Rotational Crossbreeding

What is rotational crossbreeding? #3c00dd

• Mating of individuals of 2 or more sire breeds in a cycle

• Where replacement females are kept from a generation

Heterosis in Rotational Crossbreeding #3c00dd

• What is heterosis

• AKA

• What is the formula for % retained heterosis in rotational crossbreeding

  • What is the % for 2-breed rotation

  • What is the % for 3-breed rotation

What is heterosis: The improved function of hybrid offspring compared to its parents

AKA: Hybrid vigor

What is the formula for % retained heterosis in rotational crossbreeding: (2ⁿ - 2 / 2ⁿ - 1) x 100 where n = number of breeds

• What is the % for 2-breed rotation:

  • (2² - 2 / 2² - 1) x 100

  • (4 - 2 / 4- 1) x 100

  • (2/3) x 100 = 67%

• What is the % for 3-breed rotation:

  • (2³ - 2 / 2³ - 1) x 100

  • (8 - 2 / 8 - 1) x 100

  • 0.8571 × 100 = 86%

What are the 3 advantages of rotational crossbreeding? #3c00dd

• Generates replacement females

• Uses some heterosis

• Sire breeds of similar size produces uniform offspring

What are the 4 disadvantages of rotational crossbreeding? #3c00dd

• No breed complementarity

• Loss of heterosis

• Multiple breeding groups

• Variation between generations

What is the direct breed effect in rotational crossbreeding for

• 2 breed

• 3 breed

• 4 breed

Refers to the proportion of genes in the offspring coming directly from contributing breeds

• 2 breed: ½ from each breed

  • an offspring has half of its genes from each breed → ½ = 50% from Breed A, 50% from Breed B

• 3 breed: 1/3 from each breed

• 4 breed: ¼ from each breed

What is the maternal breed effect in rotational crossbreeding for

• 2 breed

• 3 breed

• 4 breed

The influence of the dam’s breed on the offspring such as milk yield

• 2 breed: ½ from each breed

• 3 breed: 1/3 from each breed

• 4 breed: ¼ from each breed

What is the individual heterosis in rotational crossbreeding for

• 2 breed

• 3 breed

• 4 breed

It is the hybrid vigor seen in the offspring due to crossbreeding (use formula, 2ⁿ-2 / 2ⁿ-1)

• 2-breed: 2/3

• 3-breed: 6/7

• 4-breed: 14/15

What is the maternal heterosis in rotational crossbreeding for

• 2 breed

• 3 breed

• 4 breed

It is the heterosis effect seen in maternal traits like fertility and nurturing ability (use formula, 2ⁿ-2 / 2ⁿ-1)

• 2-breed: 2/3

• 3-breed: 6/7

• 4-breed: 14/15

3-Breed Rotational Crossbreeding

• May produce

• Doesn’t capitalize on

  • Why

• Benefits

May produce: Variability in progeny produced

Doesn’t capitalize on: Maternal heterosis

• Why: Because breeding females are not maintained as F1 crosses so their hybrid vigor declines as breed combinations become more mixed

Benefits:

• Generate own replacement heifers

• Heavier weight from 3-breed combination

• Higher litter size

What are the advantages of a rotational-terminal crossbreeding system?

• Utilises the advantage of heterosis (both individual and maternal heterosis are maximised) and breed complementarity

• Keep some replacement females

• Requires multiple breeding pens/pastures

• Rotate sire breed every 4 years

How to start a crossbreeding system with an existing herd

• Determine genetic makeup of herd

• Cull animals that cannot fit plan

• Choose sires to produce females replacements

• Build a herd of females that fit the environment

• Determine market possibilities

• Choose some sires that will produce offspring that match the market

• Decide what crossbreeding system is best

• Is replacement females an issue

• Are breeding facilities and manpower adequate

What are the challenges in crossbreeding

• Easier to be carried out in herds of more than 50 cows

• Rotate sire breed every 4 years

• Use composite breed

• Requires more paddocks and more breeds of bulls

• More records and identification

Summary of rotational crossbreeding

• Choosing breeds for best maternal and paternal traits

• Retained heterosis at 67% for 2-breed and 86% for 3-breed rotational systems

• Generate some replacement females

• Need adequate facilities to maintain multiple breeding groups