agricultural production guidelines
m
Veld in KwaZulu-Natal
Co-ordinated Extension KwaZulu-Natal Veld 6.2 1999
ASSESSMENT OF VELD CONDITION
M B Hardy
Western Cape Department of Agriculture
C R Hurt
ARC - Range and Forage Institute, Cedara
Planning
an Assessment of Veld Condition
Assessment of Sample Sites
Calculation of Veld Condition
INTRODUCTION
This Production Guideline describes the procedures which should be followed when conducting an assessment of veld condition on the farm. Only the grass layer is considered. To obtain a greater understanding of what is implied by veld condition and how veld condition may be used to determine grazing capacity, this Production Guideline should be read together with Production Guidelines 6.1 and 6.3 in this series.
PLANNING AN ASSESSMENT OF VELD CONDITION
Most farms, particularly those in KwaZulu-Natal with its broken topography and wide range of climatic conditions, will have a variety of vegetation types within their boundaries. These vegetation types differ due to the many environments encountered on the farm (e.g. soil form, slope, aspect and rainfall). Furthermore, each type of vegetation has its own fodder production potential, and, therefore, animal production potential. For these reasons it is important, firstly, to separate each of the major vegetation types, and, secondly, to demarcate any variations within those vegetation types where the soils, climate, slope and aspect are similar enough to support a homogeneous type of vegetation. Ideally, a 1 : 5 000 aerial photograph of the farm should be used for this purpose. The degree to which the sub-divisions are made will depend upon the size of the survey area and the degree of heterogeneity in the vegetation.
Once the Veld Type Units (VTU’s) have been delineated on the aerial photograph or some other map, they should be verified in the field (refer to Production Guideline 2.2 of this series for more detail on the delineation of VTU’s). (Note: the local extension officer could assist the farmer with the delineation of VTU’s). Field verification often permits similar areas to be combined, or results in some of the VTU’s being divided into smaller units. Veld condition is then assessed in each VTU. In the following discussion the area of a VTU that is surveyed will be referred to as the sample site.
On a well-planned farm most of the VTU’s should be fenced off from one another. In this case, a single assessment of each camp on the farm should suffice, provided the camps are not too large (experience has indicated that a sampling intensity of one survey per 60 to 70 hectares is a useful guideline for assessing veld condition on farm scale), or the vegetation too heterogeneous. However, it is often impractical to fence off the different VTU’s in a particular area of the farm. In these situations, a separate assessment should be conducted for each VTU in the camp.
A point survey of 200 observations is conducted to determine the botanical composition of the area. The survey is usually conducted along a transect across the camp, with the points being located systematically to ensure an even distribution of points on the transect. The name of the nearest plant to each point is recorded, and the results are expressed as the percentage of each species encountered in the survey. For example, if redgrass (Themeda triandra) plants were encountered 36 times out of the 200 observations, then the percentage of red grass for that camp would be:
(36/200) x 100 = 18%.
Several methods have been tested for locating points in the survey. The most practical approach is to use a metal rod approximately 1 m long, 3 mm in diameter, and which has had one end filed to a sharp point. As the observer walks along the transect, the rod is placed vertically onto the soil surface every two to three paces. Care must be taken that the placement of the point is not influenced by the observer (i.e. the observer must not consciously decide where the point should be placed). Another important consideration is that the nearest live plant to the point is defined as the plant which has its base nearest to the point. In other words, a large robust plant may appear to be nearer to the point because it has a large canopy. However, closer inspection may reveal that a less conspicuous plant must be recorded, as it is rooted closer to the point.
The number of points required in each survey is dependent on the objective of the survey and the type of vegetation. As with most estimates, the more observations recorded, the greater is the reliability of the estimate. The 200 observations per transect suggested above serves as a guideline, and will provide a reliable estimate of botanical composition where 20 or more species are likely to be encountered on the site. Some workers suggest that 100 point observations would suffice, but this is normally in areas where few species (10 to 15) make up the sward. Where the observer requires a ‘quick’ estimate of botanical composition, 100 point observations would be acceptable, as long as the results are not used for monitoring purposes (i.e. for comparing the botanical composition of the sample site over time).
A veld condition score (VCS) is determined for each sample site. The VCS provides an indication of the ‘state of health’ of the vegetation (refer to Production Guideline 6.1 in this series). In other words, it indicates whether the veld has been optimally utilized, underutilized, or overutilized, and the degree of utilization in the latter two of these categories. Various methods of indexing veld condition have been described in the past, and most have been based on the subjective classification of species into response groups (refer to Production Guideline 6.1 in this series for definitions of these groups). Furthermore, most methods make use of all species recorded in a botanical survey in order to calculate a VCS. Three approaches will be described here.
The VCS is calculated by comparing the botanical composition of a sample site to that of a benchmark site. The benchmark is an area of veld within a vegetation type (or Bioresource Group) which is considered to be in ideal condition for long-term animal production (refer to Production Guidelines 4.2 to 4.10 in this series for descriptions of the botanical composition of benchmarks for each Bioresource Group). The following procedure is used.The Benchmark method
All species recorded in the sample site are placed into species response categories (Decreaser, Increaser I, Increaser II and Increaser III species - Production Guidelines 4.2 to 4.10 and 2.4 provide a classification of common species into each of these categories). The information is recorded in tabular form as shown in the example in Appendix 1. The interpretation of this example is provided below.
- Composition score. The sample site rates a condition of only 34% relative to the best possible condition (benchmark condition).
- Decreaser species. The percentage of Decreaser species in the sample site is only 12% relative to 49% in the benchmark. Therefore, the sample site has lost a large proportion of the desirable species.
- Increaser I species. Increaser I species in the sample site are 6% of the total, compared to 25% in the benchmark, so that underutilization is probably not the cause of the decline in veld condition.
- Increaser II species. The sample site has an abnormally high percentage of Increaser II species (79% relative to only 19% in the benchmark). This shows that the veld has been overgrazed, since many of these species are pioneers and tend to predominate in veld which has been overgrazed.
The survey points to overgrazing of the sample site, and indicates that stocking rate should be reduced. In addition, appropriate resting periods should be provided to encourage the Decreaser species.
This method of assessing veld condition provides a simple and useful interpretation of the condition of the veld, provided that the species can be objectively placed into response groups. The objective identification of species responses has not been conducted in all Bioresource Groups, and the categorisation of species provided in Production Guidelines 4.2 to 4.10 in this series are largely subjective. This should be borne in mind when conducting condition assessments, as the inclusion of non-responsive species can lead to a distorted interpretation of veld condition.
Ecological Index method
In this method, the calculation of a VCS is based on the subjective subdivision of the species response groups, and the provision of weightings which are allocated to each category, depending on the ecological status (and fodder production potential) of the species. The Increaser I category is divided into:
- Increaser Ia species, which increase in abundance with moderate underutilization; and
- Increaser Ib species, which increase in abundance where defoliation is minimal or absent.
The Decreaser category remains the same as that described for the Benchmark method, while Increaser II species included:
- Increaser IIa species, which increase in abundance with moderate overutilization;
- Increaser IIb species, which increase in abundance with heavy overutilization; and
- Increaser IIc species, which increase in abundance with excessive overutilization.
In addition, a category is provided for Increaser III species, which increase in abundance with selective grazing. Species within each group are given the same weighting. Decreaser species score 10, Increaser Ia and IIa species score 7, Increaser Ib and IIb species score 4, and Increaser IIc and III species score 1. These weightings are used in conjunction with the classification of species for each Bioresource Group, provided in Production Guidelines 4.2 to 4.10 of this series.
The VCS is calculated by adding the products of the percentages of each species response group in the sample site and the weighting for that response group. The VCS of the benchmark is calculated in the same way. An example calculation is presented in Appendix 2, based on the same data set as Appendix 1. The VCS of the sample site is then expressed as a percentage of the VCS of the benchmark. The percentages of Decreaser, Increaser I and II species groups must still be examined individually to determine the causes of a low or high VCS relative to the benchmark. The VCS for the sample in Appendix 2, calculated by this method, was 397, as opposed to the benchmark score of 751, and thus the condition of the sample is approximately 53% of that of the benchmark. The technique is still disadvantaged by the fact that all species are included in the calculation of a VCS, whether they are responsive to grazing or not. In addition, the classification of the species and their weightings is done subjectively. A further shortcoming of the method is that it is difficult to determine from the VCS alone, whether veld is dominated by Increaser I or Increaser II species, as both categories have the same weightings.
Weighted Key Species method
This method was designed to overcome the disadvantages of the previous two methods. It uses a few key species which are responsive to grazing (i.e. they have been shown to increase or decrease in abundance in response to grazing). Non-responsive species are therefore omitted from calculations of the VCS, and thus the VCS provides an accurate reflection of the effect of past grazing on the species composition of the sample site. Key species abundances are multiplied by weightings, and the sum of these products represents the VCS. Key species and their weightings have been objectively identified only for Bioresource Group 8, the Moist Highland Sourveld (refer to Production Guideline 4.5 in this series), but preliminary key species and weightings are provided for the remaining Bioresource Groups (refer to Production Guidelines 4.2 to 4.4, and 4.6 to 4.10 in this series). It must be stressed that, apart from the Highland Sourveld, these species and their weightings have been selected subjectively, and might change as research provides more information on the veld of each Bioresource Group. An example VCS calculation using this method is provided in Appendix 3. The species in this example have been left in their response groups for interpretive purposes - one can see at a glance which group of species contributes the most to the VCS. The VCS calculated using this method is 320, and, when compared to that of the benchmark (664), it can be seen that the condition of sample site is approximately 49% of that of the benchmark. The highest theoretical potential score (1 000) results from the presence of only one species, and is obviously not an ideal situation in a pastoral system.The main advantage of this method is that the VCS obtained for a sample site is used to position a site along a grazing gradient. This represents an imaginary sequence of samples of veld ranging in condition from good to bad. The position of any sample in this sequence is determined by the previous grazing history of the site. The impact of grazing is considered to be a major determinant of species composition, and this is reflected in the VCS. The position of a sample of veld on this gradient can be related to that of the benchmark, and accurate assumptions regarding the previous grazing management can be made, and used to adjust the present management.
The use of veld condition assessment is clearly extremely useful in determining the ‘state of health’ of each area of the farm. One of the most useful aspects is that repeated surveys (e.g. every 5 years) will assist in evaluating the success of grazing management programmes and providing pointers to defects in such programmes. In addition, the VCS can be used to determine the current carrying capacity of the sample site. This aspect is covered in Production Guideline 6.3 of this series. Of the three methods described in this Production Guideline, the Weighted Key Species method appears to be the most versatile. Its advantages to the farmer are (1) it is quick and easy to use, with the requirement that the operator only need know a few species; (2) the VCS provides a good indication of the previous grazing history of the sample site relative to the benchmark; (3) the VCS can be used to monitor changes in veld condition over time; and (4) the VCS can be used in the calculation of grazing capacity for veld (refer to Production Guideline 6.3 in this series).