Selection for lean tissue growth in Dorset Down sheep : responses and genetic parameters : a dissertation submitted in partial fulfilment of the requirements for the degree of Bachelor of Agricultural Science (Honours) in Animal Science at Lincoln University

Performance data for Lincoln University Dorset Down sheep flock (n=1153) were available for the first five years (1985-1989) of a selection programme to improve lean tissue growth. Ewes and rams born in spring each year were run separately at pasture from late summer onwards. Animals were performance tested in autumn and winter. Measurements were made of liveweight, fat depth. and from 1988 onwards of muscle depth at each time. Prior to 1986, only rams were selected from the base population. From the 1986 mating season (autumn) two lines were established from the base population and maintained separately thereafter. One line was a randomly selected control (C) line, and the second line was selected for lean tissue growth (LTG) on the basis of an index combining information for liveweight and fat depth measured in the winter. The aggregate breeding value was lean weight and fat weight. This index was used up to 1988 after which a new index was used into which muscle depth was incorporated. The aggregate breeding value remained the same. The data indicated that there has been positive responses to selection in component traits of the index, namely liveweight (an increase) and fat depth (a decrease). However, correlated responses in muscle depth have been undesirable (a possible decrease). Inclusion of muscle depth into the index currently used is expected to result in a muscle depth increase since indices containing muscle depth have been shown to result in increase in muscle depth. The data also indicated moderate heritabilities for most component traits. Heritabilities, genetic and phenotypic correlations show variation between sexes and seasons