Bromoxynil is a selective, contact herbicide used for controlling broadleaf weeds in various crops. Applied post-emergence, it affects only the plant tissue it touches. This compound helps farmers manage common annual broadleaf weeds that compete with crops for resources. This article examines the herbicide’s chemical nature, its effect on plant physiology, and its interaction with the soil environment.
Chemical Profile and Agricultural Use
The chemical structure of Bromoxynil is 3,5-dibromo-4-hydroxybenzonitrile, classifying it into the nitrile family of herbicides. It is commonly formulated as an ester rather than its pure form. These ester forms offer better solubility and stability, making the product easier to handle and apply as an emulsifiable concentrate.
Once applied, the ester rapidly converts to the active bromoxynil phenol form, which provides the herbicidal action. Bromoxynil is utilized as a post-emergence product, sprayed directly onto the foliage of actively growing weeds. It is registered for use across crops including wheat, barley, oats, corn, sorghum, flax, and onions.
The herbicide is effective against annual broadleaf weeds like wild mustard, cocklebur, velvetleaf, and lamb’s-quarters. Its selective nature allows it to control these weeds without harming the monocot crops it is used on. The EPA classifies Bromoxynil as a Restricted Use Pesticide, ensuring only certified applicators can purchase and apply it.
Physiological Effects on Target Plants
Bromoxynil interferes with photosynthesis in susceptible plants by inhibiting Photosystem II (PSII). PSII is a protein complex located within the thylakoid membranes of plant chloroplasts, responsible for capturing light energy and initiating electron transfer during light-dependent reactions.
The herbicide binds directly to the QB-binding niche on the D1 protein subunit of the PSII complex. By occupying this site, Bromoxynil blocks the flow of electrons from the QA to the QB electron acceptors. This blockade immediately halts the electron transport chain, preventing the conversion of light energy into chemical energy.
Halting the electron transport chain prevents the production of adenosine triphosphate (ATP) and nicotinamide adenine dinucleotide phosphate (NADPH2), the energy molecules required for growth. Disruption of electron flow also leads to the accumulation of excess energy, generating reactive oxygen species (ROS). These unstable molecules cause rapid destruction of cell membranes and cellular components through lipid peroxidation.
The visible effects appear quickly, reflecting the herbicide’s rapid contact action. Within hours of application, susceptible plants show initial symptoms like blistering or mottling on the foliage. This damage progresses quickly to necrosis (physical death of the tissue), often followed by chlorosis (yellowing) around the dead spots. Susceptible weed seedlings can become brittle and disintegrate within a few days.
Environmental Fate and Soil Dynamics
After application, Bromoxynil reaches the soil where several processes determine its environmental persistence and mobility. The primary mechanism for dissipation is microbial degradation (biodegradation). Soil microorganisms actively break down the herbicide structure, often incorporating the resulting metabolites into the soil’s organic matter.
The persistence of the herbicide is generally low, with the soil half-life averaging around seven days. This half-life can vary significantly depending on local conditions, with some studies reporting dissipation rates as fast as less than one day. Factors such as soil type, temperature, and moisture content influence the activity of soil microbes and the speed of degradation.
Other processes contribute to the compound’s breakdown, including photolysis (degradation caused by sunlight exposure). The ester formulations also undergo rapid hydrolysis, converting to the more water-soluble bromoxynil phenol form.
The potential for the herbicide to leach into groundwater is considered low because the compound tends to adsorb to soil particles, limiting its downward mobility. Bromoxynil also exhibits moderate volatility, meaning a small amount of the compound can move from the soil surface into the atmosphere as a vapor.