What are the main causes of lamb mortality within 5 days of birth?
Updated: Jul 5, 2020
Lamb loses are a major cost to the sheep industry. The average lamb mortality rate between scanning and tailing in ewes is around 20%, however, in high fecund flocks with a large proportion of triplets, this rate can almost double.
Poor weather conditions during lambing can significantly increase the number of lambs lost, while the use of shelter can reduce these losses. Identifying litter size through the use of pregnancy scanning can be used to give more sheltered paddocks to multiple-bearing ewes.
The study summarised here looks at the causes of lamb deaths and the influence of cold weather on lamb deaths in the first 5 days post birth.
Deaths on the day of birth were not strongly influenced by a high chill index.
Lambs from 1 day to 3 days old were significantly influenced by a high chill index.
Losses were greater in multiple born lambs compared with single born lambs.
Causes of death
This study was completed in Australia with Merino and Merino cross sheep from eight flocks. All lamb deaths were recorded along with the corresponding weather before and after each case.
The graph below shows the number of lamb deaths and causes of lamb deaths between birth and weaning. The classification of deaths included dystocia, starvation or mismothering, predation (this was mainly from foxes which is not an issue in New Zealand), In utero or premature, exposure (deaths with no other reason than weather), infection, undiagnosed and misadventure.
Dystocia describes difficult births that are typically caused by a large or awkwardly positioned foetus. The three different dystocia subgroups, A, B and C, describe the different classifications of dystocia. Dystocia A: oedema present, B: no oedema, significant cranial and central nervous system haemorrhage but no metabolised fat, C: as for B, but not metabolised fat.
The most common cause of death was from starvation or mismothering (460 lambs), however, if all the different dystocia subgroups are considered together (A, B & C), then dystocia is the most common cause of death. The least common causes were misadventure and infection.
The graph below show only the deaths that occurred on the same day as the birth of the lamb (Day 0). Dystocia B (no oedema, significant cranial and central nervous system haemorrhage but no metabolised fat) was the greatest cause for death on the day of birth (227 lambs). Misadventure had the lowest number of deaths on the day of birth.
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The chill index takes into account the temperature, daily rainfall and wind speed. See the paper at the end of this summary for the full equation. Wind speed is the only factor that can be easily controlled by provision of shelter. Reducing the wind speed at lamb height by only 50% has shown to significantly reduce the number of days that a high chill index is recorded.
The graph below shows the mortality rate for single born lambs at different birth weights (BW). The birth weights were categorised into low (<4.0kg), medium (4.0-5.6kg) or high (>5.6kg). The medium and high birth weight single lambs had minimal effect of chill index on the mortality of these lambs. The singles of low birth weight, however, had increased mortality at a high chill index (>1000).
The graph below shows the lamb mortality rate for multiple born lambs at different birth weights against the chill index. For lambs with a birthweight above 5.6kg (high), the chill index had a low influence on lamb mortality. For medium birthweight lambs, mortality didn't begin to increase until after a chill index of above 1000. Whereas, low birthweight lambs had greater mortality at all chill index values recorded.
This means that to reduce the lamb mortality and increase lamb survival, it is best to have lamb birthweight above 4 kg and reduce the chill index by lambing in paddocks with adequate wind barriers.
High chill index scores after lambing were associated with increased mortality, especially in low birth weight lambs and multiple-born lambs. From this information, the authors predicted lamb mortality using the chill index. This was very accurate from day 1 of birth, the results from this can be found in the paper linked below. The prediction of lamb mortality for the day of lamb birth was inaccurate showing that the chill index has no effect on the lamb deaths that occur on the day of birth.
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Horton BJ, Corkrey R, Doughty AK, Hinch GN (2019). Estimation of lamb deaths within 5 days of birth associated with cold weather. Animal Production Science: 59(9) p1720-1726.