Context The availability of effective indirect rather than direct selection criteria for genetically reducing breech flystrike is crucial for the Australian wool industry, as expression of breech flystrike is routinely suppressed by management interventions to minimise the risks to sheep health, welfare and productivity. Understanding how much genetic gain can be made in reducing breech strike, while also selecting for other key traits is important in choosing breeding objectives, particularly for Merino genetic improvement programs. Aims We predicted genetic gains from within-flock selection to reduce breech flystrike incidence (FS) and improve productivity under a range of scenarios that all used indirect selection criteria in the genetic evaluation of FS. Methods Three breeding objectives were modelled, by adding FS as a trait to the breeding objective for the Dual Purpose Plus (DP+), Fibre Production Plus (FP+) and Merino Production Plus (MP+) indexes available from MERINOSELECT. A large range of economic values were examined for FS, from 0 to –$240/strike.sheep per year in $20 increments. For all scenarios, full records of productivity traits and pedigree information were assumed to be available for use as selection criteria, as well as records of breech wrinkle, dag and breech cover scores. Predictions were conducted for scenarios assuming (a) moderate heritability for FS (b) low heritability for FS, and (c) low heritability for FS, but higher heritability for dag score than assumed for scenarios (a) and (b). Key results After 10 years of selection, under assumption (a) predicted genetic gains in FS ranged from 0, up to maximum reductions of 0.19, 0.21, and 0.20/strike.sheep per year from use of the FP+FS, MP+FS and DP+FS indexes, respectively. Under assumption (b) predictions of genetic gain for FS were considerably lower and ranged from 0 to maximum reductions of 0.04, 0.06 and 0.06/strike.sheep per year based on use of FP+FS, MP+FS and DP+FS indexes, respectively. Under assumption (c), predicted genetic gains in FS were very similar to those under assumption (b). When 70% of the maximum gains in FS studied were targeted, from 70% to 93% of genetic gains in the overall index (excluding FS) were predicted to be retained when FS had moderate heritability, and from 85% to 95% when FS had low heritability. Conclusions and implications There is a practical range of economic values for FS (from –$60 to –$80/strike.sheep per year for MP+FS and DP+FS indexes and –$60 to –$140/strike.sheep per year for a FP+FS index when the heritability of FS is moderate and –$100 to –$200/strike.sheep per year for a MP+FS index and –$140 to –$240/strike.sheep per year for FP+FS and DP+FS indexes when the heritability of FS is low), that could be used in breeding programs to genetically reduce FS, while retaining competitive levels of genetic gains for other important traits. Reduction of FS is achievable to low levels, after 10–20 years of index selection, similar to levels achieved by mulesing (0.01/strike.sheep per year in average seasonal conditions).

中文翻译：

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通过使用指数选择，可以在美利奴绵羊中获得在减少臀位飞击和提高生产力性状方面的大量遗传收益

背景 对于澳大利亚羊毛业而言，有效的间接而非直接选择标准可用于从基因上减少臀飞击的选择标准至关重要，因为管理干预通常会抑制臀飞击的表达，以尽量减少对绵羊健康、福利和生产力的风险。了解在减少臀位撞击方面可以获得多少遗传收益，同时还选择其他关键性状对于选择育种目标很重要，尤其是对于美利奴基因改良计划。目标我们预测了从群内选择中获得的遗传收益，以减少臀飞击发生率 (FS) 并在一系列场景下提高生产力，这些场景都在 FS 的遗传评估中使用了间接选择标准。方法 对三个育种目标进行建模，通过将 FS 作为性状添加到 MERINOSELECT 提供的 Dual Purpose Plus (DP+)、Fiber Production Plus (FP+) 和 Merino Production Plus (MP+) 指数的育种目标中。FS 的经济价值范围很广，从 0 到 – 240 美元/strike.sheep 每年，增量为 20 美元。对于所有场景，假定生产力特征和系谱信息的完整记录可用作选择标准，以及臀皱、dag 和臀盖分数的记录。对假设 (a) FS 的中等遗传力 (b) FS 的低遗传力，和 (c) FS 的低遗传力，但 dag 分数的遗传力高于场景 (a) 和 (b) 假设的场景进行预测。主要结果 经过 10 年的选择，在假设 (a) 预测的 FS 遗传增益范围为 0、通过使用 FP+FS、MP+FS 和 DP+FS 索引，每年最多减少 0.19、0.21 和 0.20/strike.sheep。在假设 (b) 下，根据 FP+FS、MP+FS 和 DP+FS 指数的使用，对 FS 遗传增益的预测相当低，范围从 0 到每年最大减少 0.04、0.06 和 0.06/strike.sheep，分别。在假设 (c) 下，FS 中预测的遗传增益与假设 (b) 下的非常相似。当所研究的 FS 最大增益的 70% 为目标时，当 FS 具有中等遗传力时，预计整体指数（不包括 FS）中 70% 至 93% 的遗传增益将被保留，而当 FS 为 85% 至 95%遗传力低。结论和影响 FS 的经济价值范围很广（从 –$60 到 –$80/罢工。MP+FS 和 DP+FS 指数的绵羊每年，当 FS 的遗传力适中时，FP+FS 指数每年 –$60 到 –$140/strike.sheep 和 –$100 到 –$200/strike.sheep 每年MP+FS 指数和每年 –$140 至 –$240/strike.sheep 的 FP+FS 和 DP+FS 指数（当 FS 的遗传力较低时），可用于育种计划以遗传减少 FS，同时保持竞争力其他重要性状的遗传增益水平。经过 10-20 年的指数选择后，FS 的减少可以达到较低水平，类似于通过骡子达到的水平（平均季节性条件下每年 0.01/strike.sheep）。当 FS 的遗传力较低时，FP+FS 和 DP+FS 指数的绵羊每年），这可用于育种计划以减少 FS，同时保持其他重要性状的遗传增益的竞争水平。经过 10-20 年的指数选择后，FS 的减少可以达到较低水平，类似于通过骡子达到的水平（平均季节性条件下每年 0.01/strike.sheep）。当 FS 的遗传力较低时，FP+FS 和 DP+FS 指数的绵羊每年），这可用于育种计划以减少 FS，同时保持其他重要性状的遗传增益的竞争水平。经过 10-20 年的指数选择后，FS 的减少可以达到较低水平，类似于通过骡子达到的水平（平均季节性条件下每年 0.01/strike.sheep）。