Prefar was developed by Stauffer and first registered for lettuce in 1968. It was acquired by Zeneca(ICI) in 1992 and sold to Gowan in 1996. It is one of the few organophosphates that are used as a herbicide. It is also one of the few remaining EC formulations and is registered for unusually large application rates as high as 9 quarts per acre. It has changed little over the years but our understanding on how to most effectively use it has increased. Prefar works by inhibiting cell division at the growing points of the roots and does not translocate or move upward into the rest of the plant. It only works on seeds that have germinated and keeps them from developing a root system. It will only be effective if the roots contact the herbicide soon after they have started to grow. It must be incorporated into the top inch or two of soil. The only way to incorporate soil applied herbicides is mechanically or with water. Mechanical incorporation appears to dilute this herbicide too much and it does not work well when this method is used. Incorporation with water maintains a more concentrated layer of herbicide where the weed seeds are germinating. Unlike Pronamide(Kerb), which leaches with water, Prefar sticks very tightly to fine textured soils and requires high volumes of water for incorporation. This is not as critical on sandy or course textured soils. Furrow and drip irrigation do not push Prefar down into the soil and do not work well for incorporation. High volumes of water applied by sprinkler irrigation is what works best.

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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.