Dicamba is a selective, systemic herbicide used primarily to control broadleaf weeds in agricultural and non-crop settings. It belongs to the synthetic auxin class of herbicides, mimicking natural plant hormones to induce uncontrolled growth in susceptible plants. The correct ratio of Dicamba per gallon of water depends entirely on three variables determined by the product label: the specific Dicamba formulation, the required rate per unit area, and the sprayer’s application volume. Calculating the right amount converts a required dose per acre into a concentration per gallon for your specific equipment. Precision is necessary because applying too little fails to control weeds, while applying too much risks severe damage to nearby sensitive crops and can result in legal consequences.
Determining the Required Area Rate
The starting point for any application is the product label, which specifies the amount of concentrated Dicamba product needed for a defined area, usually measured per acre or per 1,000 square feet. This area rate is the true dose of the active ingredient, not the concentration in the water carrier. Different Dicamba products, often distinguished by salt formulations like dimethylamine (DMA) or diglycolamine (DGA), have varying concentrations of the active ingredient. Application rates typically range from 1 to 2 pints of product per acre, but this rate is not interchangeable between brands due to concentration differences.
The required area rate must be adjusted based on the specific weeds being targeted. Smaller, actively growing weeds require a lower rate than larger, more mature weeds. Weed density also influences the application rate, as thick infestations may require the higher end of the label’s suggested range for sufficient control. The crop itself dictates the maximum allowable rate, especially in post-emergence applications on Dicamba-tolerant crops, where exceeding the labeled limit can cause injury or violate regulatory guidelines.
Failure to strictly follow the labeled rate is considered an off-label use, carrying severe legal and financial penalties. The maximum single application rate is often restricted to 0.5 pounds of Dicamba active ingredient per acre, with strict annual limits. Exceeding the rate or using an unapproved product can lead to substantial fines and may result in the suspension of an applicator’s license. The label is a legal document, and deviation from its instructions constitutes an unlawful application.
Understanding Application Volume and Calibration
The application volume, expressed in gallons per acre (GPA), is the total liquid the sprayer distributes over the treatment area. This volume is a characteristic of the application equipment and nozzle type, not the herbicide itself. Water serves only as the medium to carry and evenly distribute the chemical dose across the acre. Label instructions typically specify an acceptable range for application volume, such as 3 to 50 GPA for ground applications, and the applicator must choose a volume within this range.
Sprayer calibration determines the actual GPA delivered by the equipment under specific operating conditions. This is a practical, on-site measurement where the operator determines the volume of spray delivered over a known distance or time at a set pressure. For example, knowing that a sprayer applies 15 gallons per acre establishes the crucial GPA variable needed for the final mixing calculation. Using a lower GPA saves time and water but requires more concentrated mixing and greater precision for adequate coverage.
Higher application volumes, such as 20 GPA or more, are often necessary when targeting dense weed canopies to ensure the spray penetrates to the lower leaves and stems. The label mandates the use of specific nozzles designed to produce extremely coarse droplets that minimize off-target drift. Maintaining a consistent, lower spray pressure, often at or below 20 pounds per square inch (psi), is also required to control droplet size and prevent drift.
Calculating the Final Mix Ratio
Once the required dose per acre and the equipment’s application volume are known, the final step is calculating the amount of concentrated Dicamba product to add to each gallon of water. This converts the area-based dose into a volume-based concentration. The general formula for the mixing ratio is: divide the required Dicamba Rate per Acre by the Application Volume in Gallons per Acre (GPA). The result is the precise amount of Dicamba product needed per gallon of water.
For example, if the label mandates 1.5 pints of Dicamba product per acre, and your calibrated sprayer delivers 15 gallons per acre, the calculation is straightforward. Dividing 1.5 pints by 15 gallons equals 0.1 pints of product per gallon of water. Since liquid herbicides are often measured in fluid ounces for small volumes, convert this amount by multiplying by 16 (16 fluid ounces per pint). The final mix ratio is 1.6 fluid ounces of Dicamba per gallon of water.
For small-scale applications, knowing that one acre equals 43,560 square feet is useful. If treating only 1,000 square feet, the required amount of Dicamba must be proportionally reduced from the per-acre rate. Precision in these conversions and the initial calibration is paramount, as even a small error results in an incorrect chemical concentration. Always double-check calculations before mixing the concentrated product into the water carrier.
Mandatory Safety and Regulatory Compliance
Regulatory compliance extends beyond the correct mixing ratio due to Dicamba’s potential for off-target movement. Dicamba has a high risk of volatility, meaning the chemical can turn into a vapor after application and move significant distances on air currents. Newer formulations use specific salts to reduce this tendency, but the risk remains under certain environmental conditions. This volatility necessitates exceptionally strict application rules.
Physical drift, where small spray droplets are carried on the wind, is managed by mandatory requirements for specific coarse-droplet nozzles and maximum wind speed limits. Applicators are prohibited from spraying when wind speeds are below 3 mph or exceed 10 to 15 mph, depending on the label. Applications are also prohibited during periods of temperature inversion, a meteorological condition that traps fine spray particles and allows them to move long distances.
Compliance protocols mandate the use of buffer zones, which are untreated strips of land around the target area to protect sensitive crops. These buffer zones can be large, sometimes exceeding 240 feet downwind, depending on the product label. Applicators must wear appropriate personal protective equipment, such as chemical-resistant gloves, eyewear, and long-sleeved clothing, when handling the concentrated product and during application. All environmental and safety parameters must be met before the mixed solution can be legally applied.