Difficult births – a push to address impacts for the sheep industry
Caroline Jacobson, Meighan Bruce, Amy Lockwood, David Miller and Andrew Thompson (Murdoch University), John Young (Farming Systems Analysis Service), Gordon Refshauge (NSW Department of Primary Industries), Paul Kenyon (Massey University) and David Masters (University of Western Australia)
Author correspondence: firstname.lastname@example.org
Dystocia refers to a difficult birth and is an important contributor to lamb and ewe mortality. Affected lambs may be born dead or die soon after birth, or may survive birth with injuries relating to the difficult birth that make them more susceptible to starvation-mismothering-exposure in the hours or days after being born.
The full extent of birth injuries can only be diagnosed with a post-mortem exam that includes careful inspection of the brain and spinal cord. In many cases lambs will have no obvious external signs to suggest dystocia was a contributing factor to death and so farmers will often underestimate the impact of dystocia in their flock.
Dystocia is often considered a problem for large single-born lambs. However, dystocia is an important cause of lamb mortality for multiples as well as single-born lambs. The dystocia risk is increased for lambs with very low or very high birthweight. Other factors that may predispose ewes to dystocia include nutrition, genetics, ewe age and stress including interruption during lambing. We conducted a review of studies to determine the impacts of dystocia for the Australian sheep industry and identify opportunities to improve lamb and ewe survival.
Lamb mortalities: systematic review
A systematic review of scientific articles found 10 studies conducted in Australia or New Zealand since 1990 that reported on cases of dystocia. These studies included more than 14,000 post-mortem exams (autopsies) of newborn lambs. Analyses were used that pooled data from across these studies to determine the proportion of lamb mortalities associated with dystocia, with weighting applied to each study based on the number of cases.
For Australian studies, 54% of lamb mortalities were related to dystocia, with the three most recent studies, representing more than 3500 lamb autopsies, reporting averages of 46-47% mortalities associated with dystocia.
There were no differences in the proportion of cases being associated with dystocia for Merino and non-Merino ewes, but there were relatively few non-Merino ewes included in the Australian studies. More data is needed to determine if the risks for dystocia vary between ewe breeds. Similarly, there was insufficient data to determine if risks are different for maiden ewes compared with adults.
Dystocia was an important contributor to mortality for lambs of all birth types (single-born lambs and multiples). The proportion of lamb deaths associated with dystocia were higher for singles compared with twins and triplet lambs, meaning that of the autopsies performed for single lambs, a higher proportion had signs of dystocia compared to the proportion of twins and triplet autopsies with evidence of dystocia.
However, total mortality (number of deaths) from all causes was higher for twin-born lambs and overall, the number of twin lambs that died from dystocia was more than double the number of single lambs that died from dystocia.
Financial impacts of lamb and ewe mortalities
MIDAS modelling was used to determine the impact on the profitability of sheep enterprises from lamb and ewe mortality related to dystocia. The MIDAS model is able to value the impact of improving ewe and lamb survival whilst accounting for the other related production and management changes that result or are necessary to maximise the value of increasing survival. Assumptions used in the model were:
- Sheep sale prices consistent with a lamb price of AU$6.50/kg carcass weight
- Wool price (eastern market indicator) of AU$14.50/kg
- Grain price of AU$286/t
- Merino ewes scanning 125% foetuses and marking 86% lambs per 100 ewes joined
- Non-Merino ewes scanning 155% foetuses and marking 112% lambs per 100 ewes joined
- Proportion of lamb mortality attributable to dystocia was based on approximately 3000 lamb autopsies conducted at Information Nucleus Flock sites where the ewe breed and lamb birth-type (single, twin or triplet) was known
- Ewe mortalities were assumed as 2.5% mortality during the lambing period (all causes), of which 35% cases are associated with dystocia.
Based on these assumptions, the impact of dystocia on national farm profit was $672 million per year or $16.00 per ewe joined. This value was sensitive to lamb sale price, where the impact on farm profit increases as lamb sale price increases above $6.50/kg. The impact of dystocia on farm profit was not strongly affected by fluctuations in wool price.
Opportunities to improve
Breeding decisions and ewe management may reduce the proportion of ewes and lambs impacted by dystocia and improve the survival of lambs that are born with birth injury (Table 1). Some of these recommendations are based on limited evidence relating directly to dystocia but have been shown to improve lamb survival and are likely to improve outcomes for lambs that are born with birth injury.
Differential management of single- and multiple- bearing ewes
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Do not mate small, immature ewes
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Address mineral nutrition during late pregnancy and lambing
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Provide edible shelter in lambing paddocks
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Provide appropriate supervision during lambing
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Manage exposure to oestrogenic pastures
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Ewe selection and culling
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Take home messages
- Dystocia is an important contributor to lamb and ewe mortality with impacts for farm profit.
- Dystocia is often under-estimated because autopsies with inspection of the brain and spinal cord are needed to find birth injuries that impact lamb viability.
- Whilst dystocia is often thought of as a problem in large single born lambs, dystocia is an important contributor to lamb mortality for all birth types. Overall, the number twin lambs that die from dystocia each year is nearly double that for singles.
- There are a number of options for reducing the risk of dystocia and impacts on lamb survival through breeding decisions, ewe nutrition and management during pregnancy and lambing.
This work was funded by Meat and Livestock Australia.