Last update: August 16, 2011 12:34:20 PM E-mail Print

 

ESTIMATING GRAZING INDEX VALUES FOR KAROO PLANTS


PCV du Toit, W v D Botha, CD Blom, HR Becker, DJ Olivier, EM Meyer, GZJ Barnard

Grootfontein Agricultural Development Institute

Private Bag X529, MIDDELBURG, 5900

 

At present, grazing capacities in the Karoo Region are calculated according to the Ecological Index Method, described by Vorster. The method employs index values based on the ecological importance of the species groups. Recalculations of the grazing capacities of sites in especially the arid areas of the Karoo, have indicated the necessity for an adjustment to the index value of the climax grasses. At the time, it was decided to compare the values of the species commonly encountered and to recommend adjustments across the board.

During the past year, a number of researchers and extension officers estimated the grazing value of the plant species most commonly found when doing botanical surveys. In order to ensure the comparability of the estimates, it was decided to score the species on the basis of six parameters. The scores took into account the ability of the species to produce fodder, to protect the soil surface against erosion, it's perenniality, forage value during the growing season and during the dormant season and, the relative ease with which the plant can be grazed. Each parameter was scored out of ten. The scores of the parameters were summed for each species, and the final score calculated to lie between one and ten. A mean value, as well as a five percent deviation from the mean was calculated for each species score submitted. Values falling outside of this range, were discarded and a new mean was then calculated. This score was vetted and accepted or rejected on a consensus basis.

Just over 60% of the scores submitted during the first round, were accepted, illustrating a high degree of concurrence between participants. During the second round, less than 15% of the scores were unacceptable. In this case species were relatively unknown or presented problems when the different parameters had to be scored.

The ElM series of scores, 1, for the annual pioneer grasses, the unpalatable karoobushes and invader plants; 4, for the perennial pioneer grasses and the less palatable karoobushes; 7, for the sub-climax grasses and the palatable karoobushes and 10, for the climax grasses changed to a continuous 0.8 to 10 for the grasses and 0.7 to 7.7 for the karoobushes. The climax grasses now range in score from 6 to 10, instead of the value of 10 used previously. When grazing capacity calculations using the old scores are compared to calculations made when using the new scores, the value of making these adjustments can be seen.

Table: Adjusted grazing capacity scores for four sample sites in the arid Karoo, using the 10, 7, 4 and 1 ElM score1 and using the continuous series of

adjusted species scores2

  ha/SSU1 ha/SSU2
Carnarvon    
“Red sand” 2.15 2.74
Shale veld 3.64 3.56
Beaufort West    
Leegte veld 2.86 2.25
Shale and sandstone veld 2.84 2.58

 

 

Published

Arid zone ecology forum 27 - 28.