Soil Health Changes Following Transition from an Annual Cropping to Perennial Management-Intensive Grazing Agroecosystem

J. Ippolito, C. Shawver, J. Brummer, J. Ahola, R. Rhoades
Colorado State University
Introduction to MiG and Soil Health
 
Management-intensive Grazing (MiG) is a very flexible, adaptive type of rotational grazing where animal nutrient demand through the grazing season is balanced with forage supply. This type of system requires frequently  manipulating the length of time animals graze (moving every 2 to 4 days; Figure 1) and space allotted (i.e., paddock size) based on available forage resources and number of animals in the herd.
 
  • Over the past decade, interest in MiG has increased steadily due to:
    • prospects of reduced production costs,
    • increased animal output,
    • land use efficiency,
    • and environmental benefits. 
  • Positive and negative impacts of grazing in perennial pasture systems can make or break the viability of a livestock enterprise. Understanding these processes is important for effective integration of MiG into animal-agrosystems in Colorado and throughout the West. As in all grazing systems, ultimate profitability is primarily driven by forage yield and quality, which is undoubtedly linked to the functionality of soil, or SOIL HEALTH. 
 
  • Correctly practicing MiG within irrigated, perennial pasture systems has the potential to produce productive, quality forage while maintaining or improving soil health factors such as soil organic matter levels, nutrient cycling, and carbon sequestration. However, if mismanaged, these animal-agroecosystems may lead to increases in compaction and bulk density to the point where water infiltration, root growth, forage productivity, and ultimately the overall ecosystem is degraded. The premise behind our research was to quantify short-term changes in health in a newly formed MiG agroecosystem.
Figure 1: Management-intensive Grazing - It is intensively managing (i.e., moving) the animals every 2 to 4 days depending on herd size, paddock size, and ultimately, removal of ~50% of forage by animals.
Materials and Methods
  • The 610 ac center-pivot irrigated, perennial pasture system was divided into paddocks that were separated by electric fence (while lines in figure below):
  • The three major soil series were sampled 1 and 2 years after project establishment.
    • In each quarter of the field, 5 replicate soil samples were obtained (blue dots in figure below).
    • The 5 replicates were comprised of 40 soil cores collected within ~ a 3m radius surrounding the blue dot.
    • Soils were separated into 0-5 and 5-15cm depths.
  • After soils were returned to the lab, they were processed following protocol outlined in the Soil Management Assessment Framework (SMAF), a tool used to quantify soil health.
  • Following analyses, soil indicators (i.e., characteristics) were entered into SMAF, along with various site characteristics (soil order, GDD, ppt, mineralogy, etc.).
  • In the background, SMAF runs algorithms based on 1) more is better (e.g., SOM), 2) less is better (e.g., Bd), or 3) somewhere in the middle is better (e.g., available P) and assigns unitless scores from 0 to 1 for all indicators.
  • Scores are combined and weighted to, ultimately, provide quantification of soil 1) physical (Bd, water stable aggregates, soil texture), chemical (pH, EC), biological (SOC, microbial biomass C, potentially mineralizable N, beta-glucosidase activity), and nutrient (available P, K) soil health scores, and an overall soil health score.
 
Results and Discussion: Soil Physical (Table 1), Biological (Table 2), Chemical (Table 3), and Nutrient Indicator Concentrations and Scores
 
 
 
Results and Discussion: Soil physical, biological, chemical, and nutrient soil health scores, and overall soil health score (Table 5)
Summary/Conclusion
  • This study illustrated that positive changes in a number of soil health factors can occur in a short period of time as land is transitioned from annual cropping to perennial pasture.
    • Microbial activity increased significantly in response to having live roots in the ground year-round.
    • Potassium levels also increased significantly as much of what cattle ingest is excreted in the urine. As urine spots are typically 3 times larger on average than manure deposits, it was not unexpected to measure an increase in potassium but not phosphorus as P is mostly excreted in manure.
    • Over time, we expect to see an increase in P as well which will reduce the need for inputs of fertilizer.
    • The increase in potentially mineralizable nitrogen will also contribute to decreased needs for added fertility.
 
  • The biggest issue is managing grazing so as not to increase bulk density above levels that will affect forage productivity.
    • Spreading animals out,
    • moving them to paddocks with coarser textured soils,
    • utilizing sacrifice paddocks, or
    • completely removing animals when soils are wet can reduce the probability of soil compaction and subsequent increases in bulk density, as shown in the two figures below following an unfortunately 500-year rain event:
 
  • This system will be monitored into the future to determine how individual, component, and overall soil health scores respond over time. We hypothesize that with proper management, the overall soil health score will eventually increase.

Abstract

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