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The Boesmanskop grazing capacity benchmark for the Nama-Karoo

 

PCV du Toit

Grootfontein Agricultural Development Institute

Private Bag X 529, MIDDELBURG 5900

 


Introduction

A benchmark for the estimation of the grazing capacities for different sites in the Nama-Karoo is described. The Nama-Karoo is one of seven biomes into which the 70 Veld Types (Acocks 1988) describing the vegetation of South Africa can be grouped (Hoffman 1966; Rutherford & Westfall 1986). This benchmark consists of rotationally grazed plains which is in a relatively stable condition, with a good species composition. The site is situated on the Boesmanskop portion of Grootfontein Agricultural Development Institute’s experimental farm, in the Middelburg District. Veld condition indices are proposed to be used in conjunction with botanical veld condition surveys of different sites, together with the different sets of index values ascribed to the plant species which are available at the moment. With the method of computation of the veld condition index, it is possible to estimate reasonably accurately the current grazing capacity applicable to the surveyed sample site, although it is still necessary to treat this circumspectly.

 

Background

During the late 1970's and early 1980's there was a tremendous countrywide interest in describing methods by which current grazing capacities for different sites in the veld could be estimated. The methods that were described in the Karoo Region (Vorster 1982), the Free State Region (Fourie & Visagie 1985)and in the Natal Region (Tainton, Foran & Booysen 1978; Tainton, Edwards & Mentis 1980) were largely based on succession theory as described by Dyksterhuis (1949). The benchmark concept was adopted by all the Agricultural Development Regions to serve as a basis with which different veld sample sites could be compared and, the relative health of the veld described, as seen from the animal grazing standpoint. Although there were marked similarities in the approaches followed by the different regions, the Karoo Region deviated slightly from the other regions in the proposed surveying methods and in the method of comparing different sites to the benchmark.

 

The Karoo benchmark

At that time, a committee consisting of pasture research officers, extension officers and leading small stock farmers from the district gathered at Grootfontein and inspected the veld at Boesmanskop, in order to estimate the grazing capacity of this portion of the Karoo plains. There was general consensus that one producing merino ewe could be carried year round on two morgen of veld in this condition. Botanical veld condition surveys were carried out and, from the species encountered and using the index values, a value of around 600 to 800 was computed for this site. This value was the veld condition index of the site and indicated the state of health of the site and its potential to provide forage on a sustainable basis to grazing animals. The index values which were used to arrive at this value, were the ecological index values proposed by Vorster (1982), e.g.

Since this exercise took place countrywide, pasture and animal scientists were involved in defining the grazing capacity norms applicable to different agro-ecological areas in the country. The grazing capacity norms were defined in terms of large stock units (Meissner et al. 1983). This action necessitated adjustments to be made, in order to express the grazing capacity of the Boesmanskop benchmark site in terms of large stock units per hectare. Thus when carrying out a veld condition survey on an unknown sample of veld, the veld condition index arrived at was divided into the value previously decided on by the committee, i.e. 650. The resultant answer then represented a grazing capacity of X morgen per small stock unit and this value had to be adjusted to obtain a value expressing the grazing capacity of the unknown sample of veld in terms of the defined large stock unit per hectare. The correction factor subsequently decided on was 7.14, this adjustment was necessary in order to change the number of producing merino ewes per morgen (from the original estimate) to the number of producing merino ewes equivalent to one large stock unit per hectare on this benchmark veld.

Revising the ecological index values

However, at approximately the same time questions were asked about the appropriateness of classifying all the climax grasses as 10-point plants. For example, Themeda triandra in the eastern high rainfall Nama-Karoo is a much more luxuriant grass with a much higher production potential than for instance Stipagrostis obtusa of the low rainfall Bushmanland. Hence, the use of an index value of 10 for Stipagrostis, led to a gross over-estimation of the grazing capacity in the Arid Karoo and Bushmanland where this grass dominates the vegetation. Research effort was subsequently focused on estimating index values for different plants which were deemed to be more in line with their productivity and grazing value.

Subjective grazing index values

Since it became obvious that the research action embarked on would take some time, a committee was formed to subjectively arrive at more appropriate index values for those plants commonly sampled when doing botanical veld condition surveys, to be used in the interim (Du Toit et al. 1995).

The following features of the plant species were considered by the committee members in arriving at a value for the specific species which was deemed to describe its grazing value to the animal, e.g.

This action led to a continuous series of index values for the Karoo subshrubs (Table 1) and grasses (Table 2). From the tables it is clear that the estimated index values of the individual species in a specific genus demonstrate their close relationships with each other much more clearly, than was the case with the ecological index values used previously. The different subjectively estimated grazing index values for a specific genus also describe a smoother series than was the case with the older disjunct series of ecological index values.

Objective grazing index values

The objectively estimated grazing index values were published eventually (Du Toit 1997, Du Toit 2000a; Du Toit 2000b). It was reasoned that these index values estimated the grazing values of the different plant species more realistically than did the previous models proposed by Vorster (1982) and Du Toit et al. (1995). From the model that was proposed to estimate the index values for the different plant species, it can be seen that the size of the plants play an important role; this aspect is also an important factor to bear in mind when conducting line-point botanical surveys. Larger plants will contribute more strikes during the survey and will presumably also produce and provide more available forage to the grazing animals. The higher volume of nutritious available forage, gleaned from the total digestible nutrient content of the species, is another important aspect to bear in mind. The model is user friendly since the size and available forage measurements were carried out in paddocks subject to grazing.

Repeated botanical line-point surveys carried out in paddocks before and after grazing has taken place, will provide a reliable indication of the current grazing capacity of the paddock. The total digestible nutrient content rates the grazing value of the grasses and subshrubs relative to each other.

An important consideration during the modeling was the effect that the ether extract content and the K÷(Ca +Mg)-ratio have on the availability of nutrients to the grazing animals during metabolism and on the physical health of the animals. In the grasses, the ether extract content is used as a multiplier to accentuate the benefit of the plant’s ability to provide essential vitamins to the animal. This is more pronounced in grasses of high colour. However, in the Karoo subshrubs, a high ether extract content indicates the existence of large amounts of aromatic oils and resins, these substances cause the animals to excrete high volumes of ethers and aldehydes, which are necessary in the metabolism of proteins, through their urine. On account of this fact, the ether extract content is used as a divider in order to negatively weight the value of the subshrubs.

 

  The grazing index value of the grasses and Karoo subshrubs are = {(the canopy spread cover + the available forage production

                                                                                                   + the total digestible nutrient content + [K÷(Ca+Mg)])

                                                                                                   x or ÷ by the ether extract content} ÷ 100

 

Testing the different sets of index values

Subsequently, all the different sets of index values that were available at the time (1996), were tested as to their ability to realistically describe the Boesmanskop benchmark, which was to be further used across the Nama-Karoo, when comparing botanical veld surveys of sample sites to the benchmark. These surveys were to be compared to the benchmark with regard to their veld condition index, percentage cover and their calculated grazing capacity.

Detailed botanical line-point surveys were conducted at the Boesmanskop site on the plain and the data tabulated and processed (Du Toit 1998a; Du Toit 1998b). Refer to Tables 3 to 6 for the botanical species composition, the veld condition indices computed by means of the three sets of index values and the percentage cover.

In Table 7 the veld condition indices arrived at by means of the different sets of index values are given as well as the average veld condition index. From this table it can be seen that the original ecological benchmark veld condition index was not far off the mark, i.e. 650 as opposed to 658. When using the subjectively estimated grazing index values to arrive at a veld condition index, the value is 612, which does not differ appreciably from that of the original veld condition index of 650. However, when the objectively estimated grazing index values are used to arrive at a veld condition index for the same site, the value is 438.

Subsequently, benchmark veld condition index values of 650, in the case of the ecological index values (EIV) and the subjectively estimated grazing index values (SGIV) were used to calculate the current grazing capacities of the research site and 500, in the case of the objectively estimated grazing index values (OGIV). This action led to an interesting phenomenon (Table 8). The mean current grazing capacity computed by means of the SGIV’s was slightly more conservative than where the EIV’s were used, 7.8 ha/LSU as opposed to 7.3 ha/LSU. Where the OGIV’s were used, once again a slightly more conservative mean grazing capacity was computed, 8.5 ha/LSU as opposed to 7.8 (SGIV) and 7.3 (EIV).

When the mean veld condition indices reported in Table 7 were rounded off and these values used in the calculation of the current grazing capacities, 660 in the case of the EIV, 610 in the case of the SGIV and 440 in the case of the OGIV, similar grazing capacities were computed within each site (Table 9). Site 1 had a mean grazing capacity of 5.9 ha/LSU, site 2 was 7.0 ha/LSU, site 3 was 7.0 ha/LSU and site 4 was 7.4 ha/LSU.

 

Conclusion

 It is, however, recommended that when the old ecological index values and the more recent subjectively estimated grazing index values are used, a benchmark value of 650 should be used when current grazing capacities are estimated. When the objectively estimated grazing index values are used, it is recommended that a benchmark value of 500 should be used to estimate current grazing capacities. Following this last recommendation, acceptably conservative current grazing capacities will be computed and, the implementation of these grazing capacities will lead to healthier more vigorous plants in the veld and ensure a healthy forage reserve to be used during times of drought. Following this recommendation, the more lenient stocking rate which will be applied, will lead to healthier, heavier animals at an earlier date.

 

References

Acocks, J.P.H. 1988. Veld Types of South Africa. Memoirs of the Botanical Survey of South Africa 57:1-146.

Du Toit, P.C.V., Botha, W.van D., Blom, C.D., Becker, H.R., Olivier, D.J., Meyer, E.M. & Barnard. G.Z.J. 1995. Estimating grazing-index values for Karoo plants. Technical Communication 239:1-15. Government Printer, Pretoria.

Du Toit, P.C.V. 1997. A model to estimate grazing index values for Karoo plants. South African Journal of Science 93:337-340.

Du Toit, P.C.V. 1998a. Description of a method for assessing veld condition in the Karoo. African Journal of Range and Forage Science 14(3):90-93.

Du Toit, P.C.V. 1998b. Grazing-index method procedures of vegetation surveys. African Journal of Range and Forage Science 14(3):107-110.

Du Toit, P.C.V. 2000a. Estimating grazing index values for plants from arid regions. Journal of Range Management 53:529-536.

Du Toit, P.C.V. 2000b. Weidingswaarde en verspreiding van Stipagrostis obtusa en die voorspelling van weidingskapasiteite in die Nama-Karoo. Grootfontein Agric 2(2):14-16.

Dyksterhuis, E.J. 1949. Condition and management of rangeland based on quantitative ecology. Journal of Range Management 2:104-115.

Fourie, J.H. & Visagie, A.J.F. 1985. Weidingswaarde en ekologiese status van grasse en Karoobossies in die Vrystaatstreek. Glen Agric 14:14-18.

Hoffman, M.T. 1966. Nama-Karoo Biome. In Vegetation of South Africa, Lesotho and Swaziland, ed. A.B. Low and A.G. Rebelo, pp. 52-61. Department of Environmental Affairs and Tourism, Pretoria.

Meissner, H.H., Hofmeyr, H.S., van Rensburg, W.J.J. & Pienaar, J.P. 1983. Classification of livestock for realistic substitution values in terms of a biologically defined Large Stock Unit. Technical Communication 175:1-40. Government Printer, Pretoria.

Rutherford,M.C. & Westfall, R.H. 1986. The biomes of southern Africa - an objective categorization. Memoirs of the Botanical Survey of South Africa 54:1-96.

Tainton, N.M., Foran, B.D. & Booysen, P.de.V. 1978. The veld condition score : An evaluation in situations of known past management. Proceedings of the Grassland Society of southern Africa 13:35-40.

Tainton, N.M., Edwards, P.J. & Mentis, M.T. 1980. A revised method for assessing veld condition. Proceedings of the Grassland Society of southern Africa 15:37-42.

Vorster, M. 1982. The development of the Ecological Index Method for assessing range condition in the Karoo. Proceedings of the Grassland Society of southern Africa 17:84-88.

 

 

Table 1. Ecological index values compared to the subjectively estimated grazing index values for the genus Pentzia

 Species

EIV

SGIV

 Pentzia albida

1

0.9

 P. annua

1

1.2

 P. cooperi

1

2.9

 P. tortuosa

4

2.9

 P. punctata

1

3.4

 P. calcarea

1

3.5

 P. elegans

4

3.8

 P. dentata

4

4.2

 P. globosa

4

4.8

 P. spinescens

4

4.8

 P. pinnatisecta

4

5

 P. incana

4

5.7

 P. lanata

4

5.7

 P. sphaerocephala

7

6.9

 

 

Table 2. Ecological index values compared to the subjectively estimated grazing index values for the genus Stipagrostis

 Species

EIV

SGIV

 Stipagrostis anomala

4

3.4

 S. namaquensis

7

3.8

 S. uniplumis

7

5.4

 S. brevifolia

7

5.8

 S. obtusa

10

6.6

 S. amabilis

10

7

 S. ciliata

10

7.2

 

 

Table 3. Species composition of site 1 in paddock 1a, comparing ecological-, subjective- and objective grazing index values and their respective veld condition indices. EIV = ecological index value, SGIV = subjectively estimated grazing index value, OGIV = objectively estimated grazing index value, VCI = veld condition index

 Species

strikes

number/100

EIV

VCI

SGIV

VCI

OGIV

VCI

 Cynodon incompletus

1

0.5

4

2.0

4.1

2.1

1.88

0.9

 Digitaria eriantha

60

30

10

300.0

8.9

267.0

6.37

191.1

 Eriocephalus spinescens

1

0.5

4

2.0

4.5

2.3

2.12

1.1

 Lycium cinerium

1

0.5

1

0.5

2.0

1.0

1.31

0.7

 Opslag

2

1

1

1.0

1.0

1.0

0.50

0.3

 Pentzia globosa

1

0.5

4

2.0

4.8

2.4

2.30

1.2

 Sporobolus fimbriatus

48

24

10

240.0

9.5

228.0

7.03

168.7

 Stachys rugosa

0

0

1

0.0

1.0

0.0

0.90

0.0

 Tetrachne dregei

1

0.5

10

5.0

10.0

5.0

7.59

3.8

 Themeda triandra

49

24.5

10

245.0

9.3

228.0

6.81

166.8

 Total

 

82

 

797.5

 

736.8

 

534.5

 

 

Table 4. Species composition of site 2 in paddock 1b, comparing ecological-, subjective- and objective grazing index values and their respective veld condition indices. EIV, SGIV, OGIV and VCI as before

 Species

strikes

number/100

EIV

VCI

SGIV

VCI

OGIV

VCI

 Cynodon incompletus

2

1

4

4.0

4.1

4.1

1.88

1.9

 Digitaria eriantha

29

14.5

10

145.0

8.9

129.1

6.37

92.4

 Dimorphotheca zeyheri

1

0.5

1

0.5

1.7

0.9

1.20

0.6

 Lycium cinerium

2

1

1

1.0

2

2.0

1.31

1.3

 Pentzia globosa

13

6.5

4

26.0

4.8

31.2

2.30

15.0

 Salvia stenophylla

2

1

1

1.0

1

1.0

0.85

0.9

 Sporobolus fimbriatus

22

11

10

110.0

9.5

104.5

7.03

77.3

 Stachys rugosa

7

3.5

1

3.5

1

3.5

0.90

3.2

 Tetrachne dregei

7

3.5

10

35.0

10

35.0

7.59

26.6

 Themeda triandra

68

34

10

340.0

9.3

316.2

6.81

231.5

 Total

 

76.5

 

666.0

 

627.5

 

450.5

 

 

Table 5. Species composition of site 3 in paddock 2a, comparing ecological-, subjective- and objective grazing index values and their respective veld condition indices. EIV, SGIV, OGIV and VCI as before

 Species

strikes

number/100

EIV

VCI

SGIV

VCI

OGIV

VCI

 Chrysocoma ciliata

1

0.5

1

0.5

1.5

0.8

1.12

0.6

 Digitaria eriantha

47

23.5

10

235.0

8.9

209.2

6.37

149.7

 Eragrostis obtusa

1

0.5

7

3.5

4

2

2.94

1.5

 Lycium cinerium

1

0.5

1

0.5

2

1

1.31

0.7

 Opslag

10

5

1

5.0

1

5

0.50

2.5

 Pentzia globosa

13

6.5

4

26.0

4.8

31.2

2.30

15.0

 Pteronia tricephala

3

1.5

1

1.5

1.7

2.6

1.20

1.8

 Rosenia humilis

1

0.5

1

0.5

3.5

1.8

1.77

0.9

 Sporobolus fimbriatus

77

38.5

10

385.0

9.5

365.8

7.03

270.7

 Themeda triandra

1

1.5

10

15.0

9.3

4.7

6.81

10.2

 Total

 

78.5

 

672.5

 

624.1

 

453.4

 

 

Table 6. Species composition of site 4 in paddock 2b, comparing ecological-, subjective- and objective grazing index values and their respective veld condition indices. EIV, SGIV, OGIV and VCI as before

 Species

strikes

number/100

EIV

VCI

SGIV

VCI

OGIV

VCI

 Chrysocoma ciliata

1

0.5

1

0.5

1.5

0.8

1.12

0.6

 Cynodon incompletus

5

2.5

4

10.0

4.1

11.3

1.88

4.7

 Digitaria eriantha

75

37.5

10

375.0

8.9

333.8

6.37

238.9

 Eragrostis lehmanniana

1

0.5

7

35.0

5.4

2.7

3.24

1.6

 Eragrostis curvula

8

4

7

28.0

6.7

26.8

3.47

13.9

 Eriocephalus spinescens

1

0.5

4

2.0

4.5

2.3

2.12

1.1

 Lycium cinerium

3

1.5

1

1.5

2.0

3.0

1.31

2.0

 Opslag

3

1.5

1

1.5

1.0

1.5

0.50

0.8

 Pentzia globosa

8

4

4

16.0

4.8

19.2

2.30

9.2

 Rosenia conferta

1

0.5

1

0.5

3.2

1.9

1.69

0.8

 Salsola calluna

1

0.5

7

3.5

7.2

3.5

3.55

1.8

 Sporobolus fimbriatus

11

5.5

10

55.0

9.5

52.3

7.03

38.7

 Total

 

59

 

497.0

 

457.9

 

313.9

 

 

Table 7. Average veld condition indices for the benchmark sample sites

   Site

EIV

SGIV

OGIV

   1

797.5

736.8

534.52

   2

666

627.5

450.54

   3

672.5

624.1

453.39

   4

497

457.9

313.9

   Mean

658.25

611.58

438.08

 

 

Table 8. Calculated grazing capacities for the three sites using the benchmark indices of 650 (EIV and SGIV) and 500 (OGIV)

   Site

EIV

SGIV

OGIV

Mean

   1

5.8

6.3

6.7

6.267

   2

7.0

7.4

7.9

7.433

   3

6.9

7.4

7.9

7.4

   4

9.3

10.1

11.4

10.267

   Mean

7.25

7.8

8.475

7.8417/7.84175

 

 

Table 9. Calculated grazing capacities for the three sites using the rounded-off benchmark indices

   Site

EIV

SGIV

OGIV

Mean

   1

5.9

5.9

5.9

5.9

   2

7.1

6.9

7.0

7.0

   3

7.0

7.0

6.9

7.0

   4

9.5

9.5

10.0

9.7

   Mean

7.4

7.3

7.5

7.4/7.4

 

Published

Karoo Agric Vol5 No1 2002 (1-6)