At a listening session on herbicide resistance, a Pennsylvania grower said “The cheapest way to control Palmer amaranth is to never get it in the first place.” We have heard similar sentiments spoken many times over the years, and for good reason.

Preventing weed infestations is an essential and cost-effective component of an Integrated Weed Management program. Prevention means avoiding the introduction of new weed seeds into a field through carriers like equipment, manure, and seed.

Particularly for herbicide resistant weeds, entry of these new species may cause abrupt, significant, and costly changes to management programs. Therefore, prevention measures that target the potential entry of new weeds to the field are often the most cost-effective way to keep weed pressure down. Even though some weeds are spread by natural means, some primary means of weed introduction are human-caused and preventable: farm equipment, crop seed, livestock feed, and manure.

Cleaning a combine before moving into a weed free field or once finished the harvest season is key to prevent weed seed dispersal. The Straw Bale Methodology helps combine deep cleaning. Photo Credit: Claudio Rubione, University of Delaware.

Ensure Weed-Free Inputs

Seed has the potential to be contaminated with weed seeds, and can become a potential vector for the spread of weed species to new farms and fields. This includes cash crop seed, cover crop seed, and conservation seed mixes. In some cases, herbicide resistant weed seeds have been transported over several states in this way. Producers can decrease or completely prevent the entry of new seeds by careful selection and monitoring of the inputs coming onto the farm. Crop seeds that have passed state certification programs are more likely to be weed-free due to the rigorous weed management requirements for certified seed. Therefore, sticking with only certified seed can help block the entry of new seeds.

A pigweed disposal bag given out by Penn State and the Pennsylvania Soybean Board. Photo Credit: William Curran, Penn State

Equipment Maintenance

Equipment that enters a field with mature herbicide-resistant weeds will easily become a vector for the spread of those weed seeds to other fields. Using proper precautions and thoroughly cleaning equipment after working in weed-infested fields can greatly reduce the spread of weed seeds to the next field.

Combines are a common cause of the introduction of herbicide resistant weeds to new fields. They’re a primary mode of weed seed spread across regions of the US including the Midwest, South and mid-Atlantic. Things that make them a vector are: they process all plants within a field, including weeds, during a time of the year when remaining mature weeds are producing viable seed; there are many small spaces within a combine where weed seeds might hide; they are time-consuming to clean.

Other pieces such as tillage equipment and tractors have also been found to carry weed seed from field to field if they are used when plants have matured and produced seed.

Because many problematic resistant weed species can produce between 50,000-1 million seeds per plant, even the spread of a few seeds can cause a serious and costly infestation within a couple of seasons.

Tips for preventing weed entry via feed and manure:

  1. Avoid purchasing feed or hay, or transporting manure, from areas where there are known infestations of invasive weeds. The product is likely to contain those weed seeds.
  2. Inspect purchased feed & hay for Palmer and waterhemp by taking samples and examining them.
  3. Do not feed contaminated grain or feed supplement.
  4. If contaminated grain must be used, first grind, roast, or ensile if possible.
  5. There is some evidence that ensiling crops containing weed seeds can destroy the seeds of some weeds if the temperature and time of ensiling are high enough. There is limited information available, mostly based on Australian weeds, about the success of these practices. The limited information that does exist suggests that ensiling may be more effective on grass weeds than broadleaf weeds. Refer to resources on ensiling to kill weed seeds, and your local extension agents, before relying on this practice.
  6. Similarly to ensiling, composting manure and plant matter may destroy weed seeds if the temperature and time are high enough. Composting must be done in a certain manner (creating large, dense, hot piles that are turned regularly) in order for temperatures to stay high enough to kill weed seeds. Refer to resources on composting, and your local extension agents, for tips on this practice before implementing it for weed control.


  • Kara Pittman
  • Annie Klodd
  • Claudio Rubione


  • Victoria Ackroyd
  • Michael Flessner
  • Barb Scott
  • Mark VanGessel
  • Steven Mirsky