California tests GreenSeeker to help with mid-season nitrogen management decisions.
By Bruce Linquist
Our recommendations call for applying all the nitrogen fertility requirements for an average yielding year at the start of the season. This N is applied as either aqua-ammonia or as part of a starter fertilizer blend (the starter blend can be applied up to four weeks after planting if scum is a problem).
At panicle initiation (PI — about 50-55 days after planting), the crop should be assessed to determine if a top-dress of nitrogen fertilizer is necessary. This is important because not applying N when needed can lead to a yield reduction; however, applying N fertilizer when it is not needed can lead to lodging, delayed maturity, increased incidence of disease and reduced yields.
The leaf color chart was developed to help with this assessment and is still a valid tool. Its limitations are that it is time consuming and limited to a relatively small area.
The GreenSeeker is a new tool we have been testing for this purpose. It measures the NDVI (Normalized Difference Vegetation Index) of the canopy.
Some preliminary results
Based on preliminary data, we have developed a response index to help growers decide when a top-dress N application is necessary.
We have found that an index of 1.1 or greater indicates the need for top-dress N application. The response index is the NDVI reading of an enriched N strip (representing a crop with unlimited N) divided by the NDVI reading from the field test area. The N enriched strip is an area where extra N was added to the field. This could be done by overlapping an area with an aqua rig.
For example, if the N enriched strip gave an NDVI value of 78 and the field test area gave an NDVI value of 68, the response index would be 1.14 (78/68=1.15). This would indicate the need for a top-dress N application.
We would like to emphasize that this is based on preliminary data and further testing may change the response index. However, it does provide a useful guide for now.
See related article about GreenSeeker use in Arkansas: A hand-held ‘fuel gauge’ for rice
The GreenSeeker has some limitations — it still tests a relatively limited area, although it is much faster to take readings and get a better assessment of the field than the color chart. You also cannot use the GreenSeeker when leaves have dew or rainfall on them. In addition, the instrument does not work well in areas with poor stand establishment or a lot of weeds.
Dr. Bruce Linquist is Cooperative Extension rice specialist in the University of California, Davis, Department of Plant Sciences. He may be reached at balinquist@ucdavis.edu.