Estimating grazing index values for plants from arid regions

Abstract

The Ecological Index Method impacted directly on the estimation of grazing capacities in the Karoo, an arid region in South Africa. Due to inherent deficiencies in the ecological index method, index values formed a disjunct series, 10, 7, 4, and 1 and the fact that different value systems were employed to score plant species, necessitated that index values of species commonly encountered during botanical surveys be subjectively adjusted by means of a species by species comparison, forming a continuous series of index values. These index values were still subjective value judgements of the agronomic value of the Karoo plant species. The author felt that a method should be developed to objectively estimate grazing index values from certain plant variables, i.e. size, animal available dry matter production, and chemical properties of the species. The use of these properties would describe the agronomic value of the plant species, which would lead to agronomically sound current grazing capacities being estimated. The following 2 models were proposed to deal with the karoo subshrubs and the grasses of the karoo separately. Grazing index value for the karoo subshrubs = ((canopy spread cover + available forage + TDN + [K divided by (Ca + Mg)]) divided by ether extract) divided by 100, and grazing index value for the grasses = ((canopy spread cover + available forage + TDN + [K divided by (Ca + Mg)]) x ether extract) divided by 100.