GENETIC TRENDS OVER 20 YEARS OF SELECTION IN THE 3 SYNTHETIC ARCOTTS,SUFFOLK AND FINNISH-LANDRACE SHEEP BREEDS .1. EARLY GROWTH TRAITS

Mixed model methodology based on a multi-trait animal model was used t o estimate genetic trends for lamb weights in three synthetic Arcott ( Canadian, Outaouais and Rideau) breeds and two random bred control (Su ffolk and Finnish Landrace) breeds. Selection in the Canadian Arcott w as primarily on lamb weight at 91 d with a lesser emphasis on litter s ize. Outaouais and Rideau Arcotts were selected primarily for litter s ize with a lesser emphasis on lamb weight at 91 d. All lambs were hous ed indoors year-round in a controlled environment under conditions of artificial rearing and 8-mo breeding cycles. Genetic improvement over 20 yr based on predicted breeding values for lamb weights at birth, 21 , 70 and 91 d of age were 0.3, 0.9, 2.1 and 2.8 kg, respectively in th e Canadian, 0.2, 0.6, 1.2 and 1.5 kg in the Outaouais, and 0.3, 0.5, 0 .9 and 1.4 kg in the Rideau breeds. Corresponding values for the unsel ected Suffolk were 0.3, 0.4, 0.4 and 0.5 kg, respectively, and for the Finnsheep were 0.2, 0.2, 0.3 and 0.5 kg. Alternative methods of estim ating genetic trends using control populations were based on deviation of the predicted mean year-season phenotype in the selected breeds fr om that in the corresponding random bred control breeds (Suffolk for C anadian, and Finnsheep for Outaouais and Rideau) and also as a deviati on from the year-season estimate in the corresponding random bred cont rol breeds. Genetic trends estimated from the alternative methods, whi ch took into account the contribution of drift, were larger than those based on predicted breeding values. These values were reflected in th e realized heritabilities estimated from regression of cumulative sele ction differential on the genetic trends. In the Arcott breeds, annual genetic trend for lamb weights varied according to breed and the esti mation procedure which amounted to 0.25-0.81% of the mean. It is evide nt selection offers one of the most practical means for rapid and perm anent improvement of early lamb growth in multi-breed synthetic popula tions under conditions of artificial rearing, 8-mo breeding cycle and a controlled environment.