Tackle Weeds Early for Top Yields

January 15, 2018

Don’t let your crop see weeds – that’s leading advice for top crop production. It hasn’t always been a rule of thumb in soybeans, but that’s quickly changing. Growers seeking to harvest the best possible crop are making early weed control a priority.

“With the introduction of Roundup Ready soybeans, it was generally regarded that you can tolerate some early weeds growing in your crop and then you go in and clean them up,” explains Steve Johns, Agronomic Sales Representitve with Syngenta Canada. “Over the years, we’ve learned that doesn’t work. Similar to corn, we don’t want soybean plants seeing weeds around them when they pop out of the ground if we want to maximize yield potential.”

According to Johns this shift in thinking is supported by some evolutionary weed research conducted at the University of Guelph.

Put down a residual program

Recent work by weed scientist Clarence Swanton demonstrates that soybean plants can sense the presence of weeds even before the plant emerges, ultimately affecting how it develops and yield. This builds on research Swanton pioneered more than 15 years ago when he first defined the critical weed-free period for soybeans as the 1st to 3rd trifoliate.

Knowing that the critical weed-free period is now sooner than before, Johns recommends that you’ll get the best results if you put down a strong residual program – surface pre-plant, pre-plant incoporated or pre-emerge – to avoid annual weed competition and irreversible yield loss.

It’s never too soon to deal with perennials

Johns also urges growers to manage perennial weeds in wheat and corn as they can impact your future soybean crop.

“If you’re an IP soybean grower, your weed control program starts back two years ago when you had wheat in the rotation. Hopefully, you used a good herbicide and were absolutely ruthless in the fall to clean up perennials in your wheat stubble,” says Johns. “That’s your golden opportunity to clean up real problematic weeds like perennial sow thistle and tufted vetch, and keep it out of the subsequent corn crop. Then, when you go to soybeans, you’ve largely managed the perennials that could have had a huge impact on your yield.”

The bottom line for perennial management is that it’s a long-term process that will pay off for years to come.

Get out and scout

Another management practice with a big return on (time) investment is putting your boots in the field. Johns advises to walk the crop looking for ‘oddball stuff’ before it becomes a problem.

“Sometimes you can nip things in the bud, but if you let them go you’re going to really regret it down the road. Just keep an eye out for oddball weeds – often you can rogue out one weed that would potentially be a real pain in the next few years if you let it go,” says Johns.

He says the best timing for scouting soybeans is about three weeks after planting, when the crop is just getting established. “Most growers would consider that too early, but it’s the sweet spot. That’s your opportunity to walk or ride your ATV through the field and make sure there is nothing coming through. If you’ve got a weed that’s emerged with the crop, that’s when you can get the best efficacy with most conventional post products.”

When it comes to scouting, Johns adds that the view from your cab doesn’t count. “By the time you see a weed in your IP soybean crop from the truck, it’s too late.”


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