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The Importance of Glyphosate Translocation   
Bob Hartzler

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December 22, 2006Dale Shaner, USDA-ARS Plant Physiologist, presented a paper at the recent NCWSS annual meeting that discussed factors that make glyphosate such a great herbicide (Shaner, 2006). He listed three things: 1) the site of action is extremely important in the plant, 2) glyphosate is translocated efficiently to sinks (rapidly growing areas), and 3) most plants are unable to metabolize glyphosate to non-toxic compounds. This article will focus on the second, translocation within plants.

Systemic herbicides are nothing new, after all, the discovery of the systemic phenoxy herbicides in the 1940's initiated the age of chemical weed control. However, glyphosate translocation is more efficient than most other herbicides and provides applicators greater flexibility in application compared to other products. The difference between glyphosate and other herbicides was illustrated in a review paper that summarized research investigating effects of application parameters on postemergence herbicides (Knoche, 1994). Reducting spray volume was more than twice as likely to cause a reduction in the performance of systemic herbicides (excluding glyphosate) as it was to increase activity (Figure 1). The activity of glyphosate, on the other hand, increased in 88% of the experiments when spray volume was reduced, and reducing the carrier volume never resulted in a decrease in activity. Contact herbicides decreased in activity at a slightly higher rate than systemic herbicides (excluding glyphosate) when spray volume was reduced.

Reductions in spray volume can affect herbicide performance in many ways. With glyphosate, low spray volumes can improve foliar absorption by increasing the concentration of the active ingredient and surfactant present in spray droplets. In addition, the quantity of antagonistic ions in the carrier is directly related to carrier volume, thus low volumes reduce the likelihood of an adverse response when hard water is used as a carrier.

The above factors are beneficial to glyphosate performance; however, a negative impact of low spray volumes is a reduction in target coverage. As the crop/weed canopy increases, the number of spray droplets, and therefore spray volume (assuming droplet size is held constant), needed to maintain target coverage also increases. However, glyphosate provides consistent control in dense vegetation even when applied with low carrier volumes using nozzles designed to minimize the formation of small droplets. Efficient translocation within plants allows glyphosate to compensate for the poor coverage that typically occurs with applications on developed canopies that often compromises performance of other herbicides..

Research at the USDA/ARS Southern Weed Science Research Unit demonstrates the unique ability of glyphosate to provide effective control when only a small fraction of a target weed is contacted by the spray solution. In one series of experiments pitted morningglory was grown in the greenhouse and allowed to climb on a stake until they reached a height of one meter (39") (Koger and Reddy, 2005). Glyhosate treatments included: 1) top third of plant contacted by spray, 2) middle third sprayed, 3) bottom third sprayed and 4) the entire plant sprayed. At the low glyphosate rate, treatments 2 through 4 provided 90 to 92% control of pitted morningglory, whereas when only the top third of the plant was sprayed the control dropped to 75%. At the high glyposate rate no difference among the treatments was observed.

The authors conducted a second series of experiments using 4-leaf pitted morningglory (Koger et al. 2004). Different levels of leaf coverage (0, 33, 66, and 100%) were obtained by wrapping leaves and cotyledons in alluminum foil. For example, in the 0% coverage treatment all leaves and cotyledons were covered with foil, leaving only stems and petioles available to be contacted by the spray solution. For 66% coverage, the cotyledons and 33% of each leaf would have been covered by foil. The rate of glyphosate applied had a much bigger impact on morningglory control than did the amount of weed contacted by the spray solution (Figure 2). At 0.75 lb ae/A, control ranged from 31% when only stems and petioles were sprayed to 40% control with complete coverage. At the high rate, morningglory control averaged 88% control and control was only reduced when all leaves and cotyledons were protected from the spray, but 76% control was still achieved in this situation. The authors concluded that rate selection was more important than spray coverage in obtaining control with glyphosate.

This research demonstrates the ability of glyphosate to compensate for poor coverage by efficiently moving to the plant's growing points. Although many other herbicides are systemic, none are as efficient at moving in the plant as glyphosate. For example, acceptable control of purple loosestrife with triclopyr (Garlon) could only be achieved with good coverage of the lower stems and leaves, and high carrier volumes were required to achieve this coverage (Katovich, 1996). This mobility of glyphosate in plants is what allows glyphosate to effectively control large weeds and be applied in ways (low spray volumes, large spray droplets) that result in control failures with other herbicides.

Literature cited
Katovich, E.J.S., R.L. Becker and B.D. Kinkaid. 1996. Influence of nontarget neighbors and spray volume on retention and efficacy of triclopyr in purple loosestrife. Weed Sci. 44:143-147.
Koger, C.H. and K.N. Reddy. 2005. Glyphosate efficacy, absorption, and translocation in pitted morningglory. Weed Sci. 53:277-283.
Koger, C.H., D.H. Poston and K.N. Reddy. 2004. Effect of glyphosate spray coverage on control of pitted morningglory. Weed Technol. 18:124-130.
Knoche, M. 1994. Effect of droplet size and carrier vollume on performance of foliage-applied herbicides. Crop. Prot. 13:163-178.
Shaner, D. 2006. An overview of glyphosate mode of action: Why is it such a great herbicide. NCWSS Proceedings 61:94.


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.