Research over many years has shown that the two most important factors affecting breech strike risk are breech wrinkle and dag.
Breech cover is the next most important trait.
The relationship between breech wrinkle score and breech strike in non-mulesed sheep is illustrated below.
Relationship between breech wrinkle score and breech strike in non-mulesed sheep.
Source: Breeding for Breech Strike Resistance Project, CSIRO Livestock Industries & AWI.
Data from three consecutive flystrike seasons of a selection experiment at Armidale, non-mulesed sheep under flystrike challenge conditions (no chemical prevention, delayed crutching).
As the breech wrinkle score increases the susceptibility to breech flystrike increases for all classes of stock. Score 5 animals (heavily wrinkled) are highly susceptible to breech strike. Score 1 animals are plain and have low susceptibility to breech strike.
In environments where dag is prevalent, they are another important determinant of breech strike risk. Dag is caused by scouring, often as a result of high intestinal worm burdens, and can be common in high winter rainfall environments. The section on Worms and Dag Management provides further details on how to reduce dag risk in the flock.
As dag score increases the risk of breech strike increases dramatically. A Dag Score 2 sheep is twice as likely to be struck on the breech compared to a Score 1 sheep. A sheep with Dag Score 4 is 7 times more likely to be struck than a Score 1 sheep.