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Use these strategies to complement herbicides for harvest weed seed control

When one more herbicide-resistant weed becomes too much to bear, chill. Herbicide-resistant weeds have been tormenting farmers for decades.

Velvetleaf first resisted atrazine in 1984 in Maryland corn and cropland. Common cocklebur resisted imazethapyr (Pursuit) in a Mississippi soybean field in 1989. Meanwhile, marestail first resisted glyphosate — the poster child for herbicide resistance — in a Delaware soybean field in 2000. This occurred just four years after glyphosate-tolerant soybeans debuted in 1996.

Since then, herbicide-resistant weeds have snowballed, spurred by the increased use of glyphosate and other herbicides used in several other herbicide-tolerant systems. Glufosinate (Liberty’s active ingredient), which was previously untouched by resistance in row crops, is now being impacted. In August, University of Missouri (MU) weed scientists confirmed glufosinate-resistant Palmer amaranth in the Missouri bootheel. Herbicide have been, are, and will continue to be a key tool to manage weeds. Still, they need help.


“There is too much reliance on herbicides alone for weed control,” says Kevin Bradley, MU Extension weeds specialist.

The good news: Several strategies can complement herbicides in the quest to control weeds.

Harvest Weed Seed Control

Though harvest weed seed control (HWSC) is new in the United States, Australian farmers have successfully used HWSC to destroy weed seed and to reduce herbicide-resistant weed populations for several years, says Bradley. Grinding seed during combining or funneling weed seed into narrow windrows or tramlines to be later burned, rot, or die has eased weed control pressure on herbicides.

HWSC works in Australia. It reduced populations of rigid ryegrass — Australia’s version of Palmer amaranth or waterhemp — by 37% to 90% in Australian university trials. This weed that infests Australian small grains and pastures resisted 13 herbicide sites of action from 1982 to 2013.

In the United States, land-grant universities are testing HWSC systems that use impact mills integrated into combines. In MU’s case, weed scientists are testing the Seed Terminator unit. Iowa State University weed scientists have been evaluating the Redekop Seed Control Unit.

“The premise is that these implements will grind up weed seeds headed out the back of combines so they are no longer viable,” Bradley says. “Over time, this depletes the weed seed soil bank.”


Pigweed seeds that pass through the combine can set the stage for future infestations. Harvest weed seed control systems, such as impact mills integrated into combines that grind weed seed at harvest, can help reduce future weeds from forming.


Limitations exist. The technology aims at weeds that form seed later in the season, such as waterhemp and Palmer amaranth. Also, not all weed seeds enter the combine in MU tests with the Seed Terminator.

“As the reel of a combine turns, it’s sometimes going to hit the seed head of a waterhemp or Palmer [amaranth] plant and cause shattering,” he says. “We get a majority of the weed seed into the combine, but around 20% of it instead lands on the soil.”

Climate is another challenge.

“We operate in a different climate compared to the arid one in Australia,” says Bradley. “Our weeds [in Missouri and neighboring states] can stay green and can be harvested before frost. There is greater potential for clogging as you harvest green nasty pigweeds that have a lot of juice in them. However, we have not clogged once in Missouri in our two years of research [with the Seed Terminator].”

Operational expense is higher in MU tests with the Seed Terminator. “It will cost you to install it on the combine,” says Bradley. “They also put more load on the engine, so it will burn a little more fuel,” Bradley says. “It will slow you down, although our tests show there is only a 0.2 mile per hour difference at harvest when it is on vs. when it is off. If you’re running a Class 7 or Class 8 combine, this has not been an issue.”

MU tests have shown the Seed Terminator renders 94% of the seed nonviable once it is in the combine.

“You will see substantial reductions in the seed bank the next spring and in subsequent years by not allowing viable seed to go back in the soil,” says Bradley.


Including wheat in a rotation can break up weed cycles and diversify herbicide sites of action.

Crop Rotation

Crop rotation is a simultaneously simple and complex tool that farmers can also use to complement herbicides in managing weeds.

“Part of the reason we are in this mess right now with multiple herbicide-resistant weeds is we are focused almost exclusively on two summer annual crops, corn and soybeans,” says Adam Davis, a University of Illinois (U of I) weed scientist. Crops that have contrasting life cycles, such as wheat or alfalfa, could help deter pigweeds such as waterhemp and Palmer amaranth that now trouble corn and soybean farmers, he adds.

Crop rotation can also enable farmers to use a different herbicide site of action on a crop such as small grains, says Lee Briese, an Edgeley, North Dakota, Centrol crop consultant. Rotating different herbicide action sites can forestall resistance in herbicides, he adds.

Diversifying away from corn and soybeans isn’t easy for Corn Belt farmers. These crops are often the most profitable and have a huge infrastructure that revolves around them.

“The marketing aspect becomes an issue,” adds Briese. “So is equipment. We’ve seen farms specialize in row crops, and they minimize their expenses as best they can, like any business would. But they end up getting rid of the pieces of equipment that they need for small grains or for a solid seeded crop."

When markets are found, though, diversity sparks costs savings and yields.

A multiyear Iowa State University study examined a corn-soybean rotation and showed that inclusion of these crops with small grain and legumes in three-and four-year rotations effectively suppressed weeds and reduced herbicide costs.

Per acre rotational herbicide costs in the ISU study were the following.

• Corn-soybeans: $28.18 
• Corn-soybean-oats/red clover: $18.17 
• Corn-soybeans-oats/alfalfa-alfalfa: $14.09 Data from a 2013 Dakota Lakes Research Farm rotational study at Pierre, South Dakota, showed the impact rotational diversity has on corn yields. 
• Continuous corn: 203 bushels per acre 
• Corn-soybean: 217 bushels per acre 
• Corn-corn-soybean-wheat-soybean: 235 bushels per acre 
The complex part, though, is creating markets for the additional crops. 

The U of I has hired two small-grain breeders to develop better wheat varieties that Illinois farmers can introduce into their rotation, says Davis. Interest also exists among wheat end users to increase markets for farmers, he adds.

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