Diets selected by Merino and Dorper sheep in Karoo veld
P. C. V. du Toit
Grootfontein Agricultural Development Institute,
Private Bag X529, Middelburg (EC) 5900
The diets that various breeds of animals select may be used to study their adaptability to the different compositions and types of herbage available for grazing in the three veld types. With this in mind, it was decided to study the diets selected by two sheep breeds, the Merino sheep, being a very selective grazer and Dorper sheep, being a fairly nonselective grazer in the Arid Karoo, in the Eastern Mixed Karoo and in the Noorsveld. Various proposals have been made concerning the suitability of combining these sheep breeds with other small-stock breeds and species in order to make better use of the vegetation. However, before recommendations regarding increases in stocking rates can be made, it should be clearly established whether the sheep are suitable for that area. Recommendations regarding increased stocking rates have been published elsewhere (Botha et al. 1983; Du Toit et al. 1995a, 1995b).
The purpose of this paper is to study the ability of Merino's and Dorpers to adapt to the herbage in these widely differing arid veld types, with the main aim to assess the suitability of keeping them on these veld types.
The studies were conducted on three research farms, at Carnarvon, Grootfontein and at Jansenville.
Carnarvon Experiment Station is situated in the Arid Karoo (Veld Type 29, Acocks 1988), while the research site is situated at 30 °57' S and 21 °59' E, at an altitude of between 1 000 and 1 300 m. The mean annual rainfall is 203,5 mm with 14 % occurring in spring, 34 % in summer, 41 % in autumn and 11 % in winter (Fig.l). The average minimum temperature (July) is 8,1 °C and the average maximum temperature (January) is 23,5 °C. A frost-free period of 240 days occurs from mid- September to mid-May.
The vegetation at Carnarvon is low growing and sparse and consists mainly of karoo bushes, with Eriocephalus ericoides, E. spinescens, Pentzia spinescens, Pteronia glomerata, P: adenocarpa, Rosenia humilis and R. glandulosa dominant, while Felicia muricata, Monechma incanum, Plinthus karooicus and Salsola tuberculata are fairly common. The grasses, mainly short grass species, of the mid to late development stages are characteristic of the soils on which they occur, with Eragrostis lehmanniana, Stipagrostis ciliata and S. obtusa occurring on the red kalahari sands, Aristida diffusa and Heteropogon contortus on doleritic soils, while Fingerhuthia africana, Eragrostis nindensis and Stipagrostis obtusa are found on the shale-derived soils. Stipagrostis namaquensis frequently occurs along the dry water-courses (Plate 1).
Grootfontein Agricultural Development Institute is located in the False Upper Karoo (Veld Type 36, Acocks 1988). It is locally referred to as the Eastern Mixed Karoo, because of the almost equal contribution of grass and karoo bush species to the species composition. The research site occurs at 31 °28' Sand 25 °02' E, at an altitude of between 1 000 and 1 500 m. The mean annual rainfall is 359 mm with 15 % falling in spring, 30 % in summer, 50 % in autumn and 5 % in winter (Fig. 1). The average minimum (July) temperature is 7,9 °C and the average maximum (January) temperature is 20,9 °C. A frost-free period of 180 days occurs from mid-October to mid-April.
Plate 1, 2
The stable vegetation at Grootfontein consists mainly of dense stands of the late development stage, tall grass species, Cymbopogon plurinodis, Digitaria eriantha, Hyparrhenia hirta, Sporobolus fimbriatus and Themeda triandra. The mid-development stage grasses Eragrostis lehmanniana and Aristida diffusa and the early development stage grasses A. congesta and Cynodon dactylon become dominant in different stages of degraded grassland. Mixed with the mid and early development stage grasses are the karoo bushes Eriocephalus ericoides, E. spinescens, Felicia muricata, E. filifolia, Pentzia incana, Plinth us karooicus, Pteronia glauco, P. tricephala, Rosenia humilis and Sutera pinnatifida (Plate 2).
The Jansenville Experiment Station is situated in the Noorsveld (Veld Type 24, Acocks 1988), while the research site is situated at 32 058' Sand 24 041' E, at an altitude of between 300 and 700 m. Mean annual rainfall is 264 mm, although in rainshadows it is as low as 170 mm. Twenty five per cent of the rain occurs in spring, 33 % in summer, 32 % in autumn and 10 % in winter (Fig. 1). The average minimum temperature (July) is 11,5 °C and the average maximum temperature (January) is 26,5 °C.
The area around Jansenville is largely frost free and as a result many trees and shrubs are found in the Noorsveld, with Euphorbia coerulescens and Portulacaria afro the dominating shrubby species, with trees such as Acacia karroo, Boscia albitrunca, Grewiaflava, G. occididentalis, Pappea capensis and Schotia afro. Tall, late development stage grasses include Cenchrus ciliaris, Cymbopogon plurinodis, Digitaria eriantha, Panicum deustum, P maximum and Themeda triandra while the following karoo bushes are found: Felicia muricata, F filifolia, Monechma spartioides, Pentzia incana and Rosenia humilis. Typical noorsveld is a rather monotonous vegetation type with a sparse covering of the karoo bushes and grasses occurring among the Euphorbia coerulescens, where the veld becomes degraded very easily, leaving large bare patches (Plate 3).
Materials and methods
Ten dry ewes and 10 castrated males of the Merino and Dorper breeds were fistulated at the oesophagus (Bredon et al. 1967). These animals were allowed to graze normally until the fistula extrusa samples were required. They would then have become accustomed to the veld plant species and would therefore graze normally during sampling.
Oesophageal fistula extrusa sampling was carried out during the winter, spring, summer and autumn of the 1984/85 to 1986/87 seasons. It was conducted over four days and took place in the early morning. Samplingtime varied between 20 and 30 minutes. Five animals per breed and sex were used to sample, following a period of fasting not exceeding 16 hours. Extrusa samples were washed in running tap water immediately after sampling (Dugmoreeta/.1991) and samples collected over a period of two days were composited per breed and per sex and fixed in a formalin, acetic acid and alcohol mixture.
Altogether 288 samples were examined microscopically (Heady et al. 1959), with 500 points identified per sample. The particle lying directly underneath the crosshairs, inserted into the eyepiece of the microscope, was removed, identified to species level, dried at 60 °C and weighed (Heady et al. 1959). The weighed particles, summed per species, were expressed as a percentage of the mass of all the points identified. This indicated the percentage of the selected diet actually consisting of a particular plant species.
As a result of the large variation found in the different veld types, the plant species were grouped into five functional groups on the basis of their growth form, i.e. karoo bushes, grasses, annual plants, trees and shrubs and noors. Because of the special interest in the karoo bushes, grass and annual (ephemeral or opslag) species groups, these were treated separately. However, no recourse was taken to further subdivide the karoo bushes into palatable, less palatable and unpalatable bushes, nor were the grasses subdivided into early, mid or late development stage grasses. Trees and shrubs were treated as a single functional group. Euphorbia coerulescens (noors) was also kept as a separate group in the Noorsveld where it occurred, because of the special interest in whether this spiny plant with its milky latex, at any stage contributed significantly to the grazable material.
The d-statistic (the index of agreement) described by Willmott (1981; 1982; Willmott & Wicks 1980; Booysen 1990; Du Toitetal.1995a; 1995b), was used in direct comparisons between breeds and in pairwise comparisons of the diets selected by the respective breeds, in the various veld types during the different seasons. Because individual values in the average diet selected by the Merino and Dorper sheep during the different years fluctuated too widely to assess the trend, individual values of the selected diets were averaged over ewes and castrated males and the resultant values were averaged over the three years of the trial. On account of the wide fluctuation it was difficult to assess trends
in the various diets selected by the two breeds, in the different veld types.
Average values enabled the comparison of trends and the assessment of whether a specific sheep breed was in fact adapted to the forage in the specific veld type. The Index of Agreement takes the form:
d = 1 - [S(Pi - Oi)2/ S / Pi'/ + / Oi’/ 2]
where n is the number of observations, Pi is a predicted observation, Oi is a measured observation, Pi' = Pi - 8 and Oi' = Oi -8 (8 is the mean of the observed variable) (Willmott & Wicks 1980).
According to the d-statistic, the closer the index value approaches one, the better the agreement between the two variables being compared and conversely. The closer the index value approaches zero, the less correspondence there is between the two compared variables.
During this study it was found that with an index value of one, the two animal groups being compared graze very similarly, and their grazing can be regarded as being nonselective when compared to the herbage as expressed by the botanical survey (cf. Table 2). As the
index value approaches zero, the animal groups being compared graze very dissimilarly, and their grazing can be regarded as being very selective.
Results and discussion
Diet selection during the different seasons
Diet selection during spring (Fig. 2 and Table 1)
During spring in the Arid Karoo the sheep concentrated on annual and ephemeral plants, such as Anchusa capensis (Cape forget-me-not, ossetongblaar), Lepidium africanum (Cape pepper cress, peperbossie) and Mesembryanthemum nodiflorum (brakslaai). During this period a high degree of selective grazing took place, with mainly these three species being selected (Du Toit, et al.1995a).
Because of the fact that spring is a fairly dry period in the Arid Karoo, animals tend to select green, succulent material such as the annuals and ephemerals which grow in response to light rainfall events. Between 50 and 60 % of the ephemeral group is selected. The Dorper sheep selected 18 % more karoo bushes than the Merino sheep, while the Merino sheep selected 12 % more grasses and 7 % more annuals.
During spring in the Eastern Mixed Karoo the sheep selected fairly large volumes of the annual plant Gnaphalium glomerulatum (cudweed, vaalbossie), with about 40 % of the palatable Felicia muricata (bloublommetjie) and Salsola calluna (swartganna) and the unpalatable Dimorphotheca zeyheri (kleinbietou) and Galenia procumbens (kraalbos), with some of the less palatable Atriplex suberecta (wildelusern) (Botha et al. 1983).
The Dorper sheep selected 13 % more karoo bushes than the Merino sheep, while the Merino sheep selected 13 % more grass and 8 % more annuals.
During spring in the Noorsveld, the sheep tended to select grass in their diet. The grasses were mainly two of the late development stage grasses, Cenchrus ciliaris (blue buffalo grass, buffelsgras) and Digitaria erantha (finger grass, vingergras). The Dorper sheep selected 9 % more karoo bushes and 1 % more trees and shrubs than the Merino sheep, while the Merino sheep selected 10 % more grass.
From Table 1 it is clear that the similarity between the diets of both Merinos and Dorpers in spring in the Arid Karoo and the Eastern Mixed Karoo is very small. This can be explained by the greater abundance of annual plants in the Arid Karoo at the time of grazing and the greater number of karoo bushes selected in the Eastern Mixed Karoo.
The low indices of similarity between the diets of both Merinos and Dorpers in the Arid Karoo and the Noorsveld, and the Eastern Mixed Karoo and the Noorsveld, can be ascribed to the greater abundance of grass selected in the Noorsveld in contrast to annual plants in the Arid Karoo and karoo bushes in the Eastern Mixed Karoo.
Diet selection during summer (Fig. 3 and Table 1)
During summer in the Arid Karoo the sheep concentrated on grass. With grass making up less than 20 % of the plant cover, the animals selectively grazed grass (about 60 % of their diet). The species selected were the annual Aristida congesta (white three-awn, steekgras) and Enneapogon desvauxii (eightdays grass, agtdaegras). During the hot, relatively dry summer, these annual grasses provided reasonably succulent, nutritious forage.
The Dorper sheep selected 12 % more karoo bushes and 4 % more annuals than the Merino sheep, while the Merino sheep selected 15 % more grass species.
During summer in the Eastern Mixed Karoo the sheep selected mainly two components, from 30 to 40 % grasses, mainly Aristida congesta, Cynodon dactylon (couchgrass, kweekgras) and Eragrostis lehmanniana (eastern province vleigrass, knietjiesgras), and between 30 to 50 % of the annual plants Gnaphalium glomerulatum and Salsola kali (Russian thistle, rolbos). The species selected indicate a fairly degraded grassland. The palatable karoo bush species made up the rest of the diet. Merino sheep selected 16 % more karoo bushes and 12 % more grasses than Dorper sheep, while Dorper sheep selected 28 % more annuals.
During summer in the Noorsveld the sheep concentrated on grass (between 50 and 75 % being included in the diet). Species selected were Cenchrus ciliaris, Digitaria eriantha, Panicum coloratum (white buffalo grass, witbuffelsgras) and Panicum deustum (broad- leaved Panicum, breeblaarbuffelsgras); these are all late development stage grasses. The Merino sheep selected 8 % more grasses and 2 % trees and shrubs than the Dorper sheep, while the latter selected 8 % more karoo bushes and 2 % more noors.
From Table 1, the indices of agreement in the diets selected in summer indicate that there is a remarkable degree of similarity in the diets of both the Merinos and Dorpers in the Arid Karoo and the Noorsveld, which can be ascribed to the grass component in the diet.
However, the similarity between the diets selected in the Arid Karoo and the Eastern Mixed Karoo for both the Merinos and Dorpers is quite low. This is ascribed to the greater volume of annuals selected at this time in the Eastern Mixed Karoo.
The Merinos show a remarkably high degree of similarity in the diets selected in the Eastern Mixed Karoo and the Noorsveld, on account of the similar diets of karoo bushes
and grasses. The Dorpers, on the other hand, display alow degree of similarity between these two veld types because of the large percentage of annuals in the diet in the Eastern Mixed Karoo in contrast to the large percentage of grass found in the diet in the Noorsveld.
Diet selection during autumn (Figure 4 and Table 1)
In autumn in the Arid Karoo, because of the fairly reliable autumn rains, the vegetation was fairly succulent and the sheep concentrated on Pentzia spinescens (doringkaroo) . However, the animals still grazed the other plant species to a certain degree (about 25 % grass). Merino sheep selected a diet containing between 60 and 65 % of the karoo bush species group. Dorper sheep, on the other hand, selected about 90 %. The Dorper sheep selected 15 % more karoo bushes than Merino sheep, while Merino sheep selected 16 % more grass species.
During autumn in the Eastern Mixed Karoo the sheep selected palatable karoo bushes Felicia muricata, Phymaspermum parvifolium (witheuningkaroo, witblommetjie) and Salsola calluna, less palatable Pentzia globosa (vaalkaroo) and unpalatable Dimorphotheca zeyheri in about equal proportions, and about 10 to 20 % grass.
Merino sheep selected 9 % more grass and 17 % more annuals than Dorper sheep, while Dorper sheep selected 17 % more karoo bushes.
During autumn in the Noorsveld sheep concentrated on the karoo bush Pentzia incana (good karoo, ankerkaroo). Merino sheep selected 2 % more karoo bushes and 5 % more noors than the Dorper sheep, while the Dorper sheep selected 6 % more grasses and 3 % more trees and shrubs.
From Table 1 it is clear that the different diets of the two sheep breeds in the various veld types are to a large degree similar. However, the diet selected by the Merinos in the Arid Karoo differs quite substantially from the diet selected in the Eastern Mixed Karoo. This condition can be ascribed to the larger percentage of karoo bushes grazed in the Arid Karoo than in the Eastern Mixed Karoo and the larger percentage of annual plants grazed in the Eastern Mixed Karoo than in the Arid Karoo.
Diet selection during winter (Fig. 5 and Table 1)
In the Arid Karoo sheep grazed mainly four species during winter. These were the grasses, Aristida congesta and Enneapogon desvauxii, the less palatable karoo bush Pentzia spinescens and the unpalatable karoo bush Eberlanzia ferox (doringvygie), which provided some moisture to the diet during the dry winter period. Dorper sheep selected 11 % more karoo bushes and 4 % more annuals than Merino sheep, while the Merino sheep selected 15 % more grasses.
In the Eastern Mixed Karoo during winter sheep mainly selected the palatable karoo bushes Salsola calluna and Felicia muricata, the less palatable Rosenia humilis (perdekaroo) and the annual plantGnaphalium glomerulatum (roerkruid, vaalbossie).
Dorper sheep selected 6 % more karoo bushes and 10 % more annuals than the Merino sheep, while the Merino sheep selected 8 % more grasses. In the Noorsveld sheep selected a diet mainly in accordance with the plants that were available, but over 50 % karoo bushes and grass while the rest was made up of trees, shrubs and noors. The annual plant component was ignored in the Noorsveld owing to the fact that it never contributed more than 1 % to either the plant cover or to the fistula extrusa samples. Dorper sheep selected 7 % more karoo bushes and 12 % more trees and shrubs than the Merino sheep, while the Merino sheep selected 3 % more grasses.
From Table 1 it is clear that as the vegetation dries off during the dormant stage, the diets selected by both sheep breeds agree to a large degree. The exception is that the diets of both breeds differ between the Arid Karoo and the Noorsveld, and the Eastern Mixed Karoo and the Noorsveld, on account of the larger contribution made by the woody plant component and the noors in the Noorsveld, supplying moister grazing to the animals than that available in the herbaceous stratum, which, at that stage, is virtually grazed out (cf. Plate 3).
On the whole over all the seasons, it can be said that Merino sheep concentrated their grazing more on grass, while Dorper sheep concentrated their grazing more on the woody plant component of the veld, i.e. karoo bushes in the Arid Karoo and the Eastern Mixed Karoo and the tree and shrub component of the Noorsveld. From the abovementioned it is clear that Dorper sheep is the more generalist grazer of the two breeds, grazing more of the woody component in the Noorsveld and more of the woody karoo bushes in the other veld types, while Merino sheep is the more selective grazer, grazing more soft-leaved grasses and karoo bush leaves and thin karoo bush twigs.
It is interesting to note that during summer and autumn both sheep breeds grazed very little noors (Euphorbia coerulescens), only about 10 to 15 %. This is ascribed to the fact that there is still sufficient, fairly soft forage available to graze. During winter both sheep breeds
grazed excess of 25 % noors (Euphorbia coerulescens), indicating drying off of the veld on the one hand, and secondly, that the soft forage became unavailable in this fairly degraded veld.
Owing to the large overlap, which ranged from 92 to 94 %, in the diets selected by the different breeds in the Arid Karoo (Table 2), the difference of 6 to 8 % is not large enough to consider combining different sheep breeds in order to better utilise the vegetation of the Arid Karoo as was indeed also reported earlier by du Toit et al. (1995a). However, the large overlap in the diets selected by the two different breeds studied, indicates that both breeds are well adapted to individually graze and produce both meat and wool on the vegetation of the Arid Karoo.
In the Eastern Mixed Karoo, during the growing season there is an overlap in the selected diets of between 75 and 89 % (Table 2). The difference of 11 to 25 % is large enough to consider the better utilisation of the vegetation by combining different sheep breeds with
cattle, following the recommendations by Botha et al. (1983). The fairly large differences in the diets selected by the two breeds in the Eastern Mixed Karoo, are indicative of the existence of special niches in the veld, where these breeds will graze preferentially, in order to satisfy their requirements.
In the Noorsveld, during the growing season there is an overlap of 97 to 98 % in the selected diets (Table 2). This difference of 2 to 3 % is not large enough to consider combining different sheep breeds to better utilise the vegetation. However, anyone of the two sheep breeds can be combined with Boer goats as was reported earlier by du Toit et al. (1995b). This combination will lead to increased meat production per hectare.
The large overlap in the diets selected by the two breeds studied, indicates that both are well adapted to the herbaceous and karoo bush vegetation component of the Noorsveld, where this veld has not become too degraded. However, because of the large contribution of the woody vegetation, more adapted genotypes, such as Boer goats in combination with Dorper sheep may graze this vegetation to advantage. It seems, however, that Merino sheep are particularly ill-adapted to graze this vegetation advantageously on their own, on account of their selection of the soft herbaceous vegetation.
ACOCKS, J.P.H. 1988. Veld types of South Africa. Memoirs of the Botanical survey of South Africa, 57:1-146. Pretoria: Government Printer.
ANONYMOUS. 1991.Agricultural Development Programme, Karoo Region. Grootfontein Agricultural Development Institute Report. Unpublished. Middelburg.
BOOYSEN, J. 1990. D.stat. Computer program developed at Potchefstroom University for CHE. Potchefstroom.
BOTHA,P., BLOM. C.D., SYKES, E. & BARNHOORN, A.S.J. 1983. A comparison between the diets of small and large stock on mixed Karoo veld. Proceedings of the Grassland Society of southern Africa, 18: 101-105.
BOTHA, W. VAN.D., DU TOIT, P.C.V., BLOM, C.D., BECKER, H.R., OLIVIER, D.J., MEYER, E.M., BARNARD, G.Z.J. & SCHOEMAN, P. 1995.Weidingswaardes vir Karoo plantspesies. Grootfontein Agricultural Development Institute Report, Unpublished. Middelburg.
BREDON, R.M., TORELL, D. T. & MARSHALL, B. 1967. Measurement of selective grazing of tropical pastures using oesophageal fistulated steers. Journal of Range Management, 20: 317-320.
DUGMORE, T.J., DU TOIT, P.C.V. & MORNING, SALLY J. 1991. Dietary selection by steers grazing kikuyu. South African Journal of Animal Science 21: 194-197.
DU TOIT, P.V.C., BLOM, C.D. & IMMELMAN, W.E 1995a. Diet selection by sheep and goats in the Arid Karoo. African Journal of Range and Forage Science, 12: 16-26.
DU TOIT, p.C.V. & BLOM, C.D. 1995b. Diet selection by sheep and goats in the Noorsveld. African Journal of Range and Forage Science, 12: 27-37.
HEADY, H.F. & TORELL, D.T. 1959. Forage preferences exhibited by sheep with oesophageal fistulas. Journal of Range Management 12:28-34.
SMITH, C.A. 1966. Common names of South African plants. Memoirs of the Botanical survey of South Africa 35: 1-642. Pretoria: Government Printer.
WILLMOTT, C.J. 1981. On the validation of models. Physical Geography, 2: 184-194.
WILLMOTT, C.J. 1982. Some comments on the evaluation of model performance. Bulletin of the American Meteorological Society, 63: 1309-1313.
WILLMOTT, C.J. & WICKS, D.E. 1980. An empirical method for the spatial interpolation of monthly precipitation within California. PhysicaI Geography, 1:59-73.