Risk factors for perinatal and postnatal mortality in lambs.

Factors associated with preweaning mortality in lambs were identified by developing risk profiles with logistic regressions for perinatal and postnatal mortality. Compared with heavy lambs, lambs of low birth weight had almost twice the risk of perinatal mortality (odds ratio [OR] = 1.9) and lambs of average weight had a slightly lower risk (OR = 0.7). Two of four lambing location categories affected perinatal mortality, with lambs born at unmonitored areas at greatest risk (OR = 2.7). Multiple births increased the risk of perinatal mortality (OR = 1.5), especially among Targhee lambs (OR = 4.0). Breed variations in perinatal mortality were significant in Suffolk lambs (OR = 1.9) and Booroola Rambouillet lambs (OR = 2.1). Lambs born weak had an increased risk of postnatal mortality while strong lambs had a decreased risk (OR = 3.7 and 0.6, respectively) if the dam had an adequate milk supply. Poor milk supply increased the risk of postnatal mortality for lambs of average vigour (OR = 3.3), but did not change the risk for weak or strong lambs. Male lambs castrated at 30 days of age were at less risk of postnatal mortality (OR = 0.3) than females. There were slight increases in the risk of postnatal mortality for intact males (OR = 1.3), low birth weight lambs (OR = 1.6), and lambs born in sheds (OR = 1.3). Suffolk lambs (OR = 1.8) and Targhee lambs (OR = 1.6) had a higher risk of postweaning mortality.