Adding pasture and grazing management to the Agro-IBIS agroecosystem simulation model

CONTEXT

Well-managed pastures support many ecosystem services, but in the Midwest US perennial pastures increasingly are converted to annual row crops mostly fed to confined livestock and ethanol refineries. This agronomic intensification increases yields but drives soil loss, climate change, water quality degradation, flooding, anti-microbial resistance, pesticide resistance, and biodiversity decline. Maintaining and expanding well-managed pastures for livestock production is key to more multifunctional agricultural landscapes, but we need better predictions about how ecosystem functions co-vary under alternative pasture management.

OBJECTIVE

Add cool-season pasture and grazing management functionality to a process-based agroecosystem model (Agro-IBIS) and compare output to observed data in southern Wisconsin, USA.

METHODS

We adapted Agro-IBIS to represent the phenology and growth of cool-season pasture under three grazing management approaches: ungrazed, rotational (livestock moved after 2 d), and continuous. Pasture phenology, growth, and carbon (C) allocation were modified to be linked processes occurring at a daily timestep. Plant functional type (PFT) C pools (root, stem, leaf, reproductive) were updated daily through dynamic feedbacks and C-allocation that responds to phenological stage, environmental conditions, and grazing management. We compared model output to pasture observations of plant growth (aboveground biomass) and phenology (leaf area index; LAI) from a two-year field experiment in southern Wisconsin comparing grazing management approaches.

RESULTS AND CONCLUSIONS

Model output aligned well with pasture observations capturing seasonal dynamics in aboveground biomass and LAI for each grazing system (R² = 0.47 to 0.90 across 2 response variables and all 3 grazing managements). Agro-IBIS predicted higher ANPP and NPP under rotational than continuous and no grazing, similar to experimental observations.

SIGNIFICANCE

Representation of pasture phenology, growth, and response to grazing management in Agro-IBIS improves our ability to explore how agricultural landscapes can be composed, configured, and managed for multifunctional outcomes under a changing climate.