Herbicide Contamination in Compost

Herbicide Contamination in Compost featured image

By Tammy Churchill

If you have livestock on your homestead, odds are you purchase hay or straw, or both. Do you know what chemicals are used in the growing of those products? If it’s been treated with a broadleaf herbicide (brand names include Grazon, Tordon, and Picloram), it’s considered safe for your animals; however, if your manure management plan includes returning their waste to your fields either fresh, aged, or composted, it could stunt or kill any plants outside of the grasses family.

This might affect you even if you don’t have livestock. Many people bring in manure to top dress their fields. If you do that, it’s vital to know the history of the food they ate, which might not be easy information to get.

How the Herbicide Spreads

Many farmers and ranchers use herbicides to keep weeds out of their fields. These chemicals can be designed to kill only specific types of plants, leaving the desired species untouched. In the case of pasture land, grasses are immune to the herbicides used.

These herbicides will get washed into the soil, where they can live for anywhere from a month to several years depending on conditions.

These chemicals also can survive on the grass itself, even through an animal’s digestive process and composting. It has been found that the herbicides remain active in stored compost even longer than in soil.

Signs of Contamination

While many of the signs of herbicide contamination are generic – low germination rates, the death of young plants, and decreased yields – the telltale sign is the curling and twisting of the leaves. The Oregon State Extension office has an excellent picture of a plant affected by herbicides.

Because so many of the symptoms are common to multiple issues, many people have only discovered what caused their low yields by noticing that sections of their garden where they hadn’t added compost (or manure or straw as mulch) were the only ones that thrived. 


If you want to test materials for herbicides before spreading them on your fields, it’s pretty simple to do. You’ll need a series of small containers – I recommend a dozen or more to help ensure another factor isn’t involved – and some bean seeds. Bean seeds are recommended because they are highly susceptible to herbicides. Grow a bean seed in each container until they all have three true leaves. 

At that time get a bucket and fill it with straw/hay, or fill halfway with aged manure, then add water to fill it to the top. Let it steep for 8 hours or so, but not more than 24 hours or you will grow some bad bacteria that might affect the test. 

Label half your containers as control and the other half as test. Then water your test containers with the water from your bucket, using regular water for your control plants. Wait a few days. Do all your plants look the same? Congratulations! The straw, hay, or manure doesn’t contain broadleaf herbicides. 

If the treated plants die, you will want to make sure not to use the contaminated material on your garden or fields. The hay or straw could still be used for feed or bedding. Afterwards, it would need to be used on pasture land or otherwise disposed of away from where you are growing anything other than grasses or grains.

You can test compost by using the above method, but instead growing the seeds directly in the compost instead of soil. You will want to make sure that the material is done composting before attempting this test. As I mentioned in the article on using chicken manure as fertilizer, this is the same test for seeing if compost is ready to be applied to the garden. If you no longer see any identifiable bits of the starting material in the compost, it’s at ambient temperature, and it has cured (rested) for at least 4-6 weeks, it should be ready for the test. 

Remediating the Issue

Should you discover that you have inadvertently introduced herbicides to your soil, what do you do? The Montana State Extension office offers the following five options for remediating Grazon contamination. They range from least to the most amount of effort and expense.

  1. Do nothing. With this option you would need to grow your sensitive plants in a non-contaminated area while the herbicides in the soil break down. This could take years, depending on the environmental conditions at your homestead. After time has passed, you can use the test above to see if the soil is still contaminated before planting non-grass crops there again.
  2. Speed up the breakdown of the herbicide with microbes. The microbes in your soil can break down the herbicides for you. In order for these microbes to thrive, they need three things: food, oxygen, and water. The herbicide is the food. You’ll need to till the affected area and water it. This will provide a microbe friendly environment and thus accelerate the breakdown of the herbicide. You might need to repeat this process multiple times.
  3. Plant a cover crop. Certain plants will absorb the herbicides in the soil. These plants would need to be fully removed from the soil and disposed of away from sensitive areas (and not composted!). The Montana State Extension Office recommended sunflowers, oats, peas, radishes, corn, and wheat.
  4. Apply a carbon-rich soil amendment. Charcoal and biochar bind with the herbicides making them inactive. Both of these options have a tendency to increase the pH of the soil, which might be an issue for certain crops. 
  5. Remove the contaminated soil. You would need to properly dispose of the old soil and bring in fresh.


How do you prevent herbicide contamination? It’s all in sourcing your materials. Hay and straw suppliers should be able to provide you with the information of what was used in the growing of the crop. If you are buying from a third party, it gets harder. If there are legumes included in the hay mix, odds are strong that no broadleaf herbicide was used. 

The mention of legumes brings up a quick side note about Roundup: While there is Roundup-ready alfalfa, current science shows it would be okay to use treated organic matter in your compost pile. 

According to the National Institute of Health, as well as numerous university extension offices, glyphosate breaks down much faster than the broadleaf herbicides, typically in under 112 days from time of application. If it had been used correctly, by the time it leaves your compost bin, there should be more than enough time for it to have been neutralized.

When getting manure or compost, it’s much more difficult. You might be told the manure is okay because the feed didn’t hurt the animals, but unfortunately that doesn’t mean there isn’t herbicide still in it. Sourcing from organic farms would be safe, but they can be hard to find. 

If you can’t find out about the possibility of the herbicides in the potential soil amendments in advance, I would recommend testing it before applying it. The testing takes time, but not nearly as much as remediation.

Homesteading is hard work. Sorry to add another thing to think about to your never ending list, but prevention in this case is by far your best defense.

More on composting

Composting with Chicken Manure: A Beginner’s Guide
Composting Without a Bin
DIY Composting Bins
Best Bins for Composting at Home
Compost: My Only Fertilizer
Rabbits and the Home Garden

If you love listening to podcasts, check out these composting episodes on our podcast.

No-Waste Composting with Michelle Balz
How to Compost a Dead Goat

Tammy Churchill is a Master Composter and enjoys sharing the joys of sustainable living as an Eco Team Docent at the Solana Center for Environmental Innovation. She spends her time spoiling her hens, wrangling worms, and creating gardening gold out of food scraps, yard trimmings, and chicken poop.

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