Tank mixing is the common practice of combining two or more crop protection products, such as herbicides, into a sprayer tank for a single application. This technique allows for the simultaneous application of multiple active ingredients, saving time and reducing the number of passes over a field. While mixing herbicides is possible, it requires careful planning and consideration to manage the inherent risks involved. The decision to combine products must be guided by a thorough understanding of how the chemicals will interact with each other and with the water carrier. Moving forward without proper precautions can lead to application failures, equipment damage, and wasted product.
Goals of Herbicide Mixtures
The primary motivation for combining different herbicides is to broaden the spectrum of weeds being targeted in a single treatment. A mixture can be designed to control both broadleaf weeds and grassy weeds, which might require two separate applications if only a single product were used. Another goal is to improve the overall effectiveness of the treatment through synergy. Synergy occurs when the combined effect of the two herbicides is greater than the sum of their individual effects, resulting in better weed control. Using herbicide mixtures is also a proactive strategy for resistance management. By combining two or more herbicides that have different mechanisms of action, the likelihood of weeds evolving resistance to both chemicals simultaneously is greatly reduced. The efficiency gained from fewer field passes also helps reduce soil compaction and the overall cost of application.
Determining Compatibility and Efficacy
Before any product is added to the tank, the first step is to consult the manufacturer’s product label. The label often lists specific approved tank mix partners, providing a reliable starting point for compatibility. Ignoring label instructions drastically increases the risk of both physical and chemical incompatibility.
Physical Incompatibility
Physical incompatibility is the visible problem where products do not dissolve correctly in the carrier solution. This can manifest as the formation of sludge, gels, or precipitates that settle at the bottom of the tank or separate into distinct layers. When this happens, the mixture becomes non-uniform and can severely clog screens, filters, and nozzles, halting the application process.
Chemical Incompatibility
Chemical incompatibility is a more insidious problem because the mixture may appear perfectly normal in the tank. This occurs when the active ingredients react with each other, resulting in either antagonism or phytotoxicity. Antagonism means that the effectiveness of one or both products is reduced, leading to poor weed control. Conversely, chemical reactions can cause a synergistic increase in potency, which can lead to phytotoxicity, or severe injury to the desirable crop or plant.
Essential Steps for Safe Tank Mixing
The most indispensable pre-mixing step is the Jar Test, which confirms the physical compatibility of the combination on a small scale. To perform this, mix the herbicides and adjuvants in a clear jar using the same ratio of water and products that will be used in the main sprayer tank. After stirring, the mixture should be allowed to sit for 15 to 30 minutes to check for any visible signs of separation, clumping, or sludge formation. If a uniform solution is not maintained, adjustments must be made before proceeding to the large-scale tank mix.
Once physical compatibility is established, the Mixing Order must be strictly followed to ensure a stable solution. The tank should be filled about halfway with clean water before any product is added, and agitation should begin immediately. Following the correct mixing order ensures that each product is fully dispersed before the next is introduced, preventing an undesirable physical reaction.
The standard mixing order is:
- Dry formulations (Wettable Powders and Dry Flowables) are added first, as they require the most time to fully dissolve and disperse.
- Liquid products follow, typically placing Suspension Concentrates before Emulsifiable Concentrates.
- Soluble liquids are added next.
- Adjuvants (surfactants, crop oils, or water conditioners) are added last.
Adjuvants are non-pesticide ingredients that enhance the herbicide’s performance, but they can significantly impact the stability of the mix.
Consequences of Incompatible Mixtures
Mixing herbicides incorrectly can lead to two major categories of negative outcomes. The most immediate consequence is an application failure, where the intended weed control is not achieved. This may be due to antagonism, where the chemical reaction between two products reduces the overall potency, requiring a costly reapplication. Alternatively, an unintended synergistic reaction can result in phytotoxicity, causing irreversible damage to the non-target crop. The second major consequence involves damage to the application equipment itself. Physical incompatibilities cause the formation of solids or gels that clog various parts of the sprayer system, including the nozzles, filters, and hoses. Clearing these blockages is time-consuming and can lead to expensive repair or replacement of components.