What is the effect of sheep grazing for cover crop termination on associated biodiversity?

Targeted sheep grazing of cover-crops could potentially benefit agriculture by enhancing nutrient cycling, soil conservation, and pest management.  Because grazing represents an ecological filter, it is important to understand the impacts this practice may have on the associated biodiversity of agroecosystems.  We compared the effects of sheep grazing and mowing for cover-crop termination on plant and carabid beetle (Coleoptera: Carabidae) community structure at Towne’s Harvest Farm in Bozeman, MT.  Metrics for plant communities included plant diversity, weed biomass, and cover-crop biomass.  Metrics for carabid beetle communities, which are beneficial generalist predators in agroecosystems, included activity-density, species richness, and diversity.  In six 10 m × 15 m plots, we seeded a cover crop of buckwheat (Fagopyrum esculentum Moench), beet (Beta vulgaris L.), sweetclover [Melilotus officianalis (L.) Lam.], and pea (Pisum sativum L.).  We allowed the sheep to graze until plots had ≥ 90% of the biomass removed, which we determined by visual inspection.  For the mowing treatment, we mowed plots using a tractor mowing deck and ensured that vegetation within each plot was homogeneously cut.  Plant biomass samples were estimated prior to cover-crop termination and again one month post termination.  Carabid beetles activity-density was assessed throughout the growing season using pitfall traps.  We did not detect any significant treatment differences in plant biomass and diversity (P > 0.10) and carabid beetle activity-density and diversity (P > 0.37).  Our results suggest that sheep grazing for cover crop termination has a similar affect on associated biodiversity as that of mowing.  Thus, farmers choosing to implement sheep grazing for cover crop termination should not experience adverse changes in plant community composition or carabid beetle assemblages.