensors. Here are few ideas to consider.

Site Selection

The best fields for any variable rate technologies are fields with a high degree of variability where an average rate of product (seed, fertilizer etc.) across the entire field may result in significant portions of the field receiving rates that are much higher or much lower than optimum. Variability in soil texture, drainage, organic matter and topography may result in variability in nitrogen availability (supply, loss) and crop yield potential, which may result in variability is spatial nitrogen requirements.

Algorithm Theory

An algorithm is the formula responsible for converting a GreenSeeker reading (NDVI) from a field to a nitrogen recommendation. While specific algorithms differ in their response to field conditions and NDVI values based on calibrations to the local regions, the basic premise of most corn algorithms is the same.

GreenSeeker Algorith Theory:  Nitrogen Application Rate in Response to NDVI

  1. NDVI values which are very low relative to the N-rich strip (less chlorophyll, less plant biomass) are limited to a minimum amount of nitrogen which may be set by the applicator. Yield potential is expected to be low in these areas and factors other than nitrogen may be limiting, thus little yield response to nitrogen is expected.
  2. As NDVI values increase, nitrogen recommendations increase. Yield potential is expected to increase relative to plants in the very low NDVI range (1). Low NDVI readings relative to the N-Rich strip suggest nitrogen may be limiting, and thus a response to nitrogen may be expected.
  3. Yield response to nitrogen is expected to increase with NDVI up until a maximum (3).
  4. Beyond the maximum point, nitrogen recommendations decrease with increasing NDVI, as NDVI values are approaching the N-rich strip where nitrogen is no longer a yield limiting factor. When NDVI is equal to or higher than the N-rich strip, nitrogen recommendations are equal to the minimum rate, as no yield response to additional nitrogen is expected.

Analysis

There are two possibilities that exist for how the GreenSeeker might improve your N management in corn.

  1. It might be able to predict what the correct average rate for your fields are based on relative NDVI scores. Can it do better than the Ontario Corn N Calculator in getting the average right rate for your field if you feed it NDVI scores from the N rich strip and the unfertilized zones in the field? If this is the key question then yield results from the whole field length strip (i.e. weigh wagon measurements) will do the job.
  2. Rather than change the average rate across the field, it may be advantageous to move N from where it is needed less to areas where it is needed more. This will improve yield per unit of N applied. In this case you must have more than whole strip data. You must have site specific GPS referenced data for both NDVI and yield all the way down the strip. This will enable you to cut up the strip and decide in a certain area whether the Green Seeker algorithm was making the right decision or not. For example, if you go through a part in the field where response to N is low and yields are quite similar between strips receiving high N fixed rates and low N fixed rates then the GreenSeeker should be automatically pulling the N rate down at those locations. Only thorough site specific analysis of the strips can you determine where the GreenSeeker is making profit improving decisions.

Go to www.gocorn.net to read the full version of this article.


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