Observations on dystocia in a romney flock

Summary

The mating and lambing records for 1964–1972 of a Romney stud that had been experiencing a serious dystocia problem were investigated. Until 1970 between 20 and 31% of ewes required assistance at lambing but this level then fell to 18% in 1971, 11% in 1972, 3.3% in 1973 and 4.0% in 1974. The decline in dystocia evident in the latter years of the investigation is considered to have been brought about largely by the culling of ewes that were repeatedly assisted at parturition and by the selection of rams that sired lambs of lower birth weight.

Dystocia in this flock was highly correlated with the mean birth weight of single lambs ( r =0.84, p<0.01). Fifty nine percent of assisted single lambs were males; ram lambs were heavier at birth than ewe lambs. Apart from the year 1966, primiparous ewes required no more assistance than older ewes and, except for that year, ewes bearing singles required the same levelof assistance as those bearing twins.

An effect of the sire on the incidence of dystocia was demonstrated for four of the eight lambings studied and was related to the birth weight of the lambs. Thirty four percent of ewes that had been assisted to lamb once had to be assisted again the following year as compared with a 16% assistance rate necessary for those ewes that had not been helped at the previous lambing, thus establishing a high level of repeatability for the condition (p<0.001).