Managing cow herd dynamics in environments of limited forage productivity and livestock marketing channels: An application to semi-arid Pacific island beef production using system dynamics

Sustainable ranching operations require access to adequate forage reserves and suitable means to market livestock, both of which are critical determinants of adaptive capacity (defined here as the ability to manipulate stocking rate). Ranch adaptive capacity is most relevant during times of forage shortages from drought. Unfortunately for island beef production systems, traditional adaptive measures used in continental systems are unavailable, such as transporting livestock to less affected areas, importing feed resources (cost prohibitive), intensive grazing practices or stockpiling forage (since forages mature too rapidly and are generally low quality), or destocking through cull cow sales (due to limited marketing and processing capacities). Located on an island of Hawaii, the case study ranch investigated here is challenged by each of these environmental and market constraints. The ranch resides on the leeward side of its island such that it receives minimal rainfall and forage productivity is similar to semi-arid rangelands in the western United States. The ranch's livestock management problem is compounded during drought, since island slaughter capacity is limited and there is no financially feasible means of marketing and transporting culled livestock off the island. Therefore, when forage is limited, managers are forced to retain ownership of culled mature cows, who are moved into a terminal herd to await the next available harvest or shipping availability. Terminal herds occupy areas with lower quality forages to conserve the most productive pastures for higher valued calves. This backlog of cull cows creates extended periods of stress on forage resources, since grazing pressure is not relieved as drought intensifies and increases operational expenses. A simulation model was created in an effort to identify key leverage points within the ranching operation that have the greatest impact on forage availability, herd size and net income. Upon completion of the model, sensitivity analyses were conducted to identify key drivers of model behaviors and several what-if scenarios were run based on questions provided by island ranch managers. Results showed that reducing terminal herd size through increased island processing capacity would not relieve forage pressure or eliminate the backlog of terminal cull cows, although net income was improved through greater cow sales. Several follow up tests were then run to evaluate changes to internal ranch decision making, which showed that reductions in heifer retention would provide a wider array of ecological and economic benefits. The ranch's ability to manipulate heifer retention rates, rather than cull cow rates, terminal herd shipping, or island processing capacity, was shown to be the critical aspect which drives ranch adaptive capacity.