Selection

SUSAN SCHOENIANSheep & Goat Specialist

University of Maryland Extensionsschoen@umd.edu - 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”