Genetic parameters for nematode resistance in periparturient ewes and post-weaning lambs in Uruguayan Merino sheep Virginia Goldberg n , Gabriel Ciappesoni, Ignacio Aguilar Instituto Nacional de Investigacio ´n Agropecuaria, INIA Las Brujas, Uruguay article info Article history: Received 14 December 2011 Received in revised form 2 May 2012 Accepted 3 May 2012 Keywords: FEC Gastrointestinal parasites Genetic correlation Genetic progress Heritability Periparturient rise abstract Gastrointestinal (GI) parasites are one of the main sanitary and economic constraints for sheep production worldwide. In Uruguay, resistance to GI parasites has been included in genetic evaluations of Merino and Corriedale breeds since 1994, using Faecal worm Egg Count measured in post-weaning lambs (lambFEC) as a selection criterion. Although adult categories are more resistant to GI parasites, a temporary loss of acquired immunity is present in the periparturient period ( ¼periparturient rise). The purpose of the present study is to estimate the genetic parameters of resistance to GI parasites in periparturient ewes and post-weaning Merino lambs. A total 2110 faecal samples of 748 periparturient ewes (eweFEC), the progeny of 107 sires, were collected in 2009 and 2010. 9458 lambFEC records from 7506 lambs born between 2001 and 2009 were analysed as well. (Co)variance components and systematic effects were estimated using a multi-trait animal model, with a Bayesian analysis using the Gibbs sampler algorithm. Direct and correlated responses (DR and CR) of eweFEC to selection using eweFEC and lambFEC respectively, were also estimated. Posterior medians (posterior standard deviation) for heritability (h 2 ) and repeatability were 0.25 (0.03) and 0.34 (0.02) for lambFEC, and 0.08 (0.03) and 0.18 (0.03) for eweFEC, respectively. Posterior median for genetic correlation between both traits was 0.81 (0.11). CR obtained by selecting for lambFEC was two times more efficient than DR from selection by eweFEC. In conclusion, eweFEC has a lower h 2 than lambFEC while their genetic correlation is moderate to high. Therefore, indirect selection by using lambFEC will be more effective than direct selection on eweFEC, and will produce ewes that eliminate less worm eggs, resulting in less pasture contamination. & 2012 Elsevier B.V. All rights reserved. 1. Introduction Gastrointestinal (GI) parasites are one of the main sanitary and economic constraints for sheep production in Uruguay and worldwide (Castells et al., 1995; Nieto et al., 2002; Perry and Randolph, 1999). Anthelmintic drugs have been used as the main control method for GI parasites, but its incorrect and continuous application, has led to problematic anthelmintic resistance (Waller, 1997). There- fore, alternative control strategies are considered, such as selection for increased genetic resistance to GI parasites. Animals with lower parasite charge, contribute to reduced pasture contamination and less need for the use of chemical drugs, thus delaying the development of anthelmintic resistance and reducing treatment costs (Bisset et al., 1996; Bishop and Stear, 2003; Nieto et al., 2002). In Uruguay, resistance to GI parasites has been included in the genetic evaluations of Merino and Corrie- dale breeds since 1994. Faecal worm Egg Count (FEC) recorded in post-weaning lambs is used as a selection Contents lists available at SciVerse ScienceDirect journal homepage: www.elsevier.com/locate/livsci Livestock Science 1871-1413/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.livsci.2012.05.003 n Corresponding author. Tel.: þ598 23677641; fax: þ598 23677609. E-mail address: goldbergvirginia@gmail.com (V. Goldberg). Livestock Science 147 (2012) 181–187