A model which simulates the operation of a pastoral farm was used to perform a 'computer experiment' on an irrigated Canterbury property. The effects of seven stocking rates, ranging from 9 to 21 stock units per hectare (SU/ha), on the performance and profitability of a self-replacing sheep flock were tested. The same experiment was performed under two scenarios (deterministic and stochastic). The deterministic scenario was equivalent to a controlled experiment, where the same pasture growth pattern occurred every year, reflecting average production in the area. The stochastic scenario introduced random variation between years, reflecting the area's natural variability in pasture production. In the deterministic experiment, maximum meat production (258 kg/ha) was obtained at 21 SU/ha, while maximum wool production (89 kg/ha) occurred at 19 SU/ha. Lambing percentage reached a maximum (135%) at 13 SU/ha. Under the assumed set of costs and prices maximum gross margin was obtained by maintaining a stocking rate of 21 SU/ha. Introducing stochastic behaviour caused maximum production levels to decrease and gross margin was maximised at a lower stocking rate (17 SU/ha), reflecting the costs associated with weather variability. Results were used to examine the trade-off between profits and risk in stocking rate decisions.
Proceedings of the New Zealand Society of Animal Production, Volume 54, , 377-382, 1994
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