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Effect of Application Timing On Herbicide Efficacy
Bob Hartzler

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December 5, 2003 - 

Rule #1:  Small weeds are easier to kill than large weeds.    

One of the basic principles taught in crop clinics is that postemergence herbicides are more effective on small weeds than large weeds.  According to this rule, a herbicide applied to a two-inch weeds will provide better control than a herbicide applied to five-inch weeds.  Although the relationship between performance and weed size is not as strong with systemic herbicides (glyphosate, Steadfast, Clarity, etc.) as with contact herbicides (UltraBlazer, Flexstar, Buctril), it is still assumed that systemic herbicides will be more consistent when applied to small weeds.  Unfortunately, life isn't as simple as we'd like, and many other factors can be as important in determining how well a herbicide kills weeds as weed size.   This article will discuss factors that influence the activity of postemergence herbicides and may result in contradictions of Rule #1.

Unlike humans, plants can’t change clothes or move indoors in response to changing weather.  Thus their physiological status continually changes in response to weather conditions.  These changes may influence how the plant responds to herbicide applications, resulting in variable herbicide performance.  An experiment investigating the influence of velvetleaf size on glyphosate performance demonstrates that environment may play as important role as weed size on herbicide efficacy.  The experiment was conducted at three locations, and at each location a different timing provided optimum control (Table 1).  For example, at Vinton velvetleaf sprayed when it was 4” tall was controlled better than 8 or 12” velvetleaf, whereas at Ames the treatment made to 12” velvetleaf was best.  The results of this experiment suggest that other factors influence herbicide performance as much as weed size.

 Table 1.  Effect of glyphosate application timing on velvetleaf control at three Iowa locations1.

Velvetleaf height
at application

% Control2





84 b

82 c

99 a


86 b

96 a

92 b


98 a

89 b

94 b

1Hartzler and Battles.  2001.  Weed Technol. 15:492-496.
2Values within columns followed by different letter are significantly different at 0.05 level.

Environment.  Several environmental factors (temperature, moisture, solar radiation, etc.) influence a plant’s tolerance to herbicides, and interactions among these factors make it difficult to develop simple generalizations on their effects on herbicide performance.   Attempts to develop tools to aid farmers or custom applicators in determining the optimum herbicide rate or optimum time to apply products have been largely unsuccessful due to the complex interactions between plants and the environment.   In one of the more in-depth studies on the influence of environment on performance of herbicides in the field, researchers identified minimum temperatures in the week prior to application, soil moisture deficits in the ten days prior to application, and maximum temperature on the day of application as factors having the most consistent effect on herbicide efficacy.

Our limited understanding of how weeds adapt to environmental fluctuations restricts how we can use weather information to optimize herbicide applications.  While we are unable to predict specific effects of weather on the physiology of weeds and how they will respond to herbicides, we can predict when weeds will be less susceptible to herbicides due to stress.  Under these conditions herbicide rates should be increased or applications delayed until more favorable conditions occur

Time of Day.  Many farmers and commercial applicators have observed differences in weed control among applications made during the same day.  Research has documented that the performance of glyphosate and other herbicides may diminish late in the day as the sun sets and not return to optimum levels until the following morning.  The potential for this response varies among weeds, herbicides, environment and other factors. 

While the time of day effect on herbicide performance is not totally understood, research has identified one reason for this response.  Some plants have the capability of moving their leaves in response to the sun.  During the day the leaves track the sun, allowing them to intercept light more efficiently.  At night leaves if these plants tend to droop and point towards the soil surface.  Leaves of plants exhibiting this diurnal leaf movement intercept significantly more herbicide when applications are made during the day rather than when applied in the evening or early morning.   Thus, reductions in herbicide interception may be responsible for poor control with night applications in some situations.  However, fluctuations in herbicide activity among applications made at different times of day have been observed in weeds that don’t have diurnal leaf movement, thus other factors may be responsible for these responses to application time.  Where feasible, increasing herbicide rates when applications must be made in the evening or early morning may reduce the potential for control failures.

Applications made in the morning often encounter plants covered with dew.   Depending upon whom you talk to, you may hear that dew enhances or decreases herbicide activity.  Research on this topic suggests that under most situations dew should not greatly affect weed control.  If sufficient dew is present that the disturbance of the sprayer causes runoff of water from leaves it would be appropriate to delay applications until the leaves dry.  However, in situations where the leaves are only slightly damp it is likely the dew will have little influence on herbicide performance.

Summary    In addition to the numerous factors that affect herbicide performance, research has shown that the relative importance these factors varies among herbicides.  Weed scientists at the University of Minnesota evaluated factors that influenced the performance of glyphosate and glufosinate (Liberty).  For glyphosate, the rank of importance of factors influencing control was: 
 rate > temperature > weed height > adjuvant > % relative humidity > time of day > dew.   Fewer factors influenced glufosinate (Liberty) activity, and their order of importance was:  rate > temperature > time of day > weed height.  It is important to note that herbicide rate was the most important factor for both herbicides.

Plants are complex organisms that respond rapidly to the environment around them.  Slight changes in a plant’s physiological status may influence how susceptible it is to a herbicide.  Due to the complexity of the interactions between plants and the environment it is impossible to predict the precise rate required to control a weed at a particular moment.  Making applications while weeds are relatively small does not guarantee optimum performance, but there are important benefits to treating small weeds.  First, spraying small weeds reduces the risk of yield losses to competition between crops and weeds.  Second, it can be difficult to obtain uniform coverage of large weeds with the carrier volumes typically used for herbicide application.  This is especially true in situations with high weed densities or a well-developed crop canopy.   Finally, delaying initial applications until weeds are greater than six to ten inches tall may complicate the implementation of secondary strategies if the weeds survive the first application.  Because of these factors, timely herbicide applications made to appropriate sized weeds provide the most consistent weed control

Rule #2:  Nothing is as simple as we would like.

Prepared by Bob Hartzler, extension weed management specialist, Department of Agronomy, Iowa State University

For more information contact:
ISU Extension Agronomy
2104 Agronomy Hall
Ames, Iowa 50011-1010
Voice: (515) 294-1923
Fax: (515) 294-9985
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Common chemical and trade names are used in this publication. The use of trade names is for clarity by the reader. Inclusion of a trade name does not imply endorsement of that particular brand of herbicide and exclusion does not imply nonapproval.