Dorper sheep were first introduced to Australia in 19961,2. The breed was developed in South Africa by crossing Dorset Horn rams over Blackhead Persian ewes1, 2. Dorpers are promoted in Australia as being easy-care, drought-tolerant, fertile and productive1,2. Attributes claimed include F1 Merino-Dorper lambs achieving 36kg liveweight at approximately 4 months of age, and a potential lambing interval of 8 months1. However there are few reports in the literature assessing the reproductive performance of Dorpers under the wide variety of environmental conditions in Australia.
The aim of this study was to record the reproductive performance of a small Dorper flock on the plains of north-western NSW.
The study flock consisted of approximately 16 x F2 and 14 x F3 Dorper ewes grazed on 16 hectares with an estimated carrying capacity of 2.5 DSE per hectare. The flock composition was typical of a growing number of similar flocks in the study area.
Ewes were joined to a single White Dorper ram. Ewes had been joined year-round until approximately 3 months prior to the study period. At the beginning of the study, restricted joinings with interlambing intervals of 7-9 months were introduced (Table 1). The ram was not subject to any reproductive soundness examination during the study period, although he tested negative for ovine brucellosis approximately 17 months after the beginning of the study.
The 16Ha property was divided into 13 grazing paddocks of native and improved pastures, some of which were irrigated.
Reproductive data was collected by the property owner and reported to the author monthly.
The reproductive performance for the study flock over the three joinings is presented in Tables 1 to 3.
a Stillbirths were defined as lambs found dead within 24 hours of birth.
b Where a ewe required intervention in order to lamb, regardless of whether the lamb was born alive or dead.
Figure 1 illustrates the lambing curves for the three lambings. Most ewes lambed during the first 3 weeks of lambing in the first and third lambing but at the second it was extended to 4 weeks.
The Dorper Sheep Society of Australia Inc. claims that the Dorper breed has a potential lambing interval of 8 months1. This claim is supported by Elias et al. (1985) who quote an inter-lambing interval of 6.2 to 7.7 months for Dorper ewes; and Elias et al. (1985) who quote lambing intervals of 6.2 and 7.7 months. In this study, there were three lambings within a 2 year period. The first inter-lambing period was 7 months, with a lambing percentage of 121%. The second inter-lambing period was 9 months, with a lambing percentage of 139%. While these figures appear to support a conclusion of improved lambing percentages with a longer inter-lambing period, other factors such as nutrition, ewe maturity and animal health may have contributed. The lambing percentage figures in the study flock were lower than that found by Elias et al. (1985) of 149% over three lambings during a 2 year period for non-maiden ewes; however when the maiden ewe figures are removed from the study flock, the lambing percentage figures average at 140%.
Dorpers are regarded as intermediate with regard to seasonal expression of oestrus activity (Cloete et al., 2000). This is supported by the data in the study flock with ewe fertility being consistent across the three joinings, one of which occurred in autumn while the other two joinings occurred during spring.
Ewe fertility in the flock was comparative with that measured in the South African Dorper. Overall ewe fertility was between 0.85 and 0.91 (ewes lambed as a proportion of ewes joined). South African figures are between 0.68 and 0.91(Cloete et al. 2000, Schoeman 2000). In the maiden portion of the study flock, fertility ranged between 0.75 and 1.0.
The average litter size in the study flock was 1.38, 1.28 and 1.45 (average 1.37) for the three lambings. These figures were lower than that found by Cloete et al. (2000) where purebred Dorpers had a litter size ranging from 1.45 to 1.60. However the study flock figures were comparable to those found by Schoeman (2000) of 1.28 for purebred Dorpers and 1.30 for cross-bred Dorpers.
The Dorper is regarded as an early maturing breed. First oestrus in ewes occurs at 7-8 months of age at a liveweight of 39-50kg (Greeff et al. 1088; Schoeman et al. 1993). Maiden ewes in the study flock with a body weight of less than 45kg at joining did not lamb. The youngest ewe to lamb was approximately 18 months of age at lambing. This is slightly younger than the average first lambing age found by Schoeman and Burger (1992) of 19.6 months.
Neonatal mortality rates within 24 hours of birth averaged at 6.6% over the three lambings. This figure is higher than 4.8% found by Olivier et al. (1984) in purebred Dorpers.
Details of the assisted lambings are not available. Possible factors involved include foeto-pelvic disproportion, nutritional stress, metabolic disturbance or management stress (eg mustering).
The lambing curves, or lambing patterns, shown by the study flock reflect typical lambing patterns for a restricted joining with the exception of the second lambing. It is unknown if the more protracted lambing pattern shown in the second lambing resulted from the 7 month inter-joining period or whether it was affected by other factors such as nutritional stress, seasonality or management factors.
This is a study of the reproductive performance of a small flock of Dorpers over two years and three lambings. While these findings should therefore be interpreted with caution the data show that the performance of these sheep is consistent with reports from South African flocks.
The author gratefully acknowledges the valuable assistance of the Leculiar family in providing the data for this study.