Compost Magazine

Composting tips, advice and science.

Manure on a blue tarpaulin before being added to compost.

Manure in Compost: A Cautionary Tale

A few years ago, a man knocked on my house door when I was at work.

My wife answered.

‘I’ve got a trailer full of shit here for you.”

(It was a big trailer.)

She wasn’t very happy.

When she asked me why I hadn’t asked her first, I asked her if she would have agreed.

‘No’, she said.

‘That’s why I didn’t ask,’ I told her.

And yes, I was in trouble.

The thing is, if you live near a farm, a trailer full of manure can be a quick way to get a lot of fertility into your garden. At just £40 ($55) for a huge trailer full, it’s also seriously cheap.

I spent the weekend happily wheelbarrowing the manure down to the garden.

I piled it into a big heap where the raised was going to be.

Manure in raised bed.

I then let it rot down over the winter, putting the leftover manure in my compost heap.

The experiment was so successful that I decided to repeat it the next year with my next raised bed.

The trouble was, nothing would grow in it for months.

Or rather, it would start, but then just stop.

Pesticides in manure and compost

I’m guessing the farmer had fed his livestock with hay treated with a herbicide such as Aminopyralid or Clopyralid.

Manufactured by the Dow chemical company, these herbicides are made for use on grassland to kill broad-leafed plants.

Unfortunately, it binds to the plant material from which hay is made. It can pass through animals’ digestive systems, and even the composting process, without breaking down.

The RHS estimates it can take two years for the herbicide to break down in manure. However, when added to the soil bacteria will start to break it down.

Regulations forbid farmers from selling manure from animals fed hay treated with herbicide. That hasn’t stopped the problem. Farmers may not know hay has been treated with herbicide, or they might not know the rules.

I’ve read dozens of reports from gardeners who have had similar problems.

And as I wrote this post, ABC news reported 200 gardeners lost their entire vegetable crop from one batch of compost.

The chemical is also now found in some garden herbicides and may be in some bought compost.

Even if the compost you buy is tested, some herbicides can cause problems in the garden at a few parts per billion. Not all labs can detect pesticides at that level.

(Another reason to make your own!)

The symptoms include:

  • stunted growth
  • pale/narrow/distorted shoots
  • cupped leavers
  • fernlike growth
  • veining on foliage

You can see examples in the video below.

Identify growth problems caused by aminopyralid residues in hay and horse manure

It makes growing very difficult – you’re essentially treating your plants with weedkiller.

What can you do about herbicides in compost?

Herbicides do break down over time and your plot will eventually return to health.

There may also be things you can do reduce the damage it causes.

A study by Singh et al looked at using activated charcoal to reduce the damage caused herbicides to vegetable.

The results were encouraging, with some crop damage reduced by nearly 80%. However, the exact results depended on both the crop grown and the herbicide causing the issue.

The RHS also recommend digging or rotavating the ground several times. This should speed up the breakdown of the residue, but that’s not really an option for no-dig gardeners.

If you are a no-dig gardener, and you have simply spread the compost over the top, it may be worth removing it.

You can choose to grow vegetables which are less affected by the herbicides while the pesticide breaks down. These include, according to Dow, sweet corn and brassicas.


These foods are almost certainly in the food supply. I don’t know how bad it is for us, but I am glad that my family (and, I am sure, many readers) grow many of our own vegetables.

I now get my manure from only one place, a local farmer who grows his own hay and doesn’t use pesticides.

If you don’t have this option, it might be best to avoid using manure. If you use manure, make sure you get it from a reputable source. You might also want to test manure compost by growing a few plants in it before using it en-masse.

Broad beans, peas and tomatoes are particularly susceptible and make good testing plants.

There’s a petition to have the herbicide banned which you can sign here. If you are in the UK can report problems with manure to Corteva here or to the Health and Safety Executive here.