Weed Seed Spread Is Accelerating Herbicide Resistance Across Farms

The most problematic weed species have an unmatched potential to spread. Left unmanaged, both waterhemp and Palmer amaranth can produce up to 1 million seeds per plant, according to Ohio State University Extension.

“Thirty years ago, weed control programs would cost $10–15 per acre,” said Josh Putman, a technical marketing manager for BASF. “Today, we are up north of $100 per acre, primarily due to the resistance and competitive nature of a lot of weed species.”

Understanding how weeds spread is the first step in building an effective control program and reducing the risk of herbicide resistance.

How Weeds Spread

Seeds are spread through a variety of methods, including wind, water, and wildlife.

For example, marestail seeds can be picked up by the wind once the plant has reached 3–4 feet, and viable Palmer amaranth seeds have been found in the stomachs of geese and deer. Farms located near rivers can see weed species from the south moved by the water current.

However, for the average farmer, weed seeds most often spread through everyday operations. Small seeded weeds, such as Palmer amaranth and waterhemp, are especially easy to spread unintentionally.

“Their seed is extremely small — about the size of the ballpoint tip in a pen,” Putman said. “That can get caught on the bottom of your boot in some mud. It can get in your truck tires, tractor tires, or sprayer tires. It can be transported in manure. It can be transported in cottonseed meal that gets sent from Texas onto dairy farms in Wisconsin and New York.”

Minimizing Seed Spread

The best ways to prevent weed spread are timely applications and proper use of herbicides. Be sure to follow all directions and restrictions on the label for best results.

“Timely applications can be difficult when you have a large number of acres to cover and the weather doesn’t cooperate,” said Karla Gage, a weed science and plant biology professor at Southern Illinois University.

Emerging technologies — such as The Weed Zapper, a tractor attachment that electrocutes late-season weed escapes — may reduce weed populations and risk of seed spread. These technologies have their own parameters for use, mainly that the weed plants need to be taller than the crop canopy. Early testing has shown these technologies to be more effective on pigweeds than grass species, Gage said.

If weed plants reach the reproductive stage, preventing natural seed spread is nearly impossible. However, some simple measures can reduce human-driven movement.

“One of the easiest ways to prevent weed seed spread is to start in your cleanest fields first,” Gage said. “Always start scouting or whatever you’re doing in the cleanest fields, and then move to fields where you have weed control issues. That way, you’re not spreading weed seeds as you move through your whole operation.”

Harvest Weed Seed Control

Weed seeds can spread substantially at harvest.

“If you have a field with shattercane problems, for example, you might spread that to other fields with the combine,” said Matt Geiger, an agronomy service representative for Syngenta. “It’s really hard to clean a combine out. It’s good practice to get in there with a leaf blower or power wash, but not always practical.”

In fields with substantial weed problems, consider introducing a harvest weed seed control method. These often involve destroying weed seeds to prevent building a weed seed bank.

• Chaff lining: This is the cheapest harvest weed seed control method. A metal chute on the back of the combine is used to separate the chaff fraction from the grain fraction. This directs the weed seeds to a location where they can be selectively controlled, usually with a follow-up herbicide application.
• Chaff tramlining: Similar to chaff lining, this involves placing the chaff fraction in the combine’s tire tracks. The repeated compaction can reduce seed viability.
• Impact mills: A more expensive option, this often involves retrofitting the combine with a seed impact mill. The chaff fraction is directed into a chamber, where seeds repeatedly ricochet, cracking the seed coat and reducing viability.

Herbicide Resistance Concerns

Prolific weed spread is particularly concerning as herbicide resistance grows. Palmer amaranth and waterhemp are of particular concern due to their common reproductive method.

“The unique thing about these species is that there are male plants and female plants in the field,” Southern Illinois University’s Gage said. “This helps them evolve very quickly because there has to be a genetic exchange of information to produce seeds.”

Herbicide resistance is a natural mutation with a one- in-a-million chance of occurring. While those would be favorable odds in most cases, with pigweed species, it’s a losing battle.

“Palmer amaranth can produce half a million seeds per mother plant,” Gage said. “It only takes one out of that half a million with the genetic mutation to build a herbicide-resistant population. It’s a probabilities game, and these plants are winning by producing so many seeds.”

Managing Herbicide Resistance

A zero-tolerance policy for weeds is the best method for managing herbicide resistance and prolonging the life of available herbicides.

“If you let weeds emerge before spraying postemergence, there’s a likelihood you’re going to find a weed that has the resistance mechanism to that particular chemistry, it sets seed, and produces offspring,” Syngenta’s Geiger said. “Letting weeds come up before trying to kill them is the fastest way to reduce the effectiveness of our available chemistries.”