RAINFALL, STOCKING RATE AND ANIMAL PRODUCTION: AN ECOLOGICAL PERSPECTIVE
Private Bag X529
Carnarvon Experimental Station
PO Box 98
The grazing animals, the pasture, the micro-organisms and the soil in any given area can be considered as an ecosystem in which all the components interact. The ecosystem is manipulated in agricultural practice, within certain constraints, to optimize profit. These manipulations are likely to be most successful if the nature of the interactions between components can be isolated and their effects determined.
Environmental factors (climatological and edaphic) have the greatest effect on pasture production and quality, but the efficiency with which this herbage is then converted into animal products depends more on animal factors and managements, especially stocking rate applied. Data from a long-term grazing trial in the Arid Karoo, designed to determine the long-term grazing capacity, was used to examine the nature of the interactions between rainfall, stocking rate and animal production. In this trial, Dry Afrino ewes were grazed on Pentzia spinescens dominated veld, at 4 stocking rates, to determine the optimum stocking rate.
The effect of rainfall on herbage production and effective stocking rate, was examined for similar periods in 1990 and 1991. The 2 periods were identical as to the length of grazing period and applied stocking rate (stocking density). The difference between the two periods was ascribed to the difference in rainfall between the 2 periods, although the effect of different camps could not be quantified. Essentially, the observed differences were interpreted as being typical of wet and dry periods.
It was found that the effective stocking rate was higher during the wet season than during the dry season. This observation is attributed to the fact that effective stocking rate is not just dependent on animal numbers, but also on average seasonal mass of the animals. Rainfall also seems to affect animal production in terms of live mass gain over the grazing period. In the wet season stocking rate had little effect on the average production per animal. In contrast, in the dry season, stocking rate had a pronounced effect on average live mass gain over the grazing season. An increase in stocking rate led to a decrease in live mass gain per animal. As a result of this, the optimum stocking rate as determined by animal production per hectare (Jones & Sandland 1974), varied markedly for the 2 periods.
The following conclusions can be drawn:
- Effective stocking rate increased naturally during wet periods as a result of an increase in average live mass of stock. This reduces the need for an increase in animal numbers during wet periods to some extent.
- Higher lambing percentages as a result of good fodder conditions could lead to overstocking in periods succeeding a wet mating season. This can happen irrespective of fodder conditions during the lambing season and could have detrimental effects on veld condition.
- The effect of stocking rate on production per animal seems to be very important in dry periods. The effect during wet periods seems to be less important.
- Grazing capacity is usually determined from optimal stocking rate in terms of animal production per hectare. Because of the effect of rainfall on optimal stocking rate, it is recommended that grazing capacity norms in arid regions be expressed as a range of stocking rates, rather than a single fixed value. In such away, adaptive management could be accommodated within official grazing capacity norms.
Arid zone ecological forum 32 - 33.