Cow manure is a popular soil amendment, but in some cases it can cause unexpected problems in gardens and fields. Trouble can occur when cows have eaten hay or grazed pastures treated with certain weed killers, especially picloram or aminopyralid. These herbicides are designed to last a long time. They pass through the animal without breaking down and end up in the manure. Even after composting, the chemicals can still be active.

When this manure is added to soil, it can injure many common garden plants. Crops like tomatoes, beans, peas, peppers, and squash are especially sensitive. Flowers and fruit trees may also be affected. Plants may look twisted or stunted, grow unevenly, or fail to produce fruit. Because the damage can resemble nutrient problems or disease, the real cause is often overlooked at first.

What makes this issue difficult is that contaminated manure looks and smells normal. There are no warning signs, and people using it often have no reason to suspect a problem. Manure may come from farms with good management practices, where herbicides were used legally and correctly. Since manure and compost are rarely tested for herbicide residues, the problem usually isn’t discovered until plants start showing symptoms.

Once these herbicides get into the soil, their effects can last much longer than expected. Sensitive plants may struggle for months or even years. This can mean lost crops, wasted effort, and a lot of frustration. To reduce risk, gardeners and growers should ask about the source of manure and the history of the hay or pasture. A simple plant test, such as growing beans or peas in treated soil, can help catch problems early. When in doubt, choosing another soil amendment is often the safest choice.

1