From crop models to tissue testing, there are a lot of variable-rate technologies available from which to choose.

Often, these different technologies will result in very different recommendations. Plus, there’s always the question of whether the technology is economically viable. University of Minnesota researchers say it’s important to do your due diligence when evaluating both the technology and the interpretation of results.

In a recent study funded by the Agricultural Fertilizer Research and Education Council, U-M researchers evaluated the use of the pre-sidedress nitrate test to determine nitrogen recommendations. They compared this strategy against split-applying nitrogen and applying the nitrogen all in one application.

Brad Carlson (Photo courtesy of University of Minnesota)

“We were sampling two times during the growing season, roughly 10 days apart, and were finding incredible variability between soil nitrate values for the two sampling times,” says Brad Carlson, Extension educator. “The variability in soil nitrate was anywhere between half and double between the two samples, which is not common in that short of a time span. So the question when you’re using this technology becomes, are there external factors that are causing problems with our interpretation?”

Carlson says in the researchers’ case, they theorized that nitrogen was being immobilized by the breakdown of soil organic matter, or denitrified before the sample was analyzed. It was still there, but no longer in the nitrate form or the ammonium form — so the soil test values didn’t show the true available nitrogen. That got the team thinking about how to evaluate the use of variable-rate technology. Is it in the technology itself, or in the interpretation?

“That really is the caution for any variable-rate nitrogen technology,” Carlson says. “Does the technology work? Is the process resulting in an accurate recommendation? Beyond that, are the numbers calibrated correctly in order to make those recommendations?”

Evaluating the technology
Primary technologies used for making variable-rate recommendations right now fall into a few different categories. Some of the oldest technologies are sensing technologies, like Greenseeker and satellite or aerial imagery, which evaluate the crop color and growth. The problem with using these technologies in corn, Carlson says, is that frequently by the time the crop expresses a change in color or light wavelength, you’ve already lost yield that is no longer able to be recovered.

Another option is in-season testing, like soil and tissue sampling. With these tests, there are many variables that can affect results and recommendations. Usually, when sampling during the growing season, fertilizer has already been applied, so the location of your sample within the field could affect the results. The breakdown of crop residue in the field, the location of nitrogen in the soil profile, and sample handling and storage after collection can alter the results. A lot of these technologies are taking wet soil samples, which are prone to conversion of nitrogen, particularly into N₂ gas if they are not handled correctly between the time of sampling and analysis.

Finally, there are crop models, which are modeling the growth of the crop during the growing season to predict nitrogen requirements by measuring factors such as growing degree days, precipitation, and date of planting to predict how much the crop has grown at a certain point of the year. Carlson says the problem with crop models is accurately predicting nitrogen gains and losses for all factors of the soil nitrogen cycle. The research isn’t there to be able to take a soil test today and predict what the nitrogen status will be next week.

“When evaluating these variable-rate technologies, it’s not simply a factor of, does the technology itself work?” says Carlson. “We also have to worry about the interpretation of what the technology is telling us. We’re doing a lot of research in all these technologies, and we’re starting to get a bit of a picture of how these things perform.”

The bottom line
Carlson suggests that growers invest time to research variable-rate technologies before incorporating them in their operations.

“We aren’t sure at this point whether these technologies are paying for themselves,” he says. “Particularly right now, with the economic state of row crop production, it might be a good time to take a step back and let the research sort itself out before investing a lot of money in these technologies.”

Research included in this article was funded by the Minnesota Agricultural Fertilizer Research and Education Council (AFREC). For more information on fertilizer and nutrient management, follow U-M on Twitter @UMNNutrientMgmt.

Source: University of Minnesota