This document presents the current knowledge and strategies to reduce lamb mortality.
Reducing Lamb Mortality
The average lamb mortality from scanning (mid-pregnancy) until weaning or sale is between 15-25% worldwide, with a significant impact on financial margins. However, significant between-flock variation is known to exist, ranging from 3% to nearly 50% mortality. Lamb mortality is highest on the day of birth, and nearly half of all deaths occur within the first week of life, but the risk of dying remains higher for lambs than for adult sheep throughout early development.
The causes of lamb mortality have been well described across many different countries. Lambs die because of: 1) a difficult birth process (dystocia) causing hypoxia (lack of oxygen) or damage, 2) an inability to adjust to postnatal life, which can lead to starvation, mis-mothering and hypothermia, 3) infectious disease, 4) congenital malformation, 5) predation and 6) accident. The relative importance of these factors will depend on lamb age: for example newborn lambs are more likely to die because of birth difficulty (which can result in stillbirth, or may contribute to losses from other causes due to lamb damage), starvation and hypothermia, whereas older lambs may be more likely to die from infectious disease. The prevalence of different causes will also be affected by farm system. Indoor lambing systems can protect the lamb from hypothermia and predation, for example, but deaths from infectious causes may be more common. In outdoor systems, lambs may be more likely to die from starvation, hypothermia and predation, but will be less exposed to the build up of infectious agents.
The risk factors for lamb mortality have also been well-studied and the most important contributors to increased lamb mortality are:
Ewe nutrition: Ewes that are underfed in pregnancy will deliver lighter lambs, show a lower amount of maternal care (e.g. spending less time licking the lamb), and produce less colostrum and milk of a poorer quality than well fed ewes. Lambs that are small at birth are slower to stand and suck, have a higher surface area-to-weight ratio so lose heat more quickly than heavier lambs and have less body reserves to maintain body temperature. The effects on ewe behaviour and milk production mean that these lambs will grow more slowly and may be more likely to succumb to infectious disease. Some trace elements or micronutrients (e.g. cobalt, selenium, iodine) reduce lamb survival if ewes are clinically deficient, however, additional supplementation of ewes which are not deficient will not improve survival.
Litter size: Lambs born from larger litters (triplets or greater) are much more likely to die than singles or twin lambs. They are usually lighter, which will cause the same effects as described above, but are also less active, less able to maintain their body temperature and slower to learn to recognise their mothers in comparison to singles and twins even at the same birth weight. This may occur due to the inability of the placenta to provide sufficient oxygen and nutrients to multiple foetuses, and potential inability of ewes to supply sufficient colostrum and milk.
Ewe inexperience: Ewes lambing for the first time are less efficient, give birth to smaller lambs, tend to lamb more slowly and may show deficits in the onset of maternal care compared to experienced ewes, regardless of their age at first lambing. They are also more vulnerable to external influences such as disturbance at lambing.
Genetics: There are breed, line and sire within breed influences on lambing difficulty and lamb vigour (ability to stand and suck quickly after birth). These traits are heritable, suggesting that selection can improve ease of delivery and lamb vigour. Breed and line differences in the amount of maternal care shown and the composition of milk and colostrum are also known to exist.
Stress: Stress or disturbance at lambing can cause the birth process to be prolonged, which increases the risk of lambs becoming damaged (hurt) during delivery and can reduce the amount of maternal care ewes show to their lamb/s. A high stocking density can cause disruption to lambing, increases the chances of mismothering and decreases ewe and lamb suckling frequency.
Lambing Environment: Ewes choose a suitable location in which to lamb (both indoors and outdoors) and the longer she spends on this site the better the bond that she develops with her lambs. If ewes choose sheltered lambing sites outdoors the lambs have more protection from rain, wind or snow, therefore providing sheltered locations, which the ewes are familiar with, can provide some protection for lambs. Poor lambing hygiene increases the chances that newborn lambs will ingest pathogens whilst their gut is immature and before passive transfer of colostrum antibodies has taken place.
Many of the causes of lamb mortality are preventable. A key goal should be for the lamb to suckle adequate amounts of colostrum from the ewe as soon as possible after birth. This will ensure a good bond between ewe and lamb, prevent starvation and hypothermia, and protect the lamb from some causes of infectious disease as gut closure will occur more quickly. Actions to improve lamb survival involve selecting animals with appropriate traits for the environment and the farm system, providing sufficient nutrition to ewes, particularly in late pregnancy, and ensuring a quiet, clean lambing environment.
Each farm or flock may have different risk factors and prevalence for different causes of mortality. Identifying the main reasons for lamb mortality, and the more important risk factors are the key steps in developing a mitigation strategy. Recording mortality and identifying when and why lambs are dying, alongside flock level risk factors, will help to improve lamb survival.
Take home messages
- Most lamb mortality is preventable, by tackling the main causes and risk factors present on farm.
- Appropriate ewe nutrition, particularly in late pregnancy, is the most important factor in improving lamb survival.
- Provide a suitable lambing environment, maintaining good hygiene practices and use of appropriate genetics for the system are also key factors in reducing lamb mortality.
Contact: This briefing has been prepared by Cathy Dwyer, SRUC, WP2 for SheepNet. For further information discuss with your local coordinator of the Scientific and Technical Working Group.