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Pelvimetry and other selected measurements in young Dohne Merino ewes

Background: Dystocia occurs when there is a failure in one or more of the three main components of birth: expulsive force, birth canal adequacy and fetal size or position [ CITATION Mee08 \l 1033 ]. Deutscher (1991) indicated that the major cause of dystocia is a disproportion between birth weight and the ewe’s pelvic area. This finding is also supported by Cook et al., 1993 and Troxel, 2008. It is concluded that an incompatibility in size between the maternal pelvis and the lamb at birth is largely responsible for the need of assistance at birth (Anderson, 1992 and Patterson & Herring, 1997). Measuring pelvic areas is not a “cure-all” against lambing problems; but it may be a useful aid in a comprehensive ewe selection program to reduce dystocia and perinatal instability
[ CITATION Tro08 \l 1033 ]. The perinatal deaths according to Cloete, (1998) are caused by stressful births ascribed to a 60% prolonged labour period which reduces lamb survival (Dwyer, 2003).

Aim: The aim of the study was to investigate dystocia in mutton sheep and to estimate correlations between the ewe’s pelvic area, birth weight of the lamb and body measurements on the lamb. The Dohne Merino breed was used as a control group.

Methodology: The study during which the pelvic areas of 434 young Dohne Merino stud ewes were measured, was conducted late 2018 at Crux Dohne Merino’s on the farm Geluksdam of Mr Wikus Bekker in the Southern Free State. The pelvic areas of the ewes were measured on 13 March 2018, approximately two weeks before mating. Thereafter, the young ewes (18.5 months old) were mated from 1 April 2018 for two cycles (34 days). Single mating was practised by mating one ram to 60 ewes. Measurements recorded at lambing included: Length of parturition, birth weight, birth status of both the lamb and the ewe, circumference of the lamb’s head and pasterns, the lamb’s shoulder width and lambing ease score. The ewes grazed the natural pastures until two weeks prior to lambing and lambing took place in lambing pens with a balanced pelleted ration.

Results and Discussion:
The pelvic areas and mating weights of the 434 ewes is indicated in Table 1. Closed-circuit television (CCTV) was used to film the young ewes during parturition (picture 1). Eventually 191 ewes out of the 434 could be successfully filmed, out of which 274 lambs were born.

The pelvic area of the young Dohne Merino ewes measured 36.61 ± 3.64 cm2 at mating and the average mating weight of the Dohne Merino ewes were 57.42 ± 3.76 kg. Dohne Merino ewes born as one of a twin or multiple, had slightly bigger measured pelvic areas than singles. The Dohne Merino ewes took 63.11 ± 47.51 minutes on average to expel the new-born lamb as measured from the start of stage 2 of parturition. The lambs weighed 4.85 kg ±1.12 at birth, and their shoulder width measured 7.04 cm ±10.06. Pastern circumference measured 9.19 ± 0.62 cm, head circumference 25.25 ± 1.55 cm and chest circumference was 36.74 ± 3.87 cm. Only one percent of the 191 ewes, filmed on CCTV in this study, needed assistance. The pelvic area to birth weight ratio of the Dohne Merino ewes were 8.02 cm2/kg lamb born. On average their gestation length was 156 days, but no specific date could be linked to specific rams due to performing natural mating.

The following graphs indicate the difference between measurements on mutton sheep and Dohne Merinos. In both cases, young maiden ewes were used which were managed according to normal management practises followed for the breed. The Dohne Merino breed is usually classified as a medium to late maturing breed while mutton breeds are normally regarded as early maturing.

Graph 1 indicates the weight of the two types investigated as 53.31kg on average for mutton sheep and 57.42kg for Dohne Merinos.

Graph 1 Average weight of the young ewes mated in 2018 season.

According to Graph 2 the age at mating for the two types, was 9.7 months and 18.54 months respectively for mutton sheep and Dohne Merino ewes. Despite this huge age difference, both breeds were physically ready to be mated.

Graph 2 Average age of the two breeds at mating.

Graph 3 shows the pelvic area measured in cm2 of the two types. The average for mutton sheep 33.43 cm2 and 36.57 cm2 for the Dohne Merino ewes.

Graph 3 Pelvic area of the different breeds in 2018 season.

Graph 4 indicates birth weights of the two types as 3.7 kg for mutton sheep and 4.85 kg for Dohne Merinos. The weights were recorded within 24 hours after lambing as allowed for by specific farm operations.

Graph 4 Birth weights of the new born lambs of the different breeds in 2018 season.

Graph 5 shows the length of parturition (minutes it took to lamb) of the two types. Phase 2 of the lambing process which can theoretically be subdivided into three phases, was recorded. The average time it took for mutton sheep, was 60.50 minutes and 63.11 minutes for Dohne Merino ewes. The big difference was that Dohne Merino ewes needed assistance in only 1% of cases whereas 67% of the mutton sheep ewes needed assistance. To limit losses, the mutton sheep ewes were provided with 60-minutes to lamb before human intervention.
Grafiek 5 toon die lengte van die geboorteproses (minute wat dit geneem het om te lam) vir die twee tipes. Fase 2 van die lamproses wat teoreties in drie fases opgedeel word, is opgeneem. Die lamproses het gemiddeld 60.50 minute geduur by vleisskape en 63.11 minute by Dohne Merinos. Die groot verskil was egter dat die Dohne Merino slegs in 1% van gevalle hulp nodig gehad het, terwyl 67% van die skaapvleis-ooie hulp moes kry. Om verliese te beperk, is die vleisskape ‘n periode van 60 minute toegelaat om te lam voor menslike ingryping.

Graph 6 shows the pelvic area in cm2 to kg lamb born. In the case of Dohne Merinos, one kg of lamb was born for every 8.02 cm2 of pelvic area. As apposed to Dohne Merinos, the mutton sheep ewes had a more favourable pelvis area of 9.69 cm2 available to produce one kg lamb but still had a greater incidence of dystocia (67 percent compared to one percent).
Conclusion: The results of the study indicate that young Dohne Merino ewes have an excellent ability to lamb without any assistance. Although the mutton sheep breed has a more favourable pelvic to birth weight ratio than the Dohne Merino ewes, Dohne Merino ewes experienced lambing difficulty in only one percent of cases, whereas 67% of the mutton breed needed assistance. It is recommended that mutton breeds investigate the possibility to mate ewes at a later age possibly as is the case with Dohne Merinos, to decrease the incidence of dystocia.
Acknowledgement: The assistance and provision of ewes by Mr Wikus Bekker is appreciated.

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