Selection and Breeding Programs
K. J. Drewry
Animal Sciences Department
1ntroduction
Genetic improvement, although slow compared to the possible environmental improvements (i.e. nutrition and management), is cumulative and permanent. The number one goal of the sheep industry must be to increase the genetic potential of sheep to produce. The genetic potential sets the upper production limit which can be obtained by improvements in environmental factors.
Methods of genetic improvement-selection and mating systems
The genetic potential of sheep can be changed by two ways:- (I) selection and (2) mating systems. Although mating system is independent of selection we must consider mating systems at the same time we consider selection. Mating system refers to the relationship between animals that are mated. Animals may be mated that are (I) alike on basis of pedigree (inbreeding or linebreeding) or
(2)	unaIike on basis of pedigree (out breeding or crossbreeding). Animals may be mated because they are: (I) alike in terms of records or (2) unlike in terms of records.
Selection is differential reproduction. For selection to be effective genetically superior animals must be chosen as parents. Genetically superior animals are those who transmit to their offspring the kind of heredity that enables the offspring to be superior in the existing environment. The transmitting ability or breeding value can only be inferred from the record(s) of the individual or its relatives.
Rate of genetic improvement from selection
The rate of genetic improvement from selection depends upon three factors; (I) accuracy, (2) intensity, and (3) variability. Accuracy in estimating the breeding value of the individual from its record is determined by the amount of the total variation that is due to genetics or heredity. When heritability of a trait is high, then more emphasis should be placed on the record of the individual. When heritability of a trait is low, emphasis should be placed on the record of the individual, its relatives and progeny if these are available.
Intensity is determined by the proportion of the animals chosen to be parents of the next generation. Intensity with a given trait, is decreased as the number of traits under selection increases. With sheep approximately 40 to 50 per cent of the ewe lambs weaned are retained for replacements compared to 2 to 5 per cent of the ram lambs.
The amount of variability observed for traits under selection determines the directional change from selection.
Traits to be emphasized
The traits of most concern in both commercial and purebred sheep production must be those of most economic importance. An industry that is struggling for its very existence cannot justify "fads and fancies" and "must devote its efforts to improving efficiency of meat and wool production." The traits which should be of concern to the U.S. sheep producers are:
Cooperative Extension Service, PURDUE UNIVERSITY, Lafayette, Indiana