Selection for Breeding for Improvement of Livestock Performance

The purpose of animal breeding is not to genetically improve individual animals (livestock performance) once an individual is conceived; it is a bit late for that-but to improve animal populations, to improve future generations of animals.

To this task breeders bring two basic tools: selection and mating. Both involve decision making. In selection, we decide which individuals become parents, how many offspring they may produce, and how long they remain in the breeding population.

In mating, we decide which of the males we have selected will be bred to which of the females we have selected.

Measuring Livestock Performance

In order to select animals, we must first measure performance (phenotype) on eligible candidates for selection. Systematic measurement of performance in a population is called performancetesting. Performance testing programs vary among species and breeders within species.

A progressive beef cattle breeder’s program, for example, might include the recording of birth date, calf birth weight and calving ease score at calving time; weaning date, calf weaning weight, cow weight, and cow pregnancy status at weaning time; feed intake from weaning to yearling age; and weigh date, yearling weight, hip height, pelvic dimensions, back fat thickness or ultrasound measurements, scrotal circumference, and breeding soundness score at yearling time.

Performance Testing: Systematic measurement of performance (phenotype) in a population.

Performance testing programs are widespread in traditional livestock species (beef and dairy cattle, swine, poultry, and sheep) in developed countries.

Seed stock producers commonly take part in such programs, reporting the data they record to breed associations or government agencies.

Commercial producers may do performance testing also. However, because of the labor and expense involved in recording performance data, commercial testing programs are typically less elaborate than seed stock programs.

Selection Using Information on Relatives

Most animal breeders are unlikely to limit themselves to individual performance information alone in making selection decisions. They will use information on relatives as well.

For example, when a dog breeder purchases an eight-week-old puppy from another breeder, she probably does not base her choice on just the conformation and personality characteristics evident in such a young puppy.

She wants to evaluate those same traits in the littermates, the dam, and the sire. She might want to see a copy of the puppy’s extended pedigree to learn more about its ancestors.

Similarly, when beef cattle breeders evaluate a sire to use via artificial insemination (A.I.), they look further than the sire’s own performance for growth rate. They want to know something about the growth performance of his progeny.

Dam: A Female Parent.

Sire: A Male Parent.

The above examples illustrate the use of two different types of information (data) on relatives: pedigree data and progeny data. By examining the young puppy’s parents, littermates, and extended pedigree, the dog breeder is using pedigree data.

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She is trying to learn something about the genes made available to the puppy through its parents. Beef cattle breeders, on the other hand, are using progeny data. They are trying to learn something about an A.I. sire’s genes by evaluating the performance of his offspring.

Pedigree Data: Information on the genotype or performance of ancestors and (or) collateral relatives of an individual.

As the above examples should make clear, the information used to make selection decisions can be subjective, objective, or something in between. The pedigree data used by the dog breeder are, for the most part, subjective.

The puppy’s Papers may include some semi objective information on show championships won by ancestors, but the breeder’s observations on conformation and personality are essentially subjective in nature.

In contrast, the progeny data used by beef cattle breeders are relatively objective. They consist of carefully measured (we hope) weights of animals taken at specific ages.

Table1.1: Commonly Measured Traits

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