Using targeted grazing to close the phenological niche exploited by invasive annual bromes

Authors: Kray, Julie; Blumenthal, Dana; STEPHENSON, MITCHELL - University Of Nebraska; Porensky, Lauren; Augustine, David; Romero, Seth; Mortenson, Matthew; Derner, Justin

Submitted to: Rangeland Ecology and Management
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/18/2024
Publication Date: 9/23/2024
Citation: Kray, J.A., Blumenthal, D.M., Stephenson, M.B., Porensky, L.M., Augustine, D.J., Romero, S.J., Mortenson, M.C., Derner, J.D. 2024. Using targeted grazing to close the phenological niche exploited by invasive annual bromes. Rangeland Ecology and Management. 98:441-453. https://doi.org/10.1016/j.rama.2024.08.024.
DOI: https://doi.org/10.1016/j.rama.2024.08.024

Interpretive Summary: The invasive annual grasses, cheatgrass and field brome begin and end growth earlier than most native competitors in the western Great Plains, providing an opportunity for targeted grazing. However, managers need a better understanding of when grazers preferentially consume or avoid annual bromes relative to desired native perennial grasses. We used genetic analysis of cattle fecal samples to quantify temporal patterns of consumption of annual bromes by cattle at two mixedgrass prairie sites in Wyoming and Nebraska. Results indicated that consumption could be predicted effectively using two simple metrics—plant height and days after seed maturation. The ideal time windows for grazing annual bromes started when plants were 3.7 inches tall, ended one after seed maturation, and lasted 38 days. Across years, consumption of annual bromes was consistently higher when they grew taller before flowering. Spring targeted grazing reduced brome seed production by 30-77%. Cattle diet quality remained high during the targeted grazing window, and cattle weight gains were similar to those from other spring grazing studies in the region. Together, these results should help rangeland managers to time grazing so that it maximizes consumption and control of annual bromes in the western Great Plains.

Technical 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 and B. arvensis begin and end growth earlier than most native competitors, providing an opportunity for targeted grazing. To take advantage of this opportunity, however, managers need a better understanding of when grazers preferentially consume or avoid annual bromes relative to desired native perennial grasses. We implemented spring targeted grazing for four years and quantified temporal patterns of consumption of annual bromes by cattle at two mixedgrass prairie sites in Wyoming and Nebraska, USA. Consumption of annual bromes was measured twice per week using DNA metabarcoding of fecal samples, together with phenology, forage quality, and biomass of bromes and coexisting native species. We found that cattle consumption of annual bromes can be predicted effectively using two phenological metrics—plant height and days after seed maturation—both within seasons and across sites. 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). Consumption of annual bromes also varied across years, ranging from 19% to 55% of total graminoid consumption. Cattle consumption of bromes and selection for bromes relative to their availability were consistently higher in years when they grew taller before flowering. Despite the fact that 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 in the spring. Cattle diet quality remained high during the targeted grazing window, and cattle weight gains were similar to those from other spring grazing studies in the region. Together, these results indicate that simple phenological metrics can predict cattle consumption of bromes during spring targeted grazing, 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.