- Effects of stocking rate on sheep management and vegetation composition at Carnarvon
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EFFECTS OF STOCKING RATE ON SHEEP MANAGEMENT AND VEGETATION COMPOSITION AT CARNARVON
L. van den Berg# & J.C.O. du Toit
Grootfontein Agricultural Development Institute, Private Bag X529, Middelburg (EC), 5900
# E-mail: Loraine vd Berg
The Arid Karoo is mostly used for extensive livestock farming from natural veld. Consequently the biggest questions and challenges are inseparably related to aspects of forage production (Meyer, 1992). The biggest effect of a decrease in veld condition is a decrease in the grazing capacity of Karooveld. Changes in Karooveld are mostly attributed to overutilisation by sheep, resulting in thinning of plants, a decrease or destruction of perennial grasses, and an increase in unwanted karroid dwarf shrubs and other woody species (Meyer, 1992). The ultimate risk of overutilisation is possible veld degradation. The direct results of this include reduced forage and animal production, higher maintenance costs (e.g. supplementation), and increased rehabilitation costs.
The selection of the correct stocking rate is the most important range management decision as it directly impacts on long-term plant species composition, animal productivity and forage production (Roberts, 1980; Holechek et al., 1999; Thorne & Stevenson, 2007). It has been shown that conservative grazing is a reliable way to increase forage production and improve vegetation composition on rangelands (Holechek et al., 1999; Hersom, 2005). Heavy stocking increases the probability of forage deficits and negatively influence the financial viability of the enterprise (Hatch & Tainton, 1995).
Individual animal performance is maximised at low stocking rates, while at high stocking rates, animal production declines as a result of poor individual animal performances (Valentine, 1990; Meyer, 1992; Hersom, 2005; Thorne & Stevenson, 2007). According to Mott and Moore (1970) increased stocking rates will generally decrease individual animal performance, but increase the production per unit of land area up to a given point.
During the 1950s to 1980s a number of different grazing systems and management guidelines were developed and implemented throughout the Karoo region (Tidmarsh, 1957; Roux, 1968). Management guidelines for these areas, based on the concept of maximum profit in the short-term while still maintaining veld condition in the long-term, suggested a slow rotational grazing system with three camps per herd. Land users, however, did not apply guidelines with regards to stocking rates, due to a number of reasons and it became evident that the best stocking rate should be determined in a more scientific way. As a result of this the Arid Karoo Stocking Rate trial was established in 1988. The objective of this investigation was to determine if different stocking rates in the Arid Karoo had a discernable effect on changes in veld condition and animal production.
MATERIALS AND METHODS
The investigation was conducted at the Carnarvon Experimental Farm located in the Northern Cape Province (30° 58’ S; 21° 58’ E) at an altitude of 1323 m above sea level. The vegetation is classified as Western Upper Karoo (Nku1) by Mucina & Rutherford (2006). According to Acocks (1988) the study area falls within the Bushmanland Nama Karoo vegetation type, on the boundaries of the Arid Karoo (Acocks 25) and the Upper Nama Karoo, also called the False Arid Karoo (Acocks 35). The Bushmanland Nama Karoo (notably the Arid Karoo) is characterised by rainfall up to 200 mm per annum, predominantly occurring in autumn.
The Arid Karoo Stocking Rate trial was established at the Carnarvon Experimental Farm in 1988, with treatments comprising stocking rates of 4 ha/SSU (very heavy), 5.5 ha/SSU (heavy), 7 ha/SSU (moderate) and 8 ha/SSU (light). A total of 12 camps (324 ha) was grouped into three blocks of four camps each. The four treatments were assigned randomly to the four blocks. The total trial area was rested for 18 months before the onset of the trial to reduce any pre-existing differences that there may have been. Since 1988 the trial has been managed as a 3-camp system, where livestock were moved every six weeks between two camps while the third camp rested for a whole year. Dry (non pregnant) Afrino ewes were used as trial animals, but were replaced by Dorper ewes in 1996 as this breed was more commonly used by land users at the time (Meyer 1992; Botha & Mellet, 2002). The Dorper ewes were, however, again replaced by Afrino ewes in 2006 when the Carnarvon Dorper flock was donated to emerging farmers.
Vegetation was sampled according to the descending point method. One thousand points per camp were taken, 500 points on each diagonal line in a camp (Roux, 1963). The canopy spread cover of grass and dwarf shrub species closest to the individual points were recorded. Surveys were conducted at the onset of the trial (1988) and then thereafter in 1991, 1998, 1999, 2001 and 2002. Non-metric multidimensional scaling (NMMS) was used to describe vegetation composition from 1988 to 2002.
The Ecological Index Method (EIM) was used to determine veld condition of the different treatments throughout the trial period (Vorster, 1982). This method is based on the subjective subdivision of species, and the provision of weightings depending on the ecological status of the species (Hardy & Hurt, 1999). A one-way analysis of variance (ANOVA) was used to compare the Veld Condition Score of the four different treatments at the onset of the trial in 1988, as well as at the last vegetation survey conducted (2001). Individual camps were treated as replicates.
All trial animals were weighed every two weeks and the results recorded. The average daily gain (ADG) per treatment as well as the gain per hectare over the total trial period were analysed by using an analysis of variance (ANOVA) analysis to determine whether significant differences occurred over the 20 year trial period. Years were treated as replicates.
RESULTS AND DISCUSSION
Rainfall data (1931 – 2013) from the Carnarvon Experimental Farm showed that annual rainfall decreased until about 1970, when there were heavy rains. Rainfall has increased from then until present. Rainfall is unimodal, peaking in March. February and April rains are similar, as are October to January and May (Figure 1). From June to September rainfall is near zero. Rainfall data from 1988 to 2011 were expressed seasonally, i.e. from the period 1 September of one year to 31 August the following year (Figure 2). The results showed that for the majority of the trial period lower than average rainfall was received. The low rainfall years were however distributed throughout the trial period and were usually followed by very good rainfall years.
Figure 1. Average monthly rainfall from Carnarvon (1931 – 2012)
Figure 2. Seasonal rainfall (September to August) at Carnarvon Experimental Farm expressed as departure from the mean
Meyer (1992) reported on the botanical composition at the onset of the trial in 1988. He found that there were no significant differences in the botanical composition between the different treatments. The vegetation consisted mainly of unpalatable dwarf shrub species, dominated by Pentzia spinescens, as well as pioneer and annual grass species such as Aristida congesta and Enneapogon desvauxii. Results of vegetation surveys conducted in 1991 indicated a general shift in composition for all treatments. This shift was characterised by a decrease in the frequency of annual grasses and an increase in dwarf shrubs. Results showed significant changes in the general botanical composition of the total trial area (P<0.01). These changes were mainly due to a decrease in the frequency of annual grass species and an increase in dwarf shrub species. The changes were consistent across all treatments and not related to stocking rate (treatment effects). The percentage of palatable dwarf shrub species, however, decreased in all treatments.
Botha & Mellet (2002) reported on vegetation surveys conducted in 2002. They found that the condition of the vegetation deteriorated under the heavy stocking rates, while the condition in the moderate and light stocking rate treatments remained approximately constant. In the heavy stocking rate treatments the plants decreased both in size and number.
The NMMS ordination of all collected data (1988, 1991, 1998, 1999, 2001 & 2002) showed that there has been a compositional shift from 1988 to 2002 (Figure 3). The primary axis reflects veld condition from poor (left) to good (right). The degree to which vegetation composition varies over years (presumably a function of rainfall) is inversely related to stocking rate, i.e. very heavy is associated with poor condition veld, and only moves slightly toward good condition, and is not responsive to rainfall. As stocking rate decreases, the degree to which the veld recovers (i.e. moves to the right in the ordination) increases. Perennial dwarf shrub species, such as Pentzia spinescens and Plinthus karooicus showed a stronger relationship with the medium and lightly stocked treatments than with the heavy and very heavy treatments. Annual grass species, such as Tragus racemosus and Aristida congesta, and the occurrence of bare ground on the other hand showed a stronger association with the heavy and very heavy treatments (Figure 3). Changes in botanical composition could have resulted from a number of factors. These include that some plants naturally have a lower utilisation than others. Furthermore animals differ in their grazing habit and may graze some species more selectively than others.
Figure 3. Non-metric multidimensional scaling (NMMS) of Bray-Curtis distances among treatments based on relative abundance (%) of plant species from 1988 to 2002 (â—‹ – Very heavy, â–¡ – Heavy, ◊ – Medium, âˆ† – Light). Bargro – Bare ground, Trarac – Tragus racemosus, Aricon – Aristida congesta, Opslag – Ephemeral species, Enndes – Enneapogon desvauxii, Penspi – Pentzia spinescens, Plikar – Plinthus karooicus
An ANOVA analysis showed no significant differences in the Veld Condition Score between the various treatments in 1988 (F3,8 = 0.410; P = 0.753). For the 2001 analysis however, there was a definite treatment effect (Figure 4). The medium and light treatments differed significantly from the very heavy and heavy treatments (F3,8 = 8.53; P = 0.0071).
Figure 4. Error-bar graph showing mean Veld Condition Score (VCS) of the stocking rate treatments at Carnarvon. Bars are 95% confidence intervals
Jones & Sandland (1974) have postulated that the gain per animal remains constant as stocking rate increases up to a critical point, after which it decreases linearly to zero. The results from this trial show a similar trend, but no significant differences were observed in gain/animal/day between the four different treatments over the total trial period (F3,68 = 0,46; P = 0.7118) (Figure 5). Lower stocking rates generally lead to less energy used to graze, higher quality diets and increased dry matter intake, which in turn lead to increased production, high lambing rates as well as relatively high quantity and quality of reserves during drought conditions (Hersom, 2005). Higher stocking rates, in contrast, result in the use of more energy to graze, low intake of nutrients in diets, poorer condition of animals and decreased reserves to be used during drought conditions. At lower stocking rates, adequate forage is usually available throughout the year, and seasonal variation in animal mass can mainly be attributed to factors other than forage availability. At higher stocking densities however, adequate forage is usually only available during the active growth phases of the vegetation and seasonal variations in animal mass can therefore be attributed to forage availability.
Figure 5. Error-bar graph showing Gain/animal/day (g) and 95% confidence intervals by different treatments
Results indicated that at variable environmental conditions, stocking rate could be a determining factor influencing botanical composition and veld condition. From this trial it is evident that higher stocking rates resulted in a weakening of the botanical composition (replacement of good, palatable plant species with less palatable species or annuals), as well as a reduction in the average daily gain of individual animals (although non-significant). It could therefore be recommended that for optimal animal performance of Afrino and Dorper sheep in the Arid Karoo, light to medium stocking rates be considered.
The authors would like to thank everybody who, over the years, has been involved in the planning, maintenance, day to day running, weighing of animals and vegetation surveys of the Arid Karoo Stocking Rate trial.
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Grootfontein Agric 14 (1) (34)