Weed control at harvest, combines are ideal vehicles for spreading weed seeds

Gloria Hafemeister
Grain farmers gathered at the Nell Farm at Juneau to learn about the importance of removing weed seeds from combines before harvest, during harvest and after harvest.  2019 will likely be a challenging year for controlling weed seeds at the time of harvest.  Ryan Nell pointed out spots that need particular attention for cleaning out seeds.

JUNEAU - Weeds have always been a challenge on farms but this wet year they seem to be a challenge in many areas of the state. Herbicide-resistant weeds are an ever-growing issue, and prevention of the spread of those plants is important all year round, including at harvest time.

Weed control isn’t normally a priority during harvest, but combines are the ideal vehicle to spread weed seeds across a field or carry them to the next. 

Dan Smith, UW-Madison NPM Southwest Regional Specialist has done six regional workshops on the topic of weed seeds and combines. 

He says, “People are talking about a 40% loss in fields infested by weeds. At that point they need to make a decision whether to combine them or mow them off.”

The problem with combining, he points out, is that studies show that many weed seeds are mature at the time the field is ready to be combined.

He pointed to one study that indicated that 80% of the ragweed seed that was in a field was on the plant when the combine went through and spread it.  Mowing the field before the weed seeds mature may be less costly than dealing with the weeds in the future.

Giant ragweed can produce more than 10,000 seeds per plant. The seed is persistent and it takes two years to deplete the seed bank.

Waterhemp also retains its seed until harvest and then spreads with the combine.  Even a small percentage of waterhemp in a field will still present a problem.

Farmers attending the workshop at the Nell Farm at Juneau on Tuesday needed no convincing of the seriousness of weeds in the field at harvest time. Ryan Nell, host of the event, produced several large weeds found in his fields despite efforts to control weeds this year. Each of the weeds was loaded with tiny weed seeds that can be easily spread at the time of harvest.

Seeds are not only spread in the field during harvest but they can be carried into other fields when they build up in the combine.

Smith notes, “Smaller seeds accumulate in the front and larger seeds tend to make it back into the combine.”

He said the feeder house area on a combine is the most common place where weed seeds are found. The rock trap also gathers some. He said when researchers started the study regarding the importance of removing weed seeds from combines he never thought they would find so many weed seeds in every combine they looked at.

After running the combine empty for a self-cleanout, use a leaf blower or air compressor to remove material from exterior of the combine, focusing on the head, feederhouse, and axle and straw spreader at the rear of the machine.

He points out that a combine can hold almost 200 pounds of grain and material other than grain, even after the machine is allowed to run empty for several minutes. He recommends regularly cleaning storage bins, augurs and legs, transport vehicles, and farm equipment to prevent weed spread and cross-crop contamination. Destroy weed seeds that are separated from grain or left in the field to keep them from entering the soil seed bank.

Also taking part in the workshop was Joe Zimbric, recently hired Crops and Soils Agricultural Educator for Dodge and Fond du Lac counties.

He points out that lake associations have promoted the need to clean boats before moving them from one lake to another to prevent the spread of creatures and weeds.  The same is true for combines moving from one field to another.

He also notes, “There is a need to communicate between counties about any herbicide resistance that farmers are seeing. Herbicide resistance is a problem that will not go away.”

Several farmers questioned the other means of spreading weed seeds from field to field including livestock manure and wildlife.

The University of Missouri has studied what weed species are being consumed and transported by ducks and geese. The study found 9% pigweeds, 30% smartweeds, and 46% corn plants emerged from seed spread by geese. Similarly ducks distributed 24% smartweeds; 38% barnyard grass and 30% pigweeds.

As for livestock manure, Smith said pig manure is hot enough to kill weed seeds but cow manure is not unless it has gone through true composting.

He points out that Wisconsin tends to have fewer problems with weeds because of having more diversity in the cropping system. Alfalfa in the rotation, for instance, breaks up the weed cycle.

The effect of dairy manure regarding seed distribution varies from farm to farm. In some cases farms are feeding existing weed seeds by adding more nutrients to the soil. In other cases they are distributing weed seeds in the manure.

This year the potential is there for having more weed seeds in livestock manure because there are more weeds in the fields.

Bob Bird of the Dodge County Land Conservation Department says many farmers have received permission to feed emergency crops raised on prevent plant land. Many of those crops have weeds in them. Even hay fields that suffered winter kill in many areas are more infested with weeds than usual because of the harsh winter and wet spring.