Prepared for 2015 Alberta Sheep Symposium

1. Genetics as a tool to
improve flock health and
productivity
SUSAN SCHOENIAN
Sheep & Goat Specialist
University of Maryland Extension
sschoen@umd.edu - www.sheepandgoat.com

2. There are many ways to improve
flock health and productivity.
• Management
• Nutrition
• Facilities
• Biosecurity
• Vaccination
• Deworming
• Genetics

3. Genetics has many advantages.
• Unlike changes in management,
genetic change is cumulative and
permanent.
• Improved animal welfare
• Reduced use of antibiotics and
anthelmintics
• Complements current approach to
disease management
• Generates knowledge for
biomedical research in animals and
humans

4. Challenges to improving disease
resistance in sheep.
• Low heritabilities
• Lack of phenotypic data
(especially with regards to
genomics)
• Lack of commercial tools
• Trade-offs between disease
resistance and performance
(negative genetic correlations)
• Ability of pathogens to evolve
• Cost-benefit relationship

5. Genetic variation to disease
• Significant genetic variation in
susceptibility to disease exists
among sheep, suggesting
that genetic selection for
improved resistance to
disease will be fruitful.
• There is variation both
between and within breeds.

6. Many sheep diseases have a
genetic component.
• Abomasal emptying effect
• Cryptorchidism
(failure of testicles to descend)
• Entropion
(inverted eye lids)
• Facial eczema
Mycotoxins
• Footrot*
• Fly strike and lice
• Heat stress
• Hernias
• Internal parasites*
• Mastitis*
Udder health
• Metabolic diseases
• Ovine progressive pneumonia (OPP)*
Maedi-visna
• Prolapses
Rectal, vaginal, uterine
• Respiratory disease*
• Ringwomb
• Scrapie*

7. Footrot
• Bacterial disease caused by
Dichelobacter nodosus.
• One of the most common diseases
affecting sheep, especially in moist
climates and/or seasons.
• Highly contagious, easily spread from
sheep to sheep, via direct contact,
pasture, handling pens, etc.
• Difficult to eradicate.
• Causes significant economic loss due to
treatment costs and premature culling
of affected animals.
• Welfare issue.

8. Footrot
TRADITIONAL SELECTION
• Average heritability of footrot resistance is
0.20 (low to moderate).
• Selection is based on phenotypic
observation.
• Hoofs can easily be scored in the field,
using a numerical scale.
• Favorably correlated or uncorrelated to
performance traits (UK study).
• There are examples of sheep being
successfully bred for resistance to footrot.
• Selection is not feasible in footrot-free
flocks.
GENETIC MARKERS
• New Zealand researchers identified a
marker for footrot resistance.
• The marker test is commercially
available.
• It did not work for sheep in UK.
WHOLE GENOME
• Genomic breeding values are being
developed in Australia.

9. Internal parasites
• Disease with the greatest economic
impact on sheep worldwide.
• Most problematic in warm, moist
climates or during periods of warm,
moist weather.
• Risk varies by year, season, geographic
area, and production practices.
• Worsening problem due to widespread
development of resistant worms.
• There is both wiithin and between breed
variation in resistance to internal
parasites.

10. Two genetic traits
RESISTANCE
• Ability of the host to reduce number of
parasites that establish, reproduce, or
survive in its body.
• Quantified by fecal egg counts (# worm
eggs per gram of feces), which are an
indirect measure of the worm burden in
the animal.
RESILIENCE
• Ability of host to tolerate parasitic
infection, i.e. maintain health, thrive,
grow, and reproduce.
• Quantified by observation or
measurement of clinical signs: packed
cell volume (PCV), weight gain/loss,
body condition, dag score.
• FAMACHA© scores are an estimate of
PCVs.

11. How to select for parasite resistance
Ram lamb test
• Select 5-10 best ram lambs
• Deworm at start of test,
especially if rams are from
different contemporary
groups.
• Sample at 4, 6, and 8 weeks
after start
• Collect FAMACHA© scores,
too
• Calculate “home” EBV by
subtracting group mean from
each lamb’s FEC and
multiply by heritability.
All lambs
• Create a valid
contemporary group
 Similar age
 Same management
 Adjust for birth type (?)
• Collect all lambs when
10-15% or more have
FAMACHA© scores of 3
and/or FECs >700-1000
epg.
• Don’t include animals
that have been
previously dewormed.
NSIP protocol
• Weaning
 When group average
FEC > 500 epg
 When 10% need
deworming, based on
FAMACHA scores or
a few pooled fecal
egg counts average
more than 500-700
epg.
• Post-weaning
 Probably have to
deworm all,
especially if <10%
have been dewormed

12. Mastitis
• Inflammation of mammary gland caused
by bacterial infection.
• Of primary importance in dairy and
other intensively-managed flocks.
• Contributes to lamb mortality.
• Usually the primary reason for culling
ewes.
• Sub-clinical mastitis can be diagnosed
using somatic cell counts (SCC).
• Unlike cattle, relationships between milk
production and mastitis traits are not
consistent in sheep (both positive and
negative relationships).

13. Selection for resistance to mastitis
• Is recommended that all ewes with
clinical mastitis be culled.
• Can use somatic cell counts (SCCs) to
select for increased resistance to
mastitis.
• Somatic cell counts are low to
moderately heritable: 0.1 to 0.2.
• Selection for udder conformation traits
may also help to reduce incidence of
mastitis.

14. Ovine progressive pneumonia (OPP)
Maedi-visna
• Viral, incurable, slow-acting, wasting
disease that affects sheep worldwide.
 26% of US sheep (higher in some flocks)
• Targets the immune system and affects
many tissues.
• Hard bag is common symptom.
• Spread via colostrum and direct
contact.
• Some breeds more susceptible.
• Researchers discovered gene that
affects OPP susceptibility.

15. Ovine progressive pneumonia
• There is a gene that affects susceptibility
to OPP.
• There are three major variants of the gene
(called haplotypes 1, 2, and 3)
• Haplotypes 2, 3 are strongly associated
with OPP infection and considered to be
susceptible alleles
• Ewes with two copies of haplotype 1 were
less likely to be infected with OPP.
• Six additional haplotypes that occur at low
frequencies may confer low susceptibility.
• By purchasing 1,1 rams, a producer may
be able to increase the frequency of less
susceptible 1,1 in the ewe flock.
OPP status POS NEG total
Hamp 55.9 (52) 49.0 (50) 102
Polypay 41.9 (36) 58.1 (50) 86
Total, n 46.8 (88) 53.2 (100) 188
OPP status 1,1
%, n
2 or 3
%, n
Total, n
POS 31.2 (35) 76.3 (45) 80
NEG 68.2 (75) 23.7 (14) 89
Total, n 110 59 169
Proceedings 10th world Congress of Genetics
Applied to Animal Breeding

16. Respiratory disease
• In cattle, breed differences in
susceptibility to respiratory disease have
been documented.
• In cattle, heritability estimates suggest
there is a genetic variation with regards to
susceptibility to respiratory disease.
• New Zealand research showed
substantial sire differences in lambs t that
were treated for or diet from pneumonia.
• New Zealand researcher speculates
“could strengthening the immune system
by breeding for disease and parasite
resistance be a factor (in reducing the
incidence of pneumonia.

17. Scrapie
• Slow-developing, always fatal,
degenerative disease that affects the
the central nervous system of sheep
and goats.
• Low incidence, but disease of public
concern.
• Believed to be caused by a prion
(mishapen protein) that is transmitted
via infected placenta.
• Genotype determines susceptibility to
scrapie, if exposed to infective agent.
• Select rams with resistant genotypes.

18. Lambing ease
• Dystocia is a primary cause of death in
lambs 0-3 days of age. It also
contributes to ewe deaths.
• Birth weight is the single most important
factor affecting lambing ease, but other
aspects play a role: lamb shape, pelvic
area, lambing “will.”
• Terminal sire breeds and those selected
for growth and conformation are more
likely to experience lambing difficulty
and poor offspring vigor compared to
crossbred sires and those selected for
hardiness.

19. Lambing ease
• Lambing ease is a heritable trait and
should be included in selection criterion,
even though heritability is low: 3.8 to 9.7%
(four Danish breeds).
• In Australia, there are EBVs for lambing
ease (direct and daughters). Components
of the trait include lambing ease scores,
birth weight, and gestation length.
 In 2008, there was a 33% difference in
lambing ease between the best and worse
terminal sires.
• EBVs are available for Texel sheep in the
UK. They measure the sheep’s ability to
produce lambs that are born without
assistance.

20. Crossbreeding to improve health
and productivity
HYBRID VIGOR (HETEROSIS) BREED COMPLEMENTARITY

21. Crossbreeding to improve health
and productivity
HYBRID VIGOR (HETOROSIS)
• Crossbred offspring are superior to their
purebred parents.
• Effects of hybrid vigor are additive:
offspring + parent.
• Hybrid vigor has the greatest impact on
traits that have low heritabilities.
• Example: effects of heterosis on
survival in crossbred lamb is 10%.
BREED COMPLEMENTARIY
• Balances the strengths and
weaknesses of different breeds.
• Some breeds are more resistant to
certain diseases: internal parasites,
• Some breeds have a greater incidence
of certain diseases.
• Some breeds are more susceptible to
certain problems.

22. Crossbreeding programs
• 2 or 3 breed rotational
Utilize dual purpose breeds
• Terminal
Market all lambs
Purchase replacements
• Roto-terminal
Terminal to produce market lambs
Rotational to produce replacements
• Upgrading
• Composite
Need to use breeds in their appropriate
roles.
Indiscriminate crossing

23. Thank you for
your attention.
Questions?
Comments?
SUSAN SCHOENIAN
Sheep & Goat Specialist
University of Maryland Extension
sschoen@umd.edu
www.sheepandgoat.com
www.wormx.info