ABSTRACT.- Gastrointestinal parasites (GIP) are one of the main causes of economic losses for sheep farmers worldwide. The need for alternative control measures comes from increasingly critical anthelmintic resistance. One alternative is to include genetic resistance to GIP in breeding programmes, by selecting for worm faecal egg count (FEC). Using this selection criterion since 1994, Uruguay has included genetic resistance to GIP in the genetic evaluation of Australian Merino and Corriedale breeds. Although FEC has been the most used selection criterion to evaluate resistant animals, data recording is time-consuming and costly and requires a nematode infection challenge. Selecting parasite resistance without the need for nematode challenge would be a less expensive alternative approach without compromising the wellbeing of the animals. Moreover, other indicator traits such as packed cell volume (PCV), FAMACHA© score, body condition score (BCS) could be included to increase genetic improvement. This paper describes the current selection programmes for GIP-resistant sheep, data recording, new criteria evaluation, selection nuclei, development and use of molecular tools, projects, as well as further approaches to enhance and improve genetic progress in Uruguay. Current databases enabled various estimations and demonstrated that genetic progress can be achieved. We can highlight the following results: (i) FEC heritability values ranging from 0.15 to 0.21; (ii) high genetic correlation between FEC in ewes at spring rise and FEC in lambs at post-weaning (0.81 ± 0.11); (iii) genetic selection by FEC is effective in different environments (low or high worm environments) and the genetic correlation between environments is high (0.87 ± 0.04); (iv) there is a moderate favorable genetic correlation between FEC and FAMACHA©; (v) the Corriedale susceptible line had up to 3.3 times higher average of FEC than resistant line; (vi) INIA Corriedales showed better genetic merit for twinning rate, greasy fleece weight, fibre diameter, and body weight at shearing in comparison with the resistant line of SUL; and (vii) in Australian Merino, it has been possible to generate heavier progeny producing more and finer wool, and also more resistant to GIP. Moreover, generating reference populations for molecular studies and selection nuclei is also very important. All strategies described in this study aim at improving the genetic resistance of sheep to GIP.
Instituto Nacional de Investigación Agropecuaria