Zimbabwe: Landuse in Dry Tropical Savannas

Cattle production in communal areas

Three scenarios

Three scenarios are envisaged. Firstly, there is the status quo, which involves a build up of the cattle population in years of good rainfall followed by massive population crashes in drought years.

Secondly, there is a conservative system which involves keeping cattle numbers at the stocking rate recommended for beef production. The stocking rates are relatively low, and calving and off-take rates are relatively high. In years of good rainfall, a considerable portion of the grass will remain unutilised, while in droughts cattle mortality will not be as much as in the status quo scenario.

Thirdly, a tracking, buffering scenario is envisaged. In this scenario, cattle numbers closely follow the abundance of the grazing resource, but in a more managed fashion than in the status quo scenario. During periods of drought, cattle numbers are maintained by stored or locally imported feed resources, where numbers are reduced, the cattle are sold at subsidised prices. After periods of drought cattle are quickly increased by purchases. During periods of good rainfall, cattle numbers are maintained at their highest possible levels as determined by the availability of feed resources.

Simulation modeling would go a long way in providng a basis for evaluating and comparing these three scenarios. But first, the constraints and limitations of each scenario have to be well understood.

Limitations and constraints

The status quo situation is of course feasible as it is what currently occurs. It has the drawback that over stocking in certain years could have negative ecological implications.

The feasibility of the conservation strategy is constrained by the low numbers of animals it maintains, perhaps, insufficient for providing the necessary drought power and other cattle functions. In addition, it presumes collective decision making in terms of maintaining cattle numbers at set stocking rates.

The tracking and buffering scenario has other kinds of constraints: i) the establishment of a system of subsidies for cattle purchase during droughts, cattle sales after droughts and feed purchases, ii) the likely negative impact of maintaining grass cover at consistently low levels due to tracking. For instance, heavy downpours, a characteristic of this region, combined with low plant cover could result in considerable erosion.

Key variables to be considered

1. Rainfall over a time series.
2. Rainfall-animal biomass relationship
3. Cattle population growth model; in relation to feed resources available
4. Value of cattle output (cash and subsistence value), taking into account supply.
5. Levels of subsidy required in the tracking and buffering scenarios (in relation to normal drought relief subsidies in other scenarios)
6. Transition costs in the conservative scenario (e.g., time required to achieve collective decisions).
7. Rainfall-grass production-erosion model
8. Cost of erosion

Ecological simulation modelling

Several such models have been developed, the BIOPAK, an ecological simulation model,(U.S. Department of Agriculture, Forest Service), SAVANNA, a landscape and Regional Ecosystem model (M.B. Coughenour, Natural Resource Ecology Laboratory, Colorado State University). We use specifically the SAVANNA model to make simulations of our own situation.