Lamb survival in Australian flocks has become a focal point of industry groups and researchers in recent years. The recent Meat and Livestock Australia (MLA) report, Priority list of endemic diseases for the red meat industry (Lane et al. 2015), listed Neonatal Lamb Mortalities as the most costly of 17 cattle, 23 sheep and nine goat diseases. The report estimates annual prevention and production losses of $540.4M, costing the average flock around $43,000. Historically, profitable sheep enterprises in Australia have placed greater focus on maximising stocking rates and pasture utilisation, rather than reproductive performance. With an increase in sheep meat prices and relative decrease in wool value in recent years, reproductive efficiency has become more important to farm profitability (Barnett 2007). An economic analysis examining proposed improvements to industry reproductive performance by Young et al. (2014) showed that increasing the survival of twin lambs would have the highest pay-off to the sheep industry ($515M). Poor lamb survival is also becoming increasingly important as an animal welfare issue (Brien et al. 2014).
Riverina and Murray Local Land Services District Veterinarians were involved in delivering several lamb survival workshops during 2015. This paper aims to give a brief overview of neonatal lamb mortality in Australian flocks, including: details of relevant papers available for further reading, discussion of current programs that have been designed for delivery to producers, and an outline of the important role District Veterinarians play in this significant industry wide problem.
Many comprehensive reviews focusing on lamb survival have been produced, several of which are referenced in this paper. Recent large studies have found that more than 80% of losses occur within the first 48 hours of life, though occasional exceptions are associated with specific disease outbreaks. Refshauge et al. (2015) categorised cause of death in 3198 newborn lambs (<5 days old) across research flocks in southern New South Wales, categorising deaths as starvation-mismothering (25%), stillbirth (21%), birth injury (18%), death in utero-prematurity (10%), dystocia (9%), predation (7%), cold exposure (5%), undiagnosed (4%), infection (1%) or misadventure (1%). The techniques used for catagorisation are based on guidelines from Mcfarlane (1965) and Haughney (1973) with a concise description provided by Holst (2004). These results are similar to previous studies including Luff (1980) who attributed 2534 deaths in the Riverina to starvation/exposure (58.2%), dystocia (17.7%), primary predation (7.8%) or infection (4.4%).
Oldham et al. (2011) lists birthweight as the single greatest influence on lamb survival in the first few days of life. Litter size has the greatest effect on birthweight, with single born lambs significantly heavier than multiple born lambs (Gardner et al. 2007). The ideal birthweight range appears to be between 3.5 and 6 kg, with slight variation between breeds, and maximum survival at approximately 4.5 kg (Oldham et al. 2011). Reshauge et al. (2015) found that lambs born at lower bodyweight are more likely to be stillborn, dead in utero or die from starvation, mismothering or exposure. Smaller lambs also have poor thermoregulatory ability relative to their size and energy reserves and are often retarded due to insufficient oxygen and energy in late gestation. Conversely, larger lambs are more prone to foetopelvic disproportion, dystocia and birthing associated injury. (Hinch and Brien 2014).
Birth type significantly affects lamb survival with losses in twin lambs typically 2-2.5 times greater than that of singles in the same flock (Hinch and Brien 2014). Litter size has the single greatest influence on birthweight (Gardner et al. 2007). Refshauge et al. (2015) found that twin lambs were more likely to die of starvation-mismothering or birth injury while single lambs were more likely to be stillborn or die from dystocia. Dutra and Banchero (2011) found that parturition duration had a significant impact on lamb viability due to asphyxia during prolonged lambing, particularly in multiple pregnancies. Dwyer (2003) found that reduced lamb survival in heavier birthweight lambs was a direct result of dystocia and prolonged parturition. In the same study, dystocia was a consistent cause of reduced survival in multiple-birth lambs. Dwyer (2003) also showed that lamb vigour, defined as how quickly the lamb stands, seeks the udder and sucks, is improved with increased lamb size but negatively affected by slow or difficult births.
Lamb vigour is also associated with ewe and ram factors. Ewes that do not rear lambs in one year have been shown to be more likely to lose lambs in subsequent years (Hatcher et al. 2009). Maternal behaviour is characterised by intense grooming of the lamb, low pitch bleating and standing to suckle within the first 6 hours, which in turn facilitates sucking and recognition by the lamb and formation of the ewe-lamb bond (Goursaud and Nowak 1999). The level of individual maternal behaviour compared to other ewes is consistent between parities, but also increases with parity as more experienced ewes exhibit stronger maternal behaviour (Dwyer 2008). Early maternal behaviour is exhibited in time spent at the birth site, which is 6 h in most breeds but has been reported as 1.3 to 3.5 h in Australian merinos (Hergenhan et al. 2014).
Feed on offer immediately before and during lambing improves ewe-lamb bonding as the ewe is able to stay at the birth site to graze (Oldham et al. 2011) and lambs born to ewes on an adequate diet stand and suck earlier (Dwyer 2003). Nutrition leading up to lambing influences colostrum production and supplementing grazing ewes with high-starch grains can double the amount of colostrum available to the newborn lamb (Banchero et al. 2015). Periconceptional ewe nutrition has recently been shown to effect lamb physiology and behaviour (Kleemann et al. 2015) with ewes on an inadequate diet at insemination producing smaller, less vigorous lambs.
The sire of the lamb also has an effect on lamb vigour by direct influence on neonatal behaviour and indirect influence on the foetal physiology by affecting placental development. These differences have been shown between lines of Merinos selected for lamb survival (Hergenhan et al. 2014) and between breeds selected for production under different environmental conditions (Dwyer and Lawrence 2005). While mothering ability and lamb survival have low within-breed variation and heritability, between-breed and between-bloodline variation is high with further gains from heterosis (Brien et al. 2014).
Exposure losses in Australia tend to be due to hypothermia rather than hyperthermia as the majority of lambs are born during colder months of the year (Hinch and Brien 2014). Cold exposure in newborn lambs is quantified as a function of wind speed, ambient temperature and rainfall (Donnelly 1984). As previously mentioned, smaller lambs have a poor thermoregulatory ability relative to their size and energy reserves, making them more susceptible to cold exposure. This is consistent with findings by Reshauge et al. (2015) and Oldham et al. (2011). Wind chill above 1000 KJ/m².h reduces lamb survival rates rapidly (Broster et al. 2012a) with mortalities approaching 50% at 1200 KJ/m².h. Shelter such as tree belts and tall perennial grass species can significantly reduce wind chill where wind speed is the predominant factor. However, large regional variation in wind speed results in large variation in the effectiveness of shelter and recommendations should be made based on site-specific weather (Broster et al. 2012b).
Lambs that stand and suck earlier have been shown to be able to produce more heat and are less likely to die from cold exposure (Hergenhan et al. 2014). Resistance to cold exposure may also be influenced by lamb sex and sire breed, particularly through skin thickness and hair coat density, though these findings are not consistent across studies (Oldham et al. 2011). Ewe mob size and stocking density during lambing has been shown to affect lamb survival but not enough data is available to draw accurate conclusions (Hinch and Brien 2014).
Predation in Australian flocks is primarily due to foxes, though the cause and significance of predation varies between locations (Refshauge et al. 2015). In most circumstances predation accounts for less than 10% of all lamb mortalities though higher losses are associated with paddocks adjacent to fox habitat or ewes with poor mothering ability (Saunders et al. 2010).
There are obvious production benefits to improving lamb survival which can be obtained by optimising nutrition, genetics and the environment. Despite efforts to improve lamb survival in Australian flocks, lamb survival figures suggest that there has been no improvement in the last 20 years (Young et al. 2014). Lamb survival has been a focus of cooperation between the Cooperative Research Centre for Sheep Industry Innovation (Sheep CRC) and industry bodies such as Meat and Livestock Australia (MLA) and Australian Wool Innovation (AWI) leading to the development of lamb survival components within extension programs including Making More for Sheep, Lifetime Wool and EverGraze. These programs improve producer awareness of ewe nutrition and lamb survival, with the aim of optimising body condition and lambing environment.
Some of the current recommendations of these programs include:
The District Veterinarian in New South Wales will continue to have a primary role in animal disease surveillance, which includes investigation of animal disease outbreaks. For this reason, it is important to understand normal levels of lamb mortality in sheep flocks so that increased mortality can be identified. Being able to perform a post-mortem examination on the neonatal lamb is an important diagnostic technique when investigating infectious abortion (Tibary 2015). Some extension material currently available includes the “Lambs Alive Post-mortem Tool” which is designed to improve producer understanding of causes of lamb mortality. These tools can be used to increase producer awareness of lamb mortality, but are unlikely to aid in diagnosing the causes on an individual farm with mortality trends requiring hundreds of post-mortems (Refshauge G, pers. comm.).
District Veterinarians have a role in improving lamb survival as it relates to Goal 2 of the Local Land Services Strategic Plan; biosecure, profitable, productive and sustainable primary industries. District Veterinarians should be proactive and reactive to opportunities that arise around lamb survival as it is a pivotal area of industry focus, research, development and extension. In addition, District Veterinarians have the unique opportunity to become involved in activities that can improve productivity, profitability and animal welfare by improving lamb survival for their respective producers.