Breeding Merinos for less breech strike

Summary

What is the likely rate of genetic gain in breeding for breech strike resistance and what are the trade-offs?

Prior to mulesing, breech strike was the main form of fly strike: 90% of all strikes in a flock. All ages were affected and ewes were especially affected.

Studies in the 1930s highlighted that breech conformation had a large impact on susceptibility.

Sheep that were very wrinkly around the breech were five to ten times more likely to be fly struck than those with a plain breech.

The mules operation was introduced to reduce the susceptibility of wrinkly breeches to fly strike.

Mulesing reduced the number of strikes and gave added protection to plain breeched sheep, but the recommendation from the Joint Blowfly Committee in 1940 was that ‘the mules operation must not be regarded as an alternative to the policy of breeding towards plain breeched sheep’.

This Primefact provides data analysis of wrinkle score as well as suggesting options for reducing breech strike.