Last update: November 26, 2010 09:42:58 AM E-mail Print

 

Predicting grazing capacities from linepoint survey- and phytomass data


P C V du Toit

National Department of Agriculture,

Grootfontein Agricultural Development Institute, Private Bag X 529, 5900 MIDDELBURG



INTRODUCTION

Predicting current grazing capacities in the Nama Karoo presents difficulties, on account of the low rainfall, the erratic nature of the rainfall, the sparse vegetative cover, the large species diversity and the different main plant guilds which have to be considered, the grasses and the Karoo bushes.  Methods based on grassland vegetation cannot be transposed easily, “as is”, onto Karoo bushes, without extensive modifications, and vice versa.  When clipping quadrats to measure grass production, for instance; the whole of the harvested material is considered to be available to grazing herbivores.  However, when clipping Karoo bushes on the other hand, it must be borne in mind that the woody fraction is unavailable.  This then immediately poses the very vexing but important question of how much of the standing crop of Karoo bush production is available to grazers on a sustainable basis?

 

METHOD

A number of trials were conducted in order to address the question of how much of the standing crop can be considered to be available to grazers; these were: 

  1. The total standing crop of Karoo bushes was measured by clipping them just above ground level (refer to Table 1)(Du Toit 1996a), 
  2. This harvested material was separated into available and unavailable fractions on the basis of a 2 mm rule of thumb (Botha 1981; Du Toit 1996b); thinner material is usually regarded as being available to grazers, while material thicker than 2 mm is regarded as not being available (refer to Table 1), approximately half of the standing crop is not available, and vice versa, practically; separation was carried out by referring to a match stick which is approximately 2 mm thick,    
  3. However, to determine the accuracy of this rule, stem wounds left on the twigs of Karoo bushes after grazing by sheep, were measured using a sliding vernier callipers, in a stocking rate trial where the veld was grazed at four stocking rates (refer to Figure 1)(Du Toit 1996b), and also,  
  4. Individual Karoo bushes were enclosed in a cocoon of chicken wire.  All the material protruding through the chicken wire was clipped monthly, in order to measure the increment in annual aboveground primary production (refer to Table 1).  It should immediately be clear that this measurement under-recovers the total increment of the bush, on account of the fact that material growing into the bush and which does not protrude outwards, is not recovered and can therefore not be measured.

 

DISCUSSION

Animal production in the stocking rate trial indicated an optimum grazing capacity of 5.7 ha SSU-1 (refer to Figure 1) and a regression of the grazed-off stem measurements, at virtually the same grazing capacity, indicated that 1.5 mm were the largest diameter stems grazed-off by non-stressed sheep (refer to Figure 1)(Du Toit 1996b).

 

Table 1 Various aboveground phytomass measurements for three Karoo bush species

 

phytomass

available (2mm)

increment

Eberlanzia ferox

170.50

96.19

35.51

Eriocephalus spinescens

125.49

59.39

17.51

Pentzia spinescens

49.76

25.05

14.57

Mean

115.25

60.21

22.53

In the Arid Karoo, the values in the table translates to grazing capacities of 3.17 ha SSU-1 (total standing crop), 6.06 ha SSU-1 (available thinner than 2 mm) and 6.62 ha SSU-1 (annual increment).  Please bear in mind that a defined small stock unit as such, does not exist, these values are quoted for comparative purposes only.  Bearing in mind that the 2 mm rule over-estimates production by 25 %, this is a rough and ready guestimate, i.e. 1.5÷2X100, the grazing capacity value of 6.06 then recalculates to 8.08 ha SSU-1, which is lower than the value of 6.62 ha SSU-1 arrived at from the annual aboveground primary plant production increment.

Referring to Table 2; the linepoint survey conducted in the experimental area, indicated a grazing capacity of 9.77 ha SSU-1.

 

It is now proposed that the two values:

  1.    The grazing capacity value arrived at by means of the linepoint survey 9.77 ha SSU-1) and, 
  2. The grazing capacity calculated from the total annual increment (6.62 ha SSU-1), be integrated to predict a realistic grazing capacity for the area.  
  3. Now extract the square root, /(6.62 X 9.77) = 8.04 ha SSU-1.  This value closely approximates the 8.08 ha SSU-1 derived from the corrected, clipped available phytomass.

 

Table 2   A linepoint survey of 500 points carried out on shaly, gravelly soil at Carnarvon, the canopy spread cover is only 25.8 %, indicative of the low plant production potential

Species

Strikes

mean

GIV

VCI

Aristida congesta

Asparagus suaveolens

Eberlanzia ferox

Eriocephalus ericoides

Eriocephalus spinescens

Galenia procumbens

Lycium cinerium

Monechma incanum

Osteospermum spinescens

Pentzia spinescens

Plinthus karrooicus

Salsola calluna

Stipagrostis obtusa

4

2

23

17

8

1

1

1

5

32

2

2

31

0.80

0.40

4.60

3.40

1.60

0.20

0.20

0.20

1.00

6.40

0.40

0.40

6.20

1.54

1.00

1.96

2.54

3.65

4.30

3.00

9.76

4.64

2.43

2.06

5.11

2.41

1.23

0.40

9.02

8.64

5.84

0.86

0.60

1.95

4.64

15.55

0.82

2.04

14.94

 

129

25.80

 

66.54

Computed current grazing capacity................9.8 ha SSU-1

 

CONCLUSIONS

Agronomically acceptable current grazing capacities can be predicted for the arid areas by making use of simultaneous linepoint survey data and clipped quadrat data.  However, care should be exercised during the computations and the results should not be applied blindly.  Since the rate of annual increment is usually unknown, the previous season’s rainfall should be taken into consideration.

 

REFERENCES

Botha P 1981.  Die invloed van spesiesseleksie deur skape, beeste en bokke op die floristiese samestelling van gemengde Karooveld.  Unpublishes D.Sc. thesis.  P.U. for C.H.E., Potchefstroom.

Du Toit PCV 1996a.  Development of a model to estimate grazing index values for Karoo plant species.  Unpublished Ph.D. thesis.  University of Pretoria, Pretoria.

Du Toit PCV 1996b.  Karoobush defoliation in the arid Karoo.  Journal of Range Management 49(2)105-111.

 

Published

Grootfontein Agric Vol 3 No 1(26-27)