Soybean herbicides have improved markedly the past 30 years. Chemists have developed several products that make weed control more certain, and growers have widely adopted herbicide-resistant technology.
But you can do everything by the book — even with Roundup — and still have a weed-control wreck.
“In more than nine cases out of 10, Mother Nature doesn't cooperate,” says Andy Kendig, University of Missouri weed specialist. “Wet or windy weather may keep you from spraying when you should. Drought stress can affect chemical activity as well as plant growth. Timing was key to weed control 30 years ago and timing is still critical in the 21st century.”
Herbicides like Roundup are less sensitive to timing than most. For one thing, they'll kill bigger weeds.
“But the rules haven't changed on how early weeds can affect crop yield,” says Kendig. “Round-up works well if you can get it on the field. But I would use a pre-emergence herbicide with some residual activity on broadleaf weeds.”
Kendig recommends using a pre-emerge tailored to the most troublesome weeds present. For example, if waterhemp is a problem, use something like Canopy XL. Sicklepod germinates virtually all summer long; a spray with good residual activity can help keep it under control.
Morning vs. evening: When's the best time to spray? “Spray whenever conditions will let you spray. And don't ignore the option of aerial application. This can let you clean up areas you otherwise couldn't get to.”
Most Roundup Ready growers spray twice and live with whatever comes up after that, he adds. Ideally, the first application should be about two weeks after planting, when weeds are 2-3" tall. The second spray should come about two weeks later.
“Some producers farther south are using three Roundup applications,” he says. “Normally, I wouldn't spray Roundup more than twice. Last year was a bad weed regrowth year, and two shots did the job for us. But I'd use a pre-emerge or a tankmix as well, especially if I had something like hemp sesbania or morningglory in the field.”
Knowing your soils and the weeds in your fields helps to plan for effective weed control.
“I would err on the side of dead weeds,” says Kendig. “It doesn't take many bushels of soybeans to buy a whole lot of weed-control chemical.”