Pyridine Contaminated Compost - A Tough Lesson

Last fall I was so excited to find a source of composted sheep manure nearby.  Raising sheep is becoming a rarity in our part of North Dakota and so, therefore, is sheep manure.  Sheep manure is my compost of choice because when a sheep ingests weed seeds, their digestive tract renders those seeds sterile before they pass through.  You can safely use composted sheep manure without worrying about an infestation of new weeds.  They, cannot, however, render herbicides harmless.

Potato leaf curling from moderate herbicide poisoning

And, thus begins the story of the alien plants in my garden this year.  My soil was absolutely beautiful this year when I went to plant.  All that compost made it loose, rich, and black.  I was looking forward to a great gardening season.  In spite of cool, dry weather, my potatoes, peas, carrots, beets, and beans quickly sprouted.  Then, a few weeks later, I started to notice that my potatoes were looking a little bizarre.  Instead of nice healthy green leaves, they started to show curling edges on the leaves.  Then, the peas started to look sickly and dried up. Some potatoes that I planted a little later for seed trials for our state agricultural university never did develop leaves.  They sprouted stalks with a "fiddlehead" on top of them.  The beans and some of the tomatoes began to look a little withered.

Fiddlehead growth on potato indicating more severe poisoning

Even more strangely, the damage was limited to the south end of the garden, and even there it was spotty.  Two rows of potatoes were affected and two were not.  The corn, carrots, cucumbers, squash, onions and cabbage on the north end of the garden were growing great guns.  I immediately thought of herbicide damage because the curling leaves and the fiddlehead growth are pretty characteristic of that.  Except that I haven't done any spraying and my vegetable garden is far enough away from anyone else's property that spray drift is highly unlikely.  It was a mystery!

Severely deformed potato plant from herbicide poisoning

So, I contacted our local Extension horticulturalist and he was able to confirm my worst fear:  the manure is definitely contaminated with pyridine herbicide.  Pyridine herbicides are used to control broadleaf weeds in pastures.  The active ingredients are aminopyralid, clopyralid, or picloram and the herbicides are marketed under the names Milestone, ForeFront, Confront, Redeem, Stinger, Imprelis, Grazon, Surmount, and Tordon.  They are widely used because they don't harm the grasses and are not absorbed by the grazing cattle.

The herbicide passes through the animal and is excreted unchanged in the urine, which mixes with the manure and contaminates it with the herbicide.  Composting does not destroy the herbicide and it can remain active in the manure for several years.  Farmers who use any of the pyridine herbicides are supposed to disclose that when selling hay or composted manure.  But, oftentimes, farmers buy hay from many sources, or if they harvest hay from the road ditches, they may not even know that the herbicide has been used on the hay or the pasture.  That was the situation with the farmer who furnished the composted manure to me.

I had asked if he used any herbicides on his sheep pasture when we made the deal for the manure and was assured he does not.  However, I've now found out that when they cleaned the corrals, they mixed horse manure in with the sheep manure, and the horses ate hay that had been baled from road ditches.  That's likely where the pyridine herbicide came from.

One possible reason that the damage in my garden is spotty is that the contaminated horse manure was not mixed in with the sheep manure, and so when we applied it to the garden, the contaminated manure was concentrated in certain parts of the pile.  It also appears that the worst damage is where we piled the composted manure on the garden before spreading it and tilling it in.  It makes sense that there is a higher concentration of the manure where the piles were dumped than in those areas that it was spread to.

Normal potato foliage

Anyway, I guess I won't be growing my own potatoes for sure, and time will tell what else will be affected and how badly.  The horticulturalist believes it is not a severe contamination, since it didn't kill things outright.  That's probably the good news.  The bad news is, that I don't know how much of my garden is really contaminated or how long it will take the herbicide to be rendered harmless.  And, I've learned a valuable lesson.

In the future, I know that before acquiring or using manure- fresh, aged, or composted- I need to be more diligent about getting the right answers to questions such as:
• What were the animals fed?
• If they were fed hay, what was the origin—on site or purchased?
• What, if any, herbicides were applied to the animal's feed and when?

Pyridine contamination can also happen when using contaminated hay as mulch in your gardens.  Pyridines are not to be used on lawn grasses anymore, but that doesn't mean they aren't, so using grass clippings for mulch or putting them in the compost bin if they come from sources where you don't know what might have been used to control broadleaf weeds is risky.  Grass clippings from golf courses are highly suspect because their turf is often sprayed heavily to control weeds.

Then, before spreading manure from any source of which I am unsure of herbicide use, I will do what's called a "pot bioassay".  I'll take samples of the manure from throughout the pile, being sure to get some from deep inside the pile.  I'll mix that 1:1 with potting mix containing fertilizer and put it in pots.  Then, I'll plant some sensitive plants, such as beans or peas and see how they do.  If they germinate and grow at least three sets of true leaves that are normal, the manure is not likely to be contaminated.

To deal with the damage that's already done, I'll pull out and destroy all the affected plants.  Then, I'll keep the soil tilled and moist throughout the summer.  The more exposure to air, light, moisture, and heat, the quicker the herbicide will dissipate. Later this fall, I'll plant an annual grass cover crop that is not sensitive to the pyridine herbicides and till that in before it goes to seed.  These actions will help remove the herbicide from my garden soil, but I will need to test using the pot bioassay method before planting vegetables again.

It may be a few years before I am able to use that garden spot again.  Thankfully, we still have one unused acre for a new vegetable garden.   But, you might run into me at the Farmer's Market this year.