Herbicide Carryover

Herbicide Carryover

Posted by: Darrin Parmenter, County Director, CSU Extension in La Plata County

For many gardeners, an application of livestock manure to the compost pile is a rite of autumn. Manures add organic matter and nitrogen to improve soil quality and vegetable production. There are good reasons to be cautious about routine manure use.  Heavy applications lead to salt and phosphorous buildup and fresh manures (less than 100 days old) may contain dangerous e. coli bacteria.

A new concern has emerged. It resembles a childhood poem and begins, “This is the garden that Jack made.” The final stanza goes, “This is the herbicide sprayed on the field that grew the hay ate by the horse that dropped the manure that lay in the garden that Jack made.” Picture Jack with a dead tomato plant.

Selective herbicides, such as the aminopyralids (brand name Milestone), clopyralids (Redeem and others), and picloram (Tordon),  used for the control of broadleaf weeds in grass-hay production and pastures, have the ability to survive cutting, baling, storage, and feeding, as well as time spent in piles of manure and compost. When these are applied to the garden, the herbicides may still be active and adversely affect subsequently planted broadleaf plants. Unfortunately, most of our important food crops are broadleaf plants. Symptoms of herbicide carryover damage to sensitive crops (beans, peas, tomatoes, potatoes, and others) are twisting, curling, and rolling of the leaves, especially the younger ones.

Manufacturers inform users of the problem on their product labels. The herbicides do not harm grass or animals that feed on them, and aged manures can be safely returned to grassy fields. But because the chemicals can persist for extended amounts of time, hay producers should disclose their herbicide selection to livestock owners. In turn livestock owners should disclose herbicide residues in manures given or sold to farmers and gardeners. The alternative, crop failure, is a liability risk for all.

If you cannot confirm their source of manure is free of this chemical, look for another source or improve your garden soil without any manure at all.Or if you are set on using manure in your garden, then performing a simple bioassay may be the easiest, cheapest, and even most effective way to determine the presence of herbicides: 

• Materials include: test material (compost, manure, topsoil), potting mix (compost-free), washed plastic pots and pea seeds.

1. Set up control pots by filling 3 pots with potting mix.
2. Prepare test pots and label pots. Use material in question and fill 3 pots. If manure needs to be tested use a 2:1 manure:soil ratio.
3. Plant 3 seeds in each prepared and labeled pot.
4. Space plants in a random order with plastic saucers under each one. Space the pots far enough apart to avoid splashing when watering. 
5. Maintain consistent growing conditions.
6. Evaluate plant growth. If twisting, curling, rolling, or death of the leaves is witnessed, it would be advisable to NOT use that product. 

For more information, go to http://whatcom.wsu.edu/ag/aminopyralid/bioassay.html.