Genetic change in South African Merino resource flocks

Genetic trends over 17-36 years for two-tooth live weight (LW), clean fleece weight (CFW) and fibre diameter (FD) are reported for three South African Merino resource flocks where these traits were included in the selection objective. The flocks were the Grootfontein Merino stud (GMS), Cradock Fine Wool stud (CMS) and the Tygerhoek Merino flock (TMF). Reproduction, namely number of lambs weaned per ewe mated (NLW) and total weight of lamb weaned per ewe mated (TWW) was reported for the Elsenburg Merino flock (EMF), which was divergently selected for NLW. Selection for overall excellence resulted in moderate positive changes in LW, CFW and FD in the GMF prior to 1986. Responding to a change in selection objective from 1986 to 1999, LW increased by 0.86% of the overall phenotypic mean p.a., while FD decreased at 0.33% p.a. Selection pressure in the CMS was initially to increase LW and CFW, resulting in responses of respectively 1.0 and 1.2% p.a. prior to 1996. From 1996, selection pressure was directed at LW and FD, resulting in respective responses of 0.35 and -0.67% p.a. Selection for CFW with a check on FD since 1969 resulted in a direct response of 0.6% p.a. in the TMF. Divergent selection for NLW resulted in direct responses of 1.5% p.a. in the upward direction (H line) and -0.8% in the downward direction (L line). Corresponding response in TWW were 1.9 (H line) and -1.0% (L line) p.a. Marked genetic progress, consistent with the selection objectives strived for, was thus attainable in all traits that were considered.