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Time of day influence on Roundup activity
by Bob Hartzler

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October 18, 1999 - A decrease in Roundup performance when applications are made in the evening has been an issue the past two seasons.  While this response has been confirmed in research, the exact mechanism responsible for the decline in activity is poorly understood.  One possible cause for this response could be that evening applications result in reduced coverage of weed foliage due to diurnal leaf movement.  Many plant species alter the angle of their leaves during the course of the day, with leaves being extended horizontal during daylight hours and then dropping the leaves to vertical during the night.  In Iowa, velvetleaf probably would be the best  example of a weed species exhibiting this phenomenon.

Weed scientists at the University of Arkansas recently published results of an experiment investigating the influence of time of day on Roundup performance and diurnal leaf movements (Norsworthy, J.K., L.R. Oliver and L. C. Purcell.   1999.  Diurnal leaf movement effects on spray interception and glyphosate efficacy.  Weed Technol.  13:466-470.).  The following table provides results of three weed species included in this study.

Relationship between time of day and leaf orientation on effectiveness of Roundup on three weed species.  (Adapted from Norsworthy et al. 1999.)

      Roundup Rate
Weed species Time
of day
Leaf angle
(degrees from horizontal)
0.5 pt 1.0 pt 2.0 pt
Prickly sida 6 AM 25 7 48 75
  11 AM 15 28 60 90
  4 PM 2 20 72 90
  9 PM 32 12 50 65
Hemp sesbania 6 AM 70 3 15 40
  11 AM 15 30 75 80
  4 PM 5 32 82 92
  9 PM 80 5 10 22
Sicklepod 6 AM 80 10 85 100
  11 AM 10 80 95 92
  4 PM 15 90 100 98
  9 PM 90 30 30 50


Prickly sida had the least diurnal leaf movement of the three species, with leaf angles varying from 2 degrees off horizontal at 4 PM to 32 degrees at 9 PM.  Spray interception by leaves was reduced by 17% with applications made at night compared to day.  Only the 2.0 pt Roundup rate provided acceptable prickly sida control, and there was a significant decrease in activity with the 9 PM application compared to the two mid-day applications.  Both hemp sesbania and sicklepod exhibited large differences in leaf orientation during the course of the day.  The 0.5 pt Roundup rate provided 80% or greater control of sicklepod when applied during the day, whereas control was 30% or less when applied in the morning or evening.  Increasing the Roundup rate to 2 pt overcame the time of day effect in the morning, but the high rate applied at 9 PM provided only 50% control.  Spray retention on sicklepod leaves was reduced 70% when applied at night compared to the day.  A significant reduction in hemp sesbania control was observed at all Roundup rates when the herbicide was applied in the morning or evening.  Increasing the rate from 1 to 2 pt per acre at 9 PM imporved control from 10 to 22%, whereas at 4 PM these rates provided 82 and 92%, respectively.  Herbicide retention by hemp sesbania leaves was reduced 67% with night applications compared to day applications.

The research indicates that a decrease in spray interception due to leaf orientation may be responsible for performance problems in some species when Roundup is applied late in the day or early in the morning.  Prickly sida had the least diurnal leaf movement of the three species, and Roundup efficacy was the least responsive to application time with this species.  However, the research also shows that other factors are involved in this response.  For example, increasing the Roundup rate on sicklepod was able to overcome the influence of sicklepod leaf orientation with morning applications but not with evening applications.   There was little difference in leaf orientation at 6 AM and 9 PM in this species.

Many physiological responses are influenced by light and it is possible that changes in plant metabolic activity between the dark and light influence herbicide activity.  One research paper reported that the amount of glyphosate required to reduce the activity of the target site (EPSPS) was more than two times greater in the dark than in the light (Tokhver,-A.K.; Pal'm,-E.V. Light-dependence of the inhibiting action of glyphosate on the shikimate pathway in cotyledon leaves of buckwheat seedlings. Sov-Plant-Physiol. 33: 748-753.).   Many people cite the presence of dew on leaves as a problem with night or morning applications.  This may be important in some situations, but did not play a role in the above studies since the Arkansas studies were conducted in a greenhouse.

Problems with reduced control when Roundup is applied in the evening or morning are most likely to occur with species that have a relatively high level of tolerance to the herbicide.  In some cases the influence of time may be overcome by increasing the rate of herbicide; however, there are no good guidelines to determine when and how much to increase rates to overcome this affect.


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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