Interspace/under-canopy foraging patterns of beef cattle in sagebrush communities : implications to sage-grouse nesting habitat Public Deposited


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  • Livestock grazing has been indirectly related to sage-grouse (centrocercus urophasianus) declines in the western United States and southern Canada; however, there is a lack of scientific research that directly relates the two. Our objective was to investigate the influence of livestock herbivory on sage-grouse nesting habitat by determining the spatial aspects of cattle forage selection patterns and possible changes in screening cover as forage utilization increases. An eighteen-day, replicated, small pasture trial was conducted in the summers of 2003 and 2004. This trial (trial 1) was developed to initially examine a null hypothesis that cattle foraged on interspace and under sagebrush canopy grasses with equivalent frequencies. If that hypothesis was deemed invalid, we hoped to determine the level of forage utilization at which cattle began to differentially access grasses adjacent to and under sagebrush canopies (Artemisia L. spp.). Understory grasses provide important cover for nesting sage-grouse. Four pastures (6.1 to 6.5 ha) were fenced in a Wyoming big sagebrush (Artemisia tridentata ssp. wyomingensis) community and each stocked with 3 to 4 yearling heifers (mean 385 kg). Within each pasture we randomly selected 30 sagebrush plants, marked a perennial grass tussock under the canopy of each sagebrush, and a second grass tussock of the same species in the adjoining interspace. Grass plants were checked every second day after turnout and given a grazed or un-grazed score. Shrub measurements (height, area, volume, and angle of accessibility of the under-canopy plants) were taken for each sagebrush to evaluate the influence of shrub morphology on grazing occurrence of under-canopy grasses. Visual obstruction for potential nest sites was evaluated using a modified Robe! pole viewed from 2m to document changes in obstruction with increasing herbaceous utilization. Changes in standing crop and utilization (by weight) were assessed weekly by clipping 20 random 1-rn2 plots in each pasture. Grazing of under-canopy plants was negligible at light to moderate levels of utilization (e.g. < 10% of under-canopy plants were grazed when 30% of the standing crop was utilized). At utilization levels >35%, under-canopy plants were used with increasing frequency (P<0.0001). Logistic regression indicated angle of accessibility (P <0.0001) was the only shrub morphology variable influencing grazing occurrence of under-canopy grasses. As angles increased grazing susceptibility increased. Visual obstruction decreased over time, but consistently across all strata. We erroneously anticipated a strata X utilization interaction as the herbaceous level was grazed and trampled by the cows (p = 0.9995). Possible explanations include 1.) ephemeral leaf fall from sagebrush and the grazing effects on the removal of the herbaceous component coincided, 2.) the physical and foraging activities of cattle affected all strata equally, or perhaps some combination of these. The fact that 75% removal of standing crop and affected only a 5% decrease in the ground level obstruction suggests that sagebrush constitutes the bulk of the obstructing cover. A larger 800ha pasture was utilized in 2003 and 2004 to test the influence of plant position on probability of grazing at larger pasture scales and to examine the modifying influence of geophysical landscape characteristics (Trial 2). We randomly selected 30 sagebrush plants and under-canopy and interspace grasses as described for trial 1. Shrub morphology variables were quantified (as per trial 1) and we derived several geospatial characteristics for each location. Grass plants were checked every 4th1 day after turnout and given a grazed or un-grazed score. Standing crop and overall herbage utilization were assessed by clipping 30 randomly located 1m2 plots at the beginning and end of the trial, and utilization was indexed at 4 day intervals by noting frequency of herbivory on 10 grass plants near each marked sagebrush. For trial 2, livestock did not display a conclusive preference for plant position as herbage utilization increased (2003: P 0.07, 2004: P 0.47, and Pooled: P = 0.28). Differences in stocking rates between the 2 years indicated that forage selection patterns and livestock distribution were most influenced (P 0.0002) by slope (P = 0.03 6), distance from water (P 0.0061), and stocking density (P < 0.000 1). When plant locations relative to sagebrush canopies were analyzed separately, distance from water affected grazing of interspace plants (P <0.000 1), while stocking intensity influenced grazing of under-canopy plants (P 0.011). In summary, cattle use of under-story plants was minimal until herbage utilization exceeded about 35% utilization for trial 1. Livestock grazing at "moderate" levels would reduce impacts on screening cover for sage-grouse nesting habitat. At larger pasture scales there are management opportunities for modifying livestock distribution to reduce grazing impacts within critical sage-grouse nesting habitat.
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