Rasali, DR; Shrestha, JNB; Crow, GH
In the 1850s, breeders established breed societies which maintained flock registries and actively participated in show rings, exhibitions and auctions promoting pure breeds that conformed to the breeders' vision of uniform and distinct morphological characteristics. At that time, the performance of the pure breeds over grade sheep was overwhelming. As a result influential government officials, specialists and scientists had to acknowledge their superiority. The supply of purebred seed-stock that was initially adequate became sparse in the years following the World War 11 because demand arising from the increasing human population exceeded the capacity for production. Thus, stimulated interest in higher productivity for commercial production resulted in the development of multi-breed synthetic populations for specific objectives e.g. reproduction, meat quality, meat and wool combinations, wool for textile, carpet wool, fur, and milk production. Today, as many as 418 sheep breeds that have been documented in nearly 75 countries are combinations of two or more distinct breeds, populations and landraces that have records on pedigree and morphological characteristics, and in some cases, production performance similar to those for long established breeds. The present day methods of forming composite breeds are based on a wealth of knowledge and creative skills passed on from breeder to breeder over the centuries, as well as the application of quantitative genetic principles that had demonstrated tremendous success in the development of hybrid corn. Research findings also suggested that in well-planned efforts to create composite breeds, the genetic background of the foundation breeds should be as broad as possible to achieve increased additive genetic variance and heterozygosity from a large proportion of segregating genes and lower probability for the expression of lethal recessive genes. There was always the possibility of lower performance in the composite breeds compared with specific crosses derived from two or more breeds, because of their inability to fully realise any benefit from individual and maternal heterosis (considered important in the mammalian species). Though unlikely, loss in performance of composite breeds could occur from recombination of non-allelic genes and the migration of undesirable genes. Nevertheless, there is opportunity for accelerating genetic response to selection as well as heterosis retention unless a rise in inbreeding occurs in the early generations causing loss in performance. The real operational advantage of a multi-breed composite population lies in raising a single breed with nearly the same productivity instead of two or more breeds for subsequent crossbreeding. Sheep in the domain of developing countries adapt well to severe climatic conditions and arduous terrains that are often not suitable for cultivation or for raising the larger livestock and are fully exploited for their ability to produce a variety of commodities without disrupting the cultural harmony. Likewise, more breeds were developed in sheep than in other livestock species with the United Kingdom, Russian Federation, Australia, Poland, China, France, the United States of America and New Zealand each having established 15 or more composite breeds that offer real advantages. In North America and worldwide, there is further prospect for complementing established breeds with exotic breeds to achieve potential biological ceiling in reproduction, lean muscle growth, feed efficiency and milk yield of sheep. In the future, application of advnces in molecular methodologies could possibly lead to the development of composite breeds for specific objectives from a combination of widely different breeds, populations and landraces, probably, even from unrelated species.