The effects of breed, breeding system and other factors on lamb mortality: 2. Factors influencing the incidence of delayed birth, dystokia, congenital defects and miscellaneous causes of early death

The incidence of a number of causes of death of lambs around the time of birth was examined in relation to breed, inbreeding, birth weight, birth type, sex of lamb, week of birth and parity. Data were from a grazing flock on an upland farm over a 6-year period. The breeds were the Scottish Blackface, Cheviot, and Welsh Mountain and the crosses among them. Each breed and cross was maintained at five levels of inbreeding ranging from zero to 59% inbreeding coefficient. The incidence of delayed birth varied significantly among the breeds and crosses with the Welsh having the lowest incidence. Compared with the pure breeds the crosses showed a lower incidence of both delayed birth and preparturient deaths, but significantly only for the latter. Dystokia was classified according to whether it arose from the relative dimensions of lamb and dam or from malpresentation. For both types the heaviest lambs had the highest incidence but the association with birth weight was much stronger for dystokia involving dimensions. Breed variation was not significant, but cross-breds had the highest frequency for the dimensions category and the lowest for malpresentation. Singles were at greater risk than twins as were lambs born to a first parity. Congenital defects as a cause of death were divided into six categories. Only skeletal defects varied significantly among breeds and the incidence increased with inbreeding, but crosses of inbred lines did not have the lowest frequency. For congenital defects overall and for hydrocephalus and hydronephrosis in particular, line crosses (noninbred) had the lowest incidence and 25% inbred lambs the highest, the frequency declining with further inbreeding. None of the other factors considered affected variation in the incidence of any of these defects to a significant extent. Liver rupture and brain haemorrhage were most frequent in heavy lambs, singles and lambs born to a first parity dam.