Gastrointestinal nematodes (GIN) constitute a problem in many sheep production systems, including those in Uruguay, causing reduced productivity and increased expenses. The main strategy to control GIN has consisted of the use of anthelmintics. However, GINs have developed resistance to anthelmintics, reducing their effectiveness. Genetic resistance to GINs has been found in flocks of different breeds. To date, there have been no reports about GINs breaking down genetic resistance in sheep. Heritability estimates of resistance to GIN within breeds are generally moderate, so that achieving genetic gain within a flock is possible. In this study, we predicted genetic gain in worm egg count (WEC), an indirect (and generally preferred) criterion of resistance to GIN, following different strategies. A multi-trait breeding objective including wool and meat traits was assumed and genetic gain over 10 years of selection in a Merino flock was estimated. We used a desired gains approach, examining situations in which the economic contribution of genetic gain in resistance to GIN in percentage terms was 0, 25, 50, 75 or 100. Except when the level of infestation with GIN was low, a considerable amount of emphasis had to be placed on selection for low WEC in order to reach the threshold below which the administration of anthelmintics is not required. High emphasis on reducing WEC lead to a reduction in genetic gain in wool and meat traits, or to their deterioration in the extreme case of 100 per cent emphasis on WEC. Given this finding, coupled with the difficulties encountered in accurately recording and selecting for WEC, we concluded that in addition to embarking upon a programme of within flock selection, sheep breeders interested in improving genetic resistance to GIN should also consider using breeding stock identified as superior for both resistance and production traits in across flock genetic evaluations.

中文翻译：

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在乌拉圭美利奴绵羊的多性状育种目标中预测胃肠道线虫抗性遗传增益的期望增益方法

胃肠道线虫 (GIN) 是包括乌拉圭在内的许多绵羊生产系统中的一个问题，导致生产力降低和费用增加。控制 GIN 的主要策略包括使用驱虫药。然而，GIN 已对驱虫剂产生抗药性，从而降低了其有效性。在不同品种的鸡群中发现了对 GIN 的遗传抗性。迄今为止，还没有关于 GIN 破坏绵羊遗传抗性的报道。品种内对 GIN 抗性的遗传力估计通常是中等的，因此在一个鸡群内实现遗传增益是可能的。在这项研究中，我们根据不同的策略预测了蠕虫卵计数 (WEC) 的遗传增益，这是对 GIN 抗性的间接（通常是首选）标准。假设了一个多性状育种目标，包括羊毛和肉类性状，并估计了美利奴羊群 10 年选择的遗传增益。我们使用了一种期望增益方法，检查了遗传增益对 GIN 抗性的经济贡献百分比为 0、25、50、75 或 100 的情况。必须强调选择低 WEC 以达到不需要施用驱虫药的阈值。高度重视减少 WEC 会导致羊毛和肉类性状的遗传增益减少，或者在 100% 强调 WEC 的极端情况下导致其恶化。鉴于这一发现，再加上在准确记录和选择 WEC 方面遇到的困难，