Proportion of annual bromes in cattle diet, with annual brome phenology, forage quality, and forage availability, measured at two mixedgrass prairie study sites, USA

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 four 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 days (± 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.