Genetic trends over 20 years of selection in the three synthetic Arcotts, Suffolk and Finnish Landrace sheep breeds. 1. Early growth traits

Abstract

Mixed model methodology based on a multi-trait animal model was used to estimate genetic trends for lamb weights in three synthetic Arcott (Canadian Outaouais and Rideau) breeds and two random bred control (Suffolk and Finnish Landrace) breeds. Selection in the Canadian Arcott was primarily on lamb weight at 91 d with a lesser emphasis on litter size. Outaouais and Rideau Arcotts were selected primarily for litter size with a lesser emphasis on lamb weight at 91 d. All lambs were housed 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 the 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 unselected 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 estimating genetic trends using control populations were based on deviation of the predicted mean year-season phenotype in the selected breeds from that in the corresponding random bred control breeds (Suffolk for Canadian, and Finnsheep for Outaouais and Rideau) and also as a deviation from the year-season estimate in the corresponding random bred control breeds. Genetic trends estimated from the alternative methods, which took into account the contribution of drift, were larger than those based on predicted breeding values. These values were reflected in the realized heritabilities estimated from regression of cumulative selection differential on the genetic trends. In the Arcott breeds, annual genetic trend for lamb weights varied according to breed and the estimation procedure which amounted to 0.25–0.81% of the mean. It is evident selection offers one of the most practical means for rapid and permanent improvement of early lamb growth in multi-breed synthetic populations under conditions of artificial rearing, 8-mo breeding cycle and a controlled environment. Key words: Sheep breeds, genetic trends, multi-trait animal model and growth traits