Quantifying the Indirect Costs of Gray Wolf – Cattle Interactions

Study Area

Like much of the Western United States, many cow-calf operations in California depend upon winter and summer range to meet their annual forage demands. The livestock studied in the proposed project move from lower-elevation (100 to 1,500 feet), private annual grassland winter ranges in June to higher-elevation (5,000 to 7,500 feet) U.S. Forest Service administered public summer grazing allotments and return to winter range in September. The spatial focus of this study is six grazing allotments on the Eagle Lake Ranger District, Lassen National Forest in northeastern California on the semi-arid east slope of the southern Cascade Mountains. The Lassen Wolf Pack [20] is well established in the western portions of the District. Climate is Mediterranean with cool moist winters and dry warm summers. The landscape consists of a mosaic of plant communities including riparian, upland meadow, sagebrush, aspen stands, and coniferous forest. Broad, open meadows separate conifer-covered buttes and are vegetated by upland grasses, shrubs, and forbs. The primary livestock forage found in the landscape is located in these open meadows. Aspen stands and riparian areas cover approximately 2% of the District, but add disproportionally to landscape diversity [21], and are targeted for conservation and enhancement. Permitted grazing by cattle occurs annually across the District from June through September. Grazing allotments range from approximately 10,000 to 30,000 acres with stocking ranging from approximately 600 to 3,200 animal unit months (typically cow-calf pairs) per allotment. Livestock distribution practices include cross-fencing, herding, and location of water and mineral supplements. U.S. Forest Service has established annual vegetation grazing utilization and streambank hoof disturbance standards for grazing in riparian and aspen habitats (e.g., <10% streambank disturbance by hoof action, <20% browse of new leader growth on riparian woody species).

Experimental Design

This cross-sectional, longitudinal study will follow eight range beef cow-calf herds on their respective summer grazing allotments over a three year period. Herd sizes range from 400 to 500 head of commercial-type angus-cross pairs. Four herds are on summer grazing allotments with no exposure to wolves and serve as negative controls. Four herds are on summer grazing allotments within wolf activity areas (WAA) of the Lassen Wolf Pack [20] and experience substantial wolf impacts, including depredation [22]. Thus, we have eight typical herds sharing similar breeding, herd management, range site, weather and climate, but with distinctly different wolf experiences.

Objective 1: Quantify the influence of wolf presence on cattle performance and grazing behavior

Approximately 20 cows from each herd will be randomly selected for GPS collars beginning with turn out on summer allotments in 2023, 2024, and 2025. GPS collars will record date, time, and spatial position at thirty-minute intervals. Collars will be removed at the end of summer annually and data analyzed for grazing behavior metrics (e.g., time spent grazing, flight response). Relevant performance metrics for collared cows will be quantified seasonally (e.g., body condition score) and annually (e.g., conception rate, weaning rate, weaning weights).

Existing research documents that wolf presence causes cattle to increase energetic costs [2], reduce time spent grazing [3], and reduce distance traveled to find forage [23]. In addition to quantifying these impacts, we will use GPS position data to flag instances where cattle traveled >500 m during a given five-minute interval representing likely flight responses [24], and situations where cattle sporadically bunch, a response to predation risk [2]. We will work with the ranchers managing each herd to record and document all grazing and herd management activities (e.g., activity type, dates, and effort) such as range riding to check cattle location and health, rotations, doctoring/removal of sick animals; this will facilitate controlling for differences in management style across ranchers that can be included in the analysis of cattle performance and behavior. Differences in performance and grazing behavior for wolf impacted and non-wolf impacted herds will be compared via linear mixed effects regression analysis. 

Objective 2: Evaluate the impacts of wolf presence driven modifications to cattle grazing behavior on environmental quality and natural resources 

As described for Research Objective 1 above, we will have thirty-minute interval GPS position data for 20 cows in each of eight herds on their respective grazing allotments over three summer seasons. Via partnership with Eagle Lake Ranger District staff and leadership we have current, high resolution spatial layers for all designated Riparian Conservation Areas (RCA) and aspen stands across all eight study allotments. Thus, we will be able to conduct GIS analysis on each allotment to quantify various metrics (e.g., duration, seasonality, frequency) of cattle presence in riparian areas and aspen stands (total and per acre) on each allotment. Differences in these metrics for wolf impacted and non-wolf impacted herds/allotments will be compared via linear mixed effects regression analysis. We will also compare cattle GPS location data to the annual cattle use metrics sampled as described below to link presence metrics to on-the-ground impacts to sensitive areas.

On-the-ground annual cattle use/disturbance standards for riparian areas across all six allotments are: 1) herbaceous vegetation biomass consumption restricted to no more than 30%; 2) required minimum residual herbaceous vegetation height of 4 inches; 3) browse on new willow growth restricted to no more than 20% for willows less than 5 feet tall; and 4) livestock hoof damage to streambanks restricted to no more than 10%. Browse on aspen terminal leaders is restricted to no more than 20% for aspen sucker less than 5 feet tall. Using standard methods adopted by U.S. Forest Service and other federal land management agencies and range academia [25–27], we will monitor each of the standards above in a stratified (across each allotment) random (within each riparian area and aspen stand) sample design such that 50% of riparian areas and aspen stands across each allotment are sampled at the end of each grazing season. Sample locations will be permanently marked to allow repeated measures each year. Differences in these use metrics for wolf impacted and non-wolf impacted herds/allotments will be compared via linear mixed effects regression analysis.

Objective 3: Evaluate the impact that wolf presence has on stress levels in cattle 

Analysis of hair cortisol concentrations (HCC) has gained acceptance as a non-invasive indicator of long-term stress in animals [28]; chronic stress impairs immune system response [29], metabolic function [30], and reproductive function [31,32]. Fear of predation stimulates cortisol production [17], suggesting that wolf presence is likely correlated with cattle performance and indirect economic consequences.

For hair cortisol concentration testing we will use the “shave-reshave” method – a specific area is clipped at the beginning of the period of interest and the re-grown hair in that same area is re-clipped at the end of the period [33]. All hair samples will be collected from the tail switch [34,35]; this location has been shown to have the best correlations to salivary cortisol and fecal glucocorticoids in beef cattle [35]. During initial hair collection (summer turn out 2023), hair will be cut as close to the skin as possible and the 3 cm closest to the skin will retained for analysis. Subsequent samples, taken each time cows are moved between summer and winter range, will be harvested from the same region; total hair regrowth will be used for analysis. Extraction of hair cortisol will be performed using methanol [34,36]; analysis of hair cortisol levels will be done using commercially available assay kits (ELISA) designed for salivary cortisol and validated for hair cortisol analysis [35,37].

Objective 4: Translate predator-related changes in performance, behavior, and stress into costs

Using data collected from Research Objectives 1 and 3, we will determine if there are quantifiable differences in performance, grazing, and stress metrics in cattle in wolf areas, compared to cattle not in wolf areas. We will employ a panel dataset that combines cattle performance, grazing behavior, and HCC metrics with herd specific husbandry practices, environmental and climatological data, and spatial data on known wolf activity areas. Having access to WAA and WAA-adjacent control herds will allow us to empirically estimate the effect of known wolf activity on the cattle performance metrics that are most correlated with profitability and economic sustainability. This experimental, difference-in-difference, empirical identification approach has been used in similar settings (e.g., changes in weaning weights due to predator activity, species recovery) [9,26]. If differences are statistically distinguishable, we will estimate cost metrics using recent prices and production costs.