Using Targeted Grazing to Close the Phenological Niche Exploited by Invasive Annual Bromes

Abstract

Phenological differences between native and invasive plants can facilitate invasion, but can also be targeted by management. In the western Great Plains of North America, the invasive annual grasses Bromus tectorum L. (cheatgrass) and B. arvensis (field brome) begin and end growth earlier than native competitors, providing an opportunity for targeted grazing. However, managers need to know when grazers preferentially consume or avoid annual bromes. We implemented spring targeted grazing for 4 years and quantified temporal cattle consumption patterns at two mixedgrass prairie sites in Wyoming and Nebraska, USA. We used fecal DNA metabarcoding to measure consumption of annual bromes and coexisting native species twice per week. Concurrently, we measured plant phenology, forage quality, and biomass. Within years, brome consumption was predicted effectively using two phenological metrics—plant height and days after seed maturation. Targeted grazing windows, defined as periods with ≥75% of maximum cattle consumption within a year, started when bromes were 9.3 cm (± 3.6 SD) tall, ended one day (± 4 SD) after seed maturation, and lasted 38 d (± 11 SD). Cattle diet quality remained high throughout these grazing windows. Across years, brome consumption ranged from 19% to 55% of total graminoid consumption, and was consistently higher in years when annual bromes grew taller before flowering. Although cattle typically selected for native perennials over annual bromes, spring targeted grazing reduced brome seed production by 30–77% relative to adjacent pastures where grazing began later. These results indicate that simple phenological metrics can predict cattle consumption of bromes during spring, both within and among years. Carefully timing grazing to align with consumption should help managers to control annual bromes and restore native mixedgrass prairie plant communities. More broadly, combining temporal analyses of livestock diets and plant phenology can be useful for precisely targeting grazing of invasive species.