A special local need registration has recently been approved for Prefar in Arizona that raises the use rate in lettuce from 4 to 6 quarts to 5 to 9 quarts. These higher rates will help reduce some of the variability in weed control that is characteristic of Prefar and will also increase the possibility of crop injury. Variable weed control of from 0 to 100% with Prefar can be frustrating and is often caused by three factors:1) weed spectrum, 2) water use and 3) timing. Weed spectrum. Prefar is active on a relatively small spectrum of weeds. These include most grasses, purslane, pigweed, goosefoot and lambsquarters. There are more than 50 weed species that are common in lettuce in this region and Prefar has no activity on most of them. It is normally used in combination with other herbicides to broaden this spectrum. Irrigation method and amount are also important variables that affect the performance of Prefar. Prefar is exactly opposite from Kerb in this regard. Prefar adheres strongly to the soil and it works best when large amounts of water are applied by sprinklers to push it down to where the weeds are germinating. It does not work well when incorporated with furrow irrigation or light amounts of water. Kerb does not adhere strongly to the soil and it works best with furrow irrigation or delayed applications applied just before or after weed germination to avoid leaching it below the germinating weeds. Time of application is another important variable affecting the performance of Prefer. Prefar only works on the roots of developing seedlings. It does not move in the plant and only stops cell division at the tip of the roots that it contacts. It must be in the soil before the weeds germinate. It again differs significantly from Kerb in this regard. Kerb does move in the plant and will still work on small seedlings of many weed species. Early postemergence applications can be effective. Partial weed control can be the result of all of these variables and the higher use rates now registered will help overcome some of this. If weed infestations are heavy, however, even 90% control can look like a complete failure without an untreated check . Lettuce has a good tolerance to Prefar at even higher rates than 9 qts. under ideal growing conditions. When any stress occurs ,this tolerance goes down. Stress can be from anything including environmental factors, pests, water, salts and many other variables. Heat and rain caused many problems earlier this season. Unfortunately, growers have a low tolerance for crop injury. This is a major reason why there are few herbicides registered for use on lettuce and this rate increase can be discontinued if many problems are reported. The SLN can be found by clicking the following link: Bensulide EPA SLN No. 120004

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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.