Sheep

Selection in Sheep

Sheep Terminology

  • Ram: Male sheep

  • Ewe: Female sheep

  • Lamb: Young sheep
      - Ram lamb: Male lamb
      - Ewe lamb: Female lamb

  • Wether: Castrated male sheep

  • Hair sheep: Breed that sheds its coat

Meat Terminology

  • Lamb: Age between 6 months to 1 year

  • Hogget: Age between 1 to 2 years

  • Mutton: Age greater than 2 years

Sheep Breeds

  • Suffolk
      - Origin: England
      - Characteristics: Fast growth, heavy lambs (meat breed)

  • Dorset
      - Origin: England
      - Characteristics: Can be horned or polled, longer breeding season (6-8 months compared to 4-6 months)

  • Merino
      - Characteristics: Fine wool

  • Booroola Merino
      - Origin: New Zealand
      - Characteristics: Litter bearing, can have 1-6 lambs per litter

  • Romanov
      - Characteristics: Litter bearing, can have 3-5 lambs per litter, record of 9 healthy lambs

  • Katahdin
      - Origin: United States
      - Characteristics: Hair sheep breed developed from African hair sheep crossed with European meat breeds, sheds its coat yearly, does not require shearing, heat tolerant, parasite resistant, smaller carcass size, flighty temperament, less flocking instinct

Canadian Breeds

  • ARCOTT: Developed at Animal Research Center in Ottawa
      - Types:
        - Rideau Arcott: High fertility
        - Finish Landrace: Specialized for lambing
        - Suffolk: Focused on meat production
        - East Friesian: Focused on milk production
        - Canadian Arcott: Primarily meat production
        - DLS (Dorset Leicester Suffolk)

Polled vs Horned Traits

  • Sex-influenced trait: Relationship between genotypes and expression of horned/polled characteristics.
      - Female genotype possibilities:
        - HH: Horned
        - Hh: Polled/scurred
        - hh: Polled
      - Male genotype possibilities:
        - Can either express horn or polled characteristics based on their genotype

Single Gene Traits

  • Wool Colour:
      - White wool: Dominant and preferred for commercial mills
      - Coloured wool: Preferred by artisan spinners and weavers

  • Spider lamb syndrome:
      - Description: Autosomal recessive disease seen in Suffolk and Hampshire breeds.
      - Genetic cause: Mutation in FGFR3 (fibroblast growth factor receptor 3)

Callipyge Trait

  • Definition: Means “beautiful buttocks.”

  • Characteristics: Increased muscling resulting in lean but tough meat.

  • Genetic Imprinting: The expression of this trait is maternally imprinted; the allele from the mother (M) is “off,” while the allele from the sire (P) is “on.”

  • Expression Condition: This trait is only expressed if inherited from the sire.

   - Genotype Examples:
      - CP/NM = Big posterior
      - NP/CM = Normal
      - NP/NM = Normal
      - CP/CM = Normal

Scrapie

  • Description: A prion disease in sheep analogous to Bovine Spongiform Encephalopathy (BSE) or Chronic Wasting Disease (CWD).

  • Symptoms: Causes itching as sheep scrape their backs against fences, etc.

Genetic Factors in Scrapie

  • Single Nucleotide Polymorphisms (SNPs) associated with increased susceptibility:
      - 136A>V
      - 171R>Q

  • Management: Flocks exposed to an infected animal will cull only those sheep exhibiting intermediate to high susceptibility.

  • Recommended: Use only 171RR rams for the selection of resistance.

Susceptibility Genotypes:

  - 136AA, 171RR: Negligible susceptibility
  - 136AA, 171QR: Very low susceptibility
  - 136AV, 171QR: Intermediate susceptibility
  - 136(any), 171QQ: High susceptibility

Scrapie Flock Certification Program

  • Purpose: A national risk management program for sheep and goats.

  • Structure: 7 certification levels - E, D, C, B, A, certified, certified plus.

  • Progression: Flocks can move up one level each year until certified.

  • Enrollment Requirements:
      - Must submit samples for testing from all animals that die on the farm.
      - If no deaths in 12 months, submit one cull animal older than 18 months.
      - Only acquire animals from herds of the same level or higher.
      - Closed to uncertified farms; however, genetically resistant rams (171RR) may be acquired.

Fertility Considerations

  • Low Heritability (h² = 0.05-0.10).

  • Indicator of Fertility: Testicle size is a good predictor.

  • Average Lambing Rate: Typically 1.5 lambs; twinning is desirable.

Litter-Bearing Breeds:

  • Romanov

  • Booroola Merino

  • BMP1B receptor

Heritability Estimates

  • Carcass Quality Measures:
      - Daily Gain: h² = 0.4-0.45
      - Yearling Weight: h² = 0.4-0.45
      - Tenderness: h² = 0.3-0.35
      - Carcass Grade: h² = 0.15-0.25

  • Wool Quality Measures:
      - Fiber Diameter: h² = 0.43
      - Staple Length: h² = 0.50-0.55
      - Fleece Weight: h² = 0.40-0.45
      - Face Wool Types:
        - Closed (woolly) face
        - Open (bare) face

Mating Systems

  • Inbreeding:
      - Consequences: Decreases fertility, weight, and fleece quality.

  • Crossbreeding:
      - Consequences: Increases fertility and weight.
      - Notable Types: Between two-way, three-way, or terminal crosses.

Selection in Breeding

  • Record Keeping: Typically focuses on individual records for assessment.

  • Selection Technique: Utilizing index selection.
      - Formula for ewe productivity:
        extEweproductivity=extlambweaningwt+2.5imesracextwoolwt.yearextewewtext{Ewe productivity} = ext{lamb weaning wt} + 2.5 imes rac{ ext{wool wt.year}}{ ext{ewe wt}}

Selection Correction Factors

  • Adjusted to the 90-day weaning weight formula:
      - extWeaningWeight=extBirthweight+ext(ADGimes90ext)ext{Weaning Weight} = ext{Birth weight} + ext{(ADG} imes 90 ext{)}

  • Age of Dam Adjustments:
      - 2 years = +7 lbs
      - 3 years = +4 lbs
      - 4-5 years = 0 lbs

  • Sexual Adjustment: Ewe lambs = +8 lbs

  • Twin Adjustment: +9 lbs for lambs from twin births.