Breeding objectives – setting and getting change

For the past three and a half years, Nick Linden from Agriculture Victoria has been working with a small group of producers on a Meat & Livestock Australia-funded producer demonstration site (PDS) project. The demonstration assessed the importance of having a breeding objective, and how well the farmer’s breeding objective aligned to the genetic potential of their sheep.

Producers involved with the demonstration were all members of Jason Trompfs’ LambsAlive network.

For a flock to make genetic gain, making selection decisions that work towards the breeding objective is essential. Identifying what traits are important (to the production system) and then measuring performance enables more informed selection decisions to be made.

Nick worked with 5 commercial merino producers who used a ‘flock profile’ to assess the genetic potential of their sheep at the start and end of the demonstration. The flock profile is a commercially available genomic test that was developed from research undertaken as part of the Sheep CRC that is now commercially delivered by Sheep Genetics.

More than half of the breeders involved in the demonstration implemented a change in ram purchasing and were keen to quantify the impacts of this change (both good and bad). Where there was a new source of rams used, additional flock profiles were undertaken to make comparisons between the genetic potential of progeny from the ‘traditional’ versus the ‘new’ sires. To further track impacts of a change in genotype, phenotypic data was collected (which included weaning weight, post-weaning weight, condition score at key times, scanning percentages, fleece weight and micron), testing performance of sheep in the field.

The use of the flock profile was quick and easy, with results easily interpreted and understood by all host farmers. The flock profile showed how quickly the genetic potential of a commercial flock can be altered to match a changed breeding objective.

Kaniva site example

One of the farmers involved with the demonstration had placed additional emphasis on carcass and reproduction traits in his traditional wool flock. The change in breeding objective was a ‘business-based’ decision, backed up with data collected over several years for this farm. After changing sire selection, the genetic potential of the progeny changed considerably (Table 1). Importantly for this farm, they had been able to increase post-weaning weight (PWT) in lambs from 4.4 (25 percentile band) to 5.7 (5 percentile band) in a single joining.

Table 1. Flock profile (FP) for progeny groups from ‘traditional’ versus ‘new’ sires at the Kaniva host farm, Australian sheep breeding values (ASBVs) and percentile band results for key traits.

While post-weaning weight and yearling weight had both improved (in line with the changed breeding objective), there was no observed difference between sire groups for other important traits such as yearling fibre diameter, yearling eye muscle depth and yearling fat depth. Yearling clean fleece weight was estimated to decrease slightly.

Yearling worm egg count had moved unfavourably in the second sire group (genotype 2), which without the flock profile would not have been identifiable to the farmer before it became a problem. Knowing the elevated risks, worm issues can now be monitored with faecal egg counts.

The phenotypic data further backed up the flock profile results – lambs from the traditional sire group had growth rates to weaning of 166 g/hd/day compared to 191 g/hd/day for progeny of the new rams. Lamb growth rates declined post-weaning (in line with reduced feed availability) – there was no difference in post-weaning growth rates between the 2 genotypes, with lambs of traditional breeding doing 121 g/hd/day compared to 123 g/hd/day for lambs from the new genotype.

Despite differences in liveweight at weaning (in line with the greater genetic potential for weaning and yearling weights), there was no difference in mature weights of the 2 genotypes at first pregnancy scanning (61.6kg traditional compared to 61.1kg for the new genotype). This result was highly rated by the host farmer. as he was seeing increased early growth with no associated increase in mature size/maintenance requirements.

While there was no difference in average liveweight between the 2 genotypes at scanning, there were differences in scanning rates, with the maidens from the new genotype scanning at 122% versus 111% for the traditional genotype.

Flock profile reports now include results for an expanded range of ASBVs, including weaning rate (WR) and condition score (CS). If tracking the genetic potential of your flock for reproduction, weaning rate will be a key consideration. When these opening flock profiles were undertaken on the progeny of the traditional and new sires, results did not include these traits. As a result, when looking at reproduction results, we investigated differences in YFAT and YEMD, which can be correlated to reproduction.

The new results are an improvement over this approach, as you can now work directly with weaning rate, as opposed to a correlated trait. We had anticipated that the difference in pregnancy scanning results may have been related to a difference in liveweight at scanning, or to differences in either YFAT or YEMD. However, there was no difference in liveweight at scanning, and no difference in genetic potential for fat or muscle between the 2 genotypes.

We believe that the heavier initial weight at weaning of ewe lambs from the new sires may have carried over to joining and contributed to the higher scanning percentages. The heavier weight at weaning, with no difference in liveweight at scanning indicates that the progeny of the new sires were at a higher percentage of mature weight at weaning (and possibly still at joining) which, being more biologically mature, would be expected to increase reproduction rates.

These examples of altered genetic potential and on-farm performance illustrate the changes on one farm. Other farms had different priorities, such as a greater emphasis on wool quality, and undertook collection of mid-side samples and fleece weights to assess the impact of a change in genotype. If you are interested in seeing more details from this, as well as the other farms involved in the demonstration, contact Nick Linden for a copy of the final report.

Rate of genetic gain

The Kaniva and other demonstration sites showed how quickly genetic improvements can be made.

The rate of genetic gain will be influenced at a flock level by both the logistics of changing a ram team over to newer genetics, as well as by the improvements that one ram team offers over another ram team. While one host farm replaced an entire ram team in a single year, other approaches included only updating replacement rams each year or replacing rams to be used over maiden ewes only. The rate of genetic gain across the whole flock is dependent on the strategy employed on-farm.

For all of the farms that introduced a new bloodline, there was considerable thought and planning for what the next steps would be. Most saw a need to find a middle ground between what the 2 tested genotypes represented. For some, the next step was to select sires that sat between the 2 tested genotypes, while for others it would involve a combination approach to breeding where progeny from sire group A would be joined to sire group B, and vice versa.

Where a host farmer had implemented the ongoing breeding strategy, resultant progeny were tested with a follow-up flock profile to identify how well the goal of finding a middle ground had been achieved.

Managing genetic change

The complexity of multifaceted breeding objectives is that the more traits being selected for, the harder it is to make gains in individual traits. It takes time and discipline to stabilise the genetic potential of the flock.

There remains a need for ongoing monitoring to check that management changes that achieve improvements in some areas don’t result in negative consequences to other key traits. This stabilisation of other traits appears harder to achieve than a one-off shift in genetic potential of an individual trait.

While host sites that introduced new sire lines were able to achieve their goal for certain individual traits, none had been able to do so for all the key traits. Indeed, some key traits had regressed from the initial starting point.

However, the demonstration showed a long-term focus and appropriate selection of rams and ewe replacements did achieve the aims of a complex breeding objective that balanced many traits, including the trade-offs required to meet growth, reproduction, carcass and wool quality goals. The demonstration also showed that considerable economic benefit could be achieved through genetic gain as measured through phenotypic data.

An economic model was developed to assess the net benefit and net present value of a change in genotype that increased reproductive rates and growth rates. In all cases, the introduced genetics through ram selection returned a positive economic result, with year 2 net benefit ranging from $499 to $2,080 per 100 ewes joined.

The lower net benefit was associated with a decrease in wool income that was offset by an increase in weaning weights, and the higher net benefit was accompanied by no decrease in wool values but an increase in both reproduction rates and weaning weight. Improvements in weaning weight delivered high net benefits through a reduction in feed costs due to quicker turnoff times.

These estimates included increased purchasing costs for improved rams and additional feed costs associated with higher reproduction rates.

Summary

Key findings from the demonstration included the following:

• A shift in genetic potential of the ewe flock was quickly achieved through selection of appropriate genetics. The limitation to the rate of whole flock improvement largely comes from the selected ram replacement strategy.

• The flock profile provided merino producers with an easy-to-use tool that estimates genetic strengths and weaknesses of their flocks – and is a useful tool for testing how well the strengths and weakness of the sheep aligns with the stated breeding objective.

• Regardless of whether the breeding objective has been in place a long time or has been recently developed or changed, the flock profile plays a part in identifying future selection needs.

• As well as tracking genetic potential, the flock profile provides valuable data to inform development of a breeding objective, as well as providing early warning when selection practices may have resulted in a negative outcome.

• Within the demonstration, the most valuable results came from producers undertaking a flock profile on progeny from a specific mob of ewes with a known sire team (as opposed to sampling lambs from multiple joining groups). This enabled –
  • comparisons of results between the initial and later flock profiles to gauge improvements
  • phenotypic data from the mob that was tested for the flock profile to be compared to phenotypic data from other mobs as a proxy for differences in genetic variation.

• The flock profile provides a simple-to-use tool for producers to benchmark the genetic merit of their merino flock. Understanding this initial starting point gives instant context for how your sheep compare to others and what sort of gains may be possible.

• Ongoing monitoring is important to establish progress towards or unexpected deviation away from the breeding objective when changing genotypes. How multiple ram types are managed in a practical sense needs consideration. Host farms in this demonstration that introduced new bloodlines largely planned to alternate between 2 ram sources, rather than change completely from one to the other.

• With diligent selection and management, even a relatively complex multi-trait breeding objective (as required in a ‘dual purpose’ merino genotype) can be met.

We would like to sincerely thank all the host farms that submitted samples for analysis, contributed or collected phenotypic data and helped with all aspects of the demonstration.

For more information, email on-farm-demos@agriculture.vic.gov.au