Determination of Current Grazing: Capacity from Veld Condition

Up to the present time, recommended stocking rates appropriate for the veld have arisen largely from subjective assessments by trained personnel. Such recommendations suffer through their subjectivity and through the shortage of trained personnel capable of estimating grazing capacities with any degree of accuracy. Therefore, the development of an objective technique which can be used by personnel with little experience in estimating current grazing capacity has decided advantages provided the method itself is realistic.

The method proposed depends on the following assumptions:

• The grazing capacity of a sample site is a function of (i) the potential for the particular bioclimatic region, and (ii) the condition of the site.
• The potential for the bioclimatic region is realised on the benchmark site. An example of potential grazing capacity (PGC) for cattle under a well-managed rotational system is given in the table below.
• The condition of the sample site is evaluated based on four factors, namely, the species composition, basal cover, topography and soil erodability. In the absence of either a means or a need to weigh these four factors differently, they are assumed to be potentially of equal importance to current grazing capacity (CGC).

Potential grazing capacity of benchmark sites for cattle under good management in the bioclimatic regions of KwaZulu-Natal

Calculation of current grazing capacity (CGC) on veld according to veld composition, basal cover, topography and soil erodability relative to the potential grazing capacity of the benchmark site

CGC = PGC) (CF + BCF + TF + SEF) AU/ha where:

• PGC = the potential grazing capacity applicable to the benchmark site in different ecological areas in KwaZulu-Natal as shown in Table 5:
• GF = the composition factor derived from the composition score;
• BCF = the basal cover factor;
• TC = the topographic factor; and
• SEF = the soil erodability factor.

Composition factor CF = 0.25 (Composition score + number units of Increaser I in excess of benchmark) / 100.

Basal cover factor BCF = 0.75 + 2 (BSC / BCG)² where BCG is the basal cover of the sample site, and CB is the basal cover of the benchmark site;  given BCS / BGB may be read from Figure 1.

Topographic factor TF – 0.25 if slope 0.5%, provided it is not a drainage channel,

                  Or 0.02 if the slope 5 – 15%

                  Or 0.10 if seasonally wet

                  Or 0.00 if the slope 15% if it is a drainage channel or severely eroded.

Soil erodability factor SEP = 0.0417F – 0.417, where F is the erodability rating of the soil series of the same site determined either from Anon (1976: Appendix C) or subjectively on the sample site and from below.

The contribution of species composition is given by a composition factor (CF) which is simply a function of the composition score except when Increaser I plants in the sample site are more common than in the benchmark.  In the case of such exceptions it is reasoned that previous under-utilisation must be rectified by a temporary increase in stocking rate over and above the achieving optimal defoliation of the Decreaser species. This is considered justified provided the Increaser I group is not dominated by unpalatable grasses such as Cymbopogon excavatus. In the case of the latter, the Increaser I term in the equation must be omitted, unless management is such that animals will graze it, as after a burn.

                        Topographic factor TF:  for a slope between 5 and 15% = 2.0

                        Soil erodability factor for a soil of moderate erodabiolity – 0.15

                        Total numerical rating for site – 0.6199

This factor is then applied to the potential grazing capacity of Bioclimatic Group 6 as follows:

Current grazing capacity            = 0.6199 x potential grazing capacity

                                                                        = 0.6199 x 0.6

                                                                        = 0.371 AU/ha or s.69 ha/AU

This method has not yet been exhaustively tested and should be applied with caution. The numerical ratings which have been recommended may need revision, but the principle behind the assessment of current grazing capacity appears sound and could form a useful basis for estimating safe recommended stocking rates for a wide range of veld types in various condition classes.