Genetic evaluation of susceptibility to gastrointestinal nematodes in sheep using sub-indices based on worm egg count records taken at different ages

Context Gastrointestinal nematodes (GIN) constitute a problem in many sheep production systems. Susceptibility to GIN (SGIN) is most commonly assessed by recording worm egg count in the faeces (WEC). The heritability for WEC records taken at different ages may vary, but is generally moderate. The genetic correlation among them is generally high, but seldom equal to 1.0, hence records taken at different ages should be treated as different traits. Aims This study aimed to model SGIN as a non-directly measurable trait genetically related to WEC at different ages, and to develop a sub-breeding objective using WEC at several ages as selection criteria. Methods We defined a sub-breeding objective with SGIN as the sole trait and WEC records at different life stages (weaning, post-weaning, yearling, hogget and adult) as selection criteria. Based on a literature review, we chose admissible phenotypic and genetic parameters. We then used selection index theory to calculate the accuracy of sub-indices with varying amount information (combinations of WEC records taken at different ages), and two genetic correlation values between the selection criteria (WEC at different ages) and SGIN. Key results A selection index combining yearling and hogget WEC records resulted in near maximum accuracy. As expected, accuracy increased when we assumed a greater genetic correlation value between WEC records and SGIN. The model effectively integrated information from multiple ages, thus improving the accuracy of estimated breeding values for SGIN. Conclusions Considering SGIN as a genetically correlated trait to WEC at different ages allowed the design of efficient selection sub-indices. Combining yearling and hogget WEC records yielded a highly informative and practical sub-index. Implications The proposed model has several advantages for sheep breeding programs: (i) it combines information from different WEC records in a single breeding value for SGIN, (ii) it accommodates records taken at different ages and overcomes the problem faced by producers when the WEC value at a particular ages does not reach the minimum threshold, and (iii) it can be easily integrated to current genetic evaluation services, and the information can be combined with that from other traits in the breeding objective.