Composite trait selection to improve reproduction and ewe productivity: a review

Abstract. Reproduction andeweproductivity are complex composite traits that are influencedby several component traits. Genetic improvement by selection for an individual component trait may not always be advantageous because adverse or neutral genetic relationships can exist among the component traits. Selection for an overall composite trait of ewe productivity, defined as litter weight weaned per ewe joined, can result in a balanced biological composite trait with favourable responses in component traits including fertility, number of lambsborn, lambsurvival, lactation and lambgrowth. Selection for litter weight weaned may also overcome adverse genetic relationships among reproductive component traits. This review examines the genetic responses of selecting directly for litter weightweaned, comparedwith selection responses for one ormore of its component traits. It is concluded that selection to improve reproductive efficiency and eweproductivity, under most production and environmental systems, would benefit from selection for a composite trait such as litter weight weaned, rather than for a single component trait