Stability, resilience and sustainability in pasture-based grazing systems

Abstract

In this paper we employ a simple dynamic simulation model to illustrate and extend the pasture envelope concept as an approach to characterising the stability, resilience and sustainability of pasture-based beef grazing enterprises. The pasture envelope is a form of phase diagram in which the trajectories over time of key biophysical variables such as pasture biomass and composition are graphed against critical thresholds established on the basis of pasture growth rates and livestock growth requirements. We extend the concept to incorporate key financial variables such as cash flow and critical financial thresholds. The model simulates a steer fattening enterprise based on a phalaris and sub-clover pasture in the Northern Tablelands of New South Wales, Australia. The model incorporates pasture growth and senescence for the two pasture species with competition between the species for soil nutrients and light, preferential grazing of the two species by the livestock with livestock growth based on pasture consumption. The model incorporates a variety of decision rules for rotating livestock among multiple paddocks. The model did not simulate changes in soil nutrients. Scaling the seasonal growth pattern of the pasture species captured the influence of rainfall and temperature on pasture growth. Two sets of simulations were run to illustrate the use of the pasture envelope concept to explore the economic and biological stability and resilience of the pasture system. The first set was designed to explore the financial and biological stability of the enterprise and involved simulating the impact of different stocking rates and rotation period on pasture production and composition, and cash flow. The second set of simulations was designed more to explore the resilience of the enterprise and involved introducing shocks to the enterprise in the form of `droughts' of varying strengths. This was achieved by, for example, reducing the maximum growth rate for both pasture species by 50% but maintaining the same seasonal pattern in the maximum growth rates of each species. The first simulation showed that at low stocking rates the enterprise was biologically stable, but cash flow was also low. Increasing stocking rates increased the cash flow, but also reduced the biological stability of the pasture until at very high stocking rates the pasture system collapsed. Changing the rotation period also affected the stability of the enterprise. In situations where the rotation period was very long, greater than 120 days (or 20 days/paddock), the biological system became unsustainable due to detrimental changes in pasture composition. The enterprise was somewhat resilient to drought at stocking rates less than 1 steer/ha. At stocking rates of 1 steer/ha, the enterprise was economically and biologically unsustainable in moderate or severe droughts. At a stocking rate of 1.25 steers/ha, the enterprise was unsustainable for droughts of any severity.