CAUSES OF PERINATAL LAMB MORTALITY AMONGST DORMER AND SA MUTTON MERINO LAMBS

Perinatal lamb deaths were investigated in Dormer (n = 290) and SA Mut ton Merino (n = 306) progeny born to 166 and 147 ewes respectively. Ca uses of mortality were investigated by autopsy in 30 Dormer and 61 SA Mutton Merino lambs that died during the perinatal period. Perinatal l amb deaths were higher (P less than or equal to 0,01) in SA Mutton Mer ino lambs than in Dormers (0,199 vs 0,103). Anteparturient deaths (APD ) were confined almost exclusively to triplet lambs, and contributed 1 0-12 % of the overall perinatal lamb mortalities in both breeds. This category was closely associated with prolonged parturition, with 86 % of the 7 ewes involved needing assistance after more than 4 h. The maj ority of parturient deaths (PD) were associated with stressful births, which contributed a higher (P less than or equal to 0,05) proportion of total perinatal deaths in SA Mutton Merino lambs than in Dormers (0 ,508 vs 0,267). Ewes giving birth to litters consisting of one or more lambs in the birth stressed PD or birth stressed post parturient deat h (PPD) categories (also including APD deaths because of their obvious relationship to prolonged births), furthermore had longer (P less tha n or equal to 0,05) parturitions than contemporaries that lost one or more lambs as a result of other causes, or ewes that reared all lambs born through the perinatal period (214 vs 98 and 74 min respectively). Uncomplicated starvation/mismothering/exposure (SME) syndromes mostly contributed to the PPD category, and constituted 21-27 % of perinatal lamb deaths in the 2 breeds. Deaths with clinical signs of SME could mainly be related to udder malfunction, which was observed in nearly 6 0 % of the cases. Very few cases of SME involved adverse weather condi tions (n = 2) and separation of ewes from more than one lamb (n = 2). The contribution of separation was underestimated since 17 lambs not r eceiving maternal care, that would have died of starvation but for hum an intervention, were excluded from the data. Lambs in the PPD categor y with signs of birth stress, constituted approximately 7 % of perinat al deaths in both breeds and appeared unable to progress normally duri ng the neonatal stage. The overall frequency of deaths classified as l ethal congenital malformations, infections acquired after birth and ot her causes were higher (P less than or equal to 0,05) in Dormer lambs than in SA Mutton Merinos (0,300 vs 0,098). The implications of these findings are discussed in relation to sheep breeding and management.