Efficacy of prescribed grazing depends on timing intensity and frequency

1. Exotic weeds and woody plants have invaded many grasslands, and prescribed grazing is one management technique used to combat these plants. Prescribed grazing entails introducing livestock such as sheep or goats that eat unwanted plants. It sometimes has desirable effects, but inconsistencies among study results discourage widespread use. Detailed studies that manipulate grazing timings, intensities and frequencies may explain inconsistencies among previous studies and identify effective weed control strategies. 2. We studied Euphorbia esula, an invasive forb avoided by cattle but eaten by sheep. We used simulated grazing (clipping) to estimate E. esula and resident plant responses to cattle and sheep grazing protocols. 3. Depending on timing, intensity and frequency, simulated grazing either: (i) did not dramatically affect the invader and/or resident species, (ii) increased the invader and decreased resident species or (iii) decreased the invader and increased resident species. These disparate results illustrate that successful prescribed grazing entails more than simply introducing animals that eat unwanted plants. 4. Our most promising finding was that removing small quantities of invader and resident species' biomass at early growth stages reduced the invader and increased resident species over time. Defoliating more intensively at later growth stages often gave the opposite response. Forage availability is lowest in spring, so a given landmass can be prescription grazed with fewer animals (or in less time) in spring compared with later in the year. 5. Synthesis and applications. Our study illustrates that responses to prescribed grazing depend heavily on the specifics of the grazing regime. Our results show that some grazing strategies have the potential to exacerbate weed problems, while other strategies help to control the invader and restore desired species. In the case of E. esula, very light prescribed grazing can be beneficial when conducted early in the growing season.