Tough (pyridate) and Pursuit (imazethapyr) were evaluated in several trials in growers' fields and at the Hyslop research farm near Corvallis during the 1988-89 season. Both crop safety and broadleaf weed control were evaluated. Trials at several locations during the previous season indicated that these two herbicides were relatively safe on small-seeded legumes such as clover, alfalfa, and trefoil. Because dinoseb has been banned from agricultural use by the Environmental Protection Agency, no herbicide is registered for use in seedling clover that will control such composite-family weeds as mayweed chamomile (Anthemis cotula) or common groundsel (Senecio vulgaris). No herbicides that are effective on broadleaf weeds are registered in crimson clover at any growth stage.
Tough controls weeds through the foliage and has a very short soil life, so crop rotations will not be affected by its use. Pursuit acts through the foliage and the soil and remains active for several months, which is a concern for certain sensitive crops grown in rotation with clover. Neither herbicide alone will control all broadleaf weeds in a typical Willamette Valley clover field. Pursuit is effective on many weeds including those in the mustard family such as shepherdspurse (Capsella bursa-pastoris), but is weak on most weeds in the composite family.
Research to date has concentrated on developing rates and timings of application that are safe to the crop and effective on the weeds. Older plants, whether weeds or crops, tend to be more tolerant of these materials, so early application is necessary to obtain weed control, but the crop must be old enough to escape serious injury. In general, weed control was best in plots treated with both herbicides, depending on the weed spectrum at a given site. Seed yield also tended to be highest in plots treated with both herbicides, but only in those treatments where the two materials were applied in two separate timings. Tank-mix applications produced lower seed yields because the herbicide treatment was relatively ineffective since treatment had to be delayed to avoid crop injury.