- Using dietary overlap to calculate animal ratios on mixed Karoo veld
|Last update: March 30, 2012 09:11:09 AM|
USING DIETARY 0VERLAP TO CALCULATE ANIMAL RATIOS ON MIXED KAROO VELD
P BOTHA, CD Blom, E Sykes and A S J Barnhoorn
Grootfontein College of Agriculture
Private Bag X529
The diets of different animal types grazing the same veld will differ as a result of behavioural preferences, anatomical specializations, physiological requirements and limitations, as well as body size of the animal. Consequently, the number of animals able to utilize a particular area will differ from animal type to animal type.
With the present emphasis on optimal utilization of natural resources, multiple use of veld has become more and more important. In order to optimize the utilization of veld by means of multiple use, the diet selection of the various animal types should be known. Flinders & Conde (1980) have indicated that if the dietary overlap between different animal types were known, the ratio between the animal types could be drastically changed.
The fact that very little was known of the dietary selection of the most important animal types in the Karoo and the possibilities of multiple use of the vegetation led to the study to determine the botanical composition of the diets of Merino and Dorper sheep, Boer goats and Afrikaner cattle on a mixed karoo veld.
When the study commenced in August 1975, the palatable karoo bush component contributed approximately 45%, the unpalatable karoo bush component approximately 20% and the grass component approximately 34% to the floristic composition of the veld expressed on a mass basis.
Oesophageally fistulated Dorper and Merino wethers and ewes; Afrikaner oxen and heifers; and neutered Boer goats were used to collect dietary samples. Four animals of each kind were used. Collection was done on four consecutive days every three months. The largest differences in the chemical composition of the diet of the animals occurred between the first and last two days (Zeeman, personal communication, 1980). The floristic composition of the diet was determined by means of a microscope point survey (Heady & Torell, 1959).
The floristic composition of the diets of the animals was compared and the percentage similarity calculated. Similarity was calculated by means of a method devised by Roux (1968) and Sorenson's Similarity Index (Flinders & Conde, 1980). Both methods were used, as the first method calculates the similarity quantitatively and the second method gives an indication of the overlap in selected species.
Tables 1 and 2 were compiled from the results of the floristic analysis of the oesophageal extrusa and the calculations of the similarity indices.
COMPARISON OF THE DIETS OF THE DIFFERENT ANIMAL BREEDS
1. Dorper versus Merino
The two breeds of sheep showed an average similarity in the floristic composition of their diet of 61 % (Table 1). Winter and spring sampling periods showed the highest degree of similarity (Table 1), when both animal groups concentrated mainly on the palatable karoo bush component during winter and the unpalatable component during spring (Fig 1 & 2).
The concurrent dietary species were mainly Salsola calluna (swartganna) and to a lesser extent Felicia muricata (bloublommetjie). The summer and autumn sampling periods were characterized by a large number of similar species (Table 2), but with a wide range of percentage occurrence. Grass was the most common species during these periods. Dimorphotheca zeyheri (bietou) was, however, an important dietary species in common during autumn.
2. Dorper versus Boer goat
The highest degree of similarity between these breeds was found during summer (Table 1), when both groups concentrated mainly on annual herbaceous plants and to a lesser extent on the grass component (Fig I & 3). Gnaphalium glomerulatum (groenbossie) and Salsola kali (rolbos) were the most important joint dietary species of the summer period. The second highest similarity in the diets of the two groups of animals was found during winter (Table 1). During this period the animals concentrated mainly on the palatable karoo bush component (Fig I & 3), particularly Salsola calluna (swartganna). The Dorper sheep concentrated mainly on the unpalatable karoo bush component during spring (Fig I), while the Boer goats concentrated mainly on the palatable karoo bush component (Fig 3). Hence there was a low degree of similarity (Table 1), although a relatively high number of concurrent species, at differing percentages, was included in the diet of both groups of animals (Table 2). In autumn both animal groups concentrated mainly on the palatable karoo bush component (Fig 1 & 3), with the unpalatable species Dimorphotheca zeyheri (bietou) and the geophyte Homeria pallida (geeltulp) being the most important common dietary species.
3. Dorper versus Afrikaner
As shown in Table 1, there was only a 30% similarity in the diets of these two animal breeds. The highest similarity was found in spring. At that particular sampling period a number of palatable karoo bush species, viz Atriplex suberecta (wilde lusern), Felicia muricata (bloublommetjie) and Salsola calluna (swartganna), and the grass component were the most important dietary species in common. The diet of the cattle was characterized by a high degree of selection for grass during summer (Fig 4), while the Dorpers concentrated mainly on annual herbaceous plants (Fig 1), hence the low degree of similarity (Table 1). During winter the diets were also relatively low in similarity and the palatable species Felicia muricata (bloublommetjie) and, to a lesser extent, Salsola calluna (swartganna) were the most important joint dietary species.
4. Merino versus Boer goat
On the average the selected diet of the Merino and Boer goat showed an approximately 50% similarity (Table 1). The highest similarity was found in winter (Table 1), when both groups concentrated mainly on the palatable karoo bush component (Fig 2 & 3) and when Salsola calluna (swartganna) was the most important concurrent species.
In summer the diets of the animal groups were similar by almost 60% (Table 1), as a result of a high degree of selection for the annual herbaceous plant Salsola kali (rolbos) by both animal groups. In spring and autumn the Boer goats showed a preference for the palatable karoo bush component (Fig 3). The Merino, on the other hand, preferred the unpalatable karoo bush component during spring and selected almost equal percentages of the two karoo bush components during autumn (Fig 2). The dissimilar selection of the two animal groups during the two seasons is revealed in the relatively low percentage similarity (Table 1).
In spring the most important concurrent species were the palatable karoo bush Salsola calluna (swartganna) and the unpalatable karoo bush Galenia procumbens (kraalbos).
In autumn the unpalatable karoo bush Pentzia globosa (vaalkaroo) and, to a lesser extent, the grass component proved to be favoured by both animal groups.
5. Merino versus Afrikaner
As was the case with the diets of Dorper sheep and cattle, there was only a 30% similarity in the diets of the Merino and Afrikaner (Table 1). Salsola calluna (swartganna), Felicia muricata (bloublommetjie) and, to a lesser extent, the grass component were the joint dietary species and components of the two animal groups. The relatively low similarity in the diets of the Merino and the Afrikaner (Table I) was expected, and is mainly due to the fact that cattle preferred the grass component (Fig 4) and sheep varied their preference during different seasons of the year (Fig 2).
6. Boer goat versus Afrikaner
An almost equal similarity exists in the diets of the two groups for winter, summer and autumn (Table 1). The winter diet of the two groups was characterized by a mutual preference for the palatable karoo bush Salsola calluna (swartganna) and the unpalatable karoo bush Rosenia humilis (blouperdekaroo). The grass component and the annual herbaceous species Gnaphalium glomerulatum (groenbossie) were the most important common elements of the summer diets. The diets in autumn showed a high degree of similarity in the amounts selected of the palatable karoo bush species Felicia muricata (bloublommetjie), Phymaspermum parvifolium (witblommetjie) and Salsola calluna (swartganna), as well as the geophyte Homeria pallida (geeltulp) and the grass component. During spring there was an increased similarity in the diets of the Boer goats and the Afrikaner (Table 1). The increase was due to a mutual preference for the annual herbaceous plants, particularly Gnaphalium glomerulatum (groenbossie), and the palatable karoo bushes, especially Salsola calluna (swartganna).
It was found that the greatest average similarity between the various breeds was between the Dorper and Merino (61%), whilst the smallest average similarity was almost 32% between Dorper sheep and Afrikaner cattle.
The large stock and the small stock (sheep and goats) showed an average similarity in their diet of almost 35%. The biggest dietary overlap was in spring when the animals concentrated mainly on the karoo bush component (Fig 1). The highest number of selected species in common between large and small stock was, however, during autumn (Table 2), when both groups selected a number of palatable and unpalatable karoo bush species. The smallest degree of similarity was during summer, viz. less than 30% (Table 1), when the large stock concentrated mainly on the grass component and the small stock on the herbaceous plants, palatable karoo bushes and the grass component.
In order to make more efficient use of the veld, especially mixed veld, by means of multiple use, it is essential to bring dietary overlap between the different animal types into consideration when calculating the number of each type. The calculated dietary overlaps for the four animal types used in this study are shown in Tables 1 & 2.
In order to eliminate the difference in diet selection by different animal types, Flinders & Conde (1980) proposed the addition of the proportion of dietary overlap to the well-known equation to calculate animal unit equivalents. This formula will then be:
AU equivalent =
For example, to determine the AU equivalent of a 47 kg Merino ewe compared to the defined AU when the dietary overlap between the two breeds is 37,3% (Table 1) the calculation will be:
AU equivalent =
= 14.6 Merino ewes/AU
Using this formula and the results from Table 1, the following AU equivalents for Dorper and Merino sheep, Boer goats and Afrikaner cattle on mixed Karoo veld have been calculated.
TABLE 4 Animal unit equivalents for four animal types on mixed Karoo vela calculated with addition of dietary overlap.
|Animal Type||Mass (kg)||Approximate no of animals/AU|
|Afrikaner steer (mature)||450||1|
|Dorper ewe (6-tooth)||53||15.7|
|Neutered Boer goat (6-tooth)||70||12.5|
From Table 4 it can be seen that 14,6 Merino sheep, grazing a mixed Karoo veld, will remove the same amount of forage of the same plant species composition as one steer on the same type of veld. Similarly 12,5 Boer goats and one steer will remove the same amount of forage when grazing similar veld.
Aucamp (1979) has determined a 40% increase in carrying capacity of the Valley Bushveld where Boer goats and Dorper sheep were run as a combined flock. In the Middelburg district the carrying capacity is estimated at 1,71 ha/SSU (0,58 SSU/ha or 0,04 MLU where I MLU = 14,6 SSU (Table 4) and I SSU = a dry Merino ewe of 45 kg). The dietary overlap between Merino sheep and Afrikaner cattle is 37,3% (Table 1). When a combined flock of these animals is to be used, this dietary overlap should be compensated for when calculating the carrying capacity for the combination. The obvious method would be to reduce the number of each type by 50% of the dietary overlap, i.e. 18,65%. This implies that, for multiple use of mixed Karoo veld, the Merino sheep and Afrikaner cattle carrying capacities will be 81,35 % of the normal carrying capacity for each type.
Merino : 81,35% of 0,58 = 0,47 SSU/ha
Afrikaner : 81,35% of 0,04 = 0,03 MLU/ha
If the 0,03 MLU/ha is multiplied by 14,6 to convert it to SSU/ha the answer is 0,44 SSU/ha. The combined carrying capacity for Merino sheep and Afrikaner cattle on mixed Karoo veld will then be:
0,47 (Merino) + 0,44 (Afrikaner) = 0,91 SSU/ha This is an increase of 56,9ro compared to the single breed carrying capacity of 0,58 SSU/ha (1,71 ha/SSU).
Using the afore-mentioned method and the appropriate figures from Table 1 & 4, the following calculation for a combination of Dorper and Merino sheep can be made:
Merino carrying capacity = 0,58 SSU/ha
1 Merino = 1,06 Dorper (Table 4)
Dorper carrying capacity = 0,55 SSU/ha
Dietary overlap = 61 % (Table 1)
To compensate for the dietary overlap, the number of each breed is reduced by 50ro of the overlap, i.e. 30,5%. This means the carrying capacity of each breed will be 69,5 % of the original carrying capacity:
Merino 69,5% of 0,58 = 0,40 SSU/ha
Dorper 69,5% of 0,55 = 0,38 SSU/ha
The combined carrying capacity will then be :
0,40 (Merino) + 0,38 (Dorper) = 0,78 SSU/ha
This represents an increase of 34,5% on the single breed carrying capacity estimated at 0,58 SSU/ha for the Middelburg district.
The number of animals of each breed and type which can simultaneously utilize the veld can be determined by using the dietary overlap between the different breeds and types. In this way the number of animal units per unit area can be increased. A factor that should however always be borne in mind is the fact that the utilization of veld by animals should be detrimental to neither the veld nor the animal.
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Karoo Agric 3 (2), 12-17