Module # 13 Component # 1 Determining Grazing Capacity

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1 1 Module # 13 Component # 1 Objective Understand the purpose and methods for calculating grazing capacity and how it relates to the management of an area Expected Outcome Define grazing capacity Understand the methods for calculating grazing capacity Relate grazing capacity to management decisions for a given area Nyala: Tragelaphus angasii

2 2 Grazing Capacity The grazing capacity of an area is determined by the productivity of the veld and indicates the number of animals that can be maintained on the veld without resulting in erosion or changes in botanical composition. Grazing capacity is also dependent on vegetation structure and varies with veld condition. It can thus be deducted that good veld condition ensures a higher grazing capacity. The grazing capacity is determined separately for each homogenous vegetation unit (HVU). The grazing capacity of an area gives not only an idea of its productivity but can also be used to determine initial stocking rates. In adaptive management, trends in vegetation change are used to adjust the stocking rate of an area according to the management objectives of that area. The grazing capacity of an area is influenced mostly by availability of water for grass growth and the condition of the herbaceous layer. The grazing capacity is therefore not static but varies with changes in rainfall and veld condition. Adaptive management principles enable management to make decisions based on rainfall and veld condition monitoring data. Various methods exist to determine grazing capacity. African buffalo: Syncerus caffer

3 3 Methods for Rainfall Method The rainfall method predicts the large herbivore biomass for an area of semi-arid savanna by using mean annual rainfall data. This method is particularly geared towards areas receiving less than 700mm per annum. The equation was derived from a regression analysis between rainfall data and aerial game counts. Although it is a useful method to obtain a quick assumption on ecological capacity, it does not address the variability in ecological capacity caused by the condition of the vegetation. In the rainfall method the short-term ecological capacity is estimated by using the annual rainfall, while the long-term capacity uses the long-term mean annual rainfall in the equation. The herbivore biomass estimated with the rainfall method includes both grazers and browsers. Herbivore biomass is then related to large stock units. The equation is: Large Herbivore Biomass (kg/ha) = 8.684(AP) where: AP = mean annual precipitation (mm) Using this method provides the wildlife manager or prospective land buyer with a very rough estimate of the capacity of a piece of land. However, it must be stressed that this method should only be used as an initial rough guide or as a check against an additional method. Its results are not accurate enough to form the basis of a proper wildlife management plan. Herbaceous Phytomass Method In this method the grazing capacity is determined for the entire year and a utilization factor is built into the equation that subtracts losses of potential grazing caused by unavailable material and losses due to environmental factors such as climate and insects. where: The equation is: y = a / (total phytomass (kg/ha) x b c y = grazing capacity (ha/lsu) a = number of days in the year (365) b = utilization factor: only 35% of the herbaceous material is grazed while 40% remains as tufts and stubbles and 25 % is lost due to environmental factors. C = feed requirement per day per LSU (10kg/day)

4 4 Danckwerts Method Danckwerts developed a grazing capacity model based on veld condition score and the mean annual rainfall. The model is applicable to areas where woody vegetation is not dominating or absent. Where woody vegetation is competing with the grass layer, a further alteration to the regression equation takes this into account. The short-term and long-term grazing capacity is derived from the annual rainfall and the mean long-term annual rainfall respectively. The sample site veld condition scores are expressed as a percentage of the benchmark veld condition score. The equation is: GC = (X) + [(Y 419.7) x ] where: GC X Y = grazing capacity in large stock units per hectare (LSU/ha) = percentage veld condition score = mean annual rainfall in mm per annum The case study that follows uses this method. Gemsbok: Oryx gazelle

5 5 Case Study: Msasa Nature Reserve The Danckwerts method was used to determine the grazing capacity of each of the six communities that have already been described for the Msasa Reserve. The mean long-term rainfall data and the veld condition percentage of each community were substituted into the equation. Community Veld condition score Grazing Capacity for Cattle (LSU/ha) Grazing Capacity for Game (LSU/ha) LSU The long-term grazing capacity for Msasa Nature Reserve The Danckwerts equation is based on maximum animal production for cattle. However, with game in this case continuous grazing is applied on the veld because no migration or rotational grazing is possible on Msasa Nature Reserve. The reasons for this is that the border fence prevents migrations and it is difficult to apply rotational grazing without internal fenced camps. The animals are also very selective in feeding and selective utilization pressure is applied. Burchell's zebra: Equus quagga burchellii

6 6 Conclusion The actual grazing capacity was therefore taken as 70% of the estimated grazing capacity using the Danckwerts equation for cattle. Table # 1. summarizes the longterm grazing capacities for the different communities Many species of game move from one community to another within a single day. Therefore, the grazing capacities of the communities were combined to give an ecological grazing capacity of LSU for the whole area. The veld condition and consequently the grazing capacity must be evaluated annually to determine the optimum stocking potential. The long-term grazing capacity is a good indication of the potential of the area. Sable antelope: Hippotragus niger