Weed science is one of the few agricultural disciplines where the focus is on killing plants. This involves not just "weeds" but can be any plant including most crops, depending upon the situation. A “weed” can be any plant that is growing where or when it is unwanted. Alfalfa that comes up in lettuce is a “weed” or conversely, lettuce that comes up in alfalfa is a “weed”. The challenge is selectively controlling (killing) all plants except the one that you want to grow. Weeds can be selectively controlled with herbicides, mechanical or biological methods. Using herbicides is commonly the most effective and efficient method. Selectivity with herbicides is relative and often dependent upon how the herbicide is used. Herbicides need to be in the right place at the right time and in the right concentration or they can either be ineffective or injure the crop. The concentrations at which an herbicide will kill weeds and not hurt the crop are sometimes difficult to establish. There are many interrelated variables that can affect this. We are currently working on a project to determine the levels of the three preemergence herbicides used in lettuce that will cause crop injury. We have conducted several field trials where we have applied from 1X to 8X rates of Pronamide (Kerb), Benefin (Balan) and Bensulide (Prefar). In some trials we have seen no crop injury at 8X rates and in other trials we have seen significant injury at the 1X rate of these same herbicides. Injury, or the lack of it, is almost always related to stress that the crop is experiencing from a variety of causes. Some of these causes od stress include weak seed, environmental conditions, pests (disease, insects, nematodes, weeds etc.), water stress, fertility stress and many other conditions.

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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.