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United States Department of Agriculture

Agricultural Research Service

Title: Seasonal Cattle Management in 3 to 5 Year Old Bitterbrush Stands

item Ganskopp, David
item Svejcar, Anthony
item Taylor, Fred - USDI-BLM
item Farstvedt, Jerry - ORE. DEPT FISH&WILDLIFE
item Paintner, Kara - E. OR. AG. RESEARCH CTR

Submitted to: Journal of Range Management
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: May 23, 1998
Publication Date: March 1, 1999

Interpretive Summary: At conservative stocking rates, cattle reluctantly forage upon grasses supporting cured stems from previous years. Forage conditioning (via fire, heavy grazing, or mowing) removes these materials and enhances acceptance of forage by stock. Our goal was to quantify livestock distribution and nutrition in pastures supporting stands of both wolfy and non-wolfy grasses. One-half of each of 4 paddocks was conditioned by high intensity grazing in year 1. The remaining half was rested (unconditioned). Cattle distribution was quantified with GPS collars, and diet quality assayed with rumen cannulated steers in a grazing trial in year 2 (May-June). Of total time at pasture, cattle occupied conditioned (201 kg/ha) and unconditioned (543 kg/ha) sectors 61 and 39 percent of the time, respectively. While grazing, 68 and 32 percent of the observations were in conditioned and unconditioned sections, respectively. Crude protein of cured grasses was 2 percent, current season's growth 11 percent, and standing crop 6.5 percent in the wolfy sectors. Cannulated steers confined to treatments, exhibited dietary crude protein averaging 13.5 percent with no difference between treatments. While cattle prefer foraging among conditioned grasses, they can at least initially sustain a high plain of nutrition in unconditioned stands via selective grazing. Recent nutritional mapping of a larger pasture (400 ha) and examination of livestock distribution patterns therein, suggest that landscape nutritional characteristic affect patterns of livestock use at landscape levels.

Technical 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 (r2=0.40), levels of forage utilization (r2=0.00), stocking pressure (r2=0.00), crude protein (r2=0.02) or neutral detergent fiber content (r2=0.59) of accompanying forages, or digestibility of the forages as measured by in-vitro organic matter disappearance (IVOMD) (r2=0.62). In stepwise 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.

Last Modified: 4/22/2015
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