Adjuvant is a broad term for anything that is added to a herbicide that helps with performance or handling. Adjuvants can be added by the manufacturer for a variety of purposes such as improving solubility, shelf life, handling, compatibility, stability and other characteristics. Most people, however, think of an adjuvant as those products that they add to the spray tank. The most common type of adjuvants are surfactants or surface-active agents that are used to improve spreading and/or absorption of the applied solution. There are many other uses for adjuvants. These include deposition agents, drift control agents, anti foam agents, buffering agents, compatibility agents, water conditioners, tank cleaners and others.

Rarely are adjuvants used with soil applied herbicides. There has been interest, however, in using adjuvants with two of the soil applied herbicides used in lettuce to improve weed control. The intent is to use an adjuvant to either increase the movement of Prefar down into the soil or to reduce the movement of Kerb too far into the soil.

Prefar(bensulide) is normally applied to the soil surface and incorporated with water. It adheres very strongly to the surface and can be difficult to move down to the germinating weed seed in many fine textured soils. Some growers and pest control advisers have used non-ionic surfactants, crop oil concentrates or other specialty adjuvants to improve movement into the soil and have reported improved weed control. Results of our trials have been variable, however, and we have not been able to measure a consistent improvement in weed control from any of the adjuvants that we have tested.

Kerb(pronamide), on the other hand, does not adhere strongly to the soil and can often be leached below germinating weed seeds by irrigation water before they germinate. Adjuvants that are used to increase the adsorption of products to foliage and soil have been tested with Kerb to reduce leaching. Results have again been variable and inconclusive.

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

On April 26, 2010 the field day How Herbicides Work took place at the University of Arizona Yuma Agricultural Center. In this event Barry Tickes, UA Agriculture Agent and his students from the applied weed science class PLS 300 explained the different herbicide modes of action directly from the experimental plots. We are frequently so busy spraying our gardens and crops for weed control that we tend to forget how these products really work. By attending this meeting we have learned for example that the Seedling Root and Shoot Growth Inhibitors kill weeds by affecting mitosis, the process in cell division by which the nucleus divides causing what is called "pruned roots" in the treated plants, this makes weeds suffer from lack of water and nutrients. The students showed us Epinasty, a symptom caused by the Growth Regulators and explained how the weeds turn white by the action of Pigment Inhibitors. Tickes also showed the symptoms of an application of a Cell Membrane Disruptor which have mostly contact activity. He explained that these herbicides produce a dark green water soaked appearance on the leaves, which means that the cell membranes were destroyed and there is leaking of intercellular fluids, which causes necrosis. Are you interested in learning more?

You can watch the videos on How Herbicides Work by visiting the Vegetable IPM Video Archive located here: http://ag.arizona.edu/crops/vegetables/videos.html. This archive contains also videos on insect management, evaluation methods, and insecticide trials that are conducted by researchers at the University of Arizona Yuma Agricultural Center, and in the future will include plant pathology videos.

To contact Marco Peña go to: marcop@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.