The genetics of bulletproof lambs

Published in P/GK Progression 2024.

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Most people agree that there is nothing wrong with the fertility or fecundity of Merino ewes: they have plenty of lambs.  We are also well past the “Merinos can’t count” era.  We now know that there is enormous variability in the maternal performance of Merino ewes and the innate survivability of Merino lambs. That said, there is more to learn and plenty of room for improvement in applying that knowledge for a better overall outcome in the paddock.

This article provides an update on what we currently know about the genetics of reproduction in Merinos (and taking Merino lambs born through to a saleable product), along with the areas that require further exploration and will be the focus of research into the future.

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Selecting for survivability

The heritability of all reproductive traits tends to be low and lamb survival is no exception. In fact, it has more or less the lowest heritability of all traits. From this, you could conclude that it is a waste of time to select for improved survival (and that you must rely on the management of ewe nutrition, lambing conditions and lambing time to improve lamb survival).  Arguably, this is exactly what we have done as an industry and there is no doubt that the lowest-hanging fruit has been manipulating management to improve overall lamb survival.  However, there are four reasons why selecting better genetics for improved lamb survival remains important.  

●     Firstly, even though the heritability is low, the variation is high. This makes it possible to make genetic gain at a useful rate.  

●     Secondly, even when you know all of the right management interventions, seasonal variability makes it difficult to hit all the targets every year. In the years when you are forced to manage sub-optimally, the animals with improved genetics for lamb survival will keep you in the game.  

●     Thirdly, not all production systems lend themselves to intensification. Because of either scale or accessibility, some sheep operations have difficulty implementing the practices that would improve lamb survival, such as improving nutrition to twin-bearing ewes, lambing in small mobs and ensuring good pasture availability during lambing.

●     And, finally, even when all appropriate actions and management strategies are implemented, short- or long-term weather events can wreak havoc on newborn lambs.  Under these conditions, the need for improved genetics for lamb survival becomes paramount (and is particularly effective, as outlined below).

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Better than ever

Over the last few years, much better ASBVs for the reproduction traits have become available and there have been a lot more breeders starting to submit the right information to allow repro data tobe generated.  As you will know, the old Number of Lambs Weaned (NLW) ASBV has been replaced by five much better breeding values:

●     Conception (CON): Did a ewe conceive or not?

●     Litter Size (LS): If she conceives, how many lambs does she conceive?

●     Ewe Rearing Ability (ERA): Of the lambs conceived, how many does she raise?

●     Weaning Rate (WR): Combines the three breeding values above into a single number.

●     Maternal Behaviour Score (MBS): A measure of the distance travelled when lambs are caught (not many measures in Merinos).

Drilling into lamb survival within these ASBVs, the two to focus on are Ewe Rearing Ability (ERA) and Maternal Behaviour Score.  However, only a small number of Merino breeders record Maternal Behaviour Score (MBS) (ie those that catch and tag at birth), leaving us with ERA.  

The variation in ERA is enormous, with a range of around 0.25 lambs per ewe across the sires used in industry over the last 10 years (Figure 1).  If lamb survival is important to you, looking for positive and high numbers for ERA should be your focus.

Usual suspects

It is useful to consider how the other aspects of your selection decisions could be impacting on ewe-rearing ability.  A quick analysis of 4500 odd sires used recently (ie sires born between 2011 and 2021), with more than 75 progeny in the database, gives us a few clues about the correlations that exist in the general fine-wool population. In Table 1, the sires have been grouped: those in the top 5% for ERA, sires that are in the middle 5% for ERA and those in the worst 5% for ERA, then the average ASBVs have been given for a range of other traits.

The usual suspects of muscle, fat, condition score and wrinkle are all having the impacts we have come to expect. The differences in the worm egg count breeding value are interesting and warrant further investigation.  Recent work from Kelsey Bentley and the wider team in West Virginia demonstrated a link between WEC breeding values, colostrum quality and lamb survival.  

Even when traits are correlated, there are always a lot of animals that don’t follow the trend and plenty of opportunities to breed animals that don’t follow the correlation.  That said, it is useful to know what will happen if you don’t pay attention to a trait and only focus hard on a trait that is unfavourably correlated with it.

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Lamb survival as a trait of the lamb

Unfortunately, the breeding values we have available in Australia don’t consider the innate ability of the lamb to survive. We know that there is significant variation in a range of lamb behaviours and these are directly linked to lamb survival (reviewed by Forbes Brien and a cricket team of co-authors, Animal Production Science 2010). We have also seen sire and sire breed effects on lamb survival, which confirms that there is useful genetic variation to work with here.  

The only way to work on lamb survival as a trait of the lamb is to select animals based on traits known to be favourably correlated with lamb survival. Interestingly, birth coat score was the trait with the highest favourablen genetic correlation with lamb survival (and was the most practical to implement) in the Brien et al. (2010)paper. Unfortunately, we know that birth coat score is also unfavourably correlated with some wool quality measures, so it is unlikely that birth coat score is going to be implemented as a selection criteria for lamb survival.  

From my own PhD, I found a favourable correlation between the genetic fat of a ewe and the birth weight of her lamb under nutrient restriction.  This flows through to higher lamb survival.  Generally selecting for high fat, high muscle, high condition score and low breech wrinkle (without excessive fleece weight) sets you up for a lamb that can survive.  

It would be far better to select directly for a lamb survival breeding value. But, for now, that is not available in Australia (although it is in New Zealand).

Learning from the South African selection lines

Schalk Cloete and multitudes of colleagues and students over the years have maintained selection lines in South Africa where they selected for or against the ability of ewes to rear multiple lambs. These lines have provided interesting insights for decades and provide us some clues into what we can expect if we get serious about selecting for improved reproductive performance in Merino sheep.  

Remarkably, the work demonstrated a 10%improvement in genetic lamb survival over a 20-year period.  That is 0.5% per year, which gives us a good upper limit to what we can expect. There has been a range of really interesting outcomes from this work.  One of these outcomes is that the time taken from the onset of labour to the successful birth of a lamb has been reduced:  lambing ease is favourably correlated with lamb survival.  

There are two major findings out of the work that we need to consider as an industry. Firstly, the difference in survival between the selection lines was not evident under good lambing conditions. But it became increasingly pronounced as the weather deteriorated (as chill index increased).   Of equal importance is the fact that the heritability of the trait changed with the weather conditions under which selection occurred.  That is, under good lambing conditions, the heritability of lamb survival is lower than it is under poor lambing conditions (high chill index).  

These findings are demonstrated in the two figures taken from the Nel et al.(2023) paper,  shown below as Figure2.  The average heritability in the graph is quite high and the paper explains why this is.  However, the important thing is that you will make more genetic gain for lamb survival when lambing under harsh conditions than you will when selecting under good conditions.  

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‍Bringing it all together

So, what is a good plan for a commercial sheep producer who is keen to be making genetic gain in lamb survival?  Here are the best options, based on what we know currently:

1. Buy rams with high ERA ASBVs: an ASBV above 0.05 puts a ram in the top 20% currently.
2. Buy rams with positive ASBVs for condition score, fat and muscle, and negative ASBVs for wrinkle.
3. Buy rams from a vendor where stud ewes are managed commercially and under a bit of pressure at lambing time.
4. If possible (and you’ve got the time to investigate), buy rams from sires that have above-average lamb survival rates. This will take a bit of homework and chatting with your ram supplier.
5. Cull (or send to the terminal mob) any ewes that fail to raise a lamb (wet/dry ewes) regardless of the circumstances.
6. Select and retain ewes that are in good condition score and have low wrinkle.
7. Ensure you class single-born and twin-born ewes separately to stop accidental bias toward selecting single-born and raised ewes.