Submitted to: Eastern Oregon Agricultural Research Center
Publication Type: Experiment Station
Publication Acceptance Date: 1/1/2005
Publication Date: 1/1/2006
Citation: Ganskopp, D.C., Svejcar, A.J., Taylor, F., Farstvedt, J. 2006. Will spring cattle grazing among young bitterbrush enhance shrub growth?. Eastern Oregon Agricultural Research Center. SR1057:64-65. Interpretive Summary: Bitterbrush is a desirable shrub for cattle and wintering big game on western US rangelands, and some research has shown that light spring cattle grazing can stimulate bitterbrush growth compared to shrubs in ungrazed stands. The objectives of this study were to determine the effects of light and heavy spring cattle grazing on the subsequent growth of young shrubs and to discover a stocking pressure threshold where cattle start foraging on bitterbrush in the spring. Grass utilization was 32 percent in lightly-grazed pastures and 59 percent in heavily grazed units. About 14 percent of bitterbrush were browsed by cattle in lightly stocked pastures and 62 percent in heavily stocked units. Cattle began foraging on bitterbrush when grasses were grazed down to about 90 to 135 pounds per acre. At the end of the growing season bitterbrush in lightly grazed units were 50 percent wider, 30 percent taller, and had 8 percent longer twigs than shrubs in ungrazed pastures. In heavily grazed pastures, shrubs were 4.2 inches wider and the same height as bitter brush in ungrazed pastures. These findings suggest that light cattle grazing can enhance bitterbrush growth and that if a manager's goal is to stimulate bitterbrush growth the stand be lightly-grazed (30 to 40 percent utilization of herbage) when bitterbrush is flowering in the spring.
Technical Abstract: Due to its palatability and forage quality, antelope bitterbrush is a desirable shrub on western US rangelands. Because little information is available on management of young bitterbrush, a study was undertaken to explore stocking pressure thresholds and quantify effects of light and heavy spring cattle grazing on shrub growth. Across years, 29% of bitterbrush endured trampling in light-grazing treatments, and 55% were trampled under heavy grazing. Linear models relating time and cattle density successfully explained (r-square= 0.84-0.86) probabilities of bitterbrush being trampled. Forage utilization averaged 32% and 59% in lightly and heavily grazed units, and 14 and 62% of bitterbrush were browsed in lightly and heavily-grazed pastures, respectively. Cattle began browsing when herbaceous standing crop declined to 100-150 kg/ha Browsing in heavily-grazed pastures reduced diameters of bitterbrush by 4.5 to 9.5 cm in 1998 and 1999, but shrub height was unaffected. Lightly-grazed stands exhibited a 50% greater increase in bitterbrush diameter, 30% greater height increment, and 8% longer twigs than shrubs in ungrazed units. At the end of 1997 and 1998 growing seasons, bitterbrush in heavily-grazed pastures were 11 cm greater in diameter than ungrazed controls and equal to shrubs in lightly-grazed units. To stimulate bitterbrush growth, young stands can be lightly-grazed (30 to 40% utilization of herbaceous forage) by cattle when bitterbrush is flowering and accompanying grasses are in vegetative to late-boot stages of growth.