Sustainable pasture management is critical to agricultural production, but the potential benefits of high intensity grazing on soil health are still largely unquantified. It is not known how soil microbial communities respond to high-intensity, short-duration grazing, and how quickly soil microbial and vegetation communities respond. This will be addressed in the proposed research by evaluating the response of soil physical, chemical, and biological parameters to intensive grazing over a growing season, and contrast those to conventional, low-intensity grazing and grazing exclusions. These responses will be measured both immediately following intensive grazing, and over a full growing season. In addition, a novel methodology will be tested for monitoring vegetation growth on short time scales, using ground-level spectral images. There is a lack of scientifically sound data on the effects of intensively managed grazing, so this study will evaluate the consequences of changing grazing management regimes, and will contribute new knowledge about the impact of intensive grazing on soil biology and soil health, in addition to using new technology to link above-and below-ground ecosystem processes in a novel and creative way. The results and management recommendations will be disseminated to producers via Extension outreach and to undergraduate students at University of Wyoming as part of soil health curriculum. To convey results to the scientific community, we will submit manuscripts to peer-reviewed journals and present the research findings at the Society of Range Management Conference.
- Do soil microbial dynamics differ in response to intensive grazing regimes as opposed to light grazing and grazing exclusions?
- Is there an immediate response of the soil microbial community to high-intensity or low-intensity grazing compared to grazing exclusions?
- What is the effect of intensive defoliation on vegetation recovery growth?
- Will an increase in microbial dynamics affect vegetation recovery growth?
- Quantify the effects of high-intensity grazing compared to conventional grazing on soil physical, chemical, and biological parameters over a growing season.
- Detect the flux of microbial community dynamics immediately following high-intensity grazing events, contrasted to fluxes that occur during conventional, low-intensity grazing.
- Document the immediate response of vegetation to homogenous, intensive defoliation and to heterogeneous, moderate defoliation.
- Synthesize the interactions of above-and below ground processes in response to high-intensity, short-duration grazing.
In summer 2017, a grazing trial was implemented at the Laramie Research and Extension Center (LREC) in Laramie, Wyoming. The site contains twelve adjacent 1/2-acre paddocks that are flood irrigated and dominated by cool-season exotic grasses. The paddocks were grazed according to three treatments in a randomized complete design: a high-intensity, short-duration treatment (HI), a low-intensity, medium-duration treatment (LO), and a no-grazing control (NG). Vegetation and soil samples were taken one week before grazing (baseline data), and 24 hours, one week, and four weeks after grazing. Microbial functional diversity was quantified by extracellular enzyme assays, which detect the activity of C, N, and phosphorus (P) cycling enzymes. Microbial biomass was measured using chloroform-fumigation incubation, and soil biogeochemical parameters included dissolved organic C and N and mineral N. Vegetation biomass and recovery were monitored with a rising plate pasture meter, a manual device that uses a large metal plate to measure the density of vegetation, which, in turn, allows the user to determine forage biomass. This method is non-destructive to pasture forage so accurate measurements can be taken before and during intensive grazing.
The grazing trial continued in summer 2018, and the experimental design and data collection remained the same. Soil microbial DNA will be extracted from both years’ samples, and analyzed to determine if the abundance or diversity of the soil microbial community differs between grazing times, grazing treatments, or the interaction of the two. Laboratory analysis and data analysis will continue through spring 2019.
Preliminary results from the summer 2017 grazing trial indicate that microbial biomass C differed significantly between the grazing treatments (HI and LO) and the no-grazing control. Extracellular enzymatic activity per unit of microbial biomass C was significantly different between the grazing treatments and the NG control for the lignolytic (enzymes that degrade lignin) and P cycling enzymes. Microorganisms release extracellular enzymes into the soil, where they degrade organic matter and perform important nutrient-cycling functions. Measures of extracellular enzymatic activity can be used as a proxy for soil microbial activity – and an indirect glimpse of soil health. Dissolved organic N differed between the LO and the HI grazing treatments, and between the HI treatment and the NG control. Vegetation recovery (the growth of vegetation over time following grazing) was higher in the HI grazing treatment than both the LO treatment and control. These results indicate that grazing may have an immediate effect — detectable 24 hours following grazing — on soil nutrient availability and soil microbial activity, and that vegetation recovers faster following high-intensity, short-duration grazing compared to low-intensity, medium-duration grazing. These results suggest that grazing management can have an immediate effect on soil biological parameters, which could impact producer management for short-term soil restoration goals.
The newly collected data from the summer 2018 grazing trial will be analyzed during fall and spring 2019, with expected final results to be published via a secondary extension bulletin, scientific publications, and thesis publication by May 2019.
Educational & Outreach Activities
During the formation of the experimental design of this project, input was solicited from the attendees of Wyoming Stockgrower’s Association Range Soil Health Workshop in Buffalo, WY. Participants were shown a Powerpoint presentation of the objectives and experimental design of the project, and verbal input was sought as to how the project could be more applicable to the participants. In November 2017, I presented a poster presentation at the Wyoming Weed & Pest Conference in Sheridan, WY. The poster explained the background, objectives, and materials & methods of the project in layperson’s terms and was designed to educate the conference attendees on the poster. In April 2018, a bulletin was published in Field Days Bulletin: University of Wyoming that is freely available online to the public, and distributed to Wyoming producers who are interested in learning about projects happening around Wyoming’s agricultural research centers.
Several outreach publications and activities are in progress or planned for the future. This project is being incorporated into an active learning 3D soils modules, where undergraduate students can manipulate management decisions, such as grazing intensity, and watch how the soil changes. A pilot study is underway and will be presented to students during the fall semester of 2018. Once final results from the summer 2018 grazing trial are analyzed a secondary extension bulletin will be published, and the results will also be shared during a future Wyoming Stockgrower’s Association meeting, where producers will be asked to share their opinion and feedback on the study. Results will be disseminated via scientific journal publications and academic conference presentations as well.