Complicated Composting: Persistent Pyridine Carboxylic Acid Herbicides

This paper reviews pyridine carboxylic acid herbicide impacts on compost. Pyridine carboxylic acid herbicides are not completely broken down during grass growth, harvest and drying of hay, in the digestive tract of livestock, or during composting. These herbicides are a popular choice for broadleaf weed control because of this persistence: they remain effective for months or years. Pyridine carboxylic acids are also more effective than the common herbicide 2, 4-dichlorophenoxyacetic acid and can be applied to pastures with grazing livestock because they have low mammalian toxicity. The growth-inhibitory action of naturally occurring pyridine compounds has been researched since the discovery of α-picoline-γ-carboxylic acid in the early 1900’s. These pyridine carboxylic acid compounds mimic plant growth hormones called auxins, causing plants to grow abnormally and then die. Plants injured by auxinic herbicides have poor seed germination, twisted growth, cupped or enlongated leaves, misshapen fruit, reduced yields, and ultimately die. Picloram (4-amino-3,5,6-trichloropyridine-2-carboxylic acid) was developed by Dow Chemical Company as a systemic herbicide for herbaceous weeds and woody plants in rights-of-way, forestry, rangelands, pastures, and small grain crops. Clopyralid (3,6-dichloropicolinic acid) was also developed by Dow Chemical Company to control annual and perennial broadleaf weeds in crops and turf. Another Dow herbicide, aminopyralid (4-amino-3,6-dichloro-2-pyridine carboxylic acid), is used for broad leaf weed control in pastures. Aminocyclopyrachlor (6-amino-5-chloro-2-cyclopropylpyrimidine-4-carboxylic acid) is the first pyrimidine carboxylic acid herbicide and was developed by DuPont for weed and brush control on uncultivated non-agricultural areas, uncultivated agricultural areas, industrial sites, and natural areas. Clopyralid compost contamination was reported in 2000 at four different facilities including Washington State University. In Vermont, compost samples were tested and found to contain aminopyralid, clopyralid, and picloram in 2012. Across the U.S. since 2000, there have been many reports of apparent plant injury from compost contaminated with auxinic herbicides. Because of the limited testing facilities and expense of chemical testing, the majority of these reports remain anecdotal. If the history of a compost feedstock is unknown, bioassays are recommended to test compost for the presence of auxinic herbicides. Even though pyridine carboxylic acid herbicides are sold with proper labeling and restrictions, compost contamination is continuing. Adjustments should be made for the registered uses of these herbicides, and herbicide applicators need improved education about the implications of contaminating compost feedstock.