Smart gardeners often incorporate organic matter into their home gardens. It’s a good way to use material that might otherwise end up in a landfill and your garden appreciates the additional nutrients and tilth.
While this practice can be very beneficial, it is essential to know the source of your organic material. This is important not only when using livestock and horse manure, but also when incorporating hay or grass clippings.
Organic materials which contain herbicides from treated plants (or manure from the animals that ate the plants) can cause inadvertent damage in your garden. Affected plants usually show twisted, cupped, and elongated leaves; misshapen fruit; reduced yield; and/or death of young plants. Sometimes the damage is evident from the get-go with poor seed germination.
Some herbicides used to control broadleaf weeds can be long-lived and may remain active in hay, grass clippings, and manure even after they are composted. Some of these herbicides have a half-life of 300 days or more and at least one product is known to remain active in compost for several years!
The herbicides of greatest concern are picloram, clopyralid, and aminopyralid, but the list of products that cause at least some concern is too long to publish here. The garden plants that are most sensitive to these types of herbicides are tomatoes, potatoes, lettuce, spinach, carrots, peas, beans, dahlias, and some roses, but even less-sensitive plants can be impacted to some degree.
Most of these herbicides have a rotational crop restriction of at least 18 months for vegetable crops. So, if these herbicides are used you would need to wait this length of time to safely plant certain vegetables on that plot of soil again. This can vary from product to product and vegetable to vegetable based on their sensitivity to that product. This gives you an idea of the time frame that these chemicals are considered active
The real problems arise when the hay, manure, grass clippings, etc. leave the hands of the individual who applied the herbicides. If you choose to use any of these sources of organic material, you should find out what herbicides have been used on the grass the animals have eaten and how old the manure or compost is. A farmer you are considering getting the manure from could probably tell you this, but someone with a few horses might not know where the hay they bought for their animals originated from or with what chemicals it was treated.
Likewise, if you plan to use grass clippings you should find out what herbicides have been used on that turf to control weeds. If you do not know what, if any, herbicides were used, do not use the hay, straw, grass clippings, manure, or compost to grow sensitive crops. Assume that the organic material contains a long-lasting herbicide and compost them for at least 18 months for other crops.
Hate to say it, but sometimes you DO have to look a gift horse in the mouth. Happy gardening — Spring is just around the corner!