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THE STOCKING RATE: GRAZING CAPACITY RELATION

 

PCV du Toit


Grootfontein Agricultural Development Institute

Private Bag X 529,MIDDELBURG EC, 5900

 


Introduction

At present there is a great deal of confusion in the minds of the farming community and various researchers which are aired in various publications, with regard to these two very important aspects dealing with the grazing management of veld.  For instance, the following quote is taken from a recently published guide on sustainable rangeland management; "The Namibian Directorate of Agriculture and Training has introduced an innovative move away from expressing carrying capacity as 'hectares per large stock unit or small stock unit' and has introduced the principle of biomass per hectare (Esler et al. 2002)".  This quote highlights the poor understanding of both the concepts of grazing capacity as well as of applied stocking rate on the veld, which has been advocated in South Africa since the 1970's.

 

For ease of reference, the stocking rate applied to veld is generally taken as the number of animals grazing a particular section of veld, without mention of their body weight.  However, on account of the fact that most farming systems deal with growing animals, i.e., producing meat and/or fibre and not with mature animals only, the applied stocking rate per farm, varies over a season and therefore, due recognition should be given to the weight of the animals when they are introduced onto the veld at the start of the grazing season.  A proper evaluation of the increase in weight of the animals with the advancing season should be made, to ensure that the applied stocking rate does not exceed the grazing capacity norm.  With regard to grazing capacity, this aspect of grazing management not only varies from farm to farm and from site to site on one farm, but also with varying climatic conditions within the different seasons.  A great deal of research activity has been concentrated on these two aspects since the late 1970's, it is still ongoing and flowing from this research, the whole of the country has been divided into grazing capacity zones.  This was done mainly on account of the varying dry matter production across South Africa in sympathy with the variation in rainfall.

 

Method and materials

 

1.  Grazing management

The research that will be briefly referred to in this paper is presently being carried out on the Boesmanskop portion of the Grootfontein Agricultural Development Institute=s experimental farm.  Thirty-six Merino sheep and 6 head of Nguni cattle are run on 206 ha of eastern mixed Karoo veld, the False Upper Karoo (Acocks 1988), comprising approximately one-third plains, one-third pediment and one-third randjiesveldt.  The cattle to sheep ratio is 1:1 on a large stock unit basis.  The cattle breeds varied over the seasons since the inception of the experiment, when Afrikaner oxen were used at first to be followed later by Bonsmaras and Drakensbergers.  Nguni cattle are being used since the start of the 2000/2001 season.  The sheep and cattle graze the veld together as one herd/flock, not as a separate flock of sheep and a separate herd of cattle in different paddocks, on a leader - follower system.  They graze the 11 different paddocks in rotation, with the period of occupation being two weeks and the period of absence on most of the paddocks being 26 weeks.  On the randjiesveldt, on account of the difficult terrain, water provision is a problem.  These paddocks are on average almost three times the size of the rest and, with an acknowledged lower grazing capacity, the period of occupation per paddock is two weeks with a period of absence of four weeks, after which the animals return for a further two weeks each.  After this short rotation period of occupation, the paddocks recover for a period of sixteen weeks before the animals return to them.

 

This experiment serves as a sheep and cattle control, stocked at the medium stocking rate of several other experiments on Grootfontein, with which it is compared (du Toit 2002a).  This medium stocking rate corresponds to the grazing capacity norm applicable to the area.  The grazing capacity norm applicable to the veld in the Middelburg District and on and around Grootfontein, is 16 ha/LSU.  Because of the fact that the veld is grazed in a multi-paddock system and is being compared to experiments with different races of sheep and different management systems, no fixed rotational resting period is incorporated.  Although this approach is not advisable, all the experiments can therefore be directly compared.

 

All the animals in the experiment are replaced annually at the start of the new grazing season, usually in late August or early September.  Grazing continues through the summer and winter into the next summer period, until the replacement animals are ready to go onto the veld.

 

2.  Veld evaluation

The veld is evaluated as to its current grazing capacity at the end of every growing season.  Line-point surveys using 500 points per survey are conducted by recording the canopy spread cover.  Two sites in each paddock are surveyed with three sites surveyed in the larger paddocks.  The overall mean of the grazing capacities for the different sites is computed and the veld stocked accordingly during the following season.  The method of calculating the grazing capacity follows the methods described and proposed by Du Toit (1997; 2000b & 2002).

 

Research results

 

Refer to Figs. one to four, where Figs. one and two depict the total seasonal weight increment for the sheep and cattle respectively over the 2000/2001 and the 2001/2002 seasons.  Figures three and four on the other hand depict the seasonal change in the applied stocking rate, with reference to the grazing capacity norm as well as with the computed grazing capacity.  When each season=s graphs are compared, i.e. Fig. 1 with Fig. 3, it will be seen that as the animals grow and put on weight over the season, so does the applied stocking rate increase and it approaches the grazing capacity norm.  This is the principle on which advice to stock farmers is based, to stock their farms fairly leniently at the start of the season with growing/producing animals.  It follows of course that they will have herds of mature animals, which grow very little over the season and in these instances, the starting stocking rate can be much closer to the grazing capacity norm.

 

Fig. 1.  Cattle and sheep weight changes over the 2000/2001 season

 

Fig. 2.  Cattle and sheep weight changes over the 2001/2002 season


Fig. 3.  Illustrating the change in the applied stocking rate over the 2000/2001 season and the way in which it approaches the grazing capacity norm, the grazing capacity norm is indicated by the top straight line, and the bottom straight line indicates the mean grazing capacity calculated for Boesmanskop by the line-point method of veld evaluation

 


Fig. 4.  Illustrating the change in the applied stocking rate over the 2001/2002 season and the way in which it approaches the grazing capacity norm, the top straight line minutely above the lower line, indicates the grazing capacity norm, the bottom straight line, only a little below the line indicating the norm, indicates the mean grazing capacity calculated for Boesmanskop by the line-point method of veld evaluation

 

The same picture emerges when Fig. 2 is compared to Fig. 4.  The only difference being the fact that the line-point evaluation of the veld indicated a mean grazing capacity very close to the grazing capacity norm.  A probable explanation for the fact that the calculated grazing capacity for 2001/2002 is lower than that for 2000/2001, is the fact that Karoo veld seems to respond to long term mean climatic variables (du Toit 2002c), not only with regard to aboveground plant production, but also in terms of animal production off this veld.

 

This is a very contentious issue, but over six seasons, the animal production results indicate that there seems to be a maximum animal production when compared to rainfall.  It is surmised that although Karoo veld has a higher dry matter production during above normal rainfall years, the animals do not respond in like manner (Table 1).

 

This argument is corroborated by referring to the animal production realized per millimetre of rain during a high and a low rainfall season.  In Table 1 animal production during the 1997/98 season, 7.4 kg/mm, is compared to the production of 3.6 kg/mm during the 2001/2002 season.  The same trend manifests itself during the 1997/98 season, when the average daily gain of 5.8 kg/day was obtained and only 2.8 kg/day during the 1996/97 season.

 

Table 1.  The various animal production measurements for the Boesmanskop experiment over six seasons

 Season

 96/97

 97/98

 98/99

 99/00

 00/01

 01/02

 Total gain (kg)

1535.4

 2014.3

 1819.1

 1930.8

 1852.4

 1690.9

 Total rain (mm)

 554.2

 273.9

 281.5

 460.3

 319.1

 472.8

 Days on veld

 337

 347

 283

 339

 357

 371

 Gain kg/ha

 7.5

 9.8

 8.8

 9.4

 9.0

 8.2

 Gain kg/day

 2.8

 5.8

 6.4

 5.7

 5.2

 4.6

 Gain kg/mm

 4.6

 7.4

 6.5

 4.2

 5.8

 3.6

 

 

In Table 2 the differences in starting weight and end-weight for the animals in the experiment over two seasons can be seen.  This data clearly show the narrow  cattle : sheep ratio, especially during the 2000/2001 season.  It also vividly illustrates the difference in total animal gain during the two seasons, quite contrary to expectations.  From this data, backed by the data used to model Karoo sub-shrub growth from climatic variables, the vegetation of the Nama-Karoo seems to be unable to make use of above normal rainfall in the production of nutritious forage, which is echoed in the lower animal production off this veld.  The slightly lower computed grazing capacity for the 2001/2002 season also seems to indicate poorer vegetative response to higher rainfall.  A higher bulk of forage is produced, but this may be of poorer species with a lower grazing value (du Toit 2000).

 

Table 2.  Animal production data for the 2000/2001 and 2001/2002 seasons

 Season

Sheep

Cattle

2000/2001

   

Starting weight (kg)

1352.5

1352

End weight (kg)

2233.9

2323

Total weight gain (kg)

881.4

971

Seasonal gain (kg)

 

1852.4

2001/2002

 

 

Starting weight (kg)

1435.9

1074

End weight (kg)

2107.8

2093

Total weight gain (kg)

671.9

1019

Seasonal gain (kg)

 

1690.9

 

General Discussion

 

1. Stocking Rate

 

The applied stocking rate on any section of land allocated to a particular flock of sheep or herd of cattle, is usually expressed as x - number of hectares per animal, since the capacity of the natural pastures of South Africa to carry stock is so low on account of the low rainfall and the arid nature of the larger part of South Africa.  If the stocking rate had to be expressed as the reciprocal, i.e. number of animals per hectare, use would have to be made of extremely small decimal numbers to express the number of large stock units per hectare.  Only in the case of irrigated, cultivated pastures in South Africa, is the stocking rate expressed as x - number of animals per hectare, where one hectare of cultivated pasture is able to carry more than one, usually three to seven or more large stock units per hectare, depending on the type of pasture, fertilization and irrigation.

 

When stock is put onto the veld therefore, the stocking rate is very carefully calculated for that particular area, taking into account the class of stock, whether large or small stock, the weight of the animals and with due regard taken of the grazing capacity norm.  As can be seen, the weight of the animals being put onto the veld is of paramount importance, applied stocking rates are therefore always calculated on the basis of animal weight (Meissner 1982; Meissner et al. 1983 & Venter 1982).  The starting stocking rate on the veld is usually determined by reference to the Large Stock Unit into which the animals to be used are classed.  Since this question is so important in the sustainable use of the South African veld, there are constant endeavours to adjust these small stock/large stock substitution values for all classes and breeds of stock (Herselman 2000).  Meissner (1993), referred to, and criticized, the inappropriateness of determining the initial stocking rate to be applied, on a purely biomass basis.

 

When Esler et al. (2002) referred to Venter=s work in Namibia (1982), where he introduced the system of calculating the desirable stocking rate in terms of biomass and in her appeal to farmers and researchers to adopt the biomass method to calculate the carrying capacity in South Africa, the enormity of the uncertainty and confusion referred to above, became evident.  Fact is, this method has been introduced early-on in South Africa amongst the pasture management and animal husbandry scientists, with Meissner et al. (1983) simply formalizing the position of all classes of stock grazing veld as their principal source of forage.  The seriousness of the confusion is evidenced by the use of the terms biomass and carrying capacity when Esler et al. (2002) refer to the sustainable use of veld by domestic stock.  The term carrying capacity and biomass will be dealt with briefly later-on.  However, this immediately highlights the important concept of grazing capacity and the need to understand exactly what is meant by it.

 

2.  Grazing Capacity


Grazing capacity is often incorrectly referred to as carrying capacity and not only by Bezuidenhout (2002) and Esler et al. (2002).  It is however, prudent not to use this expression with regard to the natural grazing lands of South Africa where the object is the sustainable management and utilization of the natural resources, by grazing domestic livestock as the harvesting machines.  Based on climatic variables, particularly rainfall, different zones have been mapped in South Africa, each with its own unique forage production capability.  These zones have each been allocated a specific grazing capacity, based not only on the forage production ability of the zone, but with due regard to the forage needs of growing animals.  These zones are then stocked with grazing animals, not to capacity, but with the long term productive ability of the veld in mind.  It is therefore, very necessary to take due note of the different classes of stock on the specific farm in relation to the overall grazing capacity of the farm, because of the differences in the number of these classes of animals constituting one large stock unit, on which the grazing capacity is based.  Lately, a short paper by Bezuidenhout (2002) and a rangeland management booklet by Esler et al. (2002) focused the attention at the poor general understanding of the grazing capacity concept.

 

The basis of all the grazing capacity zones, quoted as ha/LSU throughout the country, is the fact that the defined large stock unit consumes 10 kilogrammes of acceptable forage per day.  This is very nearly 2% of the body weight of a beast weighing 450 kg.  This animal will then grow at a rate of approximately 500 grammes per day (Meissner 1982 & Meissner et al. 1983).  As seen above and, based on the rainfall received in a specific area, the production of acceptable forage varies, necessitating a larger and larger area to supply in the forage demand of the specific large stock unit.  In combined rainfall / herbage production maps of South Africa, it is clear that with the diminishing rainfall received by the country from east to west, the expected forage production from a high of 35 tonnes/ha in the east to a low of around 3.5 tonnes/ha in the west.

 

Implicit in the definition of the large stock unit (as well as the number of animals of other races and classes of stock making up one LSU), is the fact that this unit will grow over the season (it puts on weight)(Figs. 1 and 2) and therefore, the number of hectares necessary to carry it, changes over time.  It therefore becomes necessary to make an adjustment, either to the number of these units per given area of land, or to the provision of a larger area for the flock or herd to graze, or, what is more often the case in practice, is to start off grazing at the start of the season with a conservative number of animals.  In essence, it becomes necessary to withdraw some of these animals in order not to overstock the given area, so that the remainder will still be provided with their daily quota of forage, which is still in the order of 2% of their body weight at that instant, because the 10 kg necessary at the start of the season, gets larger and larger by a fraction on a daily basis.

 

The option of withdrawing animals, is difficult to apply in practice.  Therefore, the answer lies in the calculation of a realistic, somewhat conservative stocking rate to be applied at the start of the season.  This needs to be lower than the grazing capacity norm, because as the animals grow, the applied stocking rate approaches the norm (Figs. 3 and 4), and/or the grazing capacity estimated for the area being grazed through veld evaluation.  For this reason, it is generally advised that researchers and farmers refer to the capacity of the land to carry animals, as the grazing capacity of the land, which has inherent in its definition, the implicit understanding, that of the long term conservation and sustainable utilization of the natural resources.

 

3.  Carrying capacity


The term carrying capacity, which is often used to describe the above relations is very popular with especially the plant and animal ecological fraternity.  However, there is a need to distinguish between domestic grazers and game animals.  It has been advocated for some time that the term grazing capacity should be reserved to instances where the stocking rate grazing capacity relation of domestic stock is described.  This relation is a simple question of the number of animals which can be accommodated sustainably on a given area without the deterioration of the natural resources.

 

The capacity of the land to carry game, should be referred to as carrying capacity.  This stocking rate carrying capacity relation, should be reserved for the use of the land area to game relation.  This carrying capacity is much more complex than the simple domestic stock : land area relation.  Game, carrying capacity involves such factors as, inter alia: area of suitable habitat, sufficient foraging area, appropriate cover and a large enough area to cater for social needs (Furstenburg 2002).

 

However, on account of the animal population growth rate, of the different species occupying the land at the same time, this capacity of the land to carry game often becomes overstocked, resulting in the eventual over-grazing of the vegetation.  When the area can no longer support the animal population, it crashes, leading to the inevitable, massive die-off of large numbers of game animals.  The remainder starts to recover slowly at first on account of the poor vegetative cover and low available plant production resulting in the extremely low carrying capacity.  Once the vegetation has recovered to such an extent that it attains its previous carrying capacity, animal numbers start building up again.  The whole cycle of animal number build-up and the consequent overgrazing resumes.

 

In order to combat over-grazing of the veld by game, expensive animal control measures have been instituted and such operations as culling and relocation of game are required, however, these practices seldom prove popular.  The serious reader is referred to the excellent and comprehensive exposé by Odum (1971) on the question of the intricate behaviour of wild animal populations in relation to carrying capacity.

 

The salient point which emerges from this discussion, however, is that carrying capacity as it is usually understood and used by many biological researchers, is not attainable nor sustainable in the long term without major interventions.

 

4.  Biomass

A short comment on the use of the term biomass also seems in order.  Biomass relates to the mass of all living organisms per unit area. This includes animals, from the primitive one-celled to the most complex large land mammals, as well as plant species, fungi and mosses to the most advanced higher plant life forms.  Biomass refers to all the living organisms aboveground as well as below ground.  Whenever the term biomass is used, it should very clearly state to what constituent part it refers.  If it refers to animals, it should very clearly state that, but also, to the type or class of animal.  When it refers to vegetation, it should just as clearly indicate what class of plant is meant and in the case of the higher plants, whether all the aboveground as well as the below ground plant production is meant, or only that portion of the aboveground plant production which is normally deemed available to grazing animals.  A very good description of that portion of the plant production is available, i.e. aboveground available phytomass (du Toit 2001).  This available aboveground phytomass is usually in the order of 50% and lower of the total aboveground plant production.

 

Conclusions

 

Although there has been some hiccups in the past regarding the use of grazing capacity and stocking rate, these are systematically getting ironed out.  The recommended practice is still therefore to refer to the grazing capacity of the veld in terms of hectares per large stock unit.  Likewise, the applied stocking rate on the veld is referred to in the same terms.  In South Africa, with the largest part of the country considered to have an arid to desert vegetation type with an extremely low grazing capacity, reference to the grazing capacity is done in terms of hectares per large stock unit.  This usage of the term comes about in order to circumvent the use of impossibly low decimal figures to express the animal stocking rate per hectare. 

 

References


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

Bezuidenhout, R. 2002.  The latest veld management guide.  Farmer=s Weekly, 13 December 2002, p. 24.

Du Toit, P.C.V.  1997.  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.  2000.  Estimating grazing index values for plants from arid regions.  Journal of Range Management 53 :529 - 536.

Du Toit, P.C.V.  2001.  The relation between canopy spread cover and the aboveground available phytomass of Nama-Karoo sub-shrubs and grasses.  African Journal of Range and Forage Science 18:143-146.

Du Toit, P.C.V.  2002a.  Animal production under different grazing regimes at Grootfontein.  Grootfontein Agric vol 4 (2002) :37 - 42.

Du Toit, P.C.V.  2002b.  Boesmanskop grazing capacity benchmark for the Nama-Karoo.  Grootfontein Agric vol 5 (2002) :1 - 6.

Du Toit, P.C.V.  2002c.  Using climatic variables to model Nama-Karoo subshrub dry-matter production.  Grootfontein Agric vol 4 (2002) :34 - 36.

Esler, K.J., Jones, F.E., Burke, A., Samways, M., Barnard, P., Gilbert, F. & Wissel, C.  2002.  Guidelines for sustainable rangeland management : recommendations on ecological restoration of rangelands in the semi-arid Nama Karoo biome pp. 1 - 36.  Magenta Media, Cape Town.

Furstenburg, D. 2002.  Optimising game production in a new era : the road to financial success.  Grootfontein Agric vol 5(2002): 17 - 27.

Herselman, M.J.  2000.  Toepaslikheid van huidige grootvee-eenheidekwivalente van weidende skape.  Grootfontein Agric vol 2(2)(2002) :6 - 8.

Meissner, H.H.  1982.  Substitution values of various classes of farm and game animals in terms of a biologically defined large stock unit.  Farming in South Africa.  Beef Cattle C.3/1982.

Meissner, H.H.  1993.  Carrying capacity as estimated by the animal unit and biomass methods: Pros and Cons.  Proceedings of the workshop on determining grazing norms, Pretoria.

Meissner, H.H., Hofmeyr, H.S., van Rensburg, W.J.J. & Pienaar, J.P.  1983.  Classification of livestock for realistic prediction of substitution values in terms of a biologically defined Large Stock Unit.  Technical Communication no. 175 pp. .  Government Printer, Pretoria.

Odum, E.P.  1971.  Fundamentals of ecology, 3rd edition.  Saunders College Publishing, Philadelphia.  Pp. 574.

Venter, J.P. 1982.  Die regte bestokking : =n Wenresep teen droogtes.  Swalurama, Desember 1982, pp. 18 - 33.

 

Published

Karoo Agric Vol 6 (1)