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Controlling Pigweed

The pigweed family of weeds is a late-season stalker of soybeans. It often survives your best early-control efforts and ends up towering over soybean fields in August and September. This infamous weed family includes common waterhemp, Palmer amaranth, and redroot and smooth pigweed.

Compounding pigweed control is the fact that your best herbicide programs are failing in some places due to herbicide-resistant weeds. Some states see resistance to three or four herbicide classifications, including glyphosate, PPO inhibitors, and ALS inhibitors.
All is not lost, though, says Kevin Bradley, University of Missouri Extension weed scientist. For several years, he’s been encouraging growers in his state to use a multiprong approach, focusing on the pigweeds’ most vulnerable traits that include the following.

1. The Seeds
At first glance, this would not appear to be a soft underbelly since single pigweed plants can produce 300,000 seeds or more. “They are relatively short-lived in the soil,” Bradley says. Tests have shown that over 50% of waterhemp seeds remain viable after one year. After four or five years, though, only about 10% of the initial population will be capable of emergence. That’s low compared to many weeds.

“Just tell yourself you are not going to allow any seed to get on the ground, from any plants,” he says. “If you do that for a few years, you’ll have greatly diminished the seed load in a field.”

Some Southern farmers, who have battled herbicide-resistant Palmer amaranth longer than Midwest farmers, have reverted to hand-hoeing problem fields to stop seed production. If a pigweed plant escapes their best efforts, they will cut and carry it to a burn pile before it can go through the combine.

2. Tillage
If you can use judicious tillage to bury pigweed seeds 6 inches deep or more, they won’t emerge.
“By using the right combination of techniques, you can control pigweeds in a no-till system,” says Bradley. “If you have a really nasty field of resistant pigweeds, then one way of making a lot of progress in a short period of time is to use deep-tillage one time to bury the seed. Then you can begin managing that field much more judiciously in the future.”
In one test, minimum-till and no-till resulted in pigweed plant density of 6 to 10 plants per square meter at the end of the soybean season. Deep-tillage reduced the weed density to almost zero, Bradley says.

3. Cultural Practices
While technically not pigweed weaknesses, these practices can give you advantages. Narrow rows, for instance, will normally result in less weed plant density of all weeds. Row spacings of 7.5 inches and 15 inches often provide similar weed control, and both are better than 30 inches. Quicker canopy and shade are the reasons.

“We also have done tests on seeding rates, and as you reduce the rate on soybeans, you have a greater risk of weeds coming through,” says Bradley. He tested a 130,000-soybean seeding rate per acre compared with 160,000, 190,000, and 220,000. Every time the seeding rate rose, the late-season weed density went down.

Cover crops can also curb weeds, Bradley says. In his tests, a cereal rye cover crop dramatically reduced pigweed emergence.

4. Multiple Herbicides Modes of Action
“Multiple modes of herbicide action will be my advice for the foreseeable future,” Bradley says. “There are several herbicide programs that can work to control pigweeds. But you need the multiple modes of action, rather than just a single shot, to prevent the development of resistance to one class.”

His general recommendations on this include:

  • Using full herbicide rates to reduce the likelihood of escapes.
  • Using two or more overlapping residual herbicides to help give weed control early in the growing season before canopy.
  • Consider using LibertyLink soybeans with glufosinate, as this herbicide is still effective postemergence in soybeans. You can use it in your herbicide rotation program following the residual treatments.

“We need some additional behavior change on this, as it’s a serious problem for many growers right now and could be for all growers soon,” says Bradley. “If we do the right things, it’s a solvable problem.”

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