Breeding Systems in Livestock

Lesson Objectives for This Class

• Compare and contrast the types of straightbreeding and crossbreeding systems for livestock mating:

  • Two and three breed crosses

  • Inbreeding and linebreeding

  • Grading up

  • Rotation breeding

• Assess breed system examples used for cows, swine, and sheep.

Introduction to Breeding Systems

Relationship, or Portion of Common Genes, Shared Between Individuals

• The relationship between two animals is the proportion of genes they are expected to have in common. This is represented by $R$.

• Table of Common Gene Proportions:

  • Full sibs: $0.5$

  • Half sibs: $0.25$

  • Parent-offspring: $0.5$

  • Grandparent-offspring: $0.25$

  • Great-grandparent: $0.125$

  • Great-great-grandparent: $0.0625$

  • First cousin: $0.125$

Systems of Mating

• Straightbreeding: Mating animals of the same breed, resulting in purebred offspring.

• Crossbreeding: Mating animals of different breeds to produce individuals with more heterozygous gene pairs, aiming to increase hybrid vigor or heterosis.

Straightbreeding Systems

Purebred Breeding

• A purebred animal possesses the characteristics of its specific breed.

• Purebred animals serve as foundation stock for crossbreeding to produce market animals, making it a specialized business.

• Both parents of a purebred animal must also be purebred.

• These animals are typically eligible for registry in their respective purebred associations, provided there are no breed-specific disqualifications.

• There is a tendency for purebred animals to be genetically homozygous, especially with practices like inbreeding and linebreeding.

Inbreeding (or Closebreeding)

• Definition: Mating of closely related individuals.

  • Examples: Sire to daughter, dam to son, brother to sister.

  • Can often be traced back to more than one common ancestor.

• Purpose:

  • Increase homozygosity for desired traits.

  • Decrease genetic variation within a herd or population.

• Pros and Cons:

  • Pros:

    • Breeders can more easily eliminate animals with undesirable traits from the breeding program.

    • Inbred animals uniformly transmit desirable genes to offspring due to increased homozygosity.

  • Cons:

    • Can be expensive.

    • Requires extensive culling and a carefully planned selection program to achieve best results.

• Genetic Content Progression (Example for Sire-Daughter Mating):

  • Offspring typically get half ($50\%$
    ) of their genetic content from each parent.

  • In a second-generation of inbreeding (e.g., breeding a sire to his daughter's offspring), the offspring may get $75\%$
    of the genetic content from the sire ancestor and $25\%$
    from the dam ancestor.

• Inbreeding Coefficient Calculation (Example for Brother-Sister Mating):

  • The inbreeding coefficient for the offspring can be calculated as $(0.5 ext{ from male}) imes (0.5 ext{ from female}) = 0.25$
    , which equals $25\%$
    .

Linebreeding

• Definition: Mating of less closely related individuals compared to inbreeding.

  • Ideally, relationships are beyond second-degree.

  • Examples: Grandparent-offspring, great-grandparent-offspring, cousins, half-siblings.

  • Can usually be traced back to one common ancestor.

• Purpose: Increase homozygosity for desired traits and decrease genetic variation within a herd or population.

• Pros and Cons:

  • Similar to those listed for inbreeding.

  • Generally, the average animal breeder does not find inbreeding or linebreeding desirable for common use.

  • It is often employed by universities for experimental work and by seedstock breeders who provide animals for crossbreeding programs that produce market animals.

• Genetic Content Progression (Example for Sire Mating to Two Different Dams, then Half-Siblings Mated):

  • In a second-generation offspring from half-sibling mating, the offspring may get $50\%$
    of the genetic content from the common sire ancestor and $25\%$
    of the genetic content from each of the two different dam ancestors.

• Coefficient Thresholds: The acceptable coefficient for linebreeding varies with breeds and species.

  • For racehorses, a coefficient over $5\%$
    is considered inbreeding.

  • For Wagyu cattle, a coefficient over $15\%$
    is considered inbreeding.

Outcrossing

• Definition: Mating of animals from different families within the same breed.

• Purpose: To introduce desirable traits into the breeding program that are not present in the original animals.

• Pros and Cons:

  • Pros: Reduces the chances of undesirable traits appearing in the offspring.

  • Sometimes used in combination with inbreeding programs to introduce needed traits.

Linecrossing

• Definition: Mating animals from two different lines of breeding within a breed.

• Purpose: To combine desirable traits from different lines of breeding.

• Consideration: Some lines cross better than others due to different gene combinations.

Grading Up

• Definition: Mating of purebred sires to grade females.

  • Grade animals: Any animal not eligible for registry in a purebred association.

• Purpose: To improve the quality of animals on a farm by introducing purebred genetics.

• Pros and Cons:

  • Pros:

    • More cost-effective, as only the sires or their semen need to be purchased.

    • The greatest percentage of improvement is observed in the first cross, as offspring receive $50\%$
      of their genes from the purebred sire.

  • Cons:

    • Offspring of grading up programs are generally not eligible for registry in a breed association because only one parent is registered.

    • The amount of improvement depends heavily on the quality of the purebred sire selected.

• Genetic Progression:

  • First generation offspring: Referred to as $A1G$
    (where A is the purebred sire's breed, 1 indicates first generation, and G indicates contribution from the grade female).

  • Second generation: If $A1G$
    females are mated to another purebred sire ($A2$
    ), the offspring ($(A1G A2)$) gain an additional $25\%$
    in purebred genetics.

  • Third generation: If $(A1G A2)$
    females are mated to another purebred sire ($A3$
    ), the offspring ($(A1G A2 A3)$) gain another $12.5\%$
    in purebred genetics.

Crossbreeding Systems

Purpose: Hybrid Vigor (Heterosis) and Heterozygous Gene Pairs

• Crossbreeding produces individuals with more heterozygous gene pairs, which increases hybrid vigor or heterosis.

• Heterosis: Measured by the average superiority of the hybrid offspring over the average of their parents.

  • Heritability and Heterosis:

    • Traits with a high degree of heritability show little improvement from crossbreeding.

    • Traits with low heritability usually show the greatest improvement as a result of crossbreeding.

• Livestock Usage:

  • Beef, swine, and sheep producers commonly use crossbreeding to produce market animals.

  • Rarely used by dairy producers, who are primarily interested in milk production.

  • Poultry producers typically use strains developed from crossing inbred lines.

Crossbreeding Systems with Beef Cattle

• Purpose: To produce animals for slaughter that generally result in higher profits for owners.

• Pros and Cons:

  • Pros: Potential for increased profits due to hybrid vigor.

  • Cons:

    • Calving difficulties may increase when crossing large-breed sires with small-breed dams.

    • Large-breed dams tend to have fewer calving problems but incur higher maintenance costs.

    • Artificial Insemination (AI) provides access to better bulls but can increase management complexity.

    • To avoid inbreeding in rotational systems, more than one breeding pasture or careful bull management may be required.

• Example Systems:

  • Rotate Herds: Same breed of bull is used for several years (e.g., $3-4$ years), then replaced with new bulls from a different breed.

  • Composite Breeds: Development of a new breed by crossbreeding four or more existing breeds.

  • Two, Three, Four, and/or Five-Breed Rotations: The concept is the same, only differing in the number of breeds involved.

    • Idea using a Two-Breed Rotation:

      • Start: Breed A bulls mated to Breed B cows.

      • First generation: Female offspring (AB) are mated to bulls from Breed B; males are marketed.

      • Second generation: Replacement heifers from the previous cross are mated to bulls from Breed A.

      • Third generation: Replacement heifers from the previous cross are mated to bulls from Breed B, and so on.

  • Terminal Sire Crossed with F1 Females:

    • Definitions:

      • Terminal Sire: Bull(s) used specifically for producing offspring that will go to market; these offspring are not used for breeding purposes.

      • F1 Females: First-generation crossbred females, either purchased or bred within the system.

    • System: Replacement females are either purchased or bred; all offspring from the terminal sire mating go to market.

  • Rotational-Terminal Sire System (uses at least 2 breeds):

    • Bulls from Breed A and Breed B are used on a rotation basis for $50\%$
      of the herd each.

    • This produces crossbred females, while all males are sold to market.

    • Mature cows from that herd are then mated to terminal sires, with offspring sent to market.

    • Replacement females from the herd that are not sold are also mated to terminal sires.

  • Static Terminal Sire System (uses at least 4 breeds):

    • The herd is split into four groups, each with a different set of sires.

    • Breeding Group 1: Produces purebred replacement heifers (e.g., Breed AA) for Group 1 and Group 2.

    • Breeding Group 2: Bulls of Breed B are mated with replacement heifers from Group 1. This group produces crossbred heifers (e.g., Breed AB) for Group 3.

    • Breeding Group 3 and 4: Bulls of Breed C and Terminal Sires are mated with crossbred heifers from Group 2 (AB). The terminal sire should be from a breed known for high heritability of carcass gain. Offspring from Groups 3 and 4 are primarily produced for the market.

Crossbreeding Systems with Swine

• Purpose: To produce animals for slaughter that generally result in higher profits for the owners.

• Taken into Consideration:

  • Gestating Females: Select breeds that produce a high number of piglets per litter and heavier weaning weights.

  • Females (Sows): Select breeds that possess desired maternal traits (e.g., milk production, nurturing behavior).

  • Males (Boars for breeding replacement females): Select males from sows with high productivity, good backfat measurements, and strong growth rates.

  • Terminal Boars: Select breeds known for less backfat and higher rates of weight gain, as their offspring are destined for market.

• Example Systems:

  • Rotational Crossbreeding: Similar in concept to cattle systems, using two to five different breeds in rotation.

  • Terminal Crossing System:

    • Crossbred F1 females are mated to terminal boars.

    • All offspring from these matings go to market.

    • Pros: Helps to maintain maximum heterosis advantages.

    • Cons: The breeder must either keep some replacement females (from a separate breeding program) or purchase additional replacement females, which can increase health risks due to introducing outside animals.

  • Rotaterminal System:

    • Crossbred F1 females are produced by rotating boars of different breeds, which are crossed with crossbred females originating from previous matings in the system.

    • These resulting crossbred F1 females (e.g., $1/4 A, 1/4 B, 1/2 C ext{ or } D$) are then mated to terminal boars.

    • All offspring from the terminal boar mating go to market.

Crossbreeding Systems with Sheep

• Purpose: To produce animals for slaughter and/or for their wool, generally resulting in higher profits for the owners.

• Pros and Cons:

  • Pros: Crossbred sheep are often hardier, healthier, produce more milk, and have higher fleece weights.

• Example Systems:

  • Rotational: Same concept as described for swine and beef cattle, involving the rotation of different breeds.

  • Static: Their version of terminal crosses. Replacement crossbred ewes are purchased and then bred to a terminal ram to produce market lambs.

  • Rotostatic: A combination of the rotational and static systems. Replacement ewes are produced from within the flock (rotational component), and these ewes are then mated to terminal rams to produce market lambs (static/terminal component).

What's Next?

• The transcript indicates that this lesson concludes the discussion on breeding systems, with future topics to follow.