Gramoxone / paraquat
Chemical Family: Bypirydiliums
WSSA Resistance Group: 22
Crop/non-crop registration: Non-selective, often used to control existing vegetation at planting in no-till systems such as corn, directed sprays in: apples, cherries, grapes, pears, plums established more than 1 year, inter-row spraying in strawberries, stale seedbed technique for vegetables and field crops, dormant season in established alfalfa, clover and bird'sfoot trefoil, between cuttings in established alfalfa, PRE-HARVEST in dry beans, sunflowers, soybeans, and potatoes, pre-harvest desiccation of potato vines, pas
Sensitive weeds: Non-selective, affects all green plants
Application method: PREPLANT, PRE, POST directed spray to avoid contacting leaves or bark of desirable species, PRE-HARVEST; safe on mature (non-green) bark of woody plants.
Absorption & Translocation: Rapidly absorbed by foliage and green bark and is rainfast 30 min after application;little or no translocation (any movement occurs only in apoplasm including xylem), thus foliar-applied paraquat remains in treated leaves.
Mode of Action: Disrupters of cell membranes.
Metabolic pathway inhibited: Paraquat accepts electrons from photosystem I and is reduced to form the paraquat radical, which in turn reduces molecular oxygen to form the superoxide radical; superoxide then reacts with itself in the presence of superoxide dismutase to form hydrogen peroxide; hydrogen peroxide and superoxide react to generate the hydroxyl radical; superoxide and, to a lesser extent, hydrogen peroxide may oxidize SH groups on various organic compounds within the cell; the hydroxyl radical, however, is extremely reactive and readily destroys unsaturated lipids, including membrane fatty acids and chlorophyll; the hydroxyl radical produces lipid radicals which react with oxygen to form lipid hydroperoxides; such lipid hydroperoxides destroy the integrity of cell membranes allowing cytoplasm to leak into intercellular spaces which leads to rapid leaf wilting and desiccation.
Basis of Selectivity: Non-selective, all green tissue is sensitive; less effective on species with very waxy cuticle and linear leaf shape such as yellow nutsedge.
Symptoms: Rapid wilting and desiccation beginning within several hrs of application in full sunlight; complete foliar necrosis within 1-3 days.
Residuality: Essentially none (<1month); will persist in organic material such as mulches or turf thatch; therefore do not reseed treated areas for at least 5 days.
Toxicity: Oral LD50 =112-150 mg/kg - very toxic! (GRAMOXONE EXTRA = 40-150 mg/kg - extremely - very toxic!); Restricted use herbicide because of high mammalian toxicity.
Additional comments: Apply when weeds are less than 15 cm (6") high; best results if application is made on a dull or cloudy day, or in the evening; more than 1 application per season may be necessary, especially for control of perennial weeds; inactivated by adsorption to soil particles; not available to homeowners at full strength because of toxicity concerns; a non-ionic surfactant or oil adjuvant is required for maximum efficacy; when soil moisture is low and relative humidity high, paraquat residues in desiccating potato vines may move to the tubers in response to basipetal xylem flow facilitated by water demands of the growing tubers.