Reproductive consequences of genetic improvement in clean fleece production in ultrafine Merino ewes : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Animal Science, Massey University, Palmerston North, New Zealand

Abstract

To date, genetic improvement of Uruguayan Merino sheep has focused on selection for reduced fibre diameter (FD) and increased clean fleece weight (CFW) and live weight (LW). The effects of selection for these traits on ewe productive and reproductive performance have not been studied in Uruguay. The aim of this thesis was to evaluate the impact of a long-term selection program applied in an Uruguayan Merino flock between 1999 and 2018. Phenotypic responses of yearling lambs and adult ewes across 20 years of selection demonstrated that selection strategies applied in the flock resulted in heavier animals that produced heavier and finer fleeces. These phenotypic responses were accompanied by a decrease in body condition score (BCS) at mating, with a small positive change in ewe reproductive performance. Strong positive genetic correlations were found between yearling and adult expressions of FD, CFW, and LW. The genetic relationships between FD and reproduction traits were negligible. There were generally favorable genetic correlations between LW and reproduction traits. The genetic relationships between adult CFW and ewe reproductive performance were unfavorable. Genetic trends indicated that the breeding strategies applied during the study period were effective in improving genetic merit in the traits under selection (FD, CFW, and LW) and caused only marginal genetic changes in BCS at mating and ewe reproductive performance. A total of 13, 22, 42, 22, 24 and 29 genomic regions were significantly associated with FD, CFW, LW at mating, BCS at mating, pregnancy rate, and lambing potential, respectively. The results obtained in this thesis will be used to refine the breeding programmes for Merino sheep in Uruguay in the future.