Effects of clipping and sheep grazing on dyers woad

Abstract

Dyers woad (Isatis tinctoria L.) is an introduced cruciferous forb that is rapidly expanding on intermountain rangelands and is apparently reducing production and regeneration of more desirable forage plants. Mechanical and chemical controls are expensive as well as having deleterious effects on nontarget species. Limitations to controlling this noxious weed by early spring grazing were investigated with clipping experiments at 1 site in the Wasatch foothills of northern Utah. These results were compared with actual utilization of woad by sheep at a poor condition example of that site. Significant mortality and reduction in reproductive performance occurred when at least 60% of the aboveground phytomass had been removed on or after 23 May. Clipping twice, to remove as much as 90% of aboveground tissue before 23 May, did not significantly affect woad mortality, percent flowering, or fruit production. Sheep did not graze this weed heavily enough at late enough dates to significantly affect mortality or seed production. Even on a poor condition range, these animals switched to other forages after about 15 May. Stocking rates required to restrict dyers woad at our clipping site would likely result in further range deterioration. More host-specific biological control agents should be examined.