Research shows mob size is one of the key influences in Lamb mortality
- By: "Farm Tender" News
- Ag Tech News
- Jul 29, 2018
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Lamb mortalities are a major loss of production the Australian sheep industry, estimated to cost about $1B per annum. On average, 20% of lambs born will die prior to marking, and about 80% of these deaths occur in the first three days of life. We know survival in Merino’s is lower compared with non-Merinos, related to lower birth weight and poorer vigour of lambs, and poorer maternal behaviours of Merino ewes.
Economic modelling suggests that improving the survival of twins is the highest priority for the industry to improve reproductive performance. Through the National Lambing Density project supported by AWI and MLA, the effect of ewe mob size and stocking rate on the survival of twin born lambs is being studied by Amy Lockwood, Murdoch University, using on-farm research sites and survey data.
When it comes to lambing density why would we get an effect of mob size or stocking rate on lamb survival?
Ewes can be attracted to birthing fluids and newborn lambs within a few hours of lambing. This can lead to mismothering and cross-fostering, and ewe-lamb separations which result in poorer lamb survival. Therefore, where we have a higher mob size and have more ewes lambing per day, especially twin bearing ewes where we expect twice as many lambs, there will be a greater presence of birth fluids and new born lambs, which may increase the risk of mismothering and lamb mortality.
What has the research revealed?
BestWool BestLamb (BWBL) producer surveys in South Eastern Victoria were analysed for the effect of mob size and stocking rate on lamb survival. They showed that:
* For each extra 100 ewes in the mob, there was a decrease of 3.5% in survival of twins, but the effect was lower for singles at 1.4%.
* Lamb survival decreases by 0.7% for each extra ewe/ha regardless of birth type (ie. twin or single) suggesting a small stocking rate effect.
The current recommendation to producers is that twin-bearing ewes should be lambed in a mob size between 100 and 250. However, the difference in difference in lamb mortality between these two mob sizes is over 5% and the marking rate over 10%, so there is scope to improve these guidelines (Table 1).
Table 1. Recommended mob size and changes in mortality and marking rate.
Recommended mob size for twin-bearing ewes 100 – 250
Difference in lamb mortality (%) 5.25
Difference in marking rate per 150 twin-bearing ewes (%) 10.5
The Lambing Density on-farm research sites have set out to quantify the effects of mob size and stocking rates on lamb survival, in particular for twin-bearing ewes. Research is being conducted on 70 farms across Australia, using adult twin-bearing ewes, both Merino and non-Merino.
Treatments on each farm are combinations of high or low mob size x high or low stocking rate, so each farm has four lambing paddocks. Mob size has been around 100 ewes for low mob size and 250 for high, while stocking rate has been 5-6 ewes/ha for low stocking rate, and 7-8 ewes/ha for high. Condition scores (CS) were around CS3 (range CS 2.6–3.7) at lambing and food on offer (FOO) at lambing about 1500kg DM/ha.
Initial results indicate that mob size, but not stocking rate, influence lamb survival. There was about a 3% decrease in lamb survival with the higher mob size, which equates to 2% decrease in survival for each extra 100 ewes. This means if we reduce the twin-bearing ewe mob size by 100, the number of lambs marked increases by four.
Survival of Merino lambs was poorer than non-merinos (70.4 vs 81.4%) as expected, but the effect of mob size on lamb survival was consistent, regardless of breed.
Amy suggests that you can run smaller mobs for lambing, then box mobs up after lambing finishes – protect those first few days of life, then a week after the last lamb has dropped you can box them up again.
Does seasonal variation change the mob size effect?
In 2016, Pingelly WA had an exceptional season, producing 2700kg DM/ha of FOO at lambing. In these conditions there was no effect of mob size (55 vs 130 ewes/mob). Last year, a tough season, produced only 400kg DM/ha of FOO, and ewes had to be trail fed. At lambing the survival of twins was 6.2% greater at mob size of 55 vs 210 ewes (= 4% decrease in the survival of twin born lambs for each extra 100 ewes). This indicates there could be seasonal variation of the effect of mob size on lamb survival depending on FOO, and further work is intended to explore the relationship.
Factors which may influence the benefit of reducing mob size:
* Pregnancy scanning and proportion of twins – if you’re not scanning then the ability to separate twins and benefit from increased twin lamb survival will be lower. The more twins you can identify, the greater the benefit.
* Time of year – in autumn when FOO levels are lower, the benefits could be greater – if FOO is high at the end of winter/spring then the benefits may not be as high.
* Availability of paddocks
** if you need to subdivide there will be labour and fencing costs
** if many paddocks are under crop there is less scope to reduce mob size. An option would be to lamb single-bearing ewes, who have a lower risk of metabolic issues, on crops
* Use of temporary fencing – can you use it for other purposes outside of the lambing season?
* Split lambing – greater use of extra fencing would recoup costs faster than a single lambing time
* Improved pasture utilisation of smaller paddocks – greater stocking density can improve grazing efficiency and profit from additional lambs surviving to marking.
The effect of reducing mob size appears to be the same as increasing CS at lambing by 0.1 to 0.2 – a fairly small effect but when used in combination Lifetime Ewe guidelines can be significant.
This year, the final 9 out of the 70 farm research sites will be completed, and a cost-benefit analysis conducted on the practice of reducing mob size to improve lamb survival, including the costs associated with subdividing or using temporary fencing in large paddocks.
Paddocks and mobs used for these research sites would be smaller than seen on typical Mallee and Wimmera properties, but the effect of mob size on twin-lamb survival is linear. While research hasn’t been done on larger mobs of 500-1000 ewes, Amy expects that data from the national survey from lower rainfall zones will show that the benefits of reducing mob size would still apply – that for any reduction in mob size, and consequently the number of lambs being born on any one day, there will be a benefit to lamb survival through to lamb marking, particularly for twin lambs.
Recommendations to increase lamb survival:
* Allocate ewes to mobs for lambing using the Lifetime Ewe management guidelines (based on Merinos, research is currently underway for maternals) using:
* Pregnancy scanning for singles and multiples
* Separate singles and twins into separate mobs
* Condition score – then allocate paddocks to meet CS targets
* Feed-on-offer (FOO) targets – prepare paddocks or feed to meet single and multiple-bearing ewe nutritional needs
* Paddock shelter: access and use of effective shelter, particularly for twin bearing ewes, will improve twin lamb survival, especially where chill index can be high.
* Reduce mob size at lambing to reduce mismothering and improve marking rate. If needed, box mobs up again once lambing finishes (1 week after last lamb).
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