Grazing Intensity and Seasonality Manipulate Invasive Annual Grasses and Native Vegetation

Simultaneous desires for greater livestock production and reduced purchased feed inputs are further complicated by potential for rangeland degradation and weed invasion. Grazing management that emphasizes greater use of dormant rangeland forages may provide solutions. We tested grazing season (summer or fall) and intensity (moderate or heavy) combination effects on plant community composition and productivity to determine whether rangelands can sustain heavy dormant-season use without reducing native perennials. Treatment combinations were randomly assigned to 20, 60 × 30 m plots that were grazed from 2013 through 2017. Total current-yr biomass was greatest with moderate fall grazing (1 350 ± 41 kg ha−1) and similar among heavy fall, heavy summer, and moderate summer grazing (1 206 ± 41 kg ha−1). Heavy summer grazing had the lightest native species standing crop. Heavy summer grazing reduced perennial C3 grass standing crop 22%, primarily through effects on needle-and-thread (Hesperostipa comata [Trin. & Rupr.] Barkworth) and threadleaf sedge (Carex filifolia Nutt.). Moderate dormant fall grazing increased perennial C3 grass standing crop 15% but also increased the invasive annual, Japanese brome (Bromus japonicus Thunb.). Heavy dormant fall grazing effects did not differ from that of moderate summer grazing for perennial C3 grass or total native species standing crop, but heavy fall grazing reduced cheatgrass (Bromus tectorum L.) 51%. Where B. japonicus and B. tectorum co-occur, moderate growing season grazing combined with heavy dormant fall grazing may be needed to affect both species. Diversity and native species standing crop were similar between heavy fall and moderate summer grazing, but heavy fall grazing caused the least non-native species standing crop. Heavy fall use is recommended as potential treatment to be rotated among pastures over time or focused on pastures that could benefit from effects observed. Effects likely vary with differences in climate or species composition.