Humans have used animal manures to fertilize food crops for thousands of years. Manures are an organic source of plant nutrients and are often a waste byproduct that must be properly managed when raising animals. Today, many farmers and backyard gardeners continue to use animal manures to provide nutrition to their crops. However, a recent experience at our local extension office brought to our attention the need to know what else, besides nutrients, is in the manure used.
A local backyard gardener brought in samples of tomato plants that had strange new growth. She had purchased the tomato plants, along with other vegetable plants, from a local nursery. When she repotted the tomato plants into larger pots, she added horse manure from her own horses to the soil mix. She then noticed this strange growth on the tomatoes, but not in the other vegetable plants that were repotted without adding horse manure. Herbicide damage was one of the first potential causes we suggested, since the new growth was twisted and distorted, a common symptom of plants that have been sprayed by herbicides. The gardener was sure she had not sprayed any herbicides near these plants, or in the pasture where she keeps her horses.
Photos of the tomato plants were shared with other NW District agents and an agriculture agent with livestock and hay producer experience had the probable answer – herbicide damage due to the horses being fed hay from a hayfield that was treated with a particular herbicide. Interestingly, this agent also had experience with these symptoms after their neighbor had similar issues using manure to fertilize the garden.
Herbicides with the active ingredients picloram or aminopyralid are able to cause this kind of unexpected damage to many gardeners’ crops. Herbicides containing these active ingredients are used in hayfields to control broadleaf weeds. These herbicides are especially effective at controlling hard to manage weeds such as thistle, nightshade, and nettle. They also provide long-lasting weed control. Unfortunately, the persistence of these ingredients extends into the hay, and also persists in the manure and urine of animals who eat hay from treated fields. These ingredients pass through the animal unchanged and remain active as an herbicide. Since many vegetable crops are broadleaf plants, the herbicide’s ingredients cause injury.
So what can a farmer or backyard gardener do to prevent this problem? When purchasing hay for livestock, ask the seller if they know whether the hayfield has been treated with herbicides that contain either picloram or aminopyralid. Most herbicides are known by their common names, rather than their chemical name. If they give you a common name or brand name, the active ingredient can be obtained by contacting your local extension office. If the seller can’t tell you, then, as a precaution, do not use the manure to fertilize broadleaf vegetable crops. The same question should be asked if purchasing hay for mulch as well. Composting the manure or hay does not break down the active ingredient, and may even concentrate it.
While we continue to use animal manures to fertilize our crops as our ancestors did, it’s important to remember that many of the tools and products we use today are much more advanced. These advanced products require that we stay informed of all precautions, use them responsibly, and, in this case, inform end users of any precautions. Remember to always read and follow the label and ask questions. And if a science-based answer is what you’re looking for, your local extension office is a good place to go!