Challenging spring interferes with weed control plans

CORRECTION: June 10, 2020 – 0945 CST – The original version of this story erroneously quoted Sask. Ag weed specialist Clark Brenzil as saying that Arylex was the equivalent of “florasulam”, which it is not. Brenzil reminds growers to refer to product labels, manufacturer use guides or provincial pesticide guides for proper use of registered pesticides. Do not make applications that are not listed on product labels.

Fall spraying in 2019 just didn’t happen in most prairie regions.

As well, farmers have faced a cold, muddy spring with low soil temperatures. That often means that weeds and volunteer crops are not staging in their normal fashion. Instead, they’re waiting to catch up to their calendar until… well, who knows when they’ll decide to get back on schedule. Their late-staging habit could stay with us through the entire growing season.

The fact that few sprayers got into the field last fall gave weeds a strong hold this spring. Most areas have good soil moisture, so when weeds decide to take off, they’ll be ready. Low temperatures earlier in the spring slowed the metabolism of weeds, but when soil finally warmed, weeds were set to sprint into June. This kind of un-predictable staging messes up a farmer’s seeding and weed control plans.

Down-on-the-ground field checking is a vital activity when dealing with weeds and volunteers that are ignoring the calendar, according to Breanne Tidemann, weed specialist with Agriculture Canada in Lacombe, Alta. Late-staging weeds are more difficult to find and identify a good defence mechanism, said Tidemann.

“You need to scout with your nose right down on the ground if you expect to find elusive weeds,” Tidemann explained during a webinar in mid-May.

“If you’re looking for green leaves, go back and look again. The leaves on those late weeds are often dark purple, not green, and they’re very small. They’ll hide under your trash cover so make sure you brush the straw away for a better look at the surface. There’s no point spraying if the product gets tied up on the trash.

“Once the weed leaves are in the open, make sure you don’t spray until the temperature is in the recommended range. If it’s too cool, the herbicide will get stuck on the leaves and won’t transfer into the plant. Remember that the most expensive herbicide is the one that doesn’t work.”

Tidemann reminds growers and fellow agrologists that higher seeding rates and heavier crop density should always be considered as a valid weed management tools.

Stressed weeds – Clark Brenzil is provincial weed control specialist for Saskatchewan Agriculture. He said, “Earlier this spring there was not a good long run of warm weather. We had an initial warm punch so weeds started growing, but then it turned cold again and they went back into hibernation.

“Temperatures had been up and down around the freezing point after that. That’s why so many weeds have that stress syndrome and that purple colouration. Purple leaves are the weeds’ cry for help. Once conditions get back to normal, those stressed weeds will bounce right back. The purple leaves will persist and green material will grow around them.”

Now that we’re firmly stuck in this situation, producers should ignore their calendars. The only thing that matters now is growing degree days. He said weeds will quickly catch up to their own private internal calendar once GDG have provided enough heat.

The question facing farmers is whether to spray weeds before seeding, when they may not be at the optimal stage for spraying? Or is it wiser to proceed to seed when soil temperature is in the optimal range?

“That’s a hard call. If the weeds are sitting and waiting, they’re going to be ready to grow quickly before the seeded crop has emerged. So the weeds will have the time advantage, plus they will be robbing the soil of moisture and nutrients. A weed that’s ahead of the emerging crop will be 10 times more competitive than a weed that emerges after the crop is growing. The crop wins over the late-emerging weeds.

“If you’re looking at a glyphosate application you have to wait until five degrees minimum, with expectation of eight or ten degrees for at least two or three hours. That’s more important with winter perennials than annuals.”

Brenzil said farmers should be aware that many of the so-called “new products” being advertised are simply derivatives of herbicides that have been on the market for years. He said these new products rarely contain an actual new active ingredient. He said they are combinations of active ingredients that already existed, or a stripped down version of older herbicides to get rid of residuals that could carry over to damage the crop.

“We’re also seeing a lot of generic concept products. Generic companies are interested in active ingredients that have been on the market for a long time. They register them as single active products to get a new PCP number (Pest Control Products). A stand-alone active ingredient has its own PCP, even though it’s already included in the PCP of another herbicide combination.

“A lot of these generic companies are registering stand-alone actives, then combining them as package concepts to mimic the proprietary products of the big companies. So you have stand-alone products A and B and C in a carton. You mix them together and you get product D.”

He said the proprietary companies also do this to test the water for the product D combination. If they find there’s a solid market for D, then they start working on a co-formulated product that puts all those ingredients into the same jug.

New modes of action — Soil active herbicides are an important part of a weed control portfolio, said Tammy Jones, weed specialist with Manitoba Agriculture. She said these new modes of action slow early weed growth to allow for optimal timing of in-crop herbicides and minimizing crop competition.

“Some of the more traditional soil activity herbicides are seeing a resurgence in popularity,” Jones said in an email interview.

She said these include Group 3 (ethalfluralin, trifluralin), Group 5 (atrazine, metribuzin) and Group 8 (triallate, EPTC).

She said there are also some relatively new entries such as Group 13 (clomazone), Group 14 (flumioxazin, sulfentrazone) and Group 15 (pyroxasulfone) that are effective as part of a herbicide layering strategy.

“It would be remiss of me to suggest that herbicides are the only way to widen the window of weed control. Just like older soil active herbicides that are again being used, other older weed management strategies are attracting attention.

“New tillage systems can do inter-row cultivation on very narrow row spacing. And there are other methods to help enhance crop competitiveness and slow weed growth. Things like altering your seeding dates, planting cover crops, increasing seeding rates and going to narrower row spacing. This helps time an in-crop herbicide application more effectively and possibly reduces the number of herbicide applications.”

Corteva has introduced some new chemistry, said Rory Degenhardt, Canadian integrated field sciences research leader. When asked how these new ingredients work in the kind of spring we’ve had in 2020, Degenhardt replied via email saying, “Arylex active herbicide, the active ingredient in herbicides such as Paradigm, Elevore, Pixxaro, Cirpreme and Rexade, is a Group 4 synthetic auxin that’s part of a new chemical family called Arylpicolinates.

“Arylex has a unique binding profile among auxin herbicides. Testing by Corteva’s research team and external collaborators show robust activity even under adverse conditions like we encountered at various locations across the Prairies this spring. Cool, wet conditions can lead to variability in weed and crop staging. Herbicides with Arylex have been designed to work on a wide range of weed species and stages, from very small to very large. They are safe across a range of crop staging as well. Corteva has formulated Arylex with other active ingredients to provide for multiple mode of action resistance.”

Degenhardt say the new chemistry differs from previous products. Some auxin herbicides are negatively impacted when temperatures are at 8 C or lower. On the other hand, Arylex active retains its activity at cooler temperatures above freezing.

He said Corteva is still working to understand all the mechanisms behind this difference, but they believe ingredient potency is one of the main reasons. While many synthetic auxins require hundreds of grams or even kilograms of active per acre to control target weeds, Arylex active works at exponentially lower rates of two grams or less of active per acre. Products that disrupt photosynthesis often require warm, sunny weather for full potency. However Arylex active works well under overcast conditions.

“Although Arylex itself is not effective against volunteer canola, it has been formulated with other active ingredients to provide robust canola control. Paradigm is a pre-formulated mixture of Arylex and the Group 2 herbicide florasulam. Florasulam is very effective against volunteer canola, and has the added benefit of having soil activity against late-germinating weed flushes.”

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