It is important for premergent herbicides to be at the right place at the right time to work. The right place is around the weed seeds and the right time is when these seeds are germinating. The three preemergent herbicides used in lettuce are Balan (benefin), Prefar (bensulide) and Kerb (pronamide) and the time of incorporation will be different for each of them. The recommendations for Balan are to apply prior to listing the beds and incorporate mechanically within 4 hours. Balan does not incorporate well with water. The dinitroanaline herbicides which include Trifluralin and Prowl, are considered to be subject to photo degradation. Our studies have shown, however, that these herbicides can remain on dry soil for up to a week before significant lose occurs. If moisture becomes present through dew, rain or irrigation, the lose can be more rapid. If Balan is incorporated onto listed bedtops the chance of crop injury is increased. The Prefar label recommends that it be incorporated within 36 hours of application. Mechanical incorporation of prefar delutes the herbicide too much and it does not work well. It can be either chemigated or applied after planting and incorporated with sprinklers but it is important to apply as much water as possible to move the herbicide down to where the weed seeds are located. It does not move easily with water. It also does not incorporate well with furrow irrigation. Kerb differs considerably in how and when it should be incorporated. Kerb moves readily in most soils found in this area. Because of this, it should be applied and incorporated right before or after weed seeds begin to germinate. This is normally by chemigation from 1 to 10 days after sprinkler irritation has stated. Kerb needs at least 0.5 inches of water to incorporate well but no more than 1.0 inch or it can leach below the weed seeds.

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.