Selection
SUSAN SCHOENIANSheep & Goat Specialist
University of Maryland [email protected] - www.sheepandgoat.com
2013 Winter Webinar Series: Breeding Better Sheep & Goats
Two ways to make genetic improvement1. Crossbreeding
Generally, traits which respond well to crossbreeding ( heterosis) do not respond well to selection.
2. SelectionConversely, traits which respond well to selection ( heritability) usually do not respond well to crossbreeding (heterosis).
Crossbreeding Selection
What is selection?• Choosing which animals get to be parents.• Choosing which male and female mate.
Replacement females
Top-performing bucks
Two kinds of selection1. Natural - “survival of the fittest”2. Artificial - breeding plants or animals for
specific traits (human intervention)
Merino- artificial selection
Soay - natural selection
What is the goal of selection?
• Improve the frequency of desirable alleles.
• Reduce frequency of undesirable alleles.
• Eliminate deleterious genes.
Top-selling Katahdin ram
What makes selection difficult?• Most traits of economic
significance are quantitative (polygenic) - controlled by many genes.
• Phenotype ≠ GenotypeWe can’t see genes.
• Sometimes, it’s difficult to separate genetics from environmental influences.
• Some traits cannot be directly or easily selected for.
• Quantitative (population) genetics is about probabilities.
P = G + E
Selection basics• Heritability (h2)
• Repeatability (R)
• Selection differential (SD)
• Generation interval (L)
• Genetic progress (G) Kiko bucklings from 2012 buck test
Heritability - h2
• The proportion of phenotypic variation in a population that is due to genetics (genotype).
Litter size (prolificacy)h2 = 10 percent
Milk yieldh2 = 30-35 percent
Tail lengthh2 = up to 82 percent
Heritability estimates• There are a lot of h2
estimates for most sheep traits.
• There are h2 estimates for most dairy goat traits.
• It’s harder to find h2 subjectively measured traits.
• H2 estimates usually vary by species, breed, and environment, so averages are often used.
h2• There aren’t many h2
estimates for meat goats, so we usually use sheep estimates.
Heritability of different traitsTrait type Heritability Genetics Environment
Reproductive Low 5-20% 85-100%
Growth Moderate 10-50% 50-90%
Carcass Moderate 10-45% 55-90%
Fleece Moderate to high 25-55% 45-75%
Lactation Moderate 15-35% 65-85%
Heritability of reproductive traits Most maternal traits have a low heritability (<20%).
Trait Avg. h2
Age at puberty 0.25Ewe fertility* 0.05Ewe productivity * 0.20Gestation length 0.45Lamb survival * 0.05Libido 0.22Longevity 0.27Mothering ability 0.39Out-of-season breeding 0.20Prolificacy * 0.10Scrotal circumference * 0.35Spring fertility 0.07-0.11
Katahdin *Source: Sheep Production Handbook (2002)
Heritability of growth traits Most growth traits are moderately heritable (20-40%).
Trait Avg. h2
Birth weight * 0.1560 day weight * 0.1090 day weight * 0.15120 day weight * 0.20240 day weight * 0.40Mature weight 0.50Feed efficiency 0.26Post-weaning gain 0.25Pre-weaning gain 0.15
*Source: Sheep Production Handbook (2002)South Dakota lambs
Heritability of carcass traits Most carcass traits are moderately heritable (20-40%).
Trait Avg. h2
12th rib fat thickness * 0.30Bone weight 0.30Carcass length 0.31Carcass weight * 0.35Dressing percent * 0.10Grade 0.12Lean weight 0.39Loin depth 0.15-0.38Loin eye area * 0.35Percent retail cuts * 0.40Weight of retail cuts * 0.45
Southdown lamb *Source: Sheep Production Handbook (2002)
Heritability of fleece traits Sheep fleece traits are moderate to highly heritable (>25%).
Trait Avg. h2
Character 0.33Clean fleece weight* 0.25Color * 0.45Crimp* 0.45CV of fiber diameter 0.50Fiber diameter* 0.40Grease fleece weight* 0.35Handle 0.33Staple length* 0.55Yield (%)* 0.40
Rambouillet rams in South Dakota*Source: Sheep Production Handbook (2002)
Heritability of lactation (sheep)Most lactation traits are moderately heritable (20-40%).
Trait Avg. h2
Fat percentage* 0.30Fat yield* 0.35Milk yield* 0.30Protein percentage* 0.30Protein yield* 0.45Rear udder attachment** 0.15Teat placement** 0.25Teat size** 0.20Udder height/depth** 0.15
Dairy ewe in Maryland*Source: Sheep Production Handbook (2002)** Dave Thomas, University of Wisconsin
Heritability of production traits of dairy goatsDairy goat production traits are moderate to highly heritable (20-50%).
Trait Avg. h2
Milk yield 0.35Fat yield 0.35Protein yield 0.37Protein: fat ratio 0.37Fat and protein yield 0.36Fat percentage 0.52Protein percentage 0.54Age at first kidding 0.23Kidding interval 0.05
Source: Breed differences over time and heritability estimates for production and reproduction traits of dairy goats in the United States (Journal of Dairy Science, 2012).
Sannen doe in Maryland
Disease resistanceDisease Avg. h2
Dag scores (scouring) 0.10 - 0.37
Facial eczema 0.45
Parasite resistance[fecal egg counts]
0.25 - 0.50*
Fly strike 0.20 - 0.33
Foot rot 0.20 - 0.30
Mastitis 0.13
Parasite resilience[PCV, FAMACHA© scores]
0.10
Somatic cell count(strong correlation to mastitis)
0.10-0.20
Fecal egg counting with McMaster slide * Katahdin
Heritability of defectsDefect Avg. h2
Cryptorchidism high (?)
Entropion(inverted eye lid) 0.15
Face cover(precursor to wool blindness)
0.56
Multiple nipples high (?)
Jaw position 0.13
Herniasumbilical, inguinal, scrotal high (?)
Rectal prolapse 0.14
Vaginal prolapse low (?)
Congenital scrotal herniaSource: various
Heritability of conformation and type
Trait Avg. h2
Though not well documented, the
heritability of individual conformation traits (e.g.
height and length) is considered to be high.
Hampshire ewe in Kentucky
However, these traits are usually not strongly -correlated with performance.
Repeatability (accuracy)• Correlation (reliability)
between repeated measurements.
• Indicates the upper level of heritability.
• Traits with high heritability usually have high repeatability.
Top-performing buck (Kiko) from 2009 Test
Generation interval• Average age of parents when progeny are born.
Varies by gender and management.
Yearling Katahdin ewe 11 year old Katahdin ewe
Selection differential (SD)• Difference between selected animals and the average of the
population from which they were selected.
Average of population
Average of selected
population
Selection differential
1500 epg500 epg 10000 epg
Genetic progress (G)h2 x selection differential
generation intervalG =
0.40 x [1500-500]4
G =
-100 epg per year-500 epg in 5 years
-1000 epg in 10 years
G =
This is a “simplistic” example. Other factors, such as repeatability, will affect genetic progress.
Most genetic progress when…• Heritability is high.
• Repeatability is high.
• Selection differential is wide.
• Generational interval is short (though there is a trade-off with selection differential).
Rambouillet lambs in South Dakota
Practical aspects of selection• Animal identification
a) Permanentb) Unique
• Data collection a) Reproductionb) Growthc) Milkd) Fleecee) Carcassf) Disease resistanceg) Defects
• Record keeping and analysisa) Pedigreeb) Performancec) Type
Methods of selection
• Individual• Family/pedigree• Progeny• Combined– EPD– EBV– MPPA
Methods of selection
1. Single trait Generally not recommended
2. Multiple trait Success of raising sheep
and/or goats depends upon improvement or more than one trait.
Single trait selection can have unexpected and undesirable consequences due to genetic correlations or environmental responses.
In Targhee breed, each 1 lb. increase in weaning weight resulted in a 2 ½ lb.
increase in ewe mature body size.
Dorper x Polypay lambs in Kentucky
Multiple trait selection
1. Tandem selection
2. Independent culling levels
3. Selection indexDairy goat in Brazil
Tandem selection• Focus on one trait
at a time until a satisfactory level of performance is achieved; then move on to another trait and so forth.
• Selection may result in changes (positive or negative) to correlated traits.– Milk yield vs. fat percentage
(antagonistic)– Staple length vs. fleece weight
(favorable)Scottish Blackface ram
Independent culling levels Select for two or more traits
simultaneously. Establish a minimum standard
for each trait.
Examples1) Western Maryland Pasture-Based
Meat Goat Performance Test Gold, Silver, and Bronze Standards of Performance for growth rate, parasite resistance, and parasite resilience.
2) On-farmKeep only twin-birthsDon’t keep any kids that require deworming more than once.
Top-performing buck (Kiko)
Selection index
• Combine traits for overall merit
1) On-farm2) Central performance test3) EPD’s4) Profitability
Source: Montana State University
Index examplesSimple productivity index for meat sheep and goatsPounds of (quality) lamb or kid weaned[composite trait: fertility + litter size + mothering ability + offspring survival + milk production]
“Old-time” sheep productivity index[Pounds of lamb + (2.5 x fleece weight)]
Pennsylvania Meat Goat Buck Test40% average daily gain20% weight per day of age20% loin eye area (adjusted)20% leg circumference (adjusted)
Profitability index (Targhee sheep - farm flock)$1.00 WW + $0.36 MM - $0.40 YW + $1.14 FW - $0.30 FD + $0.19 LCProfitability index (Targhee sheep - range flock)$1.00 WW + $0.26 MM - $0.26 YW + $1.92 FW - $0.47 FD + $0.36 LCEPD’s: WW=weaning weight, MM-maternal milk, YW=yearling weight, FW=fleece weight, FD=fiber diameter, and LC=lamb crop
Next webinar: Feb 12, 7 pm EST“Performance Evaluation”