agricultural production guidelines
m
Veld in KwaZulu-Natal
Co-ordinated Extension KwaZulu-Natal Veld 2.5 1999
DETERMINATION OF GRAZING AND BROWSING CAPACITY
J M B Smith
KwaZulu-Natal Department of Agriculture
M B Hardy
Western Cape Department of Agriculture
Importance of Stocking
Rate
Grazing Capacity and
Mean Annual Rainfall
Calculation of Browsing Capacity
INTRODUCTION
The development of a successful grazing management system for a farm is largely dependent on the number of animals that a certain area of veld is able to support. This concept of productivity of land is called the grazing capacity of the veld and is the most important principle of grazing management. It is dependent on a number of factors such as rainfall, veld condition, slope, aspect and soil, among others. It is obvious, therefore, that the grazing capacity of veld will vary considerably from area to area, hence the need to fence off the different Veld Type Units (VTU’s) on the farm. It is important that the grazing capacity of each of these VTU’s is determined individually.
There are numerous examples of the effect of stocking rate on production of animals. At Soutpan research station it was demonstrated that by reducing stock a greater mass of weaners were produced on the same piece of land (Table 1).
Table 1. Effect of stocking rate on weaner production
Stocking rate (ha/cow)
Number of cows
Calving percentage
Weaning mass (kg)
Total weaning mass (kg/ha)
2.7
33
56
145
2 680
4.7
18
88
186
2 946
As the stocking rate is increased, animal performance in the form of mass gain and calving percentage will decrease. Expressing this performance in gain per ha will result in an increase up to a maximum, followed by a decline. The maximum gain
per ha will occur when animal performance has already decreased significantly. When considering profit this will be maximised where performance per animal has declined by about ten percent of maximum gain per animal. Therefore the optimum stocking rate should be close to that which maximises individual animal performance.
GRAZING CAPACITY AND MEAN ANNUAL RAINFALL
Herbage yield
The grazing capacity of an area of land depends on how much useful herbage (forage) is produced on that area. In general, there is a positive correlation between veld herbage yield and rainfall during the growing season. This correlation has been shown in the production of various crops (e.g. the production of one ton of maize grain requires 100 mm of rain, as does the production of 9 ton of sugarcane). The relation between grass yield and rainfall was studied in thornveld at Matopos Research Station in Zimbabwe. These results indicated that one millimetre of annual rainfall produced approximately 3 kg of grass on a dry matter basis. A similar trial in the Orange Free State examined grass yields in response to rainfall across veld in different condition. The results of this study are presented in Table 2.
Table 2. Dry matter (DM) production of veld in different stages of succession in response to rainfall (mm) in the central Orange Free State. Mean annual rainfall was 712 mm
Veld conditions
Good/Excellent
Reasonable
Poor/Critical
Basal cover
Runoff
DM kg/ha
kg DM/mm rain
9%
6%
2 256
3.2
6%
10%
1630
2.3
3%
20%
889
1.2
The above trial indicated that, with a deterioration in veld condition and a decrease in basal cover, runoff of rainfall increases. This reduces the effective rainfall and thus the herbage yield.
To determine the yield of grass, it is necessary to use the rainfall which falls during the growing season (October to March) which, in KwaZulu-Natal, is about 80% of the mean annual rainfall. This figure decreases towards the coast and is 60% at Richards Bay.
The relation between rainfall, veld condition and grass yield is presented in Table 3.
Table 3. General relations between veld condition and grass dry matter (DM) yield and rainfall.
Veld condition
Yield factor(grass yield, kg DM per mm rain October - March)
Excellent - 100%
Good - 80%
Average - 60%
Poor - 40%
Critical - <20%
1
54
3
2
1
In the Sour Sandveld, use one unit less for each veld condition class, i.e. 4, 3, 2 and 1.1
The veld condition should be determined from the composition, palatability and vigour of the species (see Production Guideline 2.4 of this series).
Stocking rate
The determination of grazing capacity provides the farmer with a guideline of the stocking rate which should be applied to each VTU on the farm. It is necessary to express stocking rate calculations in terms of Animal Units (AU). An AU refers to an animal with a mass of 450 kg which consumes 10 kg dry matter (DM) per day (i.e. about 3500 kg DM per annum). The palatability and quality of the grass sward declines at different rates during the late summer months according to the veld type. Herbage consumption will therefore also vary. The estimated annual herbage consumed per hectare for each veld type, excluding supplements, is presented in Table 4.
Table 4. Herbage consumption by livestock for different veld types.
Veld type
Bioresource Group
Herbage consumption
(kg/AU/annum)Sourveld
Sour/mixedveld
Mixedveld
Sweetveld
3 to 11 & 15
1, 2, 12, 14 & 17
13, 16 & 18 to 20
21,22,23
2 500
2 750
3 000
3 500
One of the fundamental principles of veld management is to limit the utilization of herbage to half of the total production. The axiom "take half, leave half" is applicable in this regard. To determine the stocking rate, the herbage yield should be assessed by multiplying the average growing season rainfall (in mm) by the yield factor (Table 3). The resultant yield per hectare is halved, and then divided into the annual herbage consumption per AU to obtain the number of hectares required per AU.
In a sourveld area, for example, where the mean annual rainfall is 800 mm, the growing season rainfall would be 640 mm (80% of 800 mm). With veld condition at 80% of the benchmark, the herbage yield would be 2 560 kg DM per annum per ha (640 mm x 4 kg/mm/ha - Table 3). Only half of the yield should be utilized (i.e.
1 280 kg/ha). To determine the number of hectares required per AU, divide 1 280 kg/ha into 2 500 kg/AU (Table 4). This gives a stocking rate of 1.9 ha/AU.
In a mixed veld area, where the annual rainfall is 700 mm, the growing season rainfall would be 560 mm (80% of 700 mm). With a veld condition at 70% of the benchmark, the herbage yield would be 1 960 kg DM per annum per ha. (560 mm x 3.5 kg/mm/ha - Table 3). Only half of the yield should be utilized (i.e. 980 kg/ha). To determine the number of hectares required per AU, divide 980 kg/ha into 3 000 kg/AU (Table 4). This gives a stocking rate of 3.0 ha/AU.
In the sweetveld, where Acacia trees have encroached or thickened up, their effect on grass yield should be taken into consideration when determining grazing capacities. This effect is measured in terms of tree equivalents (TE). As the number of TE increase above 900 per hectare, grass production declines. Tree equivalents are calculated from the information collected when surveying the woody component of each VTU (see procedure in Production Guideline 2.4 of this series). Since all woody plants affect grass production, the number of tree equivalents is calculated by adding the heights of all woody species sampled and dividing by 1.5. Thus one tree, 3 m tall, is two tree equivalents, having twice the competitive effect on grass production as a tree 1.5 m tall. A mean for the camp can be calculated (as for the browse units) and converted to a per hectare figure. This is then used to calculate the reduction in grazing capacity. The effects of tree density on grass production is reflected in Table 5.
Table 5. Effect of tree density on grass production.
Tree density
(tree equivalent/ha)Portion of grass yield in relation to the
absence of trees (%)<900
900
1 200
1 500
1 800
2 100
100
90
85
70
50
30
In a sweetveld area with Acacia trees, where the annual rainfall is 600 mm, the growing season rainfall would be 480 mm (80% of 600 mm).
With a veld condition at 70% of the benchmark, the grass yield (where the TE per hectare is <900) would be 1 680 kg per ha (480 mm x 3.5 kg/mm/ha - Table 3). With 1 200 TE per ha the grass yield would be reduced to 1 428 kg per ha (85% of 1 680 kg - Table 5). Only half of this yield should be utilized (i.e. 714 kg/ha). To determine the number of hectares required per AU divide 714 kg into 3500 kg/AU (Table 4). This gives a stocking rate of 4.9 ha/AU.
If browsers, e.g. goats, are to be used in the same area they would be stocked according to the amount of browse available to them. However, even under circumstances in which adequate browse is available to the goats, approximately 20% of their intake is grass, so the grazer stocking rate must be reduced even further to allow for this consumption by goats. In addition, in areas where the A. karroo plants lose their leaves in winter, goats (or other browsers) must subsist entirely on the grass component during this period. Therefore, for example, it has been shown that for an area where A. karroo has no leaves for 3 months of the year, the grazer stocking rate must be reduced by 40% of the browser stocking rate to allow for grass eaten by browsers.
CALCULATION OF BROWSING CAPACITY
If goats are to be run as browsers and to eat the minimum of grass, the stocking rate of browsers (goats) should be based on the quantity of available browse material produced by the palatable woody plants in each camp or proposed camp. The browsing capacity of the trees and shrubs will depend on the quantity of browse that is produced in the 1.5m-high browsing zone, since anything higher is essentially out of goats’ reach.
Preliminary research in the Eastern Cape has shown that the following numbers of palatable plants 1.5 m in size are needed for each mature goat per year (Table 6).
Table 6. The number of browse units required per mature goat in the different areas of the eastern Cape according to soil depth.
Catenal position
Soil depth
(mm)Rainfall (mm)
600 - 800
450 - 600
Crest
Midslope
Footslope/valley bottom
200 - 500
500 - 1 200
>1 200
2 000
1 500
1 000
2 500
2 000
1 500
The number of plants in the camp must first be sampled (see Production Guideline 2.4 of this series). Since the goats eat only palatable plants, the number of palatable plants per hectare must be calculated. Acceptability ratings are given for a range of species in Table 7. Note that this information was obtained from studies conducted in the Eastern Cape Thornveld. Farmers in the KwaZulu-Natal bushveld areas should consult with experts or use their own experience to classify woody plants into the acceptable and unacceptable categories.
Total BU/ha can be calculated for that camp using the method given in Production Guideline 2.4 of this series, taking into account the acceptability of the different species. By multiplying the number of BU per hectare by the size of the camp the total BU for the camp will be calculated. To calculate the browsing capacity for that camp divide the total BU for the camp by the relevant number of BU/goat from Table 6. Each camp on the farm must be similarly sampled to give the browsing capacity for the whole farm.
Table 7. The acceptability of various bush species to goats in the thornveld areas of the Eastern Cape.
Acceptable
Unacceptable
Acacia caffra
Acacia karroo
Boscia oleoides
Capparis sepiaria
Carissa haematocarpa
Coddia rudis
Combretum caffrum
Cussonia spicata
Ehretia rigida
Euphorbia bothae
Euphorbia coerulescens
Grewia occidentalis
Grewia robusta
Olea africana
Pappea capensis
Phyllanthus verrucosus
Portulacaria afra
Rhigozum obovatum
Rhus lancea
Rhus lucida
Rhus macowanii
Schotia afra
Schotia brachypetala
Schotia latifolia
Scutia myrtina
Sideroxylon inerme
Tecomaria capensis
Ziziphus mucronataAloe ferox
Azima tetracantha
Brachylaena elliptica
Brachylaena ilcifolia
Diospyros lyciodes
Euclea undulata
Lippia javanica
Lycium Campanulatum
Maytenus heterophylla
Ozoroa mucronata
Plumbago auriculata
Protoasparagus setaceus
Ptaeroxylon obliquum