Efficacy of prescribed grazing depends on timing intensity and frequency

Authors: Rinella, Matthew - Matt; HILEMAN, BENJAMIN - MONTANA STATE UNIVERSITY

Submitted to: Journal of Applied Ecology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/26/2009
Publication Date: 6/2/2009
Citation: Rinella, M.J., Hileman, B.J. 2009. Efficacy of prescribed grazing depends on timing intensity and frequency. Journal of Applied Ecology 46:796-803.

Interpretive Summary: Exotic weeds and woody plants are degrading the World’s grasslands, and ecologists are attempting to reverse the degradation with prescribed grazing. Prescribed grazing entails introducing livestock (e.g. sheep, goats) that eat unwanted plants. Prescribed grazing has shown modest potential in some studies, but inconsistencies among study results have discouraged adoption by managers. We believe detailed studies that manipulate timings, intensities and frequencies of prescribed grazing could explain inconsistencies among previous study results and identify effective grazing strategies. We studied leafy spurge; an invasive weed avoided by cattle but eaten by sheep. We used simulated grazing (clipping) to estimate leafy spurge and resident plant responses to multiple cattle- and sheep-grazing treatments. Depending on grazing timing, intensity and frequency, grazing either: 1) did not affect the invader and/or resident species, 2) increased the invader and decreased resident species, or 3) decreased the invader and increased resident species. This range of results illustrates that effects of grazing on invaded communities are nuanced. Therefore, successful prescribed grazing will entail more than simply introducing animals that eat invasive plants; how the grazers are managed matters. From a management standpoint, our most exciting finding was that removing very small quantities of invader and resident species biomass in spring reduced the invader and increased resident species over time. (Defoliating more intensively later on often gave the opposite response.) Forage availability is lowest in spring, so a given land mass can be prescription-grazed with fewer animals (or in less time) in spring compared to later in the year. This is fortunate given the limited availability of sheep and other prescribed grazers in the U.S. Because grasslands generate relatively low revenues per unit area, low-cost management is a must. Given this low-cost constraint, it is unclear how managers will overcome the large-scale problems they currently face (e.g. woody plant encroachment, weed invasions). Prescribed grazing is one possibility. Compared to other strategies (e.g. herbicides, seeding), prescribed grazing is inexpensive, and our study illustrates that even very light prescribed grazing can affect positive plant community change.

Technical Abstract: 1. Exotic weeds and woody plants are degrading the World’s grasslands, and ecologists are attempting to reverse the degradation with prescribed grazing. Prescribed grazing entails introducing livestock (e.g. sheep, goats) that eat unwanted plants. Prescribed grazing has shown modest potential in some studies, but inconsistencies among study results have discouraged adoption by managers. We believe detailed studies that manipulate timings, intensities and frequencies of prescribed grazing could explain inconsistencies among previous study results and identify effective grazing strategies. 2. We studied Euphorbia esula; an invasive weed avoided by cattle but eaten by sheep. We used simulated grazing (clipping) to estimate E. esula and resident plant responses to multiple cattle- and sheep-grazing treatments. 3. Depending on grazing timing, intensity and frequency, grazing either: 1) did not affect the invader and/or resident species, 2) increased the invader and decreased resident species, or 3) decreased the invader and increased resident species. This range of results illustrates that effects of grazing on invaded communities are nuanced. Therefore, successful prescribed grazing will entail more than simply introducing animals that eat invasive plants; how the grazers are managed matters. 4. From a management standpoint, our most exciting finding was that removing very small quantities of invader and resident species biomass in spring reduced the invader and increased resident species over time. (Defoliating more intensively later on often gave the opposite response.) Forage availability is lowest in spring, so a given land mass can be prescription-grazed with fewer animals (or in less time) in spring compared to later in the year. This is fortunate given the limited availability of sheep and other prescribed grazers in the U.S. 5. Synthesis and applications. Because grasslands generate relatively low revenues per unit area, low-cost management is a must. Given this low-cost constraint, it is unclear how managers will overcome the large-scale problems they currently face (e.g. woody plant encroachment, weed invasions). Prescribed grazing is one possibility. Compared to other strategies (e.g. herbicides, seeding), prescribed grazing is inexpensive, and our study illustrates that even very light prescribed grazing can affect positive plant community change.