Factors affecting post-emergent herbicide efficacy (1)

Product efficacy is determined by many factors, not just the active ingredients or formulation. To get the best results from post-emergent herbicides, the following factors are critical;

Stressed weeds

Stressed weeds are harder to kill than healthy actively growing weeds. Stress is caused by lack of moisture, lack of oxygen due to water logging, extremes of temperature, nutrient deficiencies, insect pests, disease, sublethal doses of herbicides from prior applications or residues and mechanical damage from tillage, slashing or grazing.

Once a weed has been subject to stress it will not be adequately controlled by rates of herbicide that would otherwise be sufficient to control an unstressed weed, even after the stressed weed has apparently recovered from the stress. Never apply post emergent herbicides to stressed weeds.

Timing of application

Proper timing is critical for obtaining maximum performance from post-emergent herbicides. The herbicide label defines restrictions regarding application timing and thus is the most important consideration. The impact of application timing on effectiveness and prevention of competitive yield losses also needs to be considered. Post-emergent herbicides generally decrease in effectiveness as weeds increase in size.

The improved control provided by early applications must be balanced against the risk of late emerging weeds becoming established following application. While concerns over late flushes of weeds are valid, weeds that emerge soon after planting are the most competitive with the crop. The control of weeds that emerge soon after crop planting should not be compromised in order to allow later flushes of weeds to become established.

Rainfast period

Herbicide labels include recommendations on how much time must elapse between herbicide application and subsequent rainfall to ensure good herbicide performance. This is known as the rainfast period. Generally, herbicide rainfast ratings are based on good growing conditions. Poor conditions may require a longer interval between application and any rainfall to ensure adequate herbicide translocation within the weed before the herbicide is washed off.

For many herbicides, any amount of rainfall soon after spraying has the potential to reduce absorption, translocation, and subsequent weed control. If you apply herbicide and it rains before its rainfast, herbicide performance will be reduced. Rainfast period generally ranges from 30 minutes to 6 hours depending with the herbicide under consideration among other key factors. Product labels will indicate on the specific rainfast periods.

Effects of temperature on rainfast period

The likelihood of decreased weed control due to cool temperatures will vary, depending upon the target weed, herbicide, and rate applied. For example, glyphosate usually performs well under a wide range of temperatures. When the temperature is lower than 12°C, weed growth slows, resulting in slower herbicide uptake and translocation. This increases the required rainfast period and slows the onset of symptoms and herbicide efficacy.

Water quality

Quality of water used in spray tanks can affect herbicide efficacy. Water is the primary carrier for herbicide applications. In fact, it usually makes up over 99% of the spray solution. Considering that, it should be no surprise that the chemistry of water added to the spray tank greatly impacts herbicide effectiveness.


Turbid water or water containing suspended solids, soil or organic matter can reduce effectiveness of post-emergent herbicides. Water should be clean and clear for all pesticide applications however some pesticides are not as sensitive to turbidity as others.

Hard water

Hard water contains high levels of calcium (Ca), magnesium (Mg), sodium (Na), or iron (Fe). Other cations can cause hard water, but these are the usual suspects. Ca, Mg, Na, and Fe cations (positively charged ions) attach to negatively charged herbicide molecules. Often, the association between herbicides and these cations renders the herbicide ineffective. High pH and hard water act together to reduce herbicide effectiveness. High pH causes more of the herbicide to dissociate while high concentrations of cations bind with the dissociated herbicide to reduce its effectiveness.

Water pH

Water pH is a measure of the H+ ion concentration in water. As water pH decreases, it becomes more acidic and the number of H+ ions increases. Acidic conditions (pH 3 to 6) are generally suitable for mixing post-emergent herbicides classified as weak acids. When water pH exceeds 7, consider adding adjuvants to lower pH.

Weak acids dissociate less under acid conditions where H+ ion concentration is high. Dissociated herbicides are absorbed more slowly across plant cell membranes. Ideally, spray water pH should be low such that herbicides do not dissociate or dissociate at low levels. Avoiding herbicide dissociation is the primary reason water used in pesticide mixing should be acidic.

Role of spray adjuvants

Spray adjuvants are used with post-emergent herbicides to help overcome the barriers that impede movement of the herbicide from the leaf surface to the interior of the cell. An adjuvant is any substance in a herbicide formulation or added to the spray tank to modify herbicidal activity or application characteristics.  Some products are formulated with sufficient additives such that the user usually does not need to add them to the tank whereas other products require addition of adjuvants for all uses.

Adjuvants can enhance herbicide activity in several different ways. The effect of surfactants on the surface tension of spray droplets is well documented. The epicuticular wax on the surface of leaves repels water, resulting in beading of spray droplets as they land on leaves.  In some situations a high percentage of spray droplets may simply bounce off leaves, resulting in the herbicide falling harmlessly to the ground. Surfactants reduce the surface tension of spray droplets, increasing spray retention and allowing the spray droplets to spread over a larger area.  An increase in spray coverage is especially important with contact herbicides that do not move within plants.

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