Glyphosate is the most widely used herbicide in the US. It was discovered in the late 1960’s, patented by Monsanto in 1974 and came off of patent in 2000. It kills a broad spectrum of plants and was initially limited to non-crop and fallow use and was expensive. The use of Glyphosate increased significantly around 2000 when it came off of patent and genetically engineered crops with resistance to Glyphosate were being developed.

The popularity of Glyphosate is based on its broad spectrum weed control, lack of soil residual activity which does not limit what can be planted following use, low mammalian toxicity and its currently affordable price. Unfortunately, the heavy reliance on Glyphosate has led to an alarming increase in resistant weeds and shifts to tolerant weeds. Although no weeds in this area have yet to be documented to be resistant, weed shifts have occurred.

Glyphosate is a very broad spectrum, foliar applied and highly systemic herbicide. It is absorbed by the foliage and moves throughout the plant. It works by inhibiting the production of the EPSP enzyme that is needed for many phases of plant growth and development. It uses the same general mode of action as many other herbicides that inhibit amino acid synthesis. Most of these are ALS inhibitors such as Pursuit, Raptor, Express, Affinity, Sandea, Osprey and others. Glyphosate and Sulfosate (Touchdown) are the only ones that inhibit EPSP. It can persist in soil but is so tightly bound that it has no soil activity. Although it kills many weeds, its performance can be greatly affected by many factors. These include application variables and environmental factors as well as characteristics of the weeds. The factors affecting Glyphosate have been intensively studied, are interrelated and can be complex. A brief and simplified review of the most important of these factors follows. Click link to read full article: "Maximizing the Use of Glyphosate".

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To contact Barry Tickes go to: btickes@ag.arizona.edu.

For questions or comments on any of the topics please contact Marco Pena at the Yuma Agricultural Center.
College of Agriculture, The University of Arizona, Tucson, AZ.