There are three major factors contributing to the risk of breech strike.
To reduce these three risk factors the primary selection trait is breech wrinkle. In districts where scouring is a problem, dag is also a primary selection trait. Breech cover (the amount of natural bare skin around the perineum and breech area) is of secondary importance as a selection trait.
There is a very close relationship between breech, neck and body wrinkle. If breech wrinkle is difficult to evaluate (eg. when sheep have been mulesed) you can assess and select ewes and rams based on neck and body wrinkle.
In breeding for plainer, more flystrike resistant sheep there are a number of approaches that a breeder can take.
The following sections detail the impact of these approaches on rate of gain within the flock.
The skin wrinkle selection lines at Trangie Agricultural Research Centre (Industry & Investment NSW) were maintained between 1948 and 1961 and provide an insight into the impacts of single trait selection within flock.
The Fold+ line was selected for wrinkle, whilst the Fold-line was selected for plainness. The change in breech wrinkle score over a 14 year period (approximately 1 score) is expressed as a change relative to the control line.
A heavy emphasis on selection for breech wrinkle (e.g. 80% of classed out sheep culled on wrinkle) will have a large impact on the distribution of wrinkle scores within the flock).
At the end of 10 years heavy selection on wrinkle traits a breeder could expect to have reduced the average wrinkle score by 0.8 in the flock. The change in the distribution of wrinkle scores is substantial with almost no 4 and 5 score sheep being bred in the flock and the proportion of Score 1 sheep increased from 4% to 21%. The flystrike risk profile of this flock has been substantially reduced by getting rid of the Score 4 and 5’s. The trade off from this heavy emphasis on selection for wrinkle is an estimated 20% loss in fleece weight.
By comparison a breeder may choose a more balanced breeding objective which pays attention to production traits as well as wrinkle.
The graph below illustrates three scenarios for a breeder placing moderate emphasis on fleece weight, fibre diameter and staple strength (MERINOSELECT 10%+SS index) in addition to wrinkle over a 10 year period.
In the top line, the breeder has placed all their flock’s emphasis on the 10%+SS index and no emphasis on wrinkle in the breeding objective. After 10 years the clean fleece weight of the flock has increased by 9% and the wrinkle score has increased slightly (+0.1 score).
In the middle line, the breeder has placed 25% of their flock’s selection emphasis on wrinkle (eg 25% of classed out ewes culled on wrinkle and 75% culled on the traits in the 10%+SS index). The impact over 10 years is to reduce breech wrinkle by 0.7 score and clean fleece weight has increased by approximately 5% (compared to a potential 9% using the 10%+SS index).
In the bottom line, the breeder has placed a 66% selection emphasis on wrinkle (eg 66% of classed out ewes culled on wrinkle and 34% culled on the traits in the 10%+SS index). The impact over 10 years is to reduce breech wrinkle by 1.1 score and clean fleece weight has only increased slightly (1% compared to a potential 9% using the 10%+SS index).
The gains reported in these examples include no emphasis on other visually assessed traits which commonly reduces gains in production traits by one third.
3. Balanced selection combined with across-flock selection
The use of superior genetics from other flocks has the potential to increase the gains achieved over 10 years, using a balanced breeding objective.
ASBVs provide a common language for assessing and sourcing genetics from other studs that are plainer and/or more productive for other traits in the objective.
The benefits and impacts of externally sourced genetics will depend on the genetics sourced and their characteristics relative to your own flock. ASBVs provide a sound comparison for the measured traits. Great care needs to be taken with respect to traits that are not reported as ASBVs – the genetics introduced must also be suitable for other such traits in your breeding objective.
The image below shows the impact of sourcing different sires, with different production characteristics, in one generation.
The two groups of lambs were born from a mob of ewes of the same genetics, but joined to different sires at a Central Test Sire Evaluation site. The lambs on the right were sired by Sire A with a higher wrinkle breeding value (more wrinkle). The lambs on the left were sired by the Sire B lower wrinkle breeding value (less wrinkle) and a similar breeding value for clean fleece weight. The visual differences in terms of body wrinkle are clear. Based on breeding values both groups of lambs could be expected to have similar fleece weights as adults.