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Beyond the Bale : June 2019
FARM ON 39 NG BREECH FLYSTRIKE RE THE CONSEQUENCES? selection emphasis on reducing breech flystrike incidence with a modified index, by adding breech wrinkle, breech cover and dag score records to assess animals for the breeding program. Our results suggest that the breeder could reduce breech flystrike incidence to low levels (less than one strike per 100 ewes per year in unmulesed sheep) after 13-14 years of selection, enough to consider ceasing mulesing. This genetic improvement could also slightly reduce the need and use of flystrike prevention chemicals. Use of strategic chemical treatments may still be required. Although fleece weight is still being genetically improved in the breeding program, the rate of increasing fleece weight is reduced by about one-quarter (27%). There is however no sacrifice in genetic gains for fibre diameter and reproductive rate. SCENARIO 2 A medium wool ram breeder in a winter rainfall high dag area in Western Australia’s south-west decides to breed medium wool Merinos as fast as possible to reduce breech flystrike incidence, using a modified Merino Production Plus index. The average breech flystrike incidence in unmulesed but crutched sheep (with no preventative chemical treatment) in the local area has been recorded as six strikes per 100 ewes per year at Mt Barker. The breeder’s flock starts off with typical levels of risk for flystrike, with average wrinkle, dag and breech cover scores for medium wool sheep in the area, noting that wrinkle scores are a little lower but dags are higher than for the superfine sheep in Scenario 1. Our results suggest that after 11-12 years of selection, the stud flock can reduce incidence to below one strike per 100 ewes per year in the average year (in unmulesed sheep), without affecting genetic gains in reducing fibre diameter and improving reproductive rate, although there would be a reduction in genetic gains for fleece weight (gains down by 30%). Again, this would put the breeder’s flock in a strong position to consider ceasing mulesing. SCENARIO 3 A fine wool ram breeder in a high rainfall part of Victoria’s Western District, with high levels of dag with a high potential incidence of breech flystrike (10 strikes per 100 ewes per year in unmulesed sheep) decides to breed sheep that have a much lower flystrike risk, again with the aim of moving in time to a non-mules enterprise without increased reliance on chemical protection or crutching. The breeder uses a modified Merino Production Plus index. Unlike in summer rainfall areas, the main flystrike risk period, although challenging, is shorter and concentrated in the mid to late spring period, prior to shearing. In this case, the genetic gains in reducing flystrike incidence are slower than can be achieved in summer rainfall areas due to the lower heritability of flystrike. Again, the breeder’s stud flock starts off with average wrinkle, dag and breech cover scores for FAST FACTS • Recently completed work has predicted how long it takes to breed Merino sheep for low breech flystrike risk to a point where mulesing could be ceased without increased use of chemicals or risk of strike. The predictive study was based on flystrike research on Merinos run in Mediterranean and summer rainfall areas. • In ram breeding flocks of average flystrike risk, with efficient well recorded programs aided by modified MERINOSELECT indexes, it may take between 11 to 20 years to reduce the incidence of flystrike to less than one strike per 100 ewes per year. This is low enough to cease mulesing without increased reliance on chemical protection or crutching. • Although selection for reduced flystrike incidence can reduce genetic gains for fleece weight by up to 30% in the scenarios modelled, the gains remain positive. Genetic gains in reducing fibre diameter and reproductive rate are unaffected. • The time taken to reduce breech flystrike incidence to low enough levels to make mulesing unnecessary will depend on the starting sheep type, including the initial level of wrinkle, breech cover and dag, the environment (amount of dags, length and intensity of the strike risk period) and the amount of emphasis given to the trait relative to others being selected for. the area and the sheep type. Our results suggest that it will take 19-20 years of selection (considerably longer), in the stud flock to reduce incidence to below one strike per 100 ewes per year in the average year (in unmulesed sheep). Genetic gains in fleece weight, although still positive, are 30% less than what they would be if no selection emphasis was being given to reducing flystrike incidence. As in Scenario 1 and 2, genetic gains in reducing fibre diameter and improving reproductive rate are unaffected. MORE INFORMATION Breech wrinkle, urine stain, breech cover and dags are the key indicator traits that lead to increased risk of flystrike. View the full project report at www.wool.com/flystrikelatest