Dystocia is a common problem in lambing ewes. A definition of dystocia is lambing which takes more than one hour after rupture of the foetal membranes. There is great breed variability in the incidence of dystocia. George reported 4.2% of pregnancies in Merino ewes resulted in dystocia compared to 34% in Dorset Horn ewes. The aetiology of dystocia in ewes is equivocal and multifactorial. Contributing factors include dam age, gender of offspring, large or small birth weights (Smith 1977) small pelvic dimensions (Haughey et al 1985) and reduced uterine activity due to hypocalaemia (Robalo et al 1984). A review of the literature failed to find data relating to dystocia in Australian Dorpers. The following case report raises the possibility that Dorper flocks may have a dystocia risk similar to British breed flocks and that grazing on short cereal crops prior to lambing may also increase the likelihood of dystocia.
A mob of Dorper ewes was examined at Delungra in mid-August 2011. The ewes had two origins and had been run together since March. The first mob consisted of 40 seven year old ewes bought in pregnant. The second mob comprised 100 maiden ewes joined to proven adult rams on property. Both groups of ewes were in body condition score 3.5-4.
The two rams were introduced in early August and left in with the mobs.
For four months prior to investigation the mob was grazing lucerne pasture. During this period the seven year old ewes lambed with no apparent problems. Two weeks before the maiden ewes were due to lamb the mob was shifted onto a short oats crop. This allowed the mob to be moved into the house paddock at night. Dry salt licks were provided in the oats paddock. The ewes consumed these avidly.
Once the maiden ewes started lambing the owners began to find stillborn lambs. At the time of the investigation there had been five full term, stillborn lambs from the 45 ewes that had lambed. None of the stillborns were twins. Two were large lambs with swollen heads, and three were smaller lambs with no apparent swelling. Gender was not determined.
One of the large stillborns was presented for autopsy (See figure 1). Autopsy found cerebral congestion (See figure2) and congestion of the thoracic organs. The neck was stretched and the head, tongue and eyelids bruised and swollen.
Subsequent to the visit 50 more lambs were born to the maiden ewes. Ten of these required assistance with only one further stillborn. None of the assisted lambings were difficult.
The prevalence of dystocia in the maiden ewes was 16% with overall perinatal lamb mortality at 6%. The cause of this increased prevalence of dystocia is not clear. The ease with which the foetuses were either expelled or delivered suggests reduced uterine contractility as the primary cause of the dystocia rather than maternal conformation or foetal size.
Some studies indicate a recurrence rate of 34% for dystocia (McSporran et al 1977). The advice offered in this case was to cull all ewes which had a problem lambing. While hypocalcaemia was unproven in this case, it is frequently reported in late pregnancy or lactating ewes on grazing cereals. If grazing cereal crops prior to lambing a lime, salt and magnesium oxide lick (1:1:1) should assist in preventing hypocalcaemia. It may even be preferable to avoid lambing on such a pasture.