Ovine Observer

Lamb survival is improved by reducing mob size and stocking rate at lambing: Part 1

Amy Lockwood, S. Hancock and A. Thompson, Murdoch University WA; J. Trompf, J.T. Agri-Source VIC; L. Kubeil, DEDJTR VIC; G. Kearney, Hamilton, VIC. Corresponding author:

This is the first of a two part report investigating the impact of mob size and stocking rate on lamb survival. Part two will be published in the March 2017 edition of the Ovine Observer.


Improving the survival of lambs is a priority for the Australian sheep industry as approximately 30% of all lambs born die prior to weaning.

Importantly over 80% of these deaths occur within the first three days of life. In order to reduce these deaths, the complex factors influencing lamb mortality must be investigated.

There are indications that two management factors, mob size and stocking rate, have a relationship with lamb survival, although they are poorly understood in a commercial setting.

Research in small 0.5ha paddocks, which don’t reflect grazing conditions on commercial properties, showed:

  • The survival of twin-born lambs is 20% higher in ewes lambing at 16 ewes/ha compared to ewes lambing at 30 ewes/ha in sheltered paddocks.
  • For each extra lambed ewe per hectare, 2% more ewes became permanently separated from one or more of their lambs.

Therefore, in order to see if these relationships persisted in a commercial setting the study reported here tested the hypothesis that increasing the mob size or stocking rate of lambing ewes on commercial properties will reduce the survival of their lambs.


A total of 66 sheep producers which ran separate mobs for single and twin bearing ewes after pregnancy scanning, were surveyed in 2014 through the BESTWOOL / BESTLAMB program in Victoria. Data was collected for a total of 300 individual mobs of single- and twin-bearing ewes that lambed between 2007 and 2013. Ewe and ram breeds included both Merinos and maternal breeds.

Information on mob size, stocking rate and lamb survival was collected. Lamb survival to marking was calculated based on the total number of foetuses identified at pregnancy scanning and the number of lambs marked within each paddock. All statistical analysis were performed using GENSTAT (VSN International 2012). The survival of lambs within paddocks was assessed using linear mixed models. For all analysis, terms were only included if they were statistically significant (P≤0.05).

Ewes and lambs grazing in a paddock
Ewes and lambs grazing in a paddock
Table 1 Lamb survival (%)
  Single Twin
Merino 80.9 59.5
Maternal 87.1 73.8


1. Twin and single lamb survival

  • Single born lambs had a greater survival than twin lambs for both Merino and Maternal ewes (P<0.001) (Table 1).
  • Maternal ewes also had a greater lamb survival than Merino ewes for both singles and twins (P<0.001).

2. Ewe age and lamb survival

  • Lambs born to more mature ewes aged 2 years or older had increased survival rates than those born to inexperienced, younger ewes that were a year old (P<0.001).

3. Ewe body condition and lamb survival

  • Ewe body condition score at lambing impacted on lamb survival, with 10% more lambs surviving for every 0.5 increase in condition score (P>0.001).
  • Furthermore, for each 0.5 condition score gain between lambing and marking, lamb survival increased by 5.6% ± 2.0% (P<0.01).

4. Mob size and lamb survival

  • The survival of single- and twin-born lambs decreased by 1.4% and 3.5% for each additional 100 ewes in the mob (Figure 1; P<0.01).
  • Lamb survival also decreased by 0.7% for each additional ewe per hectare (P<0.01).
line graph showing the effect of increasing the mob size of single-bearing ewes and twin-bearing ewes at a stocking rate of 8 ewes/ha on lamb survival
Figure 1 The effect (± 95% confidence intervals) of increasing the mob size of single-bearing ewes (black) and twin-bearing ewes (grey) at a stocking rate of 8 ewes/ha on lamb survival in experiment one. Lambs were born to ewes of Merino and maternal breeds

Points to note:

  1. Lamb survival decreased with increasing mob size
  2. The decrease in lamb survival was more noticeable in twin lambs

Take home messages

Increasing the number of ewes in lambing mobs may amplify the risk of disturbances from other lambing ewes and therefore the risk of cross-fostering, ewe-lamb separation and lamb mortality.

The greater effect of mob size on twin lamb survival is likely to be a result of an increased number of lambs born per day, further compounded by the increased difficulty for ewes to mother multiple lambs.

Increasing the number of lambs born per day may also compromise the time the ewe and lamb spend at the birth site, and increase the incidences of cross-fostering and ewe-lamb separation. Twinning has been shown to decrease the ability of Merino ewes to bond with their young and delay maternal recognition in lambs.

The average mob size of twin-bearing ewes in this study was 180. We have calculated that:

  • Reducing the mob size from 180 to 100 would increase lamb survival by 2.8% for twin-bearing ewes and therefore increase marking rate by 5.6%.
  • This is equivalent to the increase in lamb survival expected from increasing the body condition score of ewes from 3.0 to 3.3 at lambing based on the effect of condition score at lambing on lamb survival in this study, and previous research.

Overall this study indicates that reducing the mob sizes and stocking rates of lambing ewes will improve lamb survival, particularly for twins.

Work is currently underway to quantify the effects of mob size and stocking rate on the survival of twin-born lambs on commercial farms across all of Australia.

BESTWOOL/BESTLAMB is supported by the Department of Primary Industries Victoria and Australian Wool Innovation Ltd. The producers who participated in the program are sincerely thanked for their contributions.