Yard & Garden Report

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Toxic manure

Know where your manure comes from. It may contain herbicide.

ManureAged manure and straw mulch are great for gardens—unless they are full of herbicide.

Pyridine herbicides are widely used to control weeds in pastures. Straw and manure from a pyridine-treated pasture are hazardous.

The herbicide can persist in straw for months or longer. When pyridine-treated grass is consumed by livestock, the pyridine can pass through the animal’s system without breaking down. When you add such manure to your garden you are adding the herbicide along with it. 

Tomato, potato, pepper, bean and pea are especially sensitive to pyridine, but other vegetables including carrot, lettuce, spinach and beet are also sensitive. Rose, dahlia, and annuals such as marigold and sunflower are sensitive.

Pyridine chemicals may break down in a few weeks or, in some cases, in a few years. Gardeners who suspect their soil is contaminated can test their manure by growing beans in pots containing a 1:1 mixture of the manure with potting soil. If the potted beans grow well, the pyridine has broken down to acceptable levels.

Another way to monitor for pyridine in your garden is to grow short rows of beans and peas scattered throughout the garden. Monitor for unusual growth (Davis et al., 2010).

If you get your manure or straw from a farmer, ask them about their use of herbicides. There are reports of suspected contamination of compost/manure from municipal recycling centers and garden centers in ND.

Written by , Extension Horticulturist, North Dakota State University. Photo courtesy of Malene Thyssen
Source: Davis, J. S.E. Johnson, K. Jennings and K. Yelverton. 2010. Herbicide carryover in hay, manure, compost, and grass clippings. North Carolina Cooperative Extension: Raleigh.

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