1.
2013 Winter Webinar Series: Breeding Better Sheep & Goats
Selection
SUSAN SCHOENIAN
Sheep & Goat Specialist
University of Maryland Extension
sschoen@umd.edu - www.sheepandgoat.com

2.
Two ways to make genetic improvement
1. Crossbreeding
Generally, traits which respond well to crossbreeding ( heterosis)
do not respond well to selection.
2. Selection
Conversely, traits which respond well to selection ( heritability)
usually do not respond well to crossbreeding (heterosis).
Crossbreeding Selection

3.
What is selection?
• Choosing which animals get to be parents.
• Choosing which male and female mate.
Top-performing bucks
Replacement females

4.
Two kinds of selection
1. Natural - “survival of the fittest”
2. Artificial - breeding plants or animals for
specific traits (human intervention)
Soay - natural selection
Merino- artificial selection

5.
What is the goal of selection?
• Improve the frequency
of desirable alleles.
• Reduce frequency of
undesirable alleles.
• Eliminate deleterious
genes.
Top-selling Katahdin ram

6.
What makes selection difficult?
• Most traits of economic
significance are quantitative
(polygenic) - controlled by
many genes.
• Phenotype ≠ Genotype
We 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

7.
Selection basics
• Heritability (h2)
• Repeatability (R)
• Selection differential (SD)
• Generation interval (L)
• Genetic progress (G)
Kiko bucklings from 2012 buck test

8.
Heritability - h2
• The proportion of phenotypic variation in a
population that is due to genetics (genotype).
Litter size (prolificacy) Milk yield Tail length
h2 = 10 percent h2 = 30-35 percent h2 = up to 82 percent

9.
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 • There aren’t many h2
environment, so averages are estimates for meat goats,
often used. so we usually use sheep
estimates.

10.
Heritability of different traits
Trait 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%

11.
Heritability of reproductive traits
Most maternal traits have a low heritability (<20%).
Trait Avg. h2
Age at puberty 0.25
Ewe fertility* 0.05
Ewe productivity * 0.20
Gestation length 0.45
Lamb survival * 0.05
Libido 0.22
Longevity 0.27
Mothering ability 0.39
Out-of-season breeding 0.20
Prolificacy * 0.10
Scrotal circumference * 0.35
Spring fertility 0.07-0.11
Katahdin *Source: Sheep Production Handbook (2002)

12.
Heritability of growth traits
Most growth traits are moderately heritable (20-40%).
Trait Avg. h2
Birth weight * 0.15
60 day weight * 0.10
90 day weight * 0.15
120 day weight * 0.20
240 day weight * 0.40
Mature weight 0.50
Feed efficiency 0.26
Post-weaning gain 0.25
Pre-weaning gain 0.15
*Source: Sheep Production Handbook (2002)
South Dakota lambs

13.
Heritability of carcass traits
Most carcass traits are moderately heritable (20-40%).
Trait Avg. h2
12th rib fat thickness * 0.30
Bone weight 0.30
Carcass length 0.31
Carcass weight * 0.35
Dressing percent * 0.10
Grade 0.12
Lean weight 0.39
Loin depth 0.15-0.38
Loin eye area * 0.35
Percent retail cuts * 0.40
Weight of retail cuts * 0.45
Southdown lamb *Source: Sheep Production Handbook (2002)

14.
Heritability of fleece traits
Sheep fleece traits are moderate to highly heritable (>25%).
Trait Avg. h2
Character 0.33
Clean fleece weight* 0.25
Color * 0.45
Crimp* 0.45
CV of fiber diameter 0.50
Fiber diameter* 0.40
Grease fleece weight* 0.35
Handle 0.33
Staple length* 0.55
Yield (%)* 0.40
*Source: Sheep Production Handbook (2002)
Rambouillet rams in South Dakota

15.
Heritability of lactation (sheep)
Most lactation traits are moderately heritable (20-40%).
Trait Avg. h2
Fat percentage* 0.30
Fat yield* 0.35
Milk yield* 0.30
Protein percentage* 0.30
Protein yield* 0.45
Rear udder attachment** 0.15
Teat placement** 0.25
Teat size** 0.20
Udder height/depth** 0.15
*Source: Sheep Production Handbook (2002)
Dairy ewe in Maryland ** Dave Thomas, University of Wisconsin

16.
Heritability of production traits of dairy goats
Dairy goat production traits are moderate to highly heritable (20-50%).
Trait Avg. h2
Milk yield 0.35
Fat yield 0.35
Protein yield 0.37
Protein: fat ratio 0.37
Fat and protein yield 0.36
Fat percentage 0.52
Protein percentage 0.54
Age at first kidding 0.23
Kidding 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

17.
Disease resistance
Disease Avg. h2
Dag scores (scouring) 0.10 - 0.37
Facial eczema 0.45
Parasite resistance 0.25 - 0.50*
[fecal egg counts]
Fly strike 0.20 - 0.33
Foot rot 0.20 - 0.30
Mastitis 0.13
Parasite resilience 0.10
[PCV, FAMACHA© scores]
Somatic cell count 0.10-0.20
(strong correlation to mastitis)
Fecal egg counting with McMaster slide * Katahdin

18.
Heritability of defects
Defect Avg. h2
Cryptorchidism high (?)
Entropion 0.15
(inverted eye lid)
Face cover 0.56
(precursor to wool blindness)
Multiple nipples high (?)
Jaw position 0.13
Hernias high (?)
umbilical, inguinal, scrotal
Rectal prolapse 0.14
Vaginal prolapse low (?)
Congenital scrotal hernia
Source: various

19.
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.
However, these traits are usually not
Hampshire ewe in Kentucky strongly -correlated with performance.

20.
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

21.
Generation interval
• Average age of parents when progeny are born.
Varies by gender and management.
Yearling Katahdin ewe 11 year old Katahdin ewe

22.
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
500 epg 1500 epg 10000 epg

23.
Genetic progress (G)
G = h2 x selection differential
generation interval
G =
0.40 x [1500-500]
4
G = -100 epg per year
-500 epg in 5 years
-1000 epg in 10 years
This is a “simplistic” example. Other factors, such as repeatability, will affect genetic progress.

24.
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

25.
Practical aspects of selection
• Animal identification
a) Permanent
b) Unique
• Data collection
a) Reproduction
b) Growth
c) Milk
d) Fleece
e) Carcass
f) Disease resistance
g) Defects
• Record keeping and analysis
a) Pedigree
b) Performance
c) Type

26.
Methods of selection
• Individual
• Family/pedigree
• Progeny
• Combined
– EPD
– EBV
– MPPA

27.
Methods of selection
In Targhee breed, each 1 lb. increase in
1. Single trait weaning weight resulted in a 2 ½ lb.
 Generally not recommended increase in ewe mature body size.
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.
Dorper x Polypay lambs in Kentucky

28.
Multiple trait selection
1. Tandem
selection
2. Independent
culling levels
3. Selection index
Dairy goat in Brazil

29.
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
Scottish Blackface ram (favorable)

30.
Independent culling levels
 Select for two or more traits
simultaneously.
 Establish a minimum standard
for each trait.
Examples
1) 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-farm
Keep only twin-births
Don’t keep any kids that require
deworming more than once.
Top-performing buck (Kiko)

31.
Selection index
• Combine traits for
overall merit
1) On-farm
2) Central performance test
3) EPD’s
4) Profitability
Source: Montana State University

32.
Index examples
Simple productivity index for meat sheep and goats
Pounds 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 Test
40% average daily gain
20% weight per day of age
20% 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 LC
Profitability index (Targhee sheep - range flock)
$1.00 WW + $0.26 MM - $0.26 YW + $1.92 FW - $0.47 FD + $0.36 LC
EPD’s: WW=weaning weight, MM-maternal milk, YW=yearling weight, FW=fleece weight, FD=fiber diameter, and LC=lamb crop

33.
Next webinar: Feb 12, 7 pm EST
“Performance Evaluation”