This document discusses using genetics to improve flock health and productivity in sheep. It notes that genetics has advantages over traditional approaches like management changes in providing permanent and cumulative improvements. Traits like disease resistance have a genetic component and selecting for improved resistance can benefit flocks. Specific diseases and traits discussed include internal parasites, footrot, mastitis, ovine progressive pneumonia, scrapie, and lambing ease. The document also covers using crossbreeding to take advantage of hybrid vigor and breed complementarity.

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 over
traditional approaches.
• Unlike changes in management,
genetic change is cumulative and
permanent.
• May result in 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. But there are challenges to improving
disease resistance in sheep.
• Low heritabilities of disease
resistance.
• 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 of disease resistance
• 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 control and/or eradicate.
• Causes significant economic loss due to
treatment costs and premature culling of
affected animals.
• Major welfare issue.

8. Footrot
TRADITIONAL SELECTION MARKERS, GENOMIC
• 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.
• New Zealand researchers
identified a marker for footrot
resistance.
• The marker test is commercially
available.
• It did not work for sheep in UK.
• 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 within and between breed
variation in resistance to internal
parasites.

10. Two genetic traits
RESISTANCE RESILIENCE
• 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.
• 20-40% heritable
• 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.
• Less heritable than resistance

11. How to select for parasite resistance
Ram lamb test NSIP protocol
• 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.
• 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
• 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

12. Mastitis
• Inflammation of mammary gland usually
caused by bacterial infection (OPP can
cause hard bag).
• Of primary importance in dairy and other
intensively-managed flocks.
• Contributes to lamb mortality.
• Usually the primary reason for culling
ewes (and reducing longevity)
• Sub-clinical disease probably most costly.
• 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 (shape,
support,etc).

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 have 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 are
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 ewes in the 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.
• Major part of scrapie eradication efforts.

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) BREED COMPLEMENTARIY
• 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%.
• 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