A nasty bit of chemistry - Picloram

Just ran into this cheery bit of news. Some of the herbicides that ranchers spray on their grazing fields are very long lived.

Here is just one story from Canadian magazine Northword:

Mean Manure & Killer Compost: Grazon after-effects in the Bulkley Valley
Cheryl and Les Harmati had been successfully gardening on their property outside Smithers for 25 years. But last year something strange was happening: plants in the garden and greenhouse were curling and wilting. The deformed growth was alarming—and mysterious. What had they done that was different from other years?

The previous summer, their neighbour had his hayfields sprayed with Grazon, a herbicide that selectively kills broadleaf plants without harming grasses. His cows grazed the treated hay later in the summer. That fall, in a gesture of neighbourly generosity, he dumped a couple tractor-loads of manure into their garden. In the spring, the Harmatis spread some on the garden and into the greenhouse soil.

It wasn’t long before their troubles began. “We didn’t know what was going on,” Cheryl recalls. “We finally figured it out by talking with others who had similar experiences and by researching online.” The manure—and now their garden and greenhouse soil—was apparently contaminated with herbicide.

Picloram is the active ingredient in the herbicide Grazon. Because of picloram’s persistence in soil—it breaks down very slowly and continues to prevent weed growth in subsequent years—it is favoured by many in the weed-control business. Here in northwest BC its main use of late has been in hayfields and pasture, prompted most recently by widespread infestations of the non-native and very invasive yellow hawkweed.

Although quite toxic to certain plants, picloram has very low toxicity to mammals, so cows and horses that consume sprayed hay appear to be unharmed. However, picloram is not broken down during digestion, and is excreted unchanged in urine and manure. Even composting does not break down the chemical, and in fact may concentrate it. Picloram is very potent: sensitive plants like potatoes, tomatoes and peppers are affected by concentrations of less than 10 parts per billion.

Emphasis mine. Yikes - I am going to have to test the hay I bought last summer. The article outlines how to run the test.

There is a lot more online including this PDF report from manufacturer Dow Chemical. None of it good.