Response to selection for ultrafine Merino sheep in New Zealand I. Wool production and wool characteristics of ultrafine fibre diameter selected and control Merino yearlings

Merino ewes were screened for low fibre diameter from 15 000 yearlings with in 12 flocks to establish a foundation flock in 1987, which was expanded to 500 ewes over the subsequent two years. Ewes were selected on fibre diamet er BLUP breeding values. For comparison, a control flock of 100 ewes was ra ndomly selected from the same sources. Data from approximately 2000 progeny over five birth years within 70 sire groups were analysed. Traits measured were live weight, greasy and clean fleece weight, yield, mean fibre diamet er (FD), staple length, staple strength, staple crimp frequency, resistance to compression, Commission International de l'Eclairange (CIE) tristimulus value for scoured wool and coefficient of variation in fibre diameter. The ultrafine progeny had 2-mu m finer FD, 0.6 units less yellow and had 0.6 m ore crimps/cm (all P <0.001) than their control counterparts. This experime nt suggests that formation of an open nucleus breeding population by intens ive screening for fibre diameter from a commercial population will improve selection response relative to traditional within flock selection without s creening or to screening rams only. (C) 1999 Elsevier Science B.V. All righ ts reserved.