Is increased fleece weight associated with reduced reproduction rate? Results from long term multi-trait selection flocks

Medium-Peppin Merino sheep were selected for increased fleece weight with independent culling for various fleece quality traits over the period 1947 to 1966 at the CSIRO, National Field Station, Cunnamulla, Qld. There were two lines selected in different ways for increased fleece weight and a random bred control flock was initiated in 1950. Data from these flocks have been analysed to estimate genetic responses in fleece weight and in reproduction rate. For animals born over the period 1950 to 1964: (i) fleece weight increased in both selection lines with an average response in 18 month clean fleece weight of around 2 % per year (ii) there was no change in fleece weight in the control line (iii) there was no change in reproduction rate (lambs weaned/ewe joined for all lambing opportunities between 3 and 7 years of age) in any of the lines. These results demonstrate that long term breeding programs for Merino sheep, which include increased fleece weight as a component of a multi-trait breeding objective, can be implemented without necessarily reducing reproduction rate.