For all of recorded history, and perhaps for the entire history of agriculture, animal manure has been used as a fertilizer to improve plant productivity. While some manure is commercially composted and bagged, it is not unusual for stables and small scale animal operations to offer free or low cost manure to home gardeners.
In recent years, however, using manure in the home garden has become a dicey proposition, and it’s not because of any change to how the animals are cared for: it’s how the hay they eat has changed.
Almost all hay pasture in this country is sprayed with broadleaf herbicides to reduce weed competition and improve the quality of the hay. These herbicides don’t kill the grass, just the thistles and other weeds, some of which can be dangerous to the animals who eat the hay. In response to the desire of pasture owners to spray their fields less often, an new class of pyridine carboxylic acid herbicides was developed, in particular aminocyclopyrachlor, aminopyralid, clopyralid, and picloram. Fewer herbicides get sprayed less often, which consumes less petroleum products and energy and is safer for the farm workers.
In many situations, these herbicides can remain active for a very long time, and they do not break down in the digestive tract of animals fed hay sprayed with these herbicides. So it’s safe for the animals to consume (and safe for us to consume the animals), but their urine and manure contains the persistent chemicals. Which brings us to:
Because these chemicals break down in aerobic conditions, this means that it may take years to break them down in low oxygen conditions like manure piles and compost piles. And while the label directions specifically state the herbicides are not to be sprayed on any hay or straw that will enter the compost stream, there is no real enforcement and frugal gardeners usually take advantage of “waste” product that wasn’t intended to be compost anyway.
And it’s not just manure. Bales of straw purchased from retail locations or scavenged by the side of the road might also have persistent herbicide residue. In the presence of persistent herbicides, most of our commonly grown garden plants will not germinate, be severely damaged or even die.
Scientific testing batches of hay and straw or manure is too expensive and time consuming to be an effective strategy. So what’s a gardener to do?
How to Protect Your Garden
- Ask the supplier. This isn’t a great solution, because unless the provider of manure grows and tends all of their own hay, which is uncommon, they probably just don’t know how it was raised.
- Inspect the hay and straw. Sometimes hay is grown with desirable forbs mixed in. If you see broadleaf plants like clover mixed in with the hay, you know it hasn’t been sprayed recently enough for the herbicides to still be present.
- Perform a bioassay. Collect aged manure in the late summer and fall. Mix it with garden soil and plant seed of a vulnerable plant like peas or beans in it. It the seeds sprout and grow in a healthy fashion to 8-10 weeks, the manure is safe to use. Spread in your garden in the winter and plant in the spring.
- Pile it up and age it. If you have contaminated manure and the space to start a big compost pile of it, you can age the herbicide residue out of it. You can speed up this process by turning the pile regularly, but it may take 3 years of aging to become safe to use. Periodically perform a bioassay to check the status.
- Spread it on a future plot of garden land. Spread as a thin layer over an area of land you hope to garden in the future. In the presence of sunlight and oxygen, the herbicides will break down more quickly, and the fertilizer goodness of the manure will be working its way into the soil.
Until and unless there is cheap and affordable testing or a tracking system developed to provide users with information about the origin of hay, straw and manure, gardeners will need to be vigilant and assume these are “dirty” unless proven otherwise.
Want more information on persistent herbicides? Check out these sources: