20 March 2023

Lamb mortality: the main causes and timing

The number of lambs reared per ewe joined is a key determinant of productivity and profitability in sheep systems and lamb mortality is a key factor influencing ewe productivity.

Nationally, if lamb mortality levels were to be reduced by 3%, it would equate to 100,000 more lambs surviving each year. A significant reduction in lamb mortality can only be achieved through identifying the underlying causes and targeting them specifically.

Research work completed in Teagasc Athenry, which included a post-mortem study to evaluate the timing and causes of mortality over two years in the Athenry flock and a risk assessment survey – which examined farmers’ perceptions of the risk factors associated with lamb mortality on their farms, was used to get an understanding of when most lamb mortality occurs and to identify the main causes.

When does lamb mortality occur?

Table 1 below indicates the time of death for lambs from birth to weaning. The first three days after birth account for 74% of lamb mortality. 80% of lamb mortality occurred in the first seven days after birth. The highest proportion of mortality occurred prior to or at birth (43%), from birth to 24 hours (15%) and from 24 to 72 hours (16%).

Table 1: Time of death for lambs from birth to weaning

What are the main causes of lamb mortality?

The main causes of lamb mortality are presented in table 2. Infection was the main cause of death, accounting for 32% of all lamb mortality. Dystocia (20%) was the second main cause of death, and when combined with infection accounted for a total of 52% of overall lamb mortality. Dystocia and infection are potentially preventable with good management practices, thus lamb mortality can be reduced.

Cause of death was not identified by post-mortem for 19% of lamb deaths. Other causes, which include hypothermia and starvation accounted for 14% of mortality. Accidents accounted for 5% of mortality.

Table 3: Cause of lamb mortality (%) and how it changes with time

¹0 = born dead; <24h = deid between birth and 24h; Day 1-3 = died between 1 and 3 days of age; Day 4-7 = died between 4 and 7 days; >Day 7 = lambs that died between day 7 and weaning.

Infection

Infection was the most prevalent cause of lamb mortality. Enteritis (i.e. scour), naval / joint ill and Chlamydophila abortus (EAE) accounted for 33, 31 and 23% of mortality attributed to infection, respectively. Infection as a major cause of lamb mortality is not evenly spread across all time points (Table 3). 39% of mortality due to infection occurred from 24 hours to seven days of age, and most were attributed to various infections e.g. E-coli infections. The remaining 38% of deaths due to infection occurred from seven days of age to weaning (at ~100 days) and these were mainly associated with enteritis and pneumonia.

Dystocia

Dystocia was the second main cause of lamb mortality. Dystocia is defined as a difficult birth due to a long, unassisted parturition or prolonged delivery requiring assistance. Dystocia can be more specifically associated with maternal and fetal dystocia (fetopelvic disproportion, i.e. the lamb is too large for the ewe’s pelvis), maternal dystocia (failure of the cervix to fully dilate causing anoxia, i.e. loss of oxygen to the brain/body), and fetal dystocia (malpresentation of the lamb, i.e. head turned back, lamb been born backways etc.). Fetal dystocia (malpresentation) of lambs accounted for 53% of all dystocia cases.

Farmer perceptions of causes of lamb mortality

The risk assessment survey concluded that farmers perceived predators (i.e. fox, mink, dog etc.) to be the main cause of lamb mortality on their farm. As the majority of the farmers in the study lambed indoors, and the ewes and their lambs were not put to pasture until a few days postpartum, it is unlikely that predators are the main cause of lamb mortality. Farmers may choose issues/factors that they have no control over (e.g., predators, weather) rather than factors that they can control (e.g., birth weight, infection, dystocia). A recent study from Australia concluded that farmers overestimated predation to be three times more likely to be the primary cause of mortality than the published data indicates. Farmers in the Irish risk assessment survey perceived lamb birth weight and diseases/infection to be the second and third main causes of lamb mortality, similar to the two main causes identified in the post-mortem study.

This article was extracted from the National Sheep Conference 2022 conference book. The full paper, which was written by Dwayne Shields, Cathy M. Dwyer and Tim Keady, is available here.