Mitigating ecosystem service tradeoffs in rangelands by using grazing duration and timing to manage water quality

In this study, we found grazing duration and timing can be used as tools to mitigate ecosystem service (ES) trade-offs between cattle production and water quality in rangeland streams. Shorter grazing durations reduced the number of days E. coli levels were above regulatory limits, as did grazing that occurred either early or late in the season. These results support the idea that rotational grazing can be an effective strategy to manage water quality in semi-arid rangelands. They also highlight the need for more grazing studies that incorporate gradients of duration and timing into study designs.

Methods

We measured stream E. coli levels over three years in rangelands employing target grazing treatments. Grazing treatments included: continuous-turnout (long-duration, no rotation), deferred-rotation (medium-duration, with rotation), and time-controlled rotation (short-duration, frequent rotation). The first two of these are common in this region, the third is used less often. All were in-use across our sampling sites prior to this study. Long durations ranged from 82-138 days, medium durations from 31-81 days, and short durations from 1-30 days. Timing for continuous-turnout spanned the entire grazing season (mid-May through mid- September). For deferred-rotation, grazing began across a range of timings including mid-May, mid-June, early July, and mid-July. For time-controlled rotation, grazing timing ranged anywhere from spring through fall. All sites were grazed with beef cattle cow-calf pairs. Stocking densities were 1.03 - 1.78 pairs · ha^-1 in time-controlled areas, 0.11 – 0.28 in deferred-rotation, and 0.02 - 0.09 pairs· ha^-1 in continuous-turnout areas. These equated to stocking rates of ~0.7 pairs · ha^-1 · month^-1 in time-controlled areas, and ~0.3 pairs · ha^-1 · month^-1 in deferred-rotation and continuous-turnout areas.

We sampled every two to three weeks from May through October/early November, a timeframe that encompasses the grazing and recreation season in this area of Utah. By sampling twice per month, we captured fluctuating E. coli levels as cattle moved in and out of pastures. We collected water grab samples according to UT Division of Water Quality’s Standard Operating Procedures for collection, handling, and quantification of E. coli samples. At each site, we collected 100 mL grab samples from flowing stream channels in sterile jars, which we stored on ice. We analyzed samples within eight hours of collection using the Idexx E. coli Quanti-Tray 2000 System (Westborook, MA), adding pre-packaged Colilert reagent to jars, sealing mixture into analysis trays, and incubating samples at 35ºC for 18 - 28 hours. E. coli concentrations were identified as most probable number (MPN) of colony forming units per 100 ml via florescence under a UV light. The Quanti-Tray System can detect E. coli concentrations to a maximum of 2419.6 MPN without dilution. We did not dilute samples because this value is above all regulatory benchmark values.