Derivation of steady-state herd productivity
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The productivity of a sustainable livestock production system can be assessed at a steady (stationary) demographic equilibrium. When it is determined for an optimal culling regime, the assessment is free of the effects of initial state, planning horizon and culling pattern. Its derivation is more rigorous and efficient than its dynamic simulation. Few herd-level models derive the steady state, while numerous ones can simulate it stochastically or deterministically. Derivation algorithms currently in use are based on differential equations, transition matrices and actuarial tables. The actuarial derivation of steady-state demography and productivity is outlined. Optimal culling ages in beef and dairy ranching, respectively, are determined for the sake of demonstration. The advantages of the actuarial approach seem to be conceptual ease, conciseness, versatility with regard to realistic culling regimes, and good prospects for the implementation of model extensions. The algorithm has been implemented as an interactive PC program (PRY) which is species- and systems-independent and has been validated against stochastically simulated population dynamics.