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The quantification of grazing capacity from grazing —and production values for forage species in semi-aridgrasslands of southern AfricaHC van der Westhuizen , HA Snyman , WLJ van Rensburg & JHJ PotgieterPublished online: 12 Nov 2009.

To cite this article: HC van der Westhuizen , HA Snyman , WLJ van Rensburg & JHJ Potgieter (2001) The quantificationof grazing capacity from grazing — and production values for forage species in semi-arid grasslands of southern Africa,African Journal of Range & Forage Science, 18:1, 43-52, DOI: 10.2989/10220110109485754

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African Journal of Range & Forage Science 2001, 18: 43–52Printed in South Africa — All rights reserved

Copyright © NISC Pty LtdAFRICAN JOURNAL OF

RANGE & FORAGE SCIENCEEISSN 1727–9380

The quantification of grazing capacity from grazing — and production val-ues for forage species in semi-arid grasslands of southern Africa

HC van der Westhuizen1*, HA Snyman2, WLJ van Rensburg2 and JHJ Potgieter3

1Department of Agriculture, FSR–E Unit, Private Bag X01, Glen 9360, South Africa2Department of Grassland Science, University of the Free State, PO Box 339, Bloemfontein 9300, South Africa3Specialist Environmental Services, South African National Defence Force, Private Bag X319, Pretoria 0001, South Africa

* Corresponding author, e-mail: mias@glen.agric.za

The relation between rangeland condition and grazing capacity was determined along a degradation gradient. In studying agro-

nomic values of forage species, the average production per tuft was combined with its grazing preferences, to link grazing values

for species in the semi-arid grasslands of southern Africa. A production index, based on mean mass per tuft, was first compiled for

species. Preference utilisation ratio of species was estimated for each of cattle and sheep, from the proportion of each species

found in the diet relative to the proportion of the species found in the forage on offer. A microhistological technique was applied to

oesophageal fistula samples to determine the proportion of each species found in the diet. Grazing values and grazing index were

determined from the production and preference utilisation ratio of species. Where grazing values of species were determined sub-

jectively in the past, in this study species were objectively classified based on estimated grazing values. Meaningful relationships

between rangeland condition and grazing capacity were determined, along a degradation gradient, with the long-term coefficient of

forecasting more than 75%. The financial implication with respect to sustainable animal production and rangeland condition varia-

tion was also calculated.

Determining grazing capacity is a controversial subject due

to an abundance of factors influencing its determination

(Vorster 1981a, 1981b, Roe 1997). When one factor

changes, grazing capacity changes to a greater or lesser

extent. The notion of rangeland carrying capacity is consid-

erably ambiguous even under conditions of high environ-

mental certainty (Roe 1997). According to Roux (1979),

Fourie et al. (1985) and Van der Westhuizen (1994) the

sequence of importance of the different factors determining

grazing capacity differs in the different ecological areas. For

example, rainfall is more important in arid areas than in

areas of high rainfall (Roux 1979, Teague et al. 1981). In

most areas of southern Africa, the sequence of importance

of factors determining grazing capacity are the following:

rainfall, available soil moisture dependent on soil type, soil

depth and evapotranspiration; rangeland condition; topogra-

phy and stock type (Fourie et al. 1985).

Though environmental factors determine inherent graz-

ing capacity (O’Connor and Bredenkamp 1997), manage-

ment factors (Danckwerts and Tainton 1996) and climatic

variation (Barnes and McNeil 1978, Rutherford 1980,

Danckwerts 1982b, Van den Berg 1983, Van der Westhuizen

1994, Snyman 1998) also influence the degree of variation

from the potential long-term grazing capacity. Selecting the

correct stocking rate is the most important of all grazing

management decisions, and is based on sustainable use of

vegetation, livestock and wildlife production, and economic

return (Danckwerts and Tainton 1996, Snyman 1998).

Though it is very difficult to determine grazing capacity of

rangeland, it is essential to estimate grazing capacity, which

can serve as a guide for sustainable rangeland utilisation.

The grassland biome occupies approximately 27%

(349 174km2) of the surface area of South Africa (Rutherford

and Westfall 1994, O’Connor and Bredenkamp 1997), and is

used almost exclusively for extensive livestock production.

It is important to apply stocking rates based on estimated

grazing capacity which will allow for the sustainable utilisa-

tion of this rangeland ecosystem as stocking rate is the most

important factor influencing: rangeland condition (Van den

Berg et al. 1975, Van der Westhuizen 1994, Danckwerts and

Tainton 1996), available grazing material (Snyman 1997),

sensitivity to drought periods (Fouché 1992, Snyman and

Fouché 1991, 1993, Snyman 2000), animal performance

(Danckwerts and King 1984) and gross income (Snyman

1998, 1999). Various attempts have been made to find a sci-

entifically based method to estimate grazing capacity in

southern Africa (Tainton et al. 1980, Du Toit et al. 1981,

Danckwerts 1982b, Vorster 1982, Kruger 1983, Fourie et al.

Introduction

Keywords: Preference utilisation ratio, preference utilization ratio, grazing value, palatability, production potential, utilisation, utilization,

rangeland condition

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Van der Westhuizen, Snyman, Van Rensburg and Potgieter44

1985). Long-term grazing capacity trials give good results,

but are time-consuming and expensive and have to be

repeated under different soil conditions. Researchers agree

that within a agro-ecological unit, rangeland condition forms

the best basis for determining grazing capacity (Kruger

1983, Van der Westhuizen 1994).

With rangeland condition assessment techniques, the

dominant approach is mainly ecological in southern Africa

(Van der Westhuizen 1994), while less attention is given to

the agronomical values (productivity, forage value and

perenniality) (Van der Westhuizen et al. 1999). Grouping

species according to palatability classes (Barnes et al. 1984,

Rethman and Kotzé 1986, Barnes 1990) can increase the

objectivity of grazing capacity recommendations based on

vegetation composition. By linking the preference grazing

value of species to aboveground phytomass, more accurate

animal performance recommendations can be made.

Grazing capacity recommendations based on utilisable pro-

duction normally does not provide for the long-term influ-

ence of animals on the ecological status of the veld (Tainton

1988). The objective with this study was to determine the

relation between rangeland condition and grazing capacity

along a degradation gradient by looking at the agronomical

values in rangeland condition quantification. This technique

must not only be able to scientifically explain rangeland con-

dition, but also correct grazing capacity recommendations

according to rangeland condition for a semi-arid climate.

Study area

The vegetation of a number of monitoring sites, evenly dis-

tributed within the semi-arid sweet grassland of southern

Africa (Rutherford and Westfall 1994), was classified by

means of a TWINSPAN classification (Hill 1979a) in order to

identify a homogeneous plant community. This was done

since the monitoring sites were located in nine different veld

types (Acocks 1988) with a heterogeneous species compo-

sition. After a larger, more homogeneous area was identi-

fied, an ordination technique ‘Detrended Correspondence

Analyses’ (DCA-Decorana) (Hill 1979b), part of the ISPD

software package (Bosch et al. 1992), was used in addition

to a TWINSPAN classification to identify a homogeneous

study area.

The study was conducted in semi-arid Cymbopogon–Themeda Veld (Acocks 1988) in an area covered by the

magisterial districts of Bloemfontein, Brandfort, Excelsior,

Theunissen and Winburg, South Africa (26° 20’ to 27° 05’ E

and 28° 24’ to 29° 02’ S). Data were gathered at seven sites

in this area. The vegetation is classified as sweet grassland

with Themeda triandra dominating when it is in an ecologi-

cally stable condition. Cymbopogon plurinodis and Digitariaeriantha are other climax species occurring less abundantly.

Eragrostis chloromelas is the dominant subclimax species,

with pioneers like Aristida bipartita dominating on heavier

soils and Cynodon hirsutus on trampled and overgrazed

rangeland.

The study area is in a summer rainfall region with mean

annual long-term rainfall varying from 512mm to 565mm for

different weather stations in the area. Annual rainfall fluctu-

ates with a standard deviation of more than 160mm.

Though major seasonal differences occur, more than 70% of

rain falls from November to March. The summers are mod-

erate to warm and winters very cold with frost from the mid-

dle of April to the middle of October. The average length of

the frost period is 175 days with the growing season varying

from 168 to 212 days (ISCW–data bank 1993).

The following soil forms are found, Milkwood (more than

35% clay and no limestone in the A-horizon), Arcadia,

Valsrivier (clay content varies from 35% to 55%), Westleigh

(clay content varies between 15% and 35%), Swartland

(clay percentage between 35% and 55%), Bonheim and

Mispah (clay percentages higher than 35%) (Macvicar et al.1977).

Methods and techniques

Layout of experimental plotsSix experimental sites were identified and laid out during the

1987 growing season in the most important landtypes of the

study area. Another five sites were laid out during the 1988

growing season on the Glen Research Station. These sites

were chosen to be representative of different rangeland con-

ditions, subjectively determined by experienced

researchers. The range in condition of various sites varied

from poor to exellent. The following plots were laid out on

every experimental site: (1) a fenced plot of 100m x 100m

where dry material production and botanical composition

were determined, and (2) an unfenced plot of 100m x 100m

where only the botanical composition was determined.

Production potential plots were only grazed once a year in

winter when the grasses were dormant. This utilisation was

such that all dry material of the previous season was

removed.

Experimental techniquesThe botanical composition were used to determined range-

land condition and was fully discussed by Van der

Westhuizen et al. (1999). Total seasonal production and the

contribution of each species to the production was deter-

mined annually during winter months on the exclusion plots.

Quadrats (30) were randomly distributed over the plots and

defoliated on a tuft basis. This sample size has been shown

to be adequate for the study area (Van der Westhuizen

1994). Every tuft cut was noted and production determined

on a species basis. The production of the few shrubs occur-

ring was determined by cutting all shoots less than 2mm

thick.

The Statgraphics Plus package was used to test produc-

tion differences between species. Due to the large effect that

environment has on the production of individual species,

these data were only used in indexing species and also to

calculate grazing values.

PalatabilityTo determine the palatability of individual plant species it is

essential to determine diet preferences of stock types.

Potgieter (1991) described a microhistological technique to

determine the botanical composition of diet samples select-

ed by oesophageal fistulated animals. In the study area

these investigations were conducted on the Glen Research

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Station with Merino whethers and Bonsmara type steers,

where five experimental sites were identified. Species selec-

tion was determined with three oesophageal fistulate steers

and three whethers to supply information on preferred

species during three phenological growth stages of the

rangeland. The proportion of dry matter contributed to pro-

duction on offer was also determined for every species on

each sampling occasion.

Results and discussion

Grazing value can be defined as the potential genetic ability

of a plant species to produce forage production (Van

Oudtshoorn 1991). In this study we concentrated on produc-

tion potential and preference rating of species.

Species productionIn the production plots the number of tufts per species

clipped in each quadrate varied greatly due mainly to the

fact that most species had low abundance values.

Therefore, there are more data on the tuft production of

more abundant species like Themeda triandra and

Eragrostis chloromelas than of rarer species like Eragrostisplana and Nenax microphylla. The number of times when a

specie was cut (n), the average number of tufts clipped in

the production plots (X), the mean mass per tuft and the

standard deviation of species are shown in Table 1. The

mean mass per tuft expressed as a percentage of the mean

mass of the highest producer (Cymbopogon plurinodis) was

used to allocate production index values , varying from zero

to ten, to every species (Table 1).

The big variation in tuft sizes created a practical problem

in determining mean production data per species. This vari-

ation is normal as tuft sizes vary greatly in the veld and

results in very large standard deviations. In rare species,

where a limited number of tufts was clipped, the sample size

contribute to this large standard deviations. Regardless of

these large variations, the data were useful because they

were previously unavailable for the area investigated and

could be used to calculate grazing values.

Diet preferencesOesophageal fistula sampling was carried out during spring

(October), when grasses begins to grow, late summer

(March), near the end of the growing season and during the

winter (July), when grasses reaches dormancy. The range of

species percentage contribution to dry mass yield on offer as

well as the range of species percentage contribution to the

diet of Merino whethers and Bonsmara type steers, collect-

ed at the same time is shown in Table 2 for one phenologi-

cal growing stage (March). The contribution of karoo shrubs

and annual herbs was combined to obtain the collective

effect of this group. Grass types like Chloris virgata,

Cynodon dactylon, Eragrostis lehmanniana and

Heteropogon contortus are very palatable, this time of the

year, as they occurred in the diet of both whethers and

steers while their contribution in the rangeland was so

insignificant that they were not noted with the cutting tech-

nique. The low creeping growth form of Tragus koelerioidesresulted in it only being utilised by whethers. Themeda trian-dra was the most important species in the rangeland as well

as in the diet of both sheep and steers. The big difference

between the contribution of Digitaria eriantha in the range-

land and in the diet indicated that this species was preferred

by cattle as well as sheep. For sheep the fistula data show

a definite tendency for the preference of Digitaria argyro-grapta, Eragrostis superba and herbs in relation with it’s con-

tribution in the rangeland (Table 2)

In Table 3, the average percentage contribution of the

most important species to the diet of Merino whethers and

Bonsmara type steers is shown for three phenological

growth stages of the rangeland. The total contribution of this

group of species is more than 74% of the diet of sheep and

91% of steers (Table 3). Based on the average contribution

over the three sampling periods Themeda triandra was the

most important species in the diet of sheep, followed by the

group of annual herbs and karoo shrubs with Eragrostischloromelas the second most important species in the diet of

cattle (Table 3). Marked changes in floristic composition can

take place in areas of lower rainfall in response to grazing

pressure. Previous research in the study area showed that

Themeda triandra and Eragrostis lehmanniana decreased,

whereas Aristida congesta and Tragus koelerioidesincreased with more intensive grazing during the summer.

These changes persisted for nine years after the withdrawal

of grazing (Potts 1923, Mostert 1958, Van den Berg et al.1975). The increase in annual herbs and karoo shrubs in the

diet of whethers can be explained by the decrease in the

African Journal of Range & Forage Science 2001, 18: 43–52

Species n X x mass (g) Std dev. I.V.

Cymbopogon plurinodis 43 30 10.91 16.56 10

Aristida vestita 6 10 10.29 5.04 9

Themeda triandra 70 234 7.92 5.36 7

Digitaria eriantha 26 36 7.81 20.20 7

Sporobolus fimbriatus 6 3 7.78 11.86 7

Heteropogon contortus 35 28 6.79 8.18 6

Walafrida saxatilis 3 10 5.86 2.36 5

Helichrysum dregeanum 14 13 5.79 18.54 5

Elionurus muticus 25 41 5.29 3.70 5

Eragrostis superba 32 20 4.81 4.56 4

Aristida bipartita 34 30 4.42 6.67 4

Eragrostis plana 4 3 4.12 2.66 4

Eragrostis chloromelas 67 63 4.00 2.76 4

Pentzia globosa 22 44 3.70 6.60 3

Eragrostis lehmanniana 25 31 3.19 1.97 3

Panicum stapfianum 44 30 3.15 2.24 3

Eragrostis obtusa 39 13 2.82 5.79 3

Digitaria argyrograpta 51 72 2.13 2.34 2

Aristida congesta 51 21 2.00 2.31 2

Brachiaria eruciformis 7 21 1.94 2.12 2

Chloris virgata 15 12 1.83 3.17 2

Cynodon hirsutus 18 38 1.82 1.74 2

Setaria pallide-fusca 18 31 1.80 1.74 2

Felicia muricata 20 12 1.75 2.44 2

Nenax microphylla 4 2 1.58 1.07 1

Aristida adscensionis 5 32 1.16 0.85 1

Cynodon dactylon 14 86 0.88 0.97 1

Microchloa caffra 5 4 0.55 0.48 1

Tragus racemosus 5 3 0.50 0.33 0

Table 1: The number of times that species were cut (n), the aver-

age number of tufts cut in n (X), mean mass per tuft, standard devi-

ation and index values (I.V.) of species for the study area.

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Van der Westhuizen, Snyman, Van Rensburg and Potgieter46

quality of perennial grasses when dormant. This trend was

not observed in steers and the percentage contribution of

annual herbs and karoo shrubs found in the diet during

March consisted of the inflorescences of Hertia pallens and

Nidorella resedifolia (Table 3).

The percentage contribution of species to the diet of ani-

mals can result in wrong observations for palatability if it is

not compared to the dry mass yield on offer for each

species. The formula used by Durham and Kothmann (1977)

and Potgieter (1991) to calculate palatability of species, was

adapted in the following way during this study to estimate

the ratio in which species will be utilised.

Preference utilisation ratio = 50+(% of diet - %available) x 50

(% of diet + %available)

A preference utilisation ratio value of 100 indicates that a

species occurred only in the diet and was not found in the

rangeland with the cutting technique. This species is there-

fore very palatable and will be utilised 100%. A preference

utilisation ratio value of zero indicates that the species only

occurred in the rangeland and was not utilised by animals.

A preference utilisation ratio value of 50 indicates utilisation

in the same ratio as occurrence of the species in the range-

land and it is accepted that 50% of this species will normal-

ly be utilised. The length of the grazing period will obviously

have a influence on utilisation of species. As the grazing

period extend the high preference species will be consumed

and lesser palatable species will be grazed. According to

Van der Westhuizen (1976) and Van der Westhuizen et al.(1978), there are no large changes in plant species selection

patterns of sheep, when camps are grazed longer and avail-

able material decreases. This behavioural pattern of sheep

results in more palatable species being maximally utilised

before relatively less palatable species are proportionally

more utilised. Bester (1977) observed the same trend with

cattle. The preference utilisation ratio of the most important

species to the diet of Merino whethers and Bonsmara type

steers as well as the most common species in the range-

land, was calculated for the three different growing stages,

using the formula above (Table 4). Large variations occurred

in the preference utilisation ratio over the season and

between whethers and steers. O’Connor and Bredenkamp

(1997) discussed the same trend of variation in seasonal

utilisation.

The preference utilisation ratio of an individual plant type

can vary seasonally (Table 4). Merino whethers will prefer

species such as Sporobolus fimbriatus and D. argyrograptaduring October (spring) and they will avoid Aristida bipartita.

During March (late summer) Merino whethers preferred

Species Range land Merino whethers Bonsmara type steers

Aristida bipartita 0.37 – 11.35 0.14 – 0.95 0.09 – 2.96

Chloris virgata not cut 0.00 – 0.81 0.00 – 0.18

Cymbopogon plurinodis 6.54 – 13.75 0.86 – 7.84 3.23 – 20.34

Cynodon dactylon not cut 0.74 – 3.01 0.31 – 1.84

Digitaria argyrograpta 0.4 – 1.46 3.41 – 21.94 1.44 – 4.87

Digitaria eriantha 0.00 – 1.11 2.89 – 12.28 0.80 – 48.94

Eragrostis chloromelas 1.87 – 9.34 5.93 – 26.92 2.57 – 11.65

Eragrostis lehmanniana not cut 0.00 – 0.76 0.00 – 2.96

Eragrostis obtusa 0.62 – 4.47 0.00 – 0.80 0.00 – 0.94

Eragrostis superba 0.00 – 0.05 1.75 – 22.59 0.00 – 3.68

Heteropogon contortus not cut 0.96 – 3.32 0.24 – 1.88

Panicum stapfianum 0.29 – 2.97 1.09 – 11.11 1.99 – 5.45

Sporobolus fimbriatus 0.00 – 5.54 1.98 – 5.83 0.00 – 9.33

Themeda triandra 56.61 – 83.50 12.90 – 39.24 15.37 – 47.86

Tragus koelerioides not cut 0.00 – 0.68 0

Karoo shrubs and Herbs 1.89 – 4.27 4.46 – 27.25 0.00 – 23.26

Table 2: The range of species contribution (percentage) in the range land within which oesophageal fistula samples were collected as well

as the range of species contribution to the diet of Merino whethers and Bonsmara type steers (percentage) during March.

Species Merino whethers Bonsmara type steers

October March July Average October March July Average

Themeda triandra 63.43 24.41 23.02 36.95 63.64 31.99 47.56 47.73

Eragrostis chloromelas 5.49 13.73 7.5 8.91 11.46 8.34 25 14.93

Karoo shrubs & Herbs 0 15.08 41.18 18.75 0 9.84 0 3.28

Digitaria eriantha 0 7.48 1.5 2.99 0 22.74 0.91 7.88

Cymbopogon plurinodis 11.56 5.06 0.44 5.69 0.89 10.39 3.05 4.78

Heteropogon contortus 3.71 2.21 0.71 2.21 14.87 0.8 3.05 6.24

Eragrostis superba 0 10.46 0 3.49 0.64 2.07 0 0.9

Total 84.19 78.43 74.35 78.99 91.5 86.17 79.57 85.74

Table 3: The percentage contribution of the most important species to the diet of Merino whethers and Bonsmara type steers.

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species such as E. superba and H. contortus (if available)

and also species such as D. eriantha and D. argyrograptawhile their utilisation of A. bipartita will still be very low. There

was also a big increase in the preference utilisation ratio of

karoo shrubs and annual herbs, during March, by sheep.

During July (winter) Merino whethers preferred species such

as D. eriantha, S. fimbriatus, P. stapfianum and karoo shrubs

and herbs while their preferences for C. plurinodis, H. con-tortus and T. triandra decreased (Table 4).

Steers avoid C. plurinodis during October and they will

prefer E. superba if this specie is available. During March

steers preferred E. superba, H. contortus, D. eriantha, D.argyrograpta and P. stapfianum while their preference for A.bipartita will be low. During winter steers preferred D. eri-antha, D. argyrograpta, E. chloromelas and H. contortus(Table 4).

Except for the rarer species observed only in the diet,

Sporobolus fimbriatus is the only species preferred by sheep

throughout the season while steers preferred Heteropogoncontortus, Digitaria argyrograpta and Eragrostis chloromelasduring the season. Digitaria eriantha was also well selected

by both stock types, but being a slow initial grower, no graze-

able material was produced and available early in the sea-

son.

The preference utilisation ratio pattern of Themeda trian-dra and Cymbopogon plurinodis emphasised that sheep

don’t prefer the stems of these species. The preference util-

isation ratio of these two species decreased as the growing

season progressed. The same trend was not observed with

the steers (Table 4). Potgieter (1991) found that stems con-

tributed a mere four percent to the dry mass composition of

the diet selected by Merino whethers while steers selected

26.29 percent stems.

In whethers, the preference utilisation ratio of E.chloromelas and Panicum stapfianum increased as the

growing season progressed. The leaves of E. chloromelasthat are relatively fine and soft during the dormant period in

comparison to other species can explain this increase while

the leaves of P. stapfianum become reasonably brittle as the

species reaches dormancy. The whethers selected annual

herbs and Felicia muricata, while other shrubs like

Helichrysum dregeanum and Pentzia globosa were also

utilised. Steers avoided these species.

Grazing value of species

To link grazing values to plants, preference utilisation ratio

and production of species were considered. Grazing values

of species were calculated by multiplying preference utilisa-

tion ratio with the mean mass per tuft (Table 1). One of the

principles of controlled selective grazing is that key plants

must not be defoliated more than 60% during the growing

season (Fourie and Van Niekerk 1985). Based on the facts

that there are no large changes in plant species selection

patterns of sheep and cattle when camps are grazed longer

and that less palatable species will be grazed proportionate-

ly more (Van der Westhuizen 1976, Bester 1977), the pref-

erence utilisation ratio of species as calculated in Table 4

was proportionally increased until utilisability of Themedatriandra reached 60% during the early summer. Themedatriandra was used as it is the most ecologically important

species in the study area (Van der Westhuizen et al. 1999)

and the assumption was made that veld deterioration will

only take place if T. triandra is over utilised. Furthermore, a

utilisation percentage of 100% indicates that this species is

utilised below the cutting height (40mm).

As there are reasonably big differences between the util-

isation patterns of whethers and steers (Table 3), grazing

values were calculated for both stock types. This calculated

grazing value cannot be used as an absolute value as tuft

size varies so much, but can be used to differentiate

between species on an index basis. The grazing values and

index values of species are shown in Table 5.

Between the two stock types, the biggest differences in

grazing values of different plants are in that Sporobolus fim-briatus and karoo shrubs have higher values for sheep,

while Heteropogon contortus has a higher value for cattle

than sheep. The lower values of Themeda triandra, in rela-

tion with other species, are due to its high abundance in the

rangeland where its contribution to production is used in cal-

culating preference utilisation ratio. The high preference util-

isation ratio of species such as D. eriantha, S. fimbriatus, E.superba and H. contortus (Table 4) and also the high tuft

production of C. plurinodis (Table 1) contribute also to the

lower values of T. triandra. Though the grazing value of

Themeda triandra is relatively low (Table 5) this species

makes the biggest contribution to the diet of both stock types

(Table 3), the biggest contribution to dry mass yield of the

rangeland (Table 2) and are ecologically the most important

species in the study area (Van der Westhuizen et al. 1999).

African Journal of Range & Forage Science 2001, 18: 43–52

Species Merino whethers Bonsmara type steers

October March July Average October March July Average

Eragrostis superba 100 100 100 100 100

Digitaria eriantha 88 100 94 79 100 90

Digitaria argyrograpta 75 83 38 65 69 77 86 77

Heteropogon contortus 47 100 17 55 73 100 79 84

Sporobolus fimbriatus 91 70 100 87 50 54 52

Eragrostis chloromelas 50 63 70 61 72 56 81 70

Panicum stapfianum 41 68 100 70 32 77 68 59

Karoo shrubs & Herbs 77 100 89 54 54

Themeda triandra 45 26 23 31 46 33 38 39

Cymbopogon plurinodis 68 29 10 36 18 45 33 32

Aristida bipartita 0 14 47 20 30 14 51 32

Table 4: The preference utilisation ratio of species for different growing stages.

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Van der Westhuizen, Snyman, Van Rensburg and Potgieter48

Grazing capacity calculationsTo determined the relationship between rangeland condition

and grazing capacity the total dry matter production of each

species for, every production site, was linked with it’s mean

percentage preference utilisation ratio during the growing

season, as determined in Table 4. The preference utilisation

ratio of species as calculated in Table 4 was again propor-

tionally increased until utilisability of Themeda triandrareached 60% during the early summer. As there are differ-

ences between the selection patterns of cattle and sheep

(Tables 2 and 3), grazing capacity was determined for both

animal types. A large stock unit (LSU), defined as the equiv-

alent of a steer (450kg) gaining 500g per day on forage with

a mean digestibility of 55% (Meissner et al. 1983) and utilis-

ing 10kg dry material per day on average, was used as the

unit.

A number of rarer species like Elionurus muticus,

Aristida adscensionis, Eragrostis plana and Walafrida pan-iculata, which did not occur in camps where fistula samples

were collected, occurred on the experimental sites. As no

data concerning their palatability were collected, estimations

concerning their palatability were based on published data

(Mostert et al. 1971, Roberts 1973, Fourie and Visagie

1985, Visagie 1985, Van Oudtshoorn 1991, Du Toit et al.1995 and Du Toit 1995). These estimations were based on

previous literature where the rarer species’ preference utili-

sation ratio was equated with that of a species for which

data were gathered. The maximum contribution to produc-

tion and equated species used to estimate their preference

utilisation ratio is shown in Table 6. Though these estimates

were based on subjective observations, the mean produc-

tion of these species, compared to the rest, is less than four

percent over all experimental plots.

The grazing capacity, as determined over seven growing

seasons of the different production sites, is shown in Table

7. The grazing capacity of Glen 5 was significantly (P<0.01)

less than the grazing capacity of the other plots, which was

expected as this plot represented rangeland in poor condi-

tion.

In all the experimental plots, the grazing capacity for

sheep was lower than that for cattle (Table 7). This is

affirmed as sheep will avoid inflorescences and stems of

grasses, while cattle will utilise them (Potgieter 1991).

Species Merino whethers Bonsmara type steers

Grazing value Index Grazing value Index

Digitaria eriantha 7.81 10 7.81 10

Sporobolus fimbriatus 7.78 10 5.28 7

Heteropogon contortus 4.98 6 6.79 9

Cymbopogon plurinodis 5.24 7 4.55 6

Eragrostis superba 4.81 6 4.81 6

Themeda triandra 3.27 4 4.03 5

Eragrostis chloromelas 3.25 4 3.65 5

Eragrostis lehmanniana 2.85 4 3.19 4

Panicum stapfianum 2.94 4 2.42 3

Digitaria argyrograpta 1.85 2 2.13 3

Chloris virgata 1.83 2 1.83 2

Helichrysum dregeanum 3.17 4 0.00 0

Aristida bipartita 1.18 2 1.84 2

Cynodon dactylon 0.88 1 0.88 1

Pentzia globosa 1.43 2 0.00 0

Felicia muricata 1.35 2 0.00 0

Eragrostis obtusa 0.11 0 0.33 0

Aristida vestita 0.00 0 0.00 0

Aristida congesta 0.00 0 0.00 0

Walafrida saxatilis 0.00 0 0.00 0

Table 5: Grazing values and index values for Merino whethers and

Bonsmara type steers.

Species Max. contr. (%) Equated species

Elionurus muticus 10.65 Aristida bipartitaSetaria pallide-fusca 8.07 Digitaria erianthaBrachiaria eruciformis 7.42 Aristida congestaCynodon hirsutus 6.52 Sporobolus fimbriatusAristida adscensionis 5.03 Aristida congestaEragrostis plana 1.67 Eragrostis chloromelasSutera atropurpurea 0.68 HelichrysumdregeanumEragrostis trichophora 0.56 Eragrostis chloromelasRuschia spp. 0.47 Pentzia globosaNenax microphylla 0.35 Felicia muricataAnthospermum pumilum 0.32 Pentzia globosaMicrochloa caffra 0.22 Aristida congestaTragus racemosa 0.21 Aristida congestaRosenia humilis 0.20 Hertia pallensOsteospermum leptolobum 0.11 Helichrysum dregeanumWalafrida paniculata 0.05 Walafrida saxatilis

Table 6: The maximum contribution to production of less abundant

species and equated species used to estimate their utilisability.

Production sites Rangeland condition Merino’s (ha LSU-1) Cattle (ha LSU-1)

Grazing capacity Standard deviation Grazing capacity Standard deviation

Riviera Excellent 6.8 2.7 5.8 2.2

Glen 3 Exellent 7.3 1.9 6.3 1.7

Voëlvlei Good 5.8 3.2 5.2 2.8

Vacant Good 7.2 4.3 6.2 3.7

Swaarkry Good 8.5 5.5 7.4 4.9

Middelplaas Good 9.1 6.6 7.9 5.5

Glen 4 Good 11.8 3.6 9.7 3.1

Glen 2 Moderate 12.2 6.8 10.4 6.1

Rama Moderate 12.1 6 10.9 5.5

Glen 1 Moderate 12.6 4.8 10.4 3.9

Glen 5 Poor 24.8 9.8 18.9 7.1

Table 7: Grazing capacity and standard deviation for different production sites.

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African Journal of Range & Forage Science 2001, 18: 43–52 49

Seemingly, Merinos are not as well adapted to climax grass-

land as are cattle.

Relationship between rangeland condition and grazingcapacityThe relationship between quantified rangeland condition for

the study area (Van der Westhuizen et al. 1999) and grazing

capacity is described as follows. Regressions were used to

quantify relationships. The rectangular hyperbole regression

as defined by Motulsky (1987) in the software package

GraphPAD gave the best results. The formula of this regres-

sion is as follows:

Y = A x X

(B + X)

where

Y = Grazing value (ha LSU-1);

X = Rangeland condition (%);

A = Constant leaning towards minimum value on y-axis

B = Constant indicating minimum value on x-axis.

The r2 values indicating the relationship between range-

land condition and grazing capacity for the growing seasons

and the long-term relationship is shown in Table 8. There

was a poor correlation between rangeland condition and

grazing capacity during the 1987 and 1988 growing seasons

(Table 8). That the Glen plots, representing different range-

land conditions, were only laid out in the 1988-growing sea-

son, explains the poor relationships in the 1987 season.

The length of the x-axis (range condition) was too short to

identify any relationships.

The poor correlation between rangeland condition and

grazing capacity during the 1988 growing season is due to

the high rainfall in that season. The rainfall was more than

twice that of the long-term mean for the area. This caused

waterlogged conditions to occur in certain production plots in

Glen 2 and also on the farm Middelplaas. After removing

these two seasons from the regression, the r2-values were

0.745 for cattle and 0.872 for sheep.

The rangeland in poor condition at Glen drastically

improved with time as the rangeland received the same

treatment as the other plots. This explains the poorer corre-

lation during the 1991growing season. To obtain a long-term

relationship, the mean grazing capacity over five years (from

1987 to 1991) was used for every experimental plot. The

long-term regression curve is illustrated in Figure 1.

If one ignores the waterlogged conditions during the

1988 season, the mean r2 values are 0.741 for cattle and

0.777 for sheep for the 1988 to 1991 period. Regardless of

other environmental variables, the long-term relationship

between rangeland condition and grazing capacity is rela-

tively good (Table 8). Therefore, it is possible to determine

the long-term grazing capacity from rangeland condition (r2

values > 0.78).

Economic implicationsGrazing capacity was calculated in Table 7, based on the

potential of the rangeland. Stocking rates higher than graz-

ing capacity do not only influence animal performance and

rangeland condition detrimentally, but also the profitability of

the farming enterprise (Aucamp et al. 1982, Gammon 1984,

Danckwerts and King 1984, Fourie 1985, Venter and

Goosen 1993).

The gross margins were calculated for different range-

land conditions. In these calculations, the gross margin per

LSU for a weaner calf production system as in the financial

economic analysis of the division of Agricultural Economics

of the Free State province (1998) was used.

For the calculations, gross margins per LSU are

assumed to stay constant as grazing capacity is calculated

on the basis of preference utilisation ratio of production. As

gross margin is subject to many variables, maximum return

ha-1 is expressed in relative terms of optimal conditions

(Figure 2). From these calculations, it is clear that maximum

return ha-1 can triple at a condition class of 40%, in compar-

ison to optimal conditions, while it can almost double at a

condition class of 50% (Figure 2).

Conclusions

Rangeland condition determinations are of little value to a

livestock farmer if not linked to rangeland management. The

long-term regression between rangeland condition and graz-

ing capacity for this study can be used to explain variation

Figure 1: Relationship between rangeland condition and grazing

capacity.

Rangeland condition (%)

Cattle

Sheep

Growing seasons Cattle Sheep

1987 0.055 NS 0.034 NS

1988 0.232 NS 0.481 NS

1989 0.929 ** 0.943 **

1990 0.786 ** 0.755 **

1991 0.502 * 0.537 **

Long-term 0.781 ** 0.806 **

** P = 0.01* P = 0.05NS P> 0.05

Table 8: r2 Values indicating the relationship between rangeland

condition and grazing capacity.

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Van der Westhuizen, Snyman, Van Rensburg and Potgieter50

between these two factors (r2 > 0.75). Less than 25% of the

variation in grazing capacity can be ascribed to environmen-

tal characteristics. The relationship between rangeland con-

dition and grazing capacity varies greatly over growing sea-

sons. Rangeland condition for the 1988/89 growing season

explained more than 90% of the variation in grazing capaci-

ty.

This study further made clear that dry matter production

differs markedly between species in semi-arid climates and

that high producers like Cymbopogon plurinodis and Aristidavestita produce more than ten times as much as low pro-

ducers like Aristida adscensionis, Cynodon dactylon,

Microchloa caffra and Tragus racemosus. The large stan-

dard deviations in the production of rarer species is one

drawback of this study. By gathering additional data regard-

ing production of these rarer species, standard deviations

could be lowered. If the surface area of tufts was measured

and production was expressed per unit area, the standard

deviation would have been lower. To calculate the grazing

value of species, the production potential per tuft is essen-

tial.

A further possible drawback in calculating the grazing

value of species is the fact that quality is ignored. Animals

prefer plant types of a relatively high quality and this char-

acteristic is therefore linked to the preference utilisation ratio

of species (Van der Westhuizen 1976, Bester 1977). This

was affirmed by the contribution of Themeda triandra to the

diet of Merinos, decreasing as this species reaches dor-

mancy.

This study showed that the plant types that animals pre-

fer vary over the growing season. The fact that Merinos

avoid long grasses and do not utilise the stems of grasses,

explains the difference between the grazing capacity recom-

mendations for sheep and cattle. It would seem that Merinos

are not well adapted to pure climax grassland and rather

concentrate on shorter subclimax and pioneer veld. This

leads to the conclusion that, for the efficient utilisation of this

semi-arid grassland, it must be utilised by both cattle and

sheep. The same conclusion was drawn by Mostert et al.

(1971), Van der Westhuizen (1976) and Bester (1977) for

the study area. One lack in the study is that the diet of mut-

ton sheep, mutton wool sheep, small frame-type cattle and

large frame-type cattle was not known and the diet of

Merinos and medium frame-type cattle was used as repre-

sentative. It was also assumed that lactating animals would

have the same selection pattern as dry animals.

The plant type making an important contribution to the

diet of sheep, was T. triandra (36.95%) and karoo shrubs

and herbs (18.75%). For cattle, T. triandra (47.73%) and

Eragrostis chloromelas (14.93%) were the two most impor-

tant species in the diet. More palatable types like Digitariaeriantha and Eragrostis superba also made an important

contribution to animal production, but due to their low occur-

rence in the rangeland, their contribution was relatively lim-

ited.

The importance of T. triandra as a key species for this

veld type is continually emphasised. This species is not only

the ecologically most important species in the study area,

but also a very good indicator of rangeland condition (r2 =

0.99) (Van der Westhuizen 1994, Van der Westhuizen et al.1999). Though utilisability of T. triandra is relatively low, it is the

most important species in the diet of both sheep and cattle.

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