Over the last three centuries, California’s semi-arid grasslands have experienced multiple changes in species composition, as native communities were replaced with exotic forage grass assemblages brought by settlers and then invaded by noxious weedy grasses with little forage value.  Today, these noxious grasses pose significant threats to native biodiversity and to the forage provisioning services the grasslands provide.   Two key invaders--Taeniatherum caput-medusae (medusahead) and Aegilops triuncialis (barbed goatgrass)--share a distinctive asynchronous phenology that allows them to utilize late-season soil moisture to reach peak biomass and set seed after resident annual forage grass populations have senesced.  To control these weeds, we tested five strategies in exclosures on working rangelands: (i) no clipping, (ii) fall clipping, (iii) spring clipping, (iv) early summer (May) clipping, and (v) herbicide application plus clover and vetch seeding in the preceding May.  We examined treatment influence on weedy and forage grasses, and on key forage forbs that grow with them: storksbill (Erodium), an early season genus of low forage value, and two late-season valuable forage groups--clover (Trifolium and Medicago) and vetch (Vicia).  Percent cover of grass and forb species was assessed in early spring and early summer 2009 using modified Daubenmire cover class ranges. 

Results/Conclusions

Early summer (May) treatments were the most effective at controlling weedy grasses, but are among the most difficult for ranchers to implement.  Fall clipping—a rancher-suggested alternative—was somewhat less effective but notably better than spring clipping in increasing forage grass cover. Treatments that benefitted forage grasses generally also benefitted forage forbs as well.  Fall clipping favored the two late-season leguminous forbs, clover and vetch, more than the poorer forage Erodium.  May clipping favored clover but also favored Erodium.  In contrast, both Erodium and weedy grasses were favored by spring clipping. In unclipped plots, vetch reached some of its peak cover, suggesting that reduced grazing could increase its cover in degraded areas.  Overall, these findings suggest that phenologically-based grazing management timed to promote forage grasses in this system may also promote desirable nitrogen-fixing forage forbs, despite the forbs’ later season phenology.  Release of resources otherwise usurped by the late-season weeds may promote increased growth of these forb species.  Fall clipping represents an easy-to-implement grazing strategy for ranchers for promoting desirable forage grasses and forbs while reducing Erodium.  Alteration of grazing timing can contribute to rangeland restoration and offers a cost-effective tool for improving stewardship.