Animal Science Unit 2b: Animal Breeding

							  Animal
Reproduction

Animal Science
Definitions

 Purebred (straight breeding): the mating of
  animals from the same breed.

 Crossbreeding: the mating of animals from
  two or more different breeds.
               Definitions
• Homozygous-two parental genes calling
  for a specific characteristic that are
  identical

• Heterozygous-two parental genes calling
  for a specific characteristic that are not
  identical. (The dominant gene will override
  the effect of the recessive gene.)
                Purebred
Advantages:
 - Ancestry is known back to the start of the
 breed.

 - genetically homozygous

 - The foundation stock of crossbreeding.
               Purebred
Disadvantages:
  - Undesirable traits are more concentrated
  because of the homozygous genes.

 - Very expensive to start.

 - Must market to get full value of animal.
               Crossbred
Advantages of Crossbreeding
 - Used to improve genetics.
 - Dominant genes mask recessive genes.
 - Hybrid vigor, or heterosis, occurs from the
 combination of genes.
 - Cheaper to get started.
             Crossbreed
Disadvantages of Crossbreeding

 - Often lack superior genes of purebreds.

 - Must settle for market value at sale barn.
      BREEDING SCHEMES
Inbreeding: Mating of related animals
  - close breeding: animals being mated are
  very closely related and can be traced
  back to more than 1common ancestor.

 i.e. Father and daughter
 - most intensive form
 - high presence of undesirable traits
      BREEDNG SCHEMES
Line breeding: mating of animals that are
  more distantly related and can be traced
  back to one common ancestor.

 i.e. Cousin, grandparent to grand
 offspring, half-brother to half-sister.
         BREEDING SCHEMS
Out crossing: the mating of animals of
 different families within the same breed.
• Animals are not closely related
• Purpose is to introduce new, desirable
  genes into the breeding program.
• Reduces the changes of recessive genes
  appearing.
• Most popular method of pure breeding.
       BREEDING SCHEMES
• Grading up: the mating of a purebred sire
  to grade females.
• Grade animal is any animal not eligible for
  registry.
• Cheap method of improving the herd
• Only one purebred sire is needed or
  artificial insemination can be done.
• Improves the quality of the herd very
  quickly.
      BREEDING SCHEMES
• Two-breed rotation
  – Bulls from breed A are crossed with Cows from breed
    B.
  – The resulting heifers are bred to bulls from breed B
    for the duration of their productive life.
  – Replacement heifers chosen from these meetings are
    bred to bulls from to bulls from breed A.
  – Each succeeding generation of replacement heifers is
    bred to a bull from the opposite breed used to sire the
    replacement heifer.
      BREEDING SCHEMES
• Three-breed rotation
  – The pattern of breeding is the same as in a
    two-breed system, except that a bull from a
    third breed is used in the rotation of sires.
• Four and Five-breed rotation:
  – Used in large herds
  – A fourth of fifth bull is added to the system
  – Requires intense management practices.
      Terminal Cross System
• Cross is made that the offspring are
  undesirable for breeding.

• Ex. Smut face lamb (Black faced male on
  white faced female.)