Ch_14

Mating Systems Overview

  • Mating systems hinge on the genetic relationships of animals involved in breeding.

  • Key terms include:

    • Inbreeding: mating animals more closely related than the breed average.

    • Outbreeding: mating animals less closely related than the breed average.

Chapter 14: Mating Systems

Breeding Practices

  • Replacement Heifers:

    • Important for maintaining herd genetics and productivity.

  • Bull Breeding Rotation:

    • Rotate bull breeds every 4 years: Breed A, Breed B, Breed C.

  • Pasture Management:

    • Manage pastures based on breeding and market demands for steers and non-replacement heifers.

Learning Objectives

  • Describe roles of animal breeders.

  • Evaluate advantages/disadvantages of different breeding practices:

    • Inbreeding

    • Linebreeding

    • Outcrossing

    • Crossbreeding

  • Understand concepts of heritability and heterosis.

  • Discuss composite and hybrid breed formation.

Animal Breeder Responsibilities

  • Critical Decisions:

    1. Choosing parent individuals.

    2. Determining reproduction rates.

    3. Selecting optimal mating systems for beneficial outcomes.

Inbreeding vs Outbreeding

  • Inbreeding:

    • Mating closely related animals increases homozygosity of gene pairs.

  • Outbreeding:

    • Mating not closely related animals leads to increased genetic variability.

Forms of Inbreeding

  • Intensive Inbreeding:

    • Involves closely related animals with prolonged inbreeding.

  • Linebreeding:

    • Mild inbreeding maintaining high genetic relationship to an ancestor, while keeping inbreeding low (below 20%).

Observations on Intensive Inbreeding

  • Increased inbreeding generally:

    • Detrimental to reproductive performance and growth.

    • Promotes susceptibility to environmental stress.

    • Slows genetic improvement (no heterosis effect).

    • Aids in identifying desirable and undesirable genes effectively.

    • Leads to uniform and superior offspring in successful lines.

    • Utilized more by seedstock producers; avoided in commercial operations.

Pedigree Examples

  • Figure 14.3:

    • Bracket and arrow pedigree for full brother-sister mating, resulting in animal A.

  • Figure 14.4:

    • Bracket and arrow pedigree for sire-daughter mating, resulting in animal X.

Linebreeding Details

  • Characteristics:

    • Low-risk inbreeding method.

    • Common among seedstock producers without access to superior sires.

    • Inbreeding thresholds: low (<20%), high (>50%).

Outbreeding Types

  • Four Types:

    1. Species Cross: Different species mating.

    2. Crossbreeding: Different established breeds mating.

    3. Outcrossing: Unrelated animals within the same breed mating.

    4. Grading Up: Using purebred sires on commercial-grade females.

Crossbreeding Advantages

  • Goals:

    1. Breed Complementation: Combining strengths/weaknesses of different breeds.

    2. Heterosis: Increased productivity in crossbred offspring, improving survival, longevity, and reproduction.

Heritability and Heterosis Insights

  • Traits Analysis:

    • Reproductive traits: Low heritability, high heterosis.

    • Growth traits: Medium heritability and heterosis.

    • Carcass traits: High heritability but lower heterosis.

  • Crossbreeding is prominent in swine, beef, and sheep, less so in dairy.

Outcrossing Strategy

  • Aims to maximize heterozygosity, masking harmful recessive alleles to limit disadvantages of inbreeding.

Grading Up Explained

  • Employing purebred sires continuously will improve commercial herds, generating higher percentages of desired breeds over generations (e.g., 3/4, 7/8).

New Beef Cattle Breeds

  • Composite breeds:

    • Examples: Brangus, Beefmaster, Santa Gertrudis, MARC1.

  • Hybrid boars in swine from crossing various breeds then inbreeding for consistency.

Summary

  • Mating systems defined by the genetic relationship of breeding animals.

  • Inbreeding leads to greater homozygosity, and outbreeding boosts heterozygosity.

  • Linebreeding maintains genetic ties to outstanding ancestors, and outcrossing involves mating unrelated individuals within the same breed.

  • Crossbreeding enhances productivity through heterosis.