18. How lambs die
Many factors can contribute to the death of lambs in the period surrounding birth. Birth weight exerts a major effect, and breed of ewe and nutrition during pregnancy exerts a strong influence on birth-weight. However, over a wide range of breeds and levels of nutrition during pregnancy, optimum lamb survival occurs in the birth-weight range of 3.0 kg to 5.0 kg.
Below 3.0 kg mortality levels are increased with the majority of losses being among twin-born lambs. Above 5.0 kg, mortality levels are again increased with the majority of the losses among single-born lambs.
How single lambs die
Most single lambs die as a result of experiencing a difficult birth, brought about because the lamb is too big, the pelvic opening of the ewe is too small or by a combination of both these influences. Known as difficult birth, dystocia, or foetal-pelvic-disproportion, these births are responsible for 10 to 30 percent of all lamb deaths.
When birth processes are prolonged beyond 30 to 40 minutes, the mothering drive of the ewe falls, the chance of the ewe deserting her lamb increases, and the deserted lamb survives a day or two before dying of starvation.
Prolonged births can result in injuries which cause death of the lamb and can also lead to death of the ewe as a result of infections. A ewe failing to give birth dies during birth and is found dead with the lamb being partially born.
A major cause of these deaths is the nutritional management of single bearing ewes in the last 50 days of pregnancy when most of the growth of the developing lamb takes place.
When a mob of lambing ewes is put into a lambing paddock, about 20 to 30 percent will not start to lamb until 2 to 3 weeks after the first lamb is born and will not finish lambing until 5 weeks after the first lamb is born. Where this is a mob of single-bearing ewes, the later lambing ewes spend 20 to 35 days grazing “high quality lambing pastures” before they give birth. This increases the growth rate of the lamb in utero, and the likelihood of death of the lamb and possibly of the ewe as well.
How twin lambs die
Most twin lambs die as a result of a combination of events termed:
"mis-mothering, starvation, exposure (MSE)".
Mis-mothering commences when a ewe on the point of lambing whose mothering drive is increasing, takes interest in a lamb which does not belong to her. Where that lamb is the first born of a twin pair, by the time the lambing ewe gives birth to her second lamb, the interfering ewe can be well away from the lambing ewe and has commenced bonding with the "stolen lamb".
The lambing ewe mothers her second born lamb and moves away from the birth-site leaving her first born lamb with the interfering ewe.
Before long, the interfering ewe starts to give birth to her own lamb and where this ewe is also a twin lambing ewe, it isn't long before this ewe has three lambs, her own two and the stolen lamb. It is then not much longer until one lamb ends up all alone as a deserted, mis-mothered lamb.
Twin born lambs with low birth weights that have been deserted before having suckled, die soon after as a result of starvation, or even more quickly of exposure in the event of rain and low ambient temperatures.