Informations et ressources scientifiques
sur le développement des zones arides et semi-arides

Accueil du site → Doctorat → Afrique du Sud → Development of a model to estimate grazing index values for Karoo plant species

University of Pretoria (1996)

Development of a model to estimate grazing index values for Karoo plant species

du Toit, Petrus Cornelis Vermeulen

Titre : Development of a model to estimate grazing index values for Karoo plant species

Auteur : du Toit, Petrus Cornelis Vermeulen

Université de soutenance : University of Pretoria

Grade : Doctor of Philosophy (PhD) 1996

The ecological index values used during veld condition assessments were found to be inappropriate for the estimation of current grazing capacities. On the one hand the index values were too high, on the other hand two different value systems were combined to provide one set of index values. 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. The possibility of objectively estimating grazing index values for the plant species by using various chemical and physical attributes of the plant species and to apply these grazing index values through the Grazing Index Method, was investigated. The studies were carried out in the Eastern Mixed Karoo, the Karoo Mountainous Areas, the Arid Karoo and the Great Karoo. Related species were studied at the different sites in order to compare species values and extrapolate findings over a wide area. After measurements of the canopy spread cover, the plants were harvested and divided into grazable and non-grazable fractions on the basis of 2 mm, thinner stems were deemed to be grazable. This fraction was analysed. A model for the objective estimation of the grazing index value is proposed. This model employs the 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. The ether extract value enhances the index value of the grasses, while in the case of the karoo bushes the value detracts from their grazing value, the higher the ether extract value of the karoo bush species, the lower the 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 current grazing capacities by means of the objectively estimated grazing index values, results in values closely approximating the grazing capacity norms set by the Department of Agriculture.

Mots clés : Veld, South Africa, vegetation, Biological sciences, Range management

Présentation (NRF)

Page publiée le 5 mars 2015, mise à jour le 5 juillet 2017