Pesticide Interactions in Crops and Weeds
When crop plants and weeds are exposed to more than one pesticide, the effects may be described as: a) additive - when no interaction occurs and effects on plants are independent and predictable, b) synergistic - when the biological activity of the pesticide mixture is greater than the sum activity of its individual components, or c) antagonistic - when the biological activity of the mixture is less than the sum activity of its individual components. Interaction of pesticide combinations in crops or weeds may be due to an alteration in the uptake, translocation, or metabolism of one or more of the active ingredients. The response of crops and weeds to mixtures is highly species-dependent, so that a given mixture might be synergistic in a weed while showing no adverse effects on the crop or vice-versa.
Herbicide Antagonism
When two or more herbicides are mixed together, the result can be a reduction in the activity of one of the herbicides on certain weeds. This is known as herbicide antagonism. The most common example of this is the reduction in grass control that can occur in soybeans when postemergence grass herbicides (Assure II, Fusion, etc) are tank-mixed with postemergence broadleaf herbicides. The degree of antagonism is dependent upon the target grass weed and the herbicides applied. Antagonism rarely is a problem when volunteer corn or shattercane is the target grass, tends to occur to some degree for giant foxtail, and can be a severe problem when the target grass is yellow foxtail or a perennial such as johnsongrass.
While all postemergence broadleaf herbicides are capable of causing antagonism to some degree in soybeans, those most likely to do so are Pursuit, Classic, Basagran, Raptor, Scepter, FirstRate, Synchrony, and HarmonyGT. Tank mixes of postemergence grass herbicides with Pursuit are generally labeled for control of volunteer corn and shattercane only. Classic, Basagran, Synchrony, and HarmonyGT can be tank-mixed with grass herbicides for control of certain grasses only (including giant foxtail), and an increased rate of the grass herbicide may be required. Under conditions when grass plants are stressed and herbicide activity is reduced, the control of grass from Pursuit and Raptor can be reduced when tank-mixed with Cobra and other contact herbicides. Herbicide labels generally indicate the grass herbicide rates required and grasses controlled when combined with other herbicides. Antagonism can sometimes be reduced by using different spray additives or including nitrogen fertilizer solution or ammonium sulfate in the spray mix.
Antagonism between postemergence grass and broadleaf herbicides is most likely to occur when grasses are stressed due to cold or dry conditions and are not actively growing. Antagonism will also tend to be more evident when grass size exceeds that indicated on the grass herbicide label. To minimize antagonism, apply when grass size is well within label guidelines and do not tank-mix grass and broadleaf herbicides when conditions are not favorable for active plant growth. Applying herbicides separately is the most effective method for avoiding problems with antagonism. However, antagonism may still occur when the grass herbicide is applied too soon after the broadleaf herbicide. In general, allow 7 days after the broadleaf herbicide appliction before applying the grass herbicide. Where the grass herbicide is applied first, the waiting period is usually only a day or so. These intervals vary with the herbicides applied, so check labels for specific directions.
Herbicide - Insecticide Interactions
Herbicide-insecticide interactions are of special concern because they usually result in synergistic action and injury to crop plants. Crop injury results because some insecticides temporarily render crop plants unable to metabolize and detoxify herbicides that otherwise cause little or no injury. Application of some organophosphate corn rootworm insecticides (Counter, Thimet, Lorsban, etc) in combination with or followed by treatment with ALS inhibitor herbicides (Accent, Beacon, Exceed, Lightning, Option, Steadfast, etc.) can injure corn significantly. Symptoms of this injury can include stunting, yellowing, and a failure of the corn leaves to properly unfurl.
The severity of injury is dependent upon environmental conditions, the insecticide used, and the method of insecticide application. Injury is most likely when insecticides are applied in-furrow, rather than T-banded. The insecticides that tend to cause the most problems are Counter and Thimet, especially when applied in-furrow. Some herbicide labels prohibit application where Counter or Thimet has been or will be applied to corn, while others prohibit in-furrow application.
Most research indicates that injury from a herbicide-insecticide interaction is likely to be most severe when rain is adequate to ensure effective insecticide and herbicide uptake and activity. Some studies have shown that significant rain during the week prior to the postemergence application of an ALS inhibitor increases the severity of injury. Injury may be more likely when the corn plant is under stress from weather or a previous herbicide application. However, conditions suitable for rapid crop growth following injury will provide an opportunity for the crop to outgrow injury.
To avoid problems with herbicide-insecticide interactions, make sure the use of an insecticide is warranted based on scouting or cropping history. A preventative insecticide treatment is generally not needed in corn that is planted following any crop other than corn. Pyrethroid-type insecticides (Force, for example) do not increase the risk of injury from a herbicide, and can be substituted for organophosphate insecticides where use of an insecticide is warranted. Apply organophosphate insecticides as a band rather than in-furrow to minimize the risk of injury. See Table 6 for a list of restrictions on insecticide use for ALS-inhibiting herbicides.
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