August 14, 2009
Crop Salvage Issues
Steve Ensley, Clinical Toxicologist, ISU-VDL
Paraquat, a bipyridylium herbicide, has been proposed as a drying agent for hail damaged corn stalks. Common trade names of compounds with paraquat are Cekupat, Dextron X, Dextrone, Gramoxone, Gramoxone Inteon, Herbaxon, Herboxone, Pillarxone, Pillarquat, Total and Toxer. The labeled post grazing interval for Gramoxone Inteon varies from 70 days in new seedling alfalfa in California, 30 days in between cuttings in alfalfa in Iowa, to 7 days in field corn. Paraquat can be present in plant tissue for months after application if the plant material remains wet. As a result, residues if fed to cattle intended for slaughter or which may go to slaughter or dairy cows producing milk may exceed residual limits. In a study published by The Food and Agriculture Organization of the United Nations, in which goats were dosed at 100 mg/kg of the diet with paraquat, there was 9 ppb of paraquat in the milk, 120 ppb in the meat, 30 ppb in the fat, 560 ppb in the liver and 740 ppb in the kidney after dosing. The residual limit in meat of cattle is 50 ppb. If paraquat is sprayed at an application rate of 2 pints/acre and the yield/ acre is approximately 8 tons of dry matter the resulting dose that an animal consumes would be approximately 43 mg/kg of diet. EPA has placed paraquat in Toxicity Category I which is the highest of the 4 levels.
Paraquat is one of the most specific pulmonary toxicants known. Most cases of paraquat poisoning in domestic animals are due to ingestion of paraquat contaminated vegetation. While if used according to the manufactures instructions, it is a safe compound where little of the diluted compound will be absorbed topically or inhaled and aerosol particles generated are too large to reach the lower airways; excessive application can lead to intoxication. No cases of residual toxicity have been reported from ingestion of paraquat treated plants in either humans or animals but spraying a high moisture crop and then using it for feed is not a common practice. Storage of these crop materials either as haylage or silage, has the potential to maintain levels of the compound in a steady state.
If high moisture vegetation has been sprayed with paraquat it is possible for the compound to be present on and in the plant and prior to adequate dry down, the compound could still be present in concentrations elevated enough to cause a toxicosis. Once ingested paraquat is absorbed rapidly and reaches peak concentrations within 75 minutes. Once absorbed it is distributed to most organs in the body with highest concentration in the kidneys and lung. Once ingested a steady state is reached between the lungs and the blood, allowing continual renal excretion of paraquat for many weeks after ingestion if signs remain subclinical. There are 3 phases in the clinical progression of paraquat toxicosis. The initial phase observed is irritation from the caustic action of paraquat. The 2nd phase occurs 2-3 days post ingestion and is renal and hepatic failure. The 3rd phase is the development of pulmonary fibrosis. Once clinical signs commence, no antidote is available for paraquate intoxication, and supportive therapy rarely successful.