Effects of multi-species rotational grazing on soil microbial communities

This research was conducted at Black Tansy Farm, a small family-owned and -operated farm located on the outskirts of Springfield, Oregon. The farm was established in 2014 and currently produces meat and fiber from cattle, sheep, and pigs for direct sale to consumers. To achieve the objectives of this project, approximately 2000 feet of new permanent fencing was constructed and additional electric fencing was installed to implement the MSRG system. New fence construction was performed in May, 2020, immediately upon notification of award. As constructed, the ~18 acre pasture was divided into 6 sections, which were further subdivided into 1-acre paddocks with electric fence/netting. The rotational grazing regime began in mid-June, slightly behind the planned timeline due to difficulties scheduling sample collection, and grazing rotations continued until mid-October. In the MSRG plan, sheep went first through each multi-species paddock, as they have the highest nutritional needs, followed by cattle, and then pigs last to break up the soil and manure clods. Instead of performing management-intensive grazing, we elected to follow a strict rotation period of 4 days. The reasoning behind this decision was to ensure sufficient time (46 days) to break the parasite cycle before a given species returned to their first paddock, and to minimize variability in impacts to the soil microbial communities due to differing lengths of time spent on each paddock. See Figure 1 for MSRG plan details.

Objective 1: Measure the impact of MSRG on soil microbial diversity, composition, and function

For soil microbial analysis we followed Earth Microbiome Project protocols. Soil cores 2.5 cm in diameter by 10 cm deep were collected from five locations in a roughly "W" shape within selected paddocks (Figure 2) using sterilized stainless steel soil probes. The sample collection paddocks were different from those in our original proposal, because comments from the technical review panel indicated that a balanced number of samples (3 each) representing multi- versus single-species would be a stronger study design than our original plan, which would have resulted in only 1 sample each for cow, pig, and sheep paddocks, but 9 for multi-species. Samples were collected prior to beginning MSRG (mid-June), to provide a baseline condition, halfway through the grazing season (mid-August), and at the end of the grazing season (mid-October) to assess changes in soil microbial community structure and function. For each paddock and time point, the five soil cores were combined and homogenized, then aliquoted into two 0.25 gram subsamples; the remainder of the bulk soil was discarded, except for the final October samples, which were delivered to the local Natural Resources Conservation Service (NRCS) office for soil physical and chemical analysis. From the two technical replicates aliquoted from each sample, we extracted metagenomic DNA using the MoBio PowerSoil DNA extraction kit and prepared libraries for shotgun metagenomic analysis using the Illumina Nextera XT DNA library prep kit. Negative and positive controls were included for quality control, and samples were randomized during extraction and DNA preparation to minimize batch effects. A total of 72 soil samples were submitted on December 10, 2020, for sequencing on the Illumina HiSeq4000 platform at University of Oregon's Genomic & Cell Characterization Core Facility. Raw data was received back from the Core on March 8, 2021, but data cleaning and analyses have not been performed yet. 

Objective 2: Assess changes in livestock health and pasture forage quality following MSRG
Body condition scores were tracked from beginning to end of the grazing season to assess any impact of MSRG on livestock productivity. We also recorded disease symptoms and mortalities that occurred during the grazing season. To evaluate how MSRG affected forage quality, we  measured forage biomass production by hand clipping samples, and recorded plant species composition in 12" squares at 3 locations (representing low, medium, and high production) for each paddock immediately prior to the final set of rotations. The original proposal also included counting grass tillers in the 12" squares, but this was quickly found to be infeasible. Hand-clipped samples were oven-dried at 140 ºF and weighed; the average of the dry weights were used to estimate forage production for each paddock following the methods in Shewmaker and Bohle (2010).

Homogenized bulk soil remaining from the final samples collected in October were also tested for pH, bulk density, percentage organic matter, plant available nitrogen (N), exchangeable cations (P, Ca, Mg, K), and micronutrients (Zn, Mn, Cu, Fe, B) at AgSource Laboratories in Junction City, Oregon. We had originally planned to send samples to Oregon State University's (OSU) Central Analytical Lab, but chose to use the same lab as our 2019 soil analyses since we learned that different labs may generate different results.