SELECTION FOR REPRODUCTIVE RATE IN RAMBOUILLET SHEEP - ESTIMATED GENETIC CHANGE IN REPRODUCTIVE RATE

Rambouillet sheep were selected for high or low reproductive rate base d on an index of reproductive rate, reproductive index = dam's total l ifetime lambs born/(age in years-1). Selected ewes were first born in 1969. A random bred control line was established from the remaining fo undation ewes with the first ewes born in 1973. Genetic variances were estimated with REML procedures for reproductive index and number of l ambs born or weaned either per ewe exposed for breeding or per ewe lam bing. Heritability of reproductive index was 0.26. Heritability for nu mber of lambs born were 0.11 and 0.12 and for number of lambs weaned w ere 0.04 and 0.04 per ewe exposed for breeding and per ewe lambing, re spectively. Breeding values for the reproductive index were estimated using the full animal model (BLUP) with the complete numerator relatio nship matrix on reproductive index, with fixed effect of year of birth of ewe and breeding values for number of lambs born or weaned either per ewe exposed for breeding or per ewe lambing, estimated with the fi xed effects of year of birth of ewe and age of ewe and the random effe ct of permanent environmental effect among repeated lambing records. G enetic change was estimated as the regression of breeding value for re productive index and number of lambs born or weaned either per ewe exp osed for breeding or per ewe lambing on year of birth of the ewe. High and low lines both responded to selection for reproductive index as c ompared to the control line (P<0.01). The high line increased at a rat e of b=0.0134+/-0.0006 reproductive index units and the low line decre ased at a rate of b=-0.0098+/-0.0005 reproductive index units per year . Response to selection for reproductive index resulted in b=-0.0074+/ -0.0007 and b=0.0163+/-0.0006 lambs per ewe exposed for breeding, and b=-0.0041+/-0.0002 and b=0.0075+/-0.0002 lambs weaned per ewe exposed for breeding in the low and high lines, respectively. There was a grea ter response to selection in the high line compared to the low line. T hese results indicated that the reproductive index did respond to sele ction.