The question of whether to combine Roundup (glyphosate) with 2,4-D is common, and these herbicides are frequently mixed together. Glyphosate is a systemic, non-selective herbicide that works by inhibiting an enzyme pathway in plants, controlling a wide spectrum of annual and perennial weeds and grasses. In contrast, 2,4-D is a selective, synthetic auxin herbicide; it mimics natural plant growth hormones to specifically target and disrupt the growth of broadleaf weeds. Tank-mixing these two distinct herbicides is a strategy employed to achieve a wider range of weed control than either product offers individually. Combining different modes of action enhances overall efficacy across diverse weed populations.
The Rationale for Combining Glyphosate and 2,4-D
The primary reason for tank-mixing glyphosate and 2,4-D is to broaden the spectrum of weed control in a single pass. Glyphosate is highly effective on grasses and many broadleaf species, but it can struggle to control certain hard-to-kill broadleaf weeds that are more susceptible to the auxin activity of 2,4-D. The addition of 2,4-D provides superior control for weeds like field bindweed, dandelions, clover, and certain species of pigweed, ensuring a more complete burndown of vegetation.
This combination is particularly useful in managing weeds that have developed tolerance or resistance to glyphosate alone, such as glyphosate-resistant horseweed or Palmer amaranth. By attacking the weed with two different chemical mechanisms simultaneously, the mixture increases the likelihood of a lethal dose. The goal is a synergistic effect, where the combined activity is greater than the sum of the herbicides applied separately.
However, a scientific consideration when combining these products is the potential for antagonism, which can occur with certain weed species. Research has shown that 2,4-D can sometimes reduce the uptake and translocation of glyphosate within the plant, particularly in grass weeds exhibiting low-level glyphosate resistance. This reduced movement can compromise the effectiveness of the glyphosate component, allowing some resistant weeds to survive the application. Applicators must carefully consider the weed species present and the level of known resistance to determine if the benefits of the tank mix outweigh the risk of antagonism.
Proper Mixing Procedures and Compatibility
Successful application of the glyphosate and 2,4-D mixture depends heavily on following the correct physical mixing procedures to ensure chemical compatibility in the spray tank. A common guide for adding different formulations is the W-A-L-E-S acronym, which outlines the sequential order for adding products to the carrier water. The tank should be filled at least halfway with clean water before any product is introduced to provide sufficient volume for dissolution.
The W-A-L-E-S order suggests the following sequence:
- Wettable powders
- Agitation (thorough)
- Liquid flowables or suspension concentrates
- Emulsifiable concentrates
- Surfactants or water-soluble liquids, such as many glyphosate formulations
This order is designed to allow the hardest-to-dissolve components to fully suspend before adding the next, preventing undesirable chemical reactions or clumping.
It is important to prevent concentrated 2,4-D and glyphosate from coming into contact with each other, as this can lead to a physical incompatibility issue known as “salting out.” This reaction forms a thick sludge or precipitate that can clog filters, lines, and nozzles in the application equipment. Adding one product and allowing full mixing and agitation before introducing the next, especially with at least fifty percent water volume, helps mitigate this risk.
Before mixing a large batch, users should always perform a small-scale compatibility or “jar test.” This involves mixing small proportional amounts in a glass jar and observing the solution for approximately 15 minutes to check for signs of incompatibility. Signs include the formation of flakes, gels, or distinct layers. The specific formulation of 2,4-D—whether an amine salt or an ester—can also influence its compatibility and should be checked against the product label for guidance.
Safety Considerations and Avoiding Damage
Because the combination of glyphosate and 2,4-D creates a potent herbicide solution, strict adherence to safety protocols and environmental precautions is necessary during mixing and application. Personal Protective Equipment (PPE) is mandatory, and users must wear chemical-resistant gloves (such as nitrile or neoprene), along with chemical goggles or a face shield when handling the concentrated products. Long-sleeved shirts, long pants, and chemical-resistant footwear are also required to protect the skin from potential exposure.
The application of this mixture carries a heightened risk of damaging desirable plants outside the target area, primarily due to the characteristics of the 2,4-D component. Certain ester formulations of 2,4-D can be volatile, meaning the chemical can vaporize after application and drift as an invisible gas cloud. This drift can potentially damage sensitive crops like grapes, tomatoes, or ornamental plants far from the application site. To minimize the risk of physical drift, applicators must monitor wind speed and direction, avoiding application when conditions are too breezy or when susceptible vegetation is nearby.
The long-term management of weed populations is also a safety consideration. While tank-mixing multiple modes of action is a recommended strategy to delay herbicide resistance, relying repeatedly on the same two-way combination can select for weeds resistant to both chemistries over time. Therefore, this mixture should be integrated into a broader weed management plan that rotates different types of herbicides and incorporates non-chemical control methods to maintain effectiveness.