Introduction
Blowflies pose a serious health threat to the wool production industry. To reduce an animal’s susceptibility to breech strike, surgical mulesing soon after birth has been a major husbandry method in Australia since the early 1950s. However, during the last decade, this husbandry method has become a contentious welfare issue. Pain relief methods are being implemented but the Australian wool industry is looking for effective breeding methods to achieve a generally acceptable long term solution to this practice.
This project evaluated the effectiveness of a range of potentially new indicator traits for resistance or susceptibility to breech strike as well as current known indicator traits (dags, wrinkles, wool colour, breech cover, urine stain). The project also focused on the volatile and non-volatile chemical components of the greasy wool and the skin, to determine why flies are attracted to some sheep but not to others.
A non-challenge based selection method will have major benefits for the sheep industry. New breeding technologies based on single nucleotide polymorphisms (SNP) offer huge opportunities to find genes for traits that are expensive or difficult to measure such as breech strike and worm resistance. The existing breech strike flock at the Katanning Research station will contribute significantly to develop this technology to develop a genomic breeding value that industry can use to select for resistant animals. Such an approach will have major welfare benefits for the Australian sheep industry.
Project aims
The aims of the project were as follows:
- phenotype a large number of sheep for breech strike and potential indicator traits in a Mediterranean environment
- identify indicator traits that could be used to breed indirectly for breech strike resistance
- estimate the inheritance of the new indicator traits
- estimate the genetic and phenotypic relationships between the indicator traits and breech strike
- establish a DNA library that can be used to develop non-challenge based selection methodologies.
Materials and methods
Ewes
The flock consisted of 600 Merino ewes which was ranked on a 7% dual purpose selection index (7%DP) as described under 'Selection procedure' below. BLUP (best linear unbiased predictions) methodology was used to estimate more accurate breeding values for breech strike. Sheep that were struck previously were not chosen for the resistance selection line. To ensure that susceptible ewes are not penalised on production because they had a higher strike rate than the resistant sheep, the average 7%DP index of the selected sheep in the resistant line was determined first. A susceptible group was then compiled from the remainder of the ewes that had the same average production index value as the resistant line.
Rams
The rams were ranked on a 7%DP index. The six most resistant rams based on their breeding value for breech strike and using their own and relatives' performance information, were identified. Six susceptible rams with similar 7%DP index rankings, but with high breeding values for susceptibility to breech struck and that have been previously struck, were identified for the susceptible line. This ensured that the average 7%DP index value of the resistant and susceptible groups were the same.
Blowfly strike management
The larvae of first and second stages of the blowfly larval development, are small and feed on readily available nutrients on the skin surface. In the initial stages of the infection, these larvae cause only a relatively minor irritant to the host. The third stage larvae, known as instars, have a sharp tooth that enables them to rasp away at the skin causing a break in the protective skin barrier, causing plasma to leak out. The instars also release a toxin that can cause a shock syndrome in the host depending on the number of larvae. During favourable environmental conditions for the fly, this stage can be reached by day three of the strike. In this experiment sheep were closely monitored to detect and treat each strike before the infection causes serious injury to sheep.
Mating
Approximately 500 Merino ewes were allocated to 12 individual sire mating groups and drafted onto mating plots in February, to be joined with their allocated ram.
Certain family groups were inseminated with semen from specific rams based on their performance. This applied where an outstanding ram may have died or been injured but its semen was available, or to generate genetic links between this flock and industry flocks.
Lambing
Two weeks prior to lambing, all pregnant ewes were drafted on to the lambing plots according to their single sire mating groups. All lambing information and dam pedigrees were recorded on a daily basis during lambing. A suite of traits was recorded on each lamb and dam at lambing.
Husbandry
Shearing of hoggets and mature animals were carried out in late November/December. Weaners were crutched and wigged after weaning. First shearing with 12 months’ wool growth occurred at approximately 16 to 18 months of age.
From 2006-2009 and again from 2014-2015, the lambs were not crutched at yearling age, while from 2010-2013 the lambs were crutched to determine whether the inheritance of breech strike is different, and whether the same indicator traits are important in crutched and uncrutched sheep. From 2014 to 2017 no crutching was carried out in order to identify genetically resistant sheep.
Blowfly control
All animals are closely monitored for flystrike. The aim was to detect strikes at the third instars stage. All animals found to be affected by flies were treated immediately with a short acting chemical. Struck animals were recorded and frequently monitored and if necessary treated again to alleviate any pain or distress during the healing process. From an experimental point the optimum number of struck animals to be struck in a mob should be 50%. However, this has never happened, and on average approximately 20 to 30 % of the hogget got struck. The highest percentage of animals struck was 39%.
Worm control
Worm control was based on monitoring faecal worm egg count (WEC) as well as general animal health observations and body condition, to ensure that animal welfare was not compromised when drenching was delayed
Selection procedure
Hogget ewes were selected at 15-16 months of age. Any animal that was deemed unacceptable by industry standards was culled. Animals were ranked on a multi trait 7%DP selection index. This index is a linear equation in which the breeding values of each animal for clean fleece weight, fibre diameter, staple strength, body weight, eye muscle and subcutaneous fat depth is appropriately weighed to achieve a balanced breeding objective where 50% of the genetic gains are from wool and 50% of the gains are from meat production. The breeding values and index rankings are provided by Sheep Genetics, the national performance evaluating and breeding scheme.
All the male progeny was kept as entire rams. Twelve hogget rams were selected for breeding each year based on the same criteria as for the ewes except that a higher selection intensity was applied. Each sire was replaced by a son that has performed better than his father based on the index and breech strike trait. Two ‘link sires’, one from each line, and which have had progeny in previous years, were used in subsequent years to create genetic links across years. A cyclic mating system was followed to prevent sires mating with their daughters.
Selection criteria
The potential to cull the lowest ranking animals depends on the reproduction rate of the flock. As only 2% males are required for breeding purposes, this allows for a higher culling percentage than in ewes.
Animals were removed from the flock under the following criteria:
- animals with undesirable wool type and conformation traits that deviate from industry standards
- infertile animals
- animals that were below ‘critical limits’
- animals that suffer from chronic illnesses such as arthritis, cancer, etc.