Breeding better sheep

Breeding “better” sheep
SUSAN SCHOENIANSheep & Goat SpecialistWestern Maryland Research & Education CenterUniversity of Maryland Extensionsschoen@umd.edu – www.sheepandgoat.com

Defining “better”
Better for what?
Nature vs. nurture
Genetics vs. environment
Ways to genetically improve sheep
Crossbreeding
Selection
Today’s topics

What is “better”?(according to Google™)
Something superior in quality, condition, or effect.
From a position of superiority or authority.
The superior one of two alternatives.
Superior to another (of the same class or kind) in excellence, quality, desirability, or suitability.

The U.S. Sheep IndustryAmerican Sheep Industry Association Survey, April 2010

Why do you raise sheep?
Commercial
Meat
Wool
Seedstock (show)
Club lambs
Dairy production

Commercial meat production
Important
Performance
Reproductive efficiency
LBS. QUALITY LAMB WEANED
Growth and carcass (sire)
Post-weaning ADG
Market suitability
Fitness
Disease-resistant
Longevity
Easy-care
Less important (ewes)
Body conformation
Wool

wool production
Important
Wool
Quality
Quantity
Color
Character
Less important
Body conformation
Fitness

Seedstock (show animals)
Important
Body conformation
Frame size
Structural correctness
Muscling
Whatever the current “fad” for your breed is.
Less important
Fitness
Wool production

Club lambs
Important
Type
Body conformation
Eye appeal (“X” factor)
Less important
Fitness
Wool

Dairy
Important
Milk production
Quantity
Quality
Udder conformation
Disposition
Less important
Body conformation
Wool
Fitness

Nature vs. nurtureHeredity (genetics) vs. environment
GENOTYPE
The genetic make-up of an individual.
The genes or alleles present in an individual.
An individual’s genetic information.
PHENOTYPE
Physical appearance of an individual.
Observable or measurable traits.
Genetics + environment

Environmental factors affecting phenotype
Nutrition
Health
Weather
Season
Management
Housing
Pasture management
Age
Genetics is permanent. The environment changes.

Nature vs. nurture
Phenotype
Genotype
What an animal looks like
Visual appraisal
How an animal performs
Actual performance levels
On-farm
Adjusted weaning weights
Adjusted litter weights
Flock EPDs (expected progeny difference).
Across-flock
EPDs or EBVs
Differences in performance at Central Ram Test Stations
Individual genes
Fecundity gene
Myostatin gene
Scrapie-resistance

There are two ways to genetically “improve” sheep.
1) CROSSBREEDING
Mating individuals of different breeds or types.
2) SELECTION
Natural
“Survival of the fittest”
Artificial
Human intervention
Choosing which individuals get to become parents.

Crossbreeding has two distinct advantages
Hybrid vigor (heterosis)
Improved performance of offspring compared to the average of their parents.
Individual
Maternal
Paternal
Effects are additive.
Greatest for reproductive and fitness traits.

Heterosis in the crossbred lamb

Heterosis in the crossbred ewe

Crossbreeding has two distinct advantages
Complementarity
Improvement in performance of crossbred offspring resulting from the crossing of breeds of different, but complementary biological types.
Strengths of sire breed offset weaknesses of dam breed and vice versa.
Suffolk x Katahdin
Finn x Dorset
Suffolk x Texel

Sheep Crossbreeding programs
Should be a planned program to produce desired types of animals or fleeces.
Should optimize performance of flock.
Should use breeds in appropriate roles.
Does not imply indiscriminate mixing of breeds or types.

Crossbreeding systems
Terminal
All offspring are marketed.Katahdin ewe x Suffolk ram = market lambs
Rotational
Dual-purpose rams are alternatedDorset x Columbia = ewes + market lambs
Roto-terminal
Rotational (to produce ewe replacements); terminal to produce market lambs.1) Dorset x Ramboullet = ewes2) D x R ewes x Suffolk ram = market lambs
Composite-terminal
New breed is formed (composite) and mated to terminal ram for market lamb production.Polypay x Polypay = composite ewesComposite x Texel ram = market lambs

Selection
Select for one trait
Can lead to problems
Select for multiple traits
Progress will be slower for individual traits, but it results in more balanced selection (and sheep).
Minimum standards
Weaning wt ratio above 100%
Twin or better
Dewormed once as lamb
Selection index
Lbs. of lamb weaned
Lbs. lamb + 3x lbs. wool
Pedigree (family selection)

Qualitative vs. quantitative traits
Qualitative
Controlled by few genes
Categorical scale
Distinct traits
Quantitative
Influenced by genetics and environment
Controlled by many genes
Continuous traits
Most traits of economic importance.

Selection: Heritability
Phenotypic variation due to genetic differences.
Relationship between phenotype and genotype.
Percent of phenotypic variation that is transferred to the next generation.
Degree to which offspring resemble parents
Sire and offspring

Selection: heritablity
Individual ewes and rams pass on traits to their offspring based on the heritability of those traits.
Traits which respond well to crossbreeding (hybrid vigor) do not respond as well to selection and vice versa.
Heritability is higher for wool traits, growth, and carcass traits, and lower for reproductive and fitness traits.
Faster progress can be made for traits with high heritability, though lowly heritable traits should not be ignored, as they still have a genetic component.

Traits with low heritability, < 20%

Traits with moderate heritability, 20-40%

Traits with high heritability, > 40 %

What else determines genetic progress?
Selection differential
Superiority of selected animals
Large population
Few animals selected
Genetic improvement is made primarily through the sire
General interval
Average age of parents when offspring are born.
Short generation interval
Replacements females have the “best” genetics in the flock.
Accuracy
Correlation between estimated and true genetic merit of animal.
High heritability
Many records or progeny

Evaluation methods
Visual appraisal (subjective)
Body conformation
Udder conformation
Visual evaluation of wool
Live evaluation/handling
Show winnings
Performance (objective)
Birth records
Growth records
Fleece weights
Milk yields
Micron testing
Ultrasound
Actual carcass measurements

Tools for genetic improvement
On-farm performance testing
Carcass contests
Central performance tests
Across-flock genetic comparisons (EPDs)

On-farm performance testing
Weaning weights
Age
Sex
Type of birth and rearing
Age of dam.
Post-weaning ADG
Parasite resistance (fecal egg counts)
Parasite resistance (FAMACHA© scores and Tx)
Carcass characteristics
Fleece characteristics

Contemporary groups
Same breed or breed type
Same environment
Same management group
Same feeding program
Same season

Central ram performance tests
Mostly for terminal sire and dual-purpose breeds.
Virginia
West Virginia
Pennsylvania
Midwest
West
Growth and feed efficiency
Ultrasound carcass
Fleece characteristics
Type scores
Scrapie and spider genotype

EPDs Expected Progeny Differences
National Sheep Improvement Association (NSIP) - www.nsip.org
Submit data through breed associations
Breed association determines which traits are important
Not all breeds participate (minor breeds)
Across-breed performance evaluation

Thank you for your attention.Do you have any questions?

Breeding better sheep

by University of Maryland Extension Small Ruminant Program

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Breeding better sheep Presentation Transcript