A comparative assessment of Dorper sheep in different
production environments and systems
S.J. Schoeman
*Department of Animal Sciences, Faculty of Agriculture, University of Stellenbosch, Stellenbosch, South Africa
Abstract
Performance of Dorper sheep was compared to other breeds in pure and cross-breeding evaluations under different production systems. The number of ewes lambed per ewe joint (EL/EJ) varied for the Dorper from 0.68 to 0.91 (average 0.81)
and for other breeds and crosses from 0.65 to 0.91 (average 0.82). Average litter size (LB/lambing) varied from 1.02 to 1.52
(average 1.28) and from 1.02 to 1.70 (average 1.30) for the two groups, respectively. In general, the Afrino and Finnish composite lines were more proli®c than the Dorper. Post-weaning mortality rates in Kenya were high (49%) among Dorper lambs compared to the local Red Maaisai breed, owing to a high susceptibility to gastrointestinal parasites. Average weaning weights (WW) (100 days) were 12.4% heavier in the Dorper than in the breeds it was compared to, but 31.7% heavier than the woolled breeds. Total WW and ef®ciency of Dorper and Dorper crosses were higher than Merino, Afrino, DoÈhne merino, South African Mutton Merino, but lower than in Finnish Landrace composite lines. Both Dorper and Dorper cross-breeds reached target slaughter weight of approx. 40 kg earlier than Merino, Afrino, Suffolk and Ile de France crosses. Average daily gain to slaughter was 44% higher in the Dorper than the average of the groups it was compared to. It is concluded that the Dorper was superior in reproductive and growth traits to woolled and other indigenous breeds. However, there is a lack of information on the performance of the Dorper in cross-breeding systems.#2000 Elsevier Science B.V. All rights reserved.
Keywords:Dorper sheep; Cross-breeding; Reproduction; Growth; Review
1. Introduction
Southern Africa has a rapid growing human popu-lation. This increase has resulted in a corresponding increase in demand for meat and mutton. The per capita consumption of mutton and lamb in South Africa is 4 kg, which is high compared to many other countries, while annual production is 107103t and annual demand 160103t (Abst. Agric. Stat., 1997).
The total number of sheep in South Africa has dropped from 36.5 million in 1965 to 25.4 million in 1995, while the largest decline was in Merino sheep numbers. For several decades sheep production was primarily aimed towards wool production. How-ever, this has changed during the past three decades owing to inter alia the high demand for mutton and lamb, the meat vs. wool price structure and input costs. The numbers of mutton and dual-purpose-types has consequently increased accordingly. Approxi-mately 70% of all sheep are still woolled sheep and mainly Merino or merino lines (e.g. the DoÈhne Merino).
*Fax:27-21-808-4750.
E-mail address: [email protected] (S.J. Schoeman)
The need of a breed suitable for lamb production in the arid, extensive areas of southern Africa, resulted in the formation of the Dorper. It became a popular breed, not only in the extensive areas, but also in other parts of southern Africa and consequently became numerically the second most important sheep breed in South Africa.
1.1. The state of cross-breeding in sheep
Combining appropriate breeds in cross-breeding systems offers the opportunity to increase productive ef®ciency through both additive and non-additive genetic effects. Breed characterization programmes have revealed large differences among breeds for a variety of economic important traits. This is especially evident in the beef cattle industry across the world (Gregory et al., 1991). Cross-breeding among sheep breeds is mostly practiced in countries of the Northern Hemisphere, while in countries of the Southern Hemi-sphere wool production plays an important part in the sheep industries.
Despite the decline in woolled sheep numbers, the wool industry is still considered of great importance. Cross-breeding woolled sheep with other breeds, and especially those having hair, kempy or black ®bres in their ¯eeces is highly opposed owing to the possibility of contamination of wool ¯eeces. The wool industry, supported by The South African National Wool Growers' Association has launched a campaign against the use of hairy breeds in cross-breeding with woolled sheep. The Dorper is one of those also being mentioned due to a high frequency of spotted and even black lambs produced in woolled sheep. Dorper rams are nevertheless being used by many farmers in cross-breeding with Merino ewes. However, owing to the above-mentioned disapproval, little information is available on the use of the Dorper in cross-breeding systems.
White Dorper rams were used in the initial cross-breeding phase during the development of the Afrino breed. Some of these results were reported by Olivier et al. (1984). In a later study Du Toit (1996) compared several sire breeds, including the White Dorper, in cross-breeding with Merino ewes in exten-sive farming conditions. In this study, 50 Merino ewes in each group were mated to ®ve fertility tested rams of each breed. Performance characteristics and
relative gross income per 100 ewes were recorded in this study.
The performance of composite lines, developed from indigenous sheep breeds and the Finnish Land-race and subsequently crossed with Dorper rams, was reported by Schoeman et al. (1993, 1995) and Schoe-man and Van der Merwe (1994).
Dorper sheep were also exported to Zimbabwe, Zambia, Kenya, Israel and Saudi Arabia. In Zambia they are mainly owned by commercial farmers on mixed farms in the central and southern provinces (Stafford and Hansson, 1991). In various parts of Africa attempts were made to assess the potential of indigenous sheep breeds when crossed with Dorper sheep. Stafford and Hansson (1991) mentioned that indigenous sheep in Zambia, known as Rhodesian sheep, were crossed with Dorper to improve produc-tivity, while Red Maaisai sheep (RM) were crossed and evaluated against the Dorper in Kenya (Inyangala et al., 1991, 1992; Baker et al., 1994, 1998).
The objective of this paper is to review comparisons in literature between Dorper and other breeds in pure and cross-breeding systems.
2. Reproductive performance
The reproductive performance of Dorper sheep and other breeds, as well as some of their crosses, were compared under a variety of environmental conditions. A summary of these results is shown in Table 1.
2.1. Age at ®rst lambing
developed from crosses between indigenous sheep breeds in South Africa and the Finnish Landrace, no difference was obtained in age at puberty (Schoe-man et al., 1993). An earlier onset of puberty was also reported by Greeff et al. (1993) in Romanov and RomanovDorper cross-bred ewes as compared to Dorper ewes.
2.2. Ewe fertility
The number of ewes lambing per ewe joint (EL/EJ)
varied in the Dorper from 0.68 (Baker et al., 1998) to 0.91 (Basson et al., 1969) (Table 1). The highest fertility of 0.91 was recorded in the Dorper (Basson et al., 1969), the (1/2)D (1/2)M cross-bred ewes (Olivier et al., 1984) and the Finn composites (Schoe-man and Van der Merwe, 1994), while the lowest value of 0.65 was that reported by Olivier et al. (1984) in the Merino. The study carried out by Olivier et al. (1984) was under harsh dry Karoo-veld conditions, while in the case of Basson et al. (1969) and Schoeman and Van
der Merwe (1994) comparisons were made where the ewes were maintained in individual pens from mating until weaning of their lambs. Baker et al. (1998) also compared the Dorper to the local Red Maaisai sheep breed (RM) in the sub-humid coastal region in Kenya and reported an approx. 12% higher performance in the RM. Generally, it appears that the Dorper has an acceptable fertility rate which tends to be higher than those of the woolled sheep breeds.
2.3. Litter size
Litter size (LB/lambing) in the Dorper varied from
1.02 (Baker et al., 1998) under extremely unfavour-able conditions to 1.52 (Eltawil and Narendran, 1969) in Saudi Arabia (Table 1). Average litter size in the Dorper was 1.28 as compared to the 1.30 recorded in the breeds it was compared to. It was higher than those of the woolled breeds (Olivier et al., 1984; Schoeman, 1990; Snyman, unpubl.), approx. the same as in the RM (Baker et al., 1998) but lower than in the Afrino Table 1
Comparative reproductive performance of Dorper ewes in pure and cross-breeding evaluations
Parameter Means for breeds/crossesa Sources
D 1/2 D
1/2 M
M MM DM Finn
Comp
A RM SA
bE
L/EJ(Fertility) 0.91 0.85 0.86 Basson et al. (1969)
0.83 0.91 0.65 Olivier et al. (1984)
0.88c 0.87c 0.80d Schoeman (1990)
0.81 0.73 Eltawil and Narendran (1969)
0.74d 0.91c Schoeman and Van der
Merwe (1994)
0.68d 0.77c Baker et al. (1998)
LB/lambing
(litter size)
1.18 1.03 1.02 Olivier et al. (1984)
1.39c 1.32d 1.32d Schoeman (1990)
1.52 1.54 Eltawil and Narendran (1969)
1.50 1.31 1.70 Snyman (unpubl.)
1.08d 1.44c Schoeman and Van der
Merwe (1994)
1.02 1.02 Baker et al. (1998)
LB/EJ/year 1.57 1.18 1.21 Basson et al. (1969)
1.46c 1.42c 1.25d Schoeman (1990)
1.80 1.26 1.76 Snyman (unpubl.)
aD ± Dorper; M ± Merino; MM ± South African Mutton Merino; DM ± DoÈhne merino; Finn Comp. ± Finnish Landrace composite;
A ± Afrino; RM ± Red Maaisai; SA ± Saudi Arabian indigenous breeds (Najdi, Awassi and Harri).
bE
(Snyman, unpubl.) and Finn composites (Schoeman and Van der Merwe, 1994), respectively. In the latter case (Schoeman and Van der Merwe, 1994), litter size in the proli®c composite was 33% larger than in the Dorper. In two other reports average litter sizes in Dorper sheep of 1.59 (Cloete and De Villiers, 1987) and 1.21 (Schoeman and Burger, 1992) were obtained, respectively.
In a comparative study among maiden ewes of Romanov, Dorper and RomanovDorper crosses, ovulation rates of 1.9, 1.0 and 1.4, respectively, were reported by Greeff et al. (1993). Age at ®rst oestrus was also signi®cantly (p0.05) lower in the Romanov and the crosses than in the Dorper. In both this study as well as that of Schoeman and Van der Merwe (1994) with the Finnish Landrace composite line, it was indicated that a substantial increase in lambs born could be obtained by the infusion of genes from proli®c populations to the Dorper via the application of appropriate cross-breeding programmes.
In an accelerated lambing system, where the ewes were joint at 8-month intervals, lambing performance per year (LB/EJ/y) was signi®cantly (p0.05) higher in
the Dorper (1.46) compared to the DM (1.25) (Schoe-man, 1990). In an earlier study Basson et al. (1969) reported corresponding values of 1.57, 1.21 and 1.18 for the Dorper, DM and Merino, respectively. In another study (Schoeman and Van der Merwe, 1994) LB/LO (every 8 months) was 54% more
(p0.05) in the proli®c Finn composites than in the Dorper (1.28 vs. 0.83). The Dorper could therefore not be designated as a high proli®c breed, although it compares favourably with most other breeds in a variety of environmental and management condi-tions.
2.4. Survival rate
Survival rates of Dorper and Dorper crosses were only compared by Olivier et al. (1984); Schoe-man (1990); Baker et al. (1994); SnySchoe-man (unpubl.) (Table 2). Higher neonatal mortalities among lambs were reported by Olivier et al. (1984) when Merino ewes were crossed with either Dorper or Dorset Horn rams (6.1 and 7.1%, respectively) compared to the Dorper (4.8%) and the Merino (3.5%), respectively. It is well-known that the use of fast growing terminal sire breeds tends to increase mortality at birth in both ewes and lambs in the Merino.
Survival rates from birth to weaning were higher (p0.05) in the Dorper than in the DM and Merino, respectively (Schoeman, 1990; Snyman, unpubl.) under fairly favourable conditions, but lower than in the RM and RMDorper crosses between birth and weaning (90 days) (Baker et al., 1994). However, from weaning to 9 months of age mortality rates amongst Dorper lambs were much higher than in the RM and RMDorper crosses. Mortality rates declined as the contribution of the RM in the crosses increased (see Fig. 1).
In a later report by the same authors (Baker et al., 1998), pre-weaning mortality rates of 10.3% for the RM and 30.0% for the Dorper were obtained, while corresponding post-weaning mortalities were 18.4 and 39.2%, respectively. These high mortality rates were mainly caused by low resistance of Dorper lambs to endoparasites (see Section 5). It was suggested by these authors that although differences in pregnancy rates and litter sizes were small, the number of lambs weaned per ewes joint in the RM was almost double that of the Dorper (0.61 vs. 0.34). It clearly shows that
Table 2
Comparative survival rates of Dorper lambs in pure and crossbreeding evaluations
Period Means for breeds/crossesa Sources
D M/DM MM A RM RMX
Birth ± 90 days 0.81 0.88 0.89 Baker et al. (1994)
Birth ± 100 days 0.84b 0.81c 0.83b Schoeman (1990)
Birth ± 120 days 0.89 0.78 0.88 Snyman (unpubl.)
90 days to 9 months 0.51 0.90 0.78 Baker et al. (1994)
the Dorper, developed for the arid South African conditions, is not suitably adapted to the humid tro-pical parts of Africa.
3. Body weights and growth performances
Body weight and growth performance are important traits in all lamb-producing sheep breeds. This applies to both the pre-weaning and post-weaning phases.
3.1. Birth weight
Birth weight (BW) of Dorper lambs and those they are compared to are in Table 3. BW of Dorper Fig. 1. The in¯uence of genetic composition in Dorper, Red
Maaisai and crosses on postweaning mortality rate (Source: Baker et al., 1994).
Table 3
Comparative birth and weaning weights of Dorper lambs in pure and crossbreeding evaluations
Parameter Means for breeds/crossesa Sources
D 1/2 D
1/2 M
M/DM MM 1/2 D 1/2 Finn Comp
A RM RMX SA
Birth weight (kg)
4.2 4.5 3.8 Olivier et al. (1984)
4.3c 4.0b 4.6d Schoeman (1990)
3.8 3.9 Eltawil and Narendran
(1969)
3.7 3.7 Inyangala et al. (1991)
4.1 4.2 Inyangala et al. (1992)
5.0b 3.4c Schoeman and Van der
Merwe (1994)
4.3 4.1 4.4 Snyman (unpubl.)
Weaning weight (kg)
16.3b 13.8d 15.5c Schoeman (1990)
50 days 17.3c 16.8b Schoeman and Van der
Merwe (1994)
26.8 21.4 Basson et al. (1969)
75 days 22.1 20.0 Eltawil and Narendran
(1969)
85 days 16.0 16.1 Inyangala et al. (1991)
90 days 19.4 19.8 Inyangala et al. (1992)
11.8 10.6 11.3 Baker et al. (1994)
32.0 28.6 22.7 Olivier et al. (1984)
100 days 28.6b 25.0c 25.8c Schoeman (1990)
26.7c 26.5c Schoeman and Van der
Merwe (1994)
33.4 23.7 32.2 Snyman (unpubl.)
120 days
lambs were in most cases heavier than in the woolled breeds, except for the MM which is also known to have high BW and a high incidence of dystocia. Average BW, 42-day weights and 100-day weights of more than 117 000 records obtained from the South African Sheep Performance Testing Scheme for various breeds are in Table 4. There is no obvious explanation for the high BW of the Dorper in these data. These results could, however, be biased since it does not take into account the effect of differences in production environments and manage-ment levels. Dorper, Afrino and Damara sheep are mostly found in the extensive grazing areas, whereas some other breeds, e.g. the Ile de France and Hamp-shire are mostly farmed within intensive high input systems.
In the comparison of the Dorper with the Finn composites (Schoeman and Van der Merwe, 1994), where both ewe groups were mated to the same Dorper rams, BW of the Dorper exceeded that of the Dor-perFinn composite lambs by 47%, probably indicat-ing a negative maternal effect restrictindicat-ing BW in the composite lines. In this case an exceptionally high BW of 5.0 kg compared to the other reports was recorded in the Dorper, probably due to the very favourable conditions in nutrition and management. In the Kenya studies (Inyangala et al., 1991, 1992) BW of Dorper lambs were almost identical to those of the Dor-perRM crosses.
3.2. Weaning weights
Weaning weights (WW) were recorded at various ages, depending on whether an accelerated lambing system or a traditional once-a-year lambing system was applied. WW of Dorper lambs (Table 3) varied considerably from 11.8 kg at 90 days (Baker et al., 1994) to 33.4 (Snyman, unpubl.). The results obtained in the Kenya study (Baker et al., 1994) are an indica-tion of the impediment of the severely stressful envir-onment prevailing in the sub-humid coastal region of Kenya on WW. It also applied, although to a lesser degree, to the results obtained by Inyangala et al. (1991) at the Ol'Magogo Research Station in Kenya. WW reported in all other investigations were almost double those which were obtained in the Kenya stu-dies. WW of Dorper lambs were also signi®cantly (p0.05) higher than those of the DM, M and MM, respectively (Basson et al., 1969; Schoeman, 1990), but not signi®cantly different from the DorperFinn composite lambs, although BW and dam weights differed by 47 and 42%, respectively (Schoeman et al., 1993). In the data of the National Sheep Performance Testing Scheme (Table 4), WW of the Dorper was lower than that of the MM, while only lambs of the Afrino and Damara were smaller than those of the Dorper.
Early weaning weights in accelerated lambing sys-tems were reported by Basson et al. (1969), Schoeman (1990) and Schoeman and Van der Merwe (1994) only (Table 3). 50 dW was higher in the Dorper than in both the DM and MM. These differences may be due to variation in milk production, although it has as yet not been investigated. Contrary to that, average pre-wean-ing weight (42 days) of Dorper lambs (Table 4) was lower than those of all other breeds with the exception of the Damara, which may be an indication of either poor milk production or composition or inferior con-ditions under which the Dorper was maintained. Despite the large differences in BW between the Dorper and the DorperFinn cross-bred lambs and mature weights of their dams, 50 dW were not sig-ni®cantly different (Table 3), which might be the result of a difference in milk composition as was suggested by Schoeman et al. (1993). Carstens (1969) compared the milk production of the Dorper with that of the Dormer and obtained a signi®cantly (p0.05) higher butterfat content in the milk of the Dormer, while yield Table 4
Comparative birth, 42- and 100-day body weights of lambs of different breedsa
Ile de France 4.2 17.9 33.4
Merino Landsheep 3.6 18.9 36.4
SA Mutton merino 3.5 18.5 33.4
Suffolk 3.4 20.0 35.0
aSource: ARC ± Animal Improvement Institute (National
was not signi®cantly (p>0.05) different between the two breeds. Dormer lambs grew 10% faster than the Dorpers, while total feed intake was not signi®cantly (p>0.05) different. Growth rate between 50 and 100 days was 20% faster in the Dorper than in the MM although mature weight was not signi®cantlyp>0.05) different between the two breeds (Schoeman, 1990). Since these lambs were weaned early, the faster growth rate could be the result of a higher growth potential only.
3.3. Weights at later ages
Body weights from 6 months to mature ewe weight are in Table 5. Dorper lambs and ewes were in most cases heavier than those they were compared to, except for the DorperRM crosses reported by Inyan-gala et al. (1991), where the opposite prevails. Body weights at 12 and 18 months between Dorper, Merino and their crosses were investigated by Olivier et al. (1984) who found the Dorper to be approx. 40±45% heavier than the Merino and approx. 10% heavier than their crosses. DorperMerino crossbred lambs were approx. 5±7% heavier than the mid-parent value, indicating the contribution of heterosis. In the Kenya studies (Inyangala et al., 1991, 1992; Baker et al.,
1998) DorperRM crosses were in most cases slightly heavier than their Dorper counterparts and concluded that the conditions under which the comparisons were carried out seem to favour the DorperRM crosses. Body weights reported by Inyangala et al. (1992) were at all ages slightly higher than those previously reported by the same authors (Inyangala et al., 1991).
Mature body weights of Dorper ewes varied from 30.1 kg (Baker et al., 1998) to 74.0 kg (Schoeman et al., 1993), a difference of 146%, resulting from the large environmental constraints in the Kenya studies. A mating weight of 72.3 kg was also reported for Dorper ewes by Cloete and De Villiers (1987). Schoe-man (1990) compared mature ewe weights of Dorper with MM and DM (Table 5). Weights of the Dorper and MM were not signi®cantly (p>0.05) different, while the Dorper was on average 10% heavier than the DM. Mature weight of the Dorper was also 42% heavier than the Finn composites (Schoeman et al., 1993), but only 15% heavier than the RM (Baker et al., 1998).
Several authors (Carstens, 1969; Du Toit, 1996; Snyman, unpubl.) compared growth performance of the Dorper with other breeds from birth to slaughter. Some of these results are summarized in Table 6. Table 5
Comparative weights from 6 months to mature ewe weight of Dorper and cross-bred sheep
Parameter Means for breeds/crossesa Sources
D 1/2 D
1/2 M
M DM MM 1/2 D 1/2
Finn. Comp
RM RMX
Body weight (kg) at:
6-months 21.1 22.0 Inyangala et al. (1991)
24.5 25.5 Inyangala et al. (1992)
9 months 26.8 27.4 Inyangala et al. (1991)
29.8 30.6 Inyangala et al. (1992)
12 months 49.8 44.7 35.4 Olivier et al. (1984)
33.7 34.8 Inyangala et al. (1991)
36.5 37.7 Inyangala et al. (1992)
19.7 18.4 Baker et al. (1998)
18 months 57.3 51.9 39.9 Olivier et al. (1984)
Mature ewe weight (kg) 61.0b 55.3b 61.8b Schoeman (1990)
74.0b 52.0b Schoeman et al. (1993))
30.1 26.2 Baker et al. (1998)
In the study by Carstens (1969) Dorper and Dormer lambs were pen-fed and slaughtered at a constant age of 14 weeks, while in the studies of Du Toit (1996) and Snyman (unpubl.) they grazed on natural veld until slaughter at a constant live weight of approx. 40 kg. Snyman (unpubl.) obtained a 10.8% faster growth rate in the Dorper compared to their Afrino counterparts and they reached target weight more than 20 days earlier than the Afrino. Carstens (1969) reported a 10% faster growth rate in the Dormer. In the study carried out by Du Toit (1996) the 1/2 Dorper 1/2 Merino lambs reached slaughter weight 76 days ear-lier than the Merino and 16 days earear-lier than the second fastest growing, viz. the 1/2 Ile de France 1/ 2 Merino lambs. Growth rate (ADG) was, however, not reported by Du Toit (1996) since he failed to record BW of lambs. The best carcass grades of the eight groups were awarded to the 1/2 Dorper 1/2 Merino cross-bred lambs.
4. Productivity and ef®ciency
Little has as yet been done on the productivity and productive ef®ciency of Dorper sheep compared to other breeds or crosses. Some results are presented in Table 7. The Dorper maintained a higher output and were more ef®cient than either the DM, M or MM (Basson et al., 1969; Schoeman, 1990). Expressed on a per dam weight basis, productivity for the Dorper, DM and MM were 57.5, 47.0 and 48.1 kg of lamb at 100 days per 100 kg of dam weight, respectively. In the comparison between the Dorper, Afrino and Merino, Snyman (unpubl.) reported total weaning weights (at 120 days) produced per ewe per year of 48.0, 43.2 and 22.3 kg, respectively. Basson et al. (1969) reported an annual lamb production of 2.4, 1.8 and 1.8 for the Dorper, DM and M, respectively with annual WW per ewe of 64.3, 42.3 and 34.7 kg for the three breeds, respectively. However, Finn composites produced Table 6
Comparative growth performance and carcass grade from birth to slaughter in Dorper sheep in pure and cross-breeding evaluations
Parameter Means for breeds/crossesa Sources
D M A 1/2 IdF 1/2 A 1/2 S 1/2MM 1/2 D DO
1/2 M 1/2 M 1/2M 1/2M 1/2 M
Slaughter weight (kg) 39.2 40.9 39.9 38.8 40.0 39.5 Du Toit (1996)
41.4 43.6 41.7 Snyman (unpubl.)
Slaughter age (days) 363 303 307 312 325 287 Du Toit (1996)
191 379 207 Snyman (unpubl.)
ADG (birth to slaughter (g/day))
297c 327b Carstens (1969)
215 102 196 Snyman (unpubl.)
Highest grade (%) 15 22 9 33 23 46 Du Toit (1996)
aD ± Dorper; M ± Merino; A ± Afrino; IdF ± Ile de France; S ± Suffolk; MM ± South African Mutton Merino; Do ± Dormer. b,cp0.05.
Table 7
Comparative productivity and ef®ciency parameters of Dorper sheep in pure and cross-breeding evaluationsa
Breed/cross Parameters
TotalbW/ewe/y (kg) CW/100 kgbDDMI/ewelamb Output at 100-day/ewe/y (kg) Output/bW0.75
Dorper 24.0 1.22 35.1 1.76
Finn Comp. 34.3 2.24
Mutton Merino 29.7 1.45
DoÈhne Merino 26.0 1.39
aSources: Schoeman (1990); Schoeman and Van der Merwe (1994); Schoeman et al. (1995).
74% more lambs than the Dorper and the total CW per ewe was also 43 % higher in the Composites than in the Dorper. Per 100 kg of dam weight the Dorper produced 36.0 kg of lamb at 100 days, while the Finn composite produced 51.0 kg (Schoeman and Van der Merwe, 1994; Schoeman et al., 1995). The Compo-sites were furthermore 84% more ef®cient in CW/ 100 kg DDMI/ewe and lamb(s) than the Dorper. This large difference in ef®ciency was obviously the result of the combined effects of a higher reproductive rate, smaller size of the dam and resulting lower mainte-nance requirements and the relatively higher growth rate of the DorperComposite cross-bred lambs, com-pared to the Dorper.
In the investigation carried out by Du Toit (1996) only gross income per 100 ewes available for mating was reported. In this case the 1/2 Dorper 1/2 Merino was 8% superior to the Merino from a lamb output point of view. However, ¯eeces of Merino ewes were contaminated with kempy ®bres to such an extent that the use of Dorper rams on Merino ewes was not recommended. This supports the earlier ®nding of Olivier et al. (1984) who also obtained unacceptable levels of kempy ®bres in the ¯eeces of White Dor-perMerino crosses.
5. Disease and parasite resistance
Resistance of Dorper ewes and lambs compared to other and mostly indigenous sheep breeds to natural and arti®cial infestation withHaemonchus contortus, a gastrointestinal parasite endemic to Africa, was studied in Kenya by Baker et al. (1994, 1998), Mugambi et al. (1996, 1997) and Wanyangu et al. (1997). In all these studies the RM indigenous breed was more resistant, measured on faeceal egg counts (FEC) and packed blood cell volume (PCV), than the Dorper, Blackheaded Somali and Romney Marsh breeds. PCV, a measure of anaemia, and FEC, a measure of endoparasite infestation obtained from faecal samples at 90 days in Dorper, RM and their crosses are in Fig. 2. The higher the genetic contribu-tion of the Dorper, the lower the PCV values and the higher the FEC values, indicating that the Dorper is not suitably adapted to these regions.
Fourie and Kok (1996) investigated tick resistance (Ixodes rubicundus) to infestation in Merino and
Dorper sheep grazing on the same Karoo veld in South Africa. Peak abundance of these ticks was reached earlier in the Dorper as compared to the Merino. It also reached signi®cantly (p0.05) higher levels in the Dorper (mean17.9) than in the Merino (mean7.3). These authors suggested that differences may be related to differences in grazing patterns between the two breeds.
6. Conclusions
Dorper sheep play an important role mainly in the extensive grazing areas of South Africa, but also in other parts of Africa. Owing to several socio-econom-ical factors, Dorper sheep become more numerous at the cost of woolled sheep numbers. Dorper sires are also being used by commercial farmers in cross-breeding with woolled sheep breeds to the detriment of the wool ¯eece quality. A lack of well-planned cross-breeding studies is evident. Reports thus far published were mostly based on small numbers in ad hoc comparisons.
In comparative reproductive and growth studies the Dorper was generally superior to woolled and other indigenous breeds. It also compares favourably with the specialized mutton breeds. Although generally considered as being `well-adapted', the Dorper, how-ever, proved to be unsuitable for commercial use in the African tropics.
limited. Therein lies a challenge for researchers to obtain more objective results in a variety of breeding systems.
References
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