Managing Guinea Grass Invasion With Cattle Grazing: Thresholds and Native Plant Recovery Across Rainfall Variability

Invasive grasses are altering ecosystems at an alarming scale, and precipitation variability will likely exacerbate change in invaded grasslands. Targeted grazing can mitigate the invasiveness of palatable invasive grasses, benefiting native plant communities and wildlife. This study examines the impact of targeted grazing on invasive Guinea grass ( Megathyrsus maximus ), a C4 perennial grass introduced for forage. Over 4 years, we studied cattle effects on Guinea grass in eight pastures on a 20,000-ha working ranch, across variable Guinea grass, precipitation, and cattle utilization characteristics. Our results demonstrate that targeted grazing surpassing 50% of Guinea grass tillers grazed reduces Guinea grass productivity, reproduction, and thatch accumulation, promoting native plant communities. We found that precipitation and cattle grazing interacted to mediate these effects. During wetter periods, grazing 50% of Guinea grass tillers reduced grass height from 85 cm to 55 cm ± 5.4SE. Guinea grass thatch was reduced by cattle grazing, likely through consumption and trampling, indirectly benefiting native plants. Precipitation and grazing together enhanced Shannon diversity more than either factor alone. For example, at 300 mm of rain and 50% tiller grazing, Shannon diversity increased by 40% (from 0.46 to 0.65 ± 0.14SE). This effect may be due to Guinea grass's resource competition and thatch production. Higher precipitation required more cattle to achieve 50% tillers grazed during wetter survey periods, as measured by camera traps. In contrast, grazing was effective during dry seasons when Guinea grass was likely water-stressed. A Canonical Correlation Analysis differentiated Guinea grass from the rest of the grassland plant community, revealing positive and negative associations with key species: Prosopis glandulosa (CCA1 = 0.0511) and Ambrosia psilostachya (CCA1 = –0.0514). Our findings suggest that targeted grazing and monitoring precipitation patterns can effectively manage Guinea grass and promote native plant diversity in invaded rangelands.