Development of A Model to Estimate Grazing Index Values for Karoo Plant Species

Abstract

The ecological index values used during the Ecological Index Method of veld condition assessment were found to be inappropriate for the estimation of current grazing capacities in the Karoo, because on the one hand the index values were too high in many instances, while on the other hand two different value systems were combined to provide one set of index values, which tended to exacerbate the problem. Estimated subjective grazing index values of plant species based on the same set of variables for all species, provided more acceptable estimates of current grazing capacities. It was decided subsequently to investigate the possibility of objectively estimating grazing index values for the plant species by making use of various agronomic i.e. chemical and physical attributes of the plant species and to apply these grazing index values through the Grazing Index Method, which is largely based on the older Ecological Index Method. The studies were carried out in the Eastern Mixed Karoo, the Karoo Mountainous Areas, the Arid Karoo and the Great Karoo. Various closely related species were studied at the different sites in order to be in a position to compare the values estimated for a specific species or genus in the different areas and to extrapolate findings over a wide area. After measurements of the canopy spread cover of the plants, they were harvested and divided into grazable and non-grazable fractions on the basis that all the stems less than 2 mm thick are deemed to be grazable. This fraction was analysed for its various chemical constituents. A model for the objective estimation of the grazing index value is proposed. This model employs the agronomic parameters; production, canopy spread cover, total digestible nutrients, the values for potassium, calcium and magnesium, and the ether extract value in the estimate of the grazing index value. The model differentiates between karoo bushes and grasses on the basis of the ether extract value. In the case of the grasses the ether extract value is taken as a variable with a positive influence on the index value of the grass, while in the case of the karoo bushes the value detracts from the grazing value of the bush species, karoo bushes, the higher the ether extract value of the karoo the lower its grazing index value. The use of these objective grazing index values yields better estimates of the current grazing capacities than either of the ecological index values or the subjectively estimated grazing index values. Estimating the current grazing capacities by means of the objectively estimated grazing index values results in values closely approximating the grazing capacity norms, as set by the Department of Agriculture.