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Crop advisor casebook: NDVI map conflicts with eye test

A Crop Advisor's Solution from the February 16, 2020 issue of Grainews

Amber Bernauer. photo: Supplied

It is routine procedure at our office to monitor NDVI satellite images of our canola variety trials using digital ag software. This helps us to react quickly to any problems as well as take note of any differences that may arise between trial treatments over the course of the season.

One day in mid-July last year, I was examining NDVI imagery when I spotted something pertaining to some trials on a farmer’s field near Humboldt, Sask., I thought warranted a call with the grower, James.

The field was planted with numerous varieties of LibertyLink and Roundup Ready canola that were all from different seed companies and had been treated with their respective herbicides (Liberty on LibertyLink varieties and glyphosate on Roundup Ready varieties).

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The NDVI map showed a fairly obvious line between the positions of the LibertyLink varieties on one part of the field and the Roundup Ready varieties on the other. The LibertyLink varieties in the images were all dark green, indicating high biomass, which is often directly correlated to high yields. The Roundup Ready varieties, on the other hand, were all yellow or red in colour, indicating a lower biomass.

When I talked to James on the phone about what I was seeing on the NDVI image, he sounded somewhat surprised.

“I guess there must be something wrong with those Roundup Ready varieties, but I sure didn’t see it,” James said. “Everything looked fine with those trial plots when I drove by the field a few days ago.”

I did not anticipate large differences due to herbicide tolerance as Roundup Ready canola varieties perform very comparably to LibertyLink varieties, so I thought the best thing to do was to drive out to the farm to see for myself what was going on.

My initial look at the trial plots was in line with James’ observation. Apart from a few minor maturity differences, the entire field appeared to be the same, regardless of variety. The canola plants were all in full bloom and the plant stands seemed fairly uniform from plot to plot as well.

I thought that made sense, since when I’d conducted plant counts in the field earlier in the season prior to spraying, the counts were consistently two to four plants per square foot for all LibertyLink and Roundup Ready varieties. That was lower than the anticipated plant count of five to seven plants per square foot, but because the crop looked good, this wasn’t a concern.

While performing those early plant counts, I noted there was a heavy population of volunteer canola plants throughout the entire field in between the seeded rows. As I walked through the field now, I could see there were still lots of canola volunteers, but they were only growing in those areas planted with LibertyLink canola.

Crop advisor’s solution: Canola volunteers cause NDVI discrepancy

The fact I was now seeing the volunteers only in those trial plots seeded with LibertyLink canola varieties confirmed what I suspected was causing the discrepancy with the NDVI image. It was clear the Liberty herbicide applied in the LibertyLink treatments didn’t work against the Liberty-tolerant canola volunteers, but in the treatments treated with glyphosate, the volunteer canola plants were susceptible to the herbicide application and were controlled.

This, then, is what caused the NDVI map to indicate reduced biomass in the Roundup Ready varieties — there were no volunteer “weed” canola plants to contribute biomass, unlike those areas with LibertyLink varieties.

Mistaking weed patches or trees for high biomass in a field is a common error for those unfamiliar with NDVI maps. It does take some skill to manipulate and interpret them correctly, and it’s important to ground truth the imagery with boots in the field.

Come harvest time, the LibertyLink trial plots in James’ field containing lots of LibertyLink canola volunteers didn’t yield as well as the varieties in the Roundup Ready plots. Research shows that having too many canola plants creates more competition for resources and can reduce yields as a result, which is obviously what happened here.

To avoid situations like this, it’s important to consider what kind of impact volunteer canola can have on yield in tighter rotations, especially if you’re using the same herbicide-tolerant canola system each year. In addition to a comprehensive weed management program utilizing different groups and pre-burn applications, it is also worth considering a rotation of Roundup Ready and LibertyLink canola to thin out the volunteers every few years.

Amber Bernauer, CCA, PAg, works for Richardson Pioneer Ltd. in Humboldt, Sask.

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