Lamb mortalities are estimated to cost the Australian sheep industry about one billion dollars each year, so improving marking rate is an important driver of productivity and profitability for sheep enterprises.
A National Lambing Density project, launched in 2016 and supported by AWI and Meat & Livestock Australia, has been investigating the effects of mob size and stocking rate on the survival of twin-born lambs between birth and marking.
The research is being conducted at 70 on-farm research sites across Western Australia, South Australia, Victoria and NSW using Merino and non-Merino ewes. A large network of producers is also providing data from their own farms – across a broad a range of environmental and management conditions – to validate the data from the research sites.
The project aims to develop guidelines for sheep producers regarding the allocation of ewes to mobs and paddocks at lambing to improve lamb survival.
A survey of sheep producers in Victoria run a few years ago by the AWI state grower network BestWool/BestLamb showed that for each additional 100 twin-bearing ewes in a mob at lambing, lamb survival decreased by 3.5%, but in single-bearing ewes the effect was quite a bit lower at 1.4%. The survey also found that lamb survival decreased by 0.7% for each additional ewe per hectare, regardless of birth type.
This suggested an opportunity for producers to improve lamb survival by both reducing mob size and potentially also reducing stocking rate.
The existing guideline of 100 to 250 twin-bearing adult ewes per mob at lambing is broad and could represent a range in marking rate of at least 10% for twin-bearing mobs. So our research team set out with the aim to quantify the effects of mob size and stocking rates on lamb survival and provide some more credible recommendations for allocating ewes to mobs at lambing.
We were particularly interested in the effect of mob size in twin-bearing ewes given that improving the survival of twins is a priority for the industry.
At the Lambing Density on-farm research sites, the low mob size comprised about 100 ewes and the high mob size about 240 ewes; stocking rates were generally 5-6 ewes per hectare for the low stocking rate and 7-8 ewes per hectare for the high stocking rate.
The initial results show that lower mob sizes equate to higher lamb survival. However, there was no effect of stocking rate on lamb survival, therefore reducing stocking rates is unlikely to be an effective strategy for improving lamb survival.
On average, survival was just under 3% greater for lambs born at the lower mob sizes compared to the higher mob sizes. This equated to a 2% decrease in the survival of twin-born lambs for each extra 100 ewes in the mob at lambing. To put this simply, reducing mob size by 100 twin-bearing ewes increased the number of lambs marked by four.
The effect is linear – as we increase mob size, lamb survival decreases. For the on-farm research sites, this effect was consistent between Merino and non-Merinos, and for ewes in a condition score of 2.6-3.7 at lambing.
At larger mob sizes there will be more ewes lambing per day, particularly for twin-bearing mobs, so there will be a greater presence of birthing fluids and newborn lambs. Previous research has indicated higher lambing densities increase the risk of mis-mothering and cross-fostering, resulting in ewe-lamb separations and therefore poor lamb survival. However, most of this research has been performed on a small experimental scale.
We also performed experiments at Pingelly east of Perth in two separate years to investigate the effects of mob size on ewe-lamb behaviour and lamb survival at a larger scale, more reflective of commercial conditions.
In 2016 we had an exceptional season in WA and we ended up with Feed On Offer (FOO) levels of >2400kg DM/ha throughout lambing, and under those conditions we saw no effect of mob size on the survival of single- or twin-born lambs, when mob size was 55 compared to 130 ewes.
In comparison, last year we had quite a tough season. Our FOO levels were <400kg DM/ha at lambing and we were also supplementary feeding the ewes throughout lambing. Under those conditions the survival of twin-born lambs was 6.2% higher at a mob size of 55 compared to 210 ewes. If we're assuming that effect is linear, that equates to a 4% decrease in the survival of twin born lambs for each extra hundred twin-bearing ewes in the mob.
We are looking to do some further work to investigate the relationship between mob size, FOO and lamb survival given these contrasting results.
We know through the Lifetime Ewe Management guidelines that pregnancy scanning and separating single- and twin-bearing ewes to optimise maternal nutrition is key to improving ewe and lamb survival and performance. The guidelines also provide condition score and FOO targets for use during pregnancy and during lactation.
We also know that providing access to shelter at lambing, particularly for twin mobs, can improve lamb survival if the shelter is effective and utilised, particularly where chill index is high.
From our new work we've indicated that reducing mob size is going to also improve lamb marking rates. The effect of mob size appears to be about the same as increasing condition score for ewes at lambing by 0.1 to 0.2. While it's a fairly small effect, the overall results can be significant when used in combination with the other existing guidelines.
The Lambing Density project is in its final year. We’ve completed work on 61 of 70 on-farm research sites, so this year we’ll undertake work on the remaining nine sites. Our next step will be to undertake a full benefit-cost analysis in terms of reducing mob size to improve lamb survival, including the costs associated with subdividing or putting up temporary fencing in large paddocks. We’ll then run some workshops to communicate project outcomes and develop some practical guidelines for producers which can be implemented to increase their marking rate.
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