Sources of bias, genetic trend and changes in genetic correlation in carcass and ultrasound traits in the icelandic sheep population

Genetic parameters for carcass conformation, carcass fat, ultrasound eye muscle depth and ultrasound fat depth over eye muscle were estimated with data from Icelandic farms during three periods, 2001-2003, 2006-2008 and 2011-2013. Heritability ranged from 0.30 to 0.42. Genetic correlation between carcass conformation and carcass fat was 0.41, 0.29 and 0.26 in 2001-2003, 2006-2008 and 2011-2013, respectively. Breeding values based on carcass scoring records of 5,796,474 lambs in 2000-2013 were estimated with a bivariate model and compared to the results of a multitrait model, including 715,771 ultrasound records. The genetic merit of rams for carcass conformation was underestimated in the bivariate analysis in the cases where many offspring were kept for replacement. Applying the multitrait model reduced this bias and gave more accurate results for both traits. The genetic trend was -0.05 and 0.08 genetic standard deviations per year for carcass fat and carcass conformation, respectively, in the period 2000-2013.