Waterhemp Could Be The Thing That Pushes Midwest Into HWSC

Tall waterhemp plants in soybean field. Photo: Ohio State University

Tall waterhemp plants in soybean field. Photo: Ohio State University

In our cropping system, nothing disperses weed seeds quite as efficiently as a combine. While a small amount of seed may leave the field with the grain, most of it stays close to where it took form.

Combines readily pick up seeds from weedy patches and distribute them throughout the field, then carry some seeds to the next field where they can establish populations there.



Any newly acquired herbicide resistance goes along for the ride.

A relatively new focus of herbicide-resistance management now involves collecting those seeds during harvest and destroying as many as possible.

The phrase that describes this is harvest weed seed control (HWSC)., and it can take several forms. One high-profile approach involves modifying combines to prevent weed seed collected with the grain from spilling out the rear of the combine and building the next year’s weed population.

Learning From Down Under

In the U.S., researchers are evaluating Australian-type impact mills that grind up the chaff stream, which carries the bulk of weed seeds to the rear of the combine.

The retrofitted mills obliterate seeds or at least damage them to the point that they won’t germinate when the time comes.

Let me add that these mills aren’t an immediate option in the U.S. quite yet, although I suspect the concept will gain ground here.

At this point, the add-on mills aren’t available in our mainstream equipment distribution channels.

Beyond that, the development of mills has largely focused on cropping conditions in Australia – plenty of small grain production and a drier climate. Certainly, that also describes a sprawling amount of America’s wheat belt. But U.S. farmers also grow soybeans and corn. With soybeans, harvest starts before plants have thoroughly dried down, so how well will mills cope with green material?

These mills also draw horsepower from the combine, which might reduce running speeds. Plus, there’s the cost of the mill, itself, and prices seem to run $60,000 and up.

Cheaper Alternatives, Too

Again, I see a fit for the mills down the road. But in the meantime, simpler solutions already are at hand, based on how HWSC has evolved in Australia. With most combines, it’s relatively easy to modify the back end to prevent the chaff from going into the straw spreader.

Keep in mind that the chaff carries much of the weed seed collected by the combine. With minor tweaks, the combine deposits the chaff in a narrow, concentrated row behind the combine.

Inside that band, you will find most of the weed seed the combine collected, and it’s easier to deal with the seed in these tramlines compared to spreading them all over the field. Innovative farmers will figure out what to do with those seeds. In Australia, farmers either burn those narrow chaff bands to destroy seeds or leave them to compost between seasons.

This system of concentrating chaff is actually the dominant HWSC approach in Australia right now.

Waterhemp: It’s The One

In Iowa, waterhemp is the weed likely to drive the adoption of HWSC in one form or another. Researchers in the Midwest are already looking ahead to strategies that make the most of HWSC.

For starters, waterhemp must retain the majority of seed before harvest for HWSC to pay off.

Keep in mind that many weeds tend to shed seeds before the combine reaches them. The sooner a farmer can harvest grain, the less time a weed has to drop its first seeds on the ground.

I conducted a simple demonstration to evaluate seed retention on waterhemp growing in soybeans at Iowa State University’s Agricultural Engineering/Agronomy Research Farm near Boone. Here’s the approach:

We selected three waterhemp plants, ranging from 42 to 48 inches in height. Two trays went under each plant on Sept. 17

Click chart to enlarge Retention of waterhemp seed, Boone, IA, 2019. USDA Crop Progress Reports.

Every 7 days, we emptied the trays and sorted seed from other material the trays picked up.

After four weeks of seed collection (Oct. 15), we harvested the plants and collected and cleaned any seed remaining on the plants. I assumed that the traps captured 50% of the seed shed by plants.

We determined seed quantity by weight rather than counts, and seed loss was calculated by the cumulative amount of seed in the trays compared to total seed production.

Here is what I found:

  • Plants shed a little more than 20% of their seeds by Oct. 15, the final collection date (see chart).
  • The rate of seed drop increased over time, so delays in harvest would give plants more time to shed seed. By that reasoning, less waterhemp seeds would enter the combine where the mill could pulverize them.
  • Essentially, seeds that fell on the ground ahead of harvest have a chance to sprout and reproduce next year.

Weed scientists at the University of Illinois conducted more detailed research with waterhemp over three growing seasons. Seed retention declined to 80% on Sept. 24, Oct. 6, and Oct. 12 in the three years. To put that another way, 20% of the seeds already were in the seed bank by those dates.

One Extra Advantage – Minimizing Herbicide Resistance

Here’s another way to look at the benefits of HWSC…

The more seed a farmer destroys in the combine, the longer it takes for full-blown herbicide resistance to take shape in a field.



Researchers in Australia used computer models to simulate the benefit of HWSC in herbicide-resistance management. Capturing 50% of weed seed via HWSC delayed the evolution of resistance by 10 years compared to when all weed seed entered the seed bank.

These findings indicate that HWSC could be a useful tool for managing waterhemp here and throttling back the spread of resistance.

To put the need for seed collection into perspective, a full-season waterhemp plant that goes undamaged by herbicides can produce about 300,000 seeds. Under favorable conditions, the count might hit a million seeds. In a more typical situation, waterhemp emerges several weeks after soybeans are up and breaks through the canopy in August, so seed counts may be less dramatic.

In my evaluation with the trays, I estimate that each plant produced 20,000 to 50,000 seeds. In a soybean field with one plant like that every 10 feet of row, the waterhemp seed count could hit about 50 million per acre.

So, the imperative is to destroy as many seeds as possible.

The continued evolution of herbicide-resistant weeds is driving the need for alternative management tactics. HWSC is a tool that can be incorporated into our current production system easier than many other strategies.

Research at Iowa State University will evaluate how to mesh this new strategy with Iowa’s production system.

Source URL: https://crops.extension.iastate.edu/blog/bob-hartzler/waterhemp-seed-retention-harvest-weed-seed-control