Clomazone is a selective herbicide used in agricultural settings to manage unwanted vegetation. It belongs to the isoxazolidinone chemical family and functions primarily as a pre-emergent treatment. This compound helps control specific annual grasses and broadleaf weeds before they sprout from the soil. Its application aims to protect various crops by preventing early weed competition.
How Clomazone Works
Clomazone acts as a pigment inhibitor within susceptible plants, disrupting their ability to produce certain compounds. The herbicide specifically blocks the synthesis of carotenoids, which are natural pigments found in plants. Carotenoids are responsible for protecting chlorophyll, the green pigment necessary for photosynthesis, from damage caused by sunlight.
Without sufficient carotenoids, chlorophyll molecules become vulnerable to photodegradation when exposed to light. This breakdown of chlorophyll leads to a distinct visual symptom: the affected weeds turn white or bleached. This bleaching effect prevents the weeds from carrying out photosynthesis, leading to their death. The visible impact on weeds appears within weeks after application as seedlings emerge but fail to thrive.
The mechanism involves inhibiting the non-mevalonate isoprenoid synthetic pathway, which plants use to create carotenoids. This disruption prevents the plant from producing protective pigments. Consequently, even partial inhibition of carotenoid synthesis can result in significant chlorophyll degradation, especially under strong light conditions.
Common Uses in Agriculture
Clomazone is used in agriculture to protect various crops from weed infestations. It is commonly applied to fields growing soybeans, rice, cotton, sugarcane, and vegetables like pumpkins, peppers, sweet potatoes, and melons. It also protects crops such as peas, tobacco, corn, canola, asparagus, and cilantro.
This herbicide is effective against a broad spectrum of annual grasses and broadleaf weeds, including species like barnyardgrass, velvetleaf, and lambsquarters. Farmers apply clomazone as a pre-emergence or pre-plant incorporated herbicide, meaning it is applied directly to the soil before the crop is planted or weeds emerge.
Applying clomazone to the soil ensures direct contact with germinating weed seeds or young seedlings. This method provides early-season weed control, reducing competition for water, nutrients, and sunlight that would otherwise hinder crop growth. Common trade names include Command, Gamit, and Caravel.
Environmental and Safety Profile
Clomazone is known for its volatility. The herbicide can vaporize from treated soil surfaces and drift off-target, leading to whitening symptoms in non-target plants, even those located a mile or more away. This off-site movement is a primary concern, particularly with certain formulations and application conditions.
To mitigate drift, micro-encapsulated formulations of clomazone were developed, which reduce vapor movement compared to older emulsifiable concentrate formulations. Factors such as high temperatures, strong winds, and elevated soil moisture can increase the potential for volatility and off-target effects. Regulations often specify application restrictions, such as avoiding use near sensitive areas or when wind speeds exceed 10 mph.
Clomazone has a soil half-life ranging from one to four months, depending on soil type and environmental conditions. It is stable to hydrolysis in various pH conditions and does not readily photodegrade in water or on soil surfaces. While groundwater contamination is unlikely, surface water contamination through runoff and spray drift remains a possibility.
Clomazone has low acute toxicity to mammals, with an oral LD50 for female rats reported at 1369 mg/kg. However, it can be moderately toxic to freshwater invertebrates (LC50 of 5.2 ppm) and highly to moderately toxic to estuarine/marine invertebrates (LC50 or EC50 ranging from 0.567-5.3 ppm). It is slightly toxic to freshwater fish (LC50 between 19-34 ppm).
Government agencies, such as the U.S. Environmental Protection Agency (EPA), regulate the use of clomazone. These regulations include establishing guidelines for safe application, setting maximum residue limits on crops, and outlining specific use restrictions. Such measures aim to balance effective weed control with minimizing environmental impact and protecting non-target organisms.