Herbicides and bioassays

A dry fall followed by a dry spring may leave much higher levels of residual herbicides

I bioassayed soil in January from a field treated twice with the herbicide tribenuron-methyl in 2020 to determine if the field was suitable for a canola crop this year. It was not — the potted field soil fried the canola seedlings, which is shown in the photo at bottom. The photo at top is the check. I recommended the farmer stay with wheat this year on this quarter.

When you use a soil-persistent herbicide, there are many factors you must consider very seriously. The first thing you have to do is read the label that comes with the herbicide. It will tell you how to mix and apply, water source, what weeds are controlled and so on. Read it very carefully. Of course, it will also tell you how it works but to be a safe and competent user, you should follow up on all and any restrictions.

Generally speaking, the herbicide label will tell you most of what you need, but can you remember everything? Not really. This is where those hard copies of the 2021 crop protection books, such as the 2021 edition of Alberta Blue Book, Manitoba’s Guide to Field Crop Protection 2021 and Saskatchewan’s 2021 Guide to Crop Protection, come in very handy — good farm managers keep one or more in the truck cab.

These books are accessible online but who wants to fiddle with an iPad out in the field. Bring out the old “book,” it’s much easier. The Manitoba, Saskatchewan and Alberta books are updated annually with new pesticides or pesticide withdrawals if they are not in the book.

When you first purchase a pesticide, read and reread the label since there are optimum times of application and methodology. If you have to store extra pesticide, remember some of these pesticides may lose most, if not all, of their effectiveness if they are frozen. It will be listed on the label — do not allow this product to freeze.

In many instances, farmers will like the results of a particular product but they may, unfortunately, use this same product too often on the same cropland or quarter. In other words, some of these soil-applied herbicides will build up on short rotations or, as in some instances, no rotations — such as canola, snow, canola or wheat, wheat. Again, the product label may say use only once in two years or they may specify the soil type where the herbicide may break down faster. Believe the herbicide label.

Farmers may apply a herbicide such as tribenuron-methyl and spray it twice on the same field because of an unsatisfactory kill or for a second regrowth before seeding. I bioassayed such a field this January to see if it was suitable for a canola crop. No way. The potted field soil fried the canola seedlings — stay with wheat this year for sure on this quarter.

In another instance, last spring, imazamox was applied twice to a couple of pea crops that year. The bioassay in January showed there was a definite slowdown of growth on the treated soil samples. The canola on the treated soils in the growth chamber bioassay showed about 60 per cent of the growth of the untreated check soil.

Facts to consider

Yes, you can sow canola on these fields but ensure that you use Clearfield canola if possible. If you use non-Clearfield canola, you may get away with a crop close to an expected yield if the soil profile remains good and wet. If the cropping season stays dry or even in a drought with a dry subsoil and the odd half-inch or so of rain, expect disaster with the non-Clearfield-type canolas, since their roots will remain stranded in the top few inches of soil and they will be in prolonged contact with the herbicide.

The only salvation for such a crop would be two to three inches of rain before the canola bolts. This will allow the canola roots to move into the “clean” herbicide-free subsoil and grow fairly normally. This may be the situation with the mystery decline and damage with the chickpea crop in Saskatchewan in 2020, (i.e. prolonged contact in dry soils with the top few moist inches that have the herbicide residues). Herbicide residues do not generally move down the soil profile, they stay put in the top few inches. Therefore, in very dry springs, these residual herbicides can be very destructive.

Soil pH and organic matter

Herbicides, even if persistent, differ in their speed of breakdown depending on soil pH. Some herbicides persist at low soil pH (i.e. less than 6), whereas others persist if the pH is above 7 or 7.5. Soils high in organic matter (i.e. greater than five per cent), significantly reduce the effect of soil-persistent herbicides. Sandy, dry soils with organic matter less than two per cent offer little protection.

Nevertheless, most herbicide damage to field crops in non-dry years is not that obvious. What I mean by this is your canola crop or wheat crop comes in at 80 to 90 per cent of expected yield and you put it down to weather when, in fact, it’s the “drag” often caused by overusing soil-persistent herbicides contrary to the label.

In another field I bioassayed this year, the soil residual herbicides from previous years were at disastrous levels. This field was newly acquired from another farmer and was to be seeded to wheat this spring. The top three inches (15 cm) of this soil were loaded with seedling growth inhibitors. The canola was less than 10 per cent of the check, the flax was about 20 per cent of the check and the wheat was about 30 per cent of the check. All I could say was the soil was “loaded” with Group 2-type residues. Only time will download this “toxic” field.

It’s a good lesson when you buy or lease new cropland to always inquire about the yield of the previous crops rather than rely on the regular soil analysis and average yields for cropland in the vicinity. Soil analysis could show good nutrient levels but the herbicide cocktail was doing its expected job by inhibiting all crop growth resulting in disastrous yields.

We need feet on the ground

As a footnote, I am shocked by the fact that Alberta Agriculture has discontinued publishing the Blue Book, the most popular publication on the Prairies. A book to be found in every farmer’s truck cab.

Could I say that “common sense is a flower that does not grow in everyone’s garden”? Does the minister understand Alberta farmers?

It is fortunate the grain commissions in Alberta have agreed to continue with this bedrock publication. It’s fine to fund basic research at the federal and university levels but provincial agriculture on the Prairies needs feet on the ground — applied researchers such as cereal, oilseed and pulse specialists.

Doing away with provincial feet on the ground is like watching a hockey match between Edmonton and Calgary without any referees, since all of the players should be totally familiar with the rules and infringements. So, why would you need any referees? Chopping down provincial applied feet on the ground for field crop survey research is like having a front yard apple tree and pruning it hard every spring and complaining about the lack of apples from the sprouted stumps.

A big point to remember is an expensive chemical soil analysis may tell you what herbicide is present in the soil but it could well be inactive. It’s only a properly done bioassay that will tell you the soil-borne herbicide(s) that can seriously damage the crop you intend to seed and hammer your expected financial returns. Many herbicides require moisture to break down, so a dry fall followed by a dry spring may leave much higher levels of residual herbicides. If the label says to be careful, just be extra careful of the crop you choose.

I am supposed to respect my elders, but it’s getting harder and harder for me to find one now.

About the author

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Dr. Ieuan Evans is a forensic plant pathologist based in Edmonton, Alta. He can be reached at [email protected]

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