Isoxaflutole is a selective herbicide used in agriculture for effective weed control. It manages unwanted plant growth in various crops, contributing to better yields and efficient farming practices. This herbicide is a tool in integrated weed management strategies, addressing challenging weed populations.
Understanding Isoxaflutole
Isoxaflutole is classified as an HPPD inhibitor herbicide, belonging to the isoxazole class of chemical compounds. It is often applied as a “pre-emergent” treatment, meaning it is used before weeds sprout, or as an “early post-emergent” application, targeting very young weeds. First marketed by Rhône-Poulenc in 1996, its development marked an advancement in selective weed management by offering control against weeds resistant to older herbicides.
How Isoxaflutole Works
Isoxaflutole functions by disrupting a specific biochemical pathway in plants. Once absorbed, it converts into an active compound, a diketonitrile (DKN) derivative, through the opening of its isoxazole ring. This DKN then inhibits the enzyme 4-hydroxyphenylpyruvate dioxygenase (HPPD). HPPD is essential for the biosynthesis of plastoquinone and tocopherol, precursors for carotenoid synthesis.
Carotenoids are pigments that protect chlorophyll from sunlight damage. When HPPD is inhibited, carotenoid production ceases, leading to chlorophyll breakdown. This results in a “bleaching” effect on developing plant tissues, followed by photo-oxidation and eventual plant death from excessive sun exposure. The parent isoxaflutole is readily taken up by plants from soil treatments, transforming into the active DKN, which is mobile within the plant’s vascular system, ensuring widespread disruption.
Agricultural Applications
Isoxaflutole is widely used in agricultural crops, including corn (maize), sugarcane, and isoxaflutole-resistant soybeans. In the United States, its annual use in corn was approximately 600,000 pounds (270,000 kg) in 2018. It controls a broad spectrum of weeds, including broadleaf species like Amaranthus, Datura stramonium, and ragweed, as well as annual grass weeds such as Alopecurus, Eriochloa, and Panicum species.
The herbicide provides broad-spectrum control and residual activity, meaning it suppresses weeds for a period after application. Its flexible application timing, either before weed emergence or in early growth stages, offers farmers adaptability in their weed management programs. Some isoxaflutole formulations are combined with herbicide safeners, such as cyprosulfamide, to enhance crop tolerance.
Environmental Considerations
Isoxaflutole’s environmental behavior involves its degradation and potential movement. The parent compound degrades relatively rapidly through abiotic hydrolysis and microbial metabolism, with a soil half-life ranging from 0.5 to 14 days, depending on factors like soil type, pH, and moisture. Despite this rapid degradation, its primary transformation products, such as diketonitrile (RPA 202248) and RPA 203328, are more persistent.
These metabolites are mobile and can leach into groundwater, particularly in permeable soils or where the water table is shallow. Research indicates that heavy rainfall can move isoxaflutole through the soil profile in a pulse pattern. Isoxaflutole and its metabolites have been detected as emerging contaminants and persistent pollutants in surface and drinking waters. Concerns also exist regarding its potential toxicity to certain aquatic species, although its impact on honeybees is considered practically non-toxic.
Human Safety and Handling
When handling isoxaflutole, potential human exposure routes include dermal contact, inhalation, and ingestion. Acute toxicity studies show isoxaflutole is minimally toxic via oral, dermal, and inhalation routes. It is not considered a dermal irritant, eye irritant, or dermal sensitizer. However, carcinogenicity studies indicate it induced liver and thyroid tumors in rats and liver tumors in mice, leading to its classification as “likely to be a human carcinogen” by the HED CPRC in 1997.
To minimize risks, applicators must adhere to safety precautions, including the use of personal protective equipment (PPE). This involves wearing long pants, a long-sleeved shirt, shoes, socks, and chemical-resistant gloves. Depending on the task and product formulation, additional PPE like protective eyewear, a face shield, or a full-face respirator may be required, especially when spills or splashes are a risk. Regulatory bodies, like the United States Environmental Protection Agency (EPA), establish tolerance levels for isoxaflutole residues on crops and in soil to ensure safe application.