Seasonal cattle management in 3 to 5 year old bitterbrush stands.

Abstract

Because of its high palatability and sustained levels of forage quality, antelope bitterbrush (Purshia tridentata Pursh DC) is one of the most desired shrubs on western U.S. rangelands. Bitterbrush has decreased in abundance in many areas, and efforts to foster its restoration have met with limited success. Because little information is available regarding the grazing management of newly established stands of bitterbrush, this study was undertaken to: 1) determine the effects of early and late-season cattle grazing on bitterbrush, 2) determine when cattle were most likely to forage on these shrubs, and 3) relate use of shrubs to the quantity, quality, and phenology of accompanying herbaceous forages. Ungrazed (control), early-grazed, and dormant-grazed paddocks supporting 3+ year old bitterbrush (randomized complete block design, N = 3) were monitored for 3 years to accomplish this task. When grasses were green and growing, cattle grazed about 6% of the shrubs per day. When grasses and forbs were dormant, about 13% of the shrubs were grazed each day. Rates of use of shrubs were not significantly (P > 0.05) correlated with amounts of accompanying herbage available (r(2) = 0.40), levels of forage utilization (r(2) = 0.00), stocking pressure (r(2) = 0.00), crude protein (r(2) = 0.02) or neutral detergent fiber content (r(2) = 0.59) of accompanying forages, or digestibility of the forages as measured by in-vitro organic matter disappearance (IVOMD) (r(2) = 0.62). In step-wise regression analyses Julian date alone accounted for 92% of the variation in rates of use of shrubs and the addition of IVOMD accounted for 98% of the variation. This suggested that bitterbrush was grazed more heavily as the growing season advanced and forage quality of the grasses declined. Shrub height, diameter, and volume were reduced by early grazing in 1 of 3 years when turn out was delayed until grasses were entering anthesis. Cattle grazing when grasses were dormant caused reductions in height, diameter, and volume of the shrubs in all 3 years. Rates of shrub mortality were unaffected by treatment. Bitterbrush in all treatments experienced significant reductions in height, diameter, and volume from wildlife use during the winters of 1993 and 1994 but not 1995. When trials were terminated, shrubs in early-grazed paddocks exceeded (P < 0.10) their counterparts in the dormant grazed paddocks in height, diameter, and volume.