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Beyond the Bale : December 2012
Environmental aspects While quicker progress can be made through breeding to improve wrinkle and breech cover scores, improving dag score in Mediterranean winter-rainfall dominant regions is much more difficult as many other environmental, non-genetic factors come into play. In this region of Australia, producers must contend with an environment that causes significant dag levels in sheep, making it 'high-dag country'. Dags in southern Australia can swamp genetic progress made by selecting low wrinkle and low breech cover sheep. The selection of low-dag and low worm egg count in sheep in high-dag country is still recommended, despite the slow progress which will be made. Good worm control is important and research into dags from hypersensitivity reactions to larvae continues. Dags also cause other production issues such as high numbers of crutching cuts and levels of stain in the wool clip. Management issues In high-dag country, crutching prior to the 'high-dag season' of mid-winter to late spring, and chemical prevention are often required in addition to mulesing to achieve adequate breech strike control. For the remaining months of the year or in low-dag country, mulesing provides very effective control. Since mulesing was first adopted, there has also been an increase in improved pastures and use of fertiliser leading to much higher stocking rates and prevalence of dags in these southern winter-rainfall dominant areas. Facts and figures There has been a rapid adoption of breech flystrike indicator traits by MERINOSELECT studs throughout Australia, expressed as Australian Sheep Breeding Values (ASBVs). The correlations between breech score and the key production traits of fleece weight, fibre diameter and fertility are low. Fortunately for commercial producers, there are good numbers of sheep which have low wrinkle, cover and dag scores and also have high fleece value and high fertility. The studs who have focused on breeding 'curve benders' for low wrinkle and high fleece weight, have bred sheep with lower correlations between these traits. This further increases the number of sheep good for all the key breech strike and production traits in these studs. The ASBVs of one such sire, with over 300 evaluated progeny in the MERINOSELECT database, are shown in Table 1. The best way for commercial producers to manage multiple trait correlations (both favourable and unfavourable) is to understand and use ASBVs when selecting sires. Preventative measures Research has shown high-wrinkle sheep in high-dag country that have been treated with breech clips need to be managed as though they were unmulesed. The breech clips are not providing sufficient reduction in wrinkle and dags compared to mulesing in high dag country. There has not been any formal breech strike clip trials on low to moderate wrinkle sheep in low-dag areas, although some breeders in these areas are reporting adequate reductions in 41 ON-FARM December 2012 BEYOND THE BALE breech strike if moderate to low-wrinkle sheep are treated with clips. Chemical prevention using Dicyclanil (Clik®) has been shown to provide good protection from breech strike for all sheep irrespective of wrinkle and dags. Reducing dags in high-dag country is the topic of ongoing research and development, with only limited success to date, other than continued good worm control and breeding for lower worm egg counts and lower dags. Reducing resistance To reduce the risk of future resistance to chemicals in Australian sheep blowfly (Lucilia cuprina) populations, it is important to balance the chemical and non-chemical means of making sheep less attractive to flies. This approach is commonly referred to as Integrated Pest Management (IPM) and typically involves best practice combination use of chemical prevention, crutching and breeding. It is important producers use different chemicals when treating fly-struck sheep and preventing flystrike. Applying chemicals effectively is critical to slowing resistance. This sire has very low ASBVs for breech wrinkle, cover, dags and worm egg count yet has a Merino Production Plus Index in the top 10 per cent and Dual Purpose Plus Index in the top 1 per cent (see Table 1). TABLE 1. The ABSVs and percentiles for a Merino sire with 350 progeny (pictured below) Tr ait ASBV Merino ASBV Percentile Yearling body weight 5.6 Top 20% Yearling fat 0.9 Top 1% Yearling eye muscle 2.5 Top 1% Yearling greasy fleece weight 11 Top 30% Yearling fibre diameter -1 Top 50% Yearling staple strength 0.5 Top 50% Yearling worm egg count -15 Top 10% Number lambs weaned 7 Top 10% Breech wrinkle -0.4 Top 20% Breech cover -0.8 Top 1% Dags -0.2 Top 10% Merino production plus index 155 Top 10% Dual purpose plus index 166 Top 1% Source: MERINOSELECT database