Biological efficiency, seasonality of feed demand, and economic optima within pastoral sheep production systems: a demand-driven linear programming model

Models of pastoral livestock strategies are typically driven by feed-supply assumptions. An alternative approach is to focus on the requirements necessary to meet specific animal performance levels and the implications thereof for biological efficiency, feed demand seasonality, and systems economics relative to alternative feed supply functions. A demand-driven deterministic steady-state enterprise level linear programming model of a self-contained breeding-finishing sheep meat livestock production system was developed in Microsoft ExcelTM incorporating SolverTM. For each week starting from ewe mating date, individual animal energy requirements (MJME) were simulated relative to specified performance levels. With the objective function being total carcass output and assuming a fixed but arbitrary system constraint of 10 million MJME, these simulation outputs were then brought together within a whole-of-system linear programming framework. Calculated outputs were animal numbers, flock structure, total meat carcass production and whole-of-system weekly feed requirements. Relative to a baseline scenario developed from industry average animal-performance levels, model experimentation was undertaken to investigate how key livestock-production parameters impact on feed conversion efficiency (g carcass/MJME) and feed demand seasonality (MJME/week). Parameters were tested both one-at-a-time and in-combination. In relation to feed conversion efficiency (FCE), parameters that reduced the maternal feed overhead cost per kg of production had the greatest effect with lamb-carcass weight and lambing percentage having the highest impact. Many system parameters had a curvilinear relationship to FCE with declining marginal returns. Effects were generally not additive with combined parameter effects either being less than additive or multiplicative. Overall system benefits tended to be lower than was superficially apparent from consideration of system components. Accordingly, major improvements require a focus on multiple system parameters. In relation to feed-demand seasonality, several system parameters had large impacts. Through combined changes to system parameters, the feed-demand profile could be changed from almost flat to highly seasonal with high peak feed demands. Within a fixed feed constraint, it was also shown that changes to system parameters had a large impact on ewe numbers. Although the main focus of this study was on FCE and feed demand seasonality, the base model was extended to incorporate the impact on economic returns. The purpose was primarily illustrative. With system feed demands still driven by assumed animal performance, in relation to a specified pastoral feed supply environment, initial feed deficits were calculated and then met, via transfer activities, by transferring feed from periods of excess to shortage at a pre-determined cost. Along with product pricing and animal production cost assumptions, system gross margin (GM) was then calculated per unit of system MJME. Testing four system parameters for illustration, livestock parameters were shown to have very different relationships between FCE and GM return. Maximisation of biological efficiency will therefore not maximise economic efficiency and vice versa. In comparison to biological efficiency, which can be characterised by a smooth non-linear response curve, changes in a system parameter such as carcass weight can result in abrupt and step-wise responses to economic outcomes. Lastly, optimal systems were shown to be highly dependent on the specific feed-supply context