David L. Thomas

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Basics of Sheep Breeding for Commercial Flocks

• David L. Thomas
• Department of Animal Sciences
• University of Wisconsin-Madison

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Production (P) of a sheep is dependent upon
- The genes of the sheep (genotype, G) - The
environment in which the sheep is raised (E),
i.e. nutrition, health program, housing,
temperature, humidity, parasite challenge,
etc. Production Genotype Environment P G
E


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Production (P) of a sheep is dependent upon
Production Genotype Environment P G E
Genotype Breeding Value (BV) Gene Combination
Value (GCV) Therefore Production Breeding
Value Gene Combination Value Environment P
BV GCV E

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BV breeding value sum of the independent
effects of each allele affecting the trait ( or
deviation)
Individual alleles, and therefore independent
allele effects, are passed from parent to
offspring, so a sheeps BV is used to predict the
performance of its progeny (i.e. EPD).
The proportion of differences between sheep in
performance that is due to the differences in
their breeding values is heritability (h2) of
that trait.
h2 BV variation / Performance variation
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Visual Traits
Characteristics that you can see, such
as Conformation Height Length Wool
quality are highly heritable.
If such traits are of great economic importance
in your flock, select sheep that have the
appearance you desire.
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• Selection for Conformation is Effective

In Changing Conformation
U.S. Southdowns Then and Now
However, conformation is not highly correlated
with performance for many production traits.
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U.S. Shropshires 1946 1961 - 2005
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Production Traits Traits that result in improved
lamb, wool, and/or milk production
Litter size, lamb survival, milk production,
weaning weight, postweaning gain, loin eye area,
fat thickness, fleece weight, disease resistance,
etc.
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Production Traits
Steps in the selection of replacement ewes for
production traits

• Initial selection on performance records
  without looking at the animals. Select 10 15
  more animals than needed.
• 2. Visual appraisal of only the animals selected
  on the basis of records. Cull the poorest 10
  15 on visual appraisal.

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Selection of Replacement Ewe Lambs
Minimum record-keeping requirements

• Record birth date ID, dam, and sex of each lamb

3. Record number of lambs born and raised by each
ewe and birth-rearing type of each lamb
UW
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Adjust Litter Size for Age of Ewe
Adjust litter size for age of the ewe. Select
replacement ewe lambs from dams with high average
adjusted litter size.
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Example of Adjusted Litter Size
Even though both ewes produced 3 lambs in two
years, Ewe 722 is expected to be genetically
superior, and her ewe lambs should be considered
as replacements.
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Estimated Breeding Value (EBV)
Difference in EBV between 722 and 205 based on
their first record EBV h2 (1.48 1.00) .10
(.48) .048 lambs 722 is estimated to have a
genetic superiority of .05 more lambs per litter
than 205. Difference in EBV between 722 and 205
based on the average of their two records
EBV h2average of 2 records (1.91 1.52) .17
(.61) .104 lambs 722 is estimated to have a
genetic superiority of .10 more lambs per litter
than 205 based on two records.
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Selection of Replacement Rams
Most commercial flocks purchase their rams Since
fewer rams are selected than ewes, rams should
have higher genetic values than ewes Most of
the genetic improvement in flocks is the result
of ram selection rather than ewe
selection Select rams from flocks that are
serious about genetic improvement for production
traits The most accurate estimate of genetic
merit is the Expected Progeny Difference (EPD)
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Suffolk Genetic Trend and Performance


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Almost all commercial flocks should be utilizing
crossbreeding
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Crossbreeding Why?

• 1. Breed complementarity Utilize the strong
  points of two or more breeds in a crossbreeding
  system to maximize performance.
• Good example
• Suffolk ram x Polypay ewe
• Poor example
• Polypay ram x Hampshire ewe

Paternal male x Maternal female
Maternal male x Paternal female
2. Hybrid vigor or heterosis increased
performance of crossbreds compared to the
purebreds that make up the cross.
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Hybrid Vigor Example 60-day Weaning Weight
Suffolk x Suffolk lambs 60 lb.
Polypay x Suffolk and Suffolk x Polypay lambs
58 lb.
Hybrid vigor 58 55 3 lb.
Average of purebred Suffolk and Polypay lambs
(60 50) / 2 55 lb.
HV ((crossbred purebred) / purebred) x 100
((58 55) / 55) x 100 (3 / 55) x 100
.05 x 100 5
Polypay x Polypay lambs 50 lb.
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Maternal and Paternal Hybrid Vigor
Maternal HV - increased performance of an
individual due to its dam being crossbred (or
from a mating utilizing a crossbred
dam) Paternal HV - increased performance of an
individual due to its sire being crossbred (or
from a mating utilizing a crossbred sire)
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Cross rams x Cross ewes 4-breed cross lambs
Paternal HV 3
Pure rams x Cross ewes 3-breed cross lambs
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Maternal HV 18
Pure rams x Pure ewes 2-breed cross lambs
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Individual HV 18
Pure rams x Pure ewes Pure lambs
0
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Crossbreeding Systems
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Crossbreeding Systems
100 ewe flock All ewe replacements are produced
within the flock All rams are purebred and
purchased. No pure Finnsheep ewes are used.
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3-Breed Terminal Crossbreeding System
Market Lambs
Dorset rams x 20 Dorset ewes
Finnsheep rams x 25 Dorset ewes
Male lambs and a few cull ewe lambs
Hampshire rams x 55 FinnxDorset ewes
All Hampshire-sired lambs
Advantages Good breed complementarity Disadvantag
e Too many purebred ewes reduced maternal
hybrid vigor
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3-Breed Rotational Crossbreeding System
Finnsheep rams x Dorset ewes
Market Lambs
Hampshire rams x FxD ewes
Dorset rams x H(FD) ewes
Finnsheep rams x D(HFD) ewes
Hampshire rams x F(DHF) ewes
(system continues to rotate sire breed)
Advantages All ewes (after start) and all lambs
are crossbred good use of individual and
maternal hybrid vigor (86 of maximum) Disadvantag
e Poor breed complementarity
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3-Breed Roto-Terminal Crossbreeding System
Market Lambs
Terminal (65-75 ewes)
Rotation (25-35 ewes)
F rams x D ewes
Hamp-sired lambs, F- and D-sired male lambs, some
F- and D-sired ewe lambs
FD ewes x Hamp rams
D rams x FD ewes
F rams x DF ewes
DF ewes x Hamp rams
D rams x FD ewes
FD ewes x Hamp rams
(system continues)
Advantages 1) All ewes (after start) and all
lambs are crossbred good use of individual and
maternal hybrid vigor (67 of Mat HV, 67 100
Ind HV) 2) Good breed complementarity
Disadvantage ?
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Final Comments on Crossbreeding
1. Good crossbreds result from good
purebreds. 2. Dont expect hybrid vigor to
compensate for poor or inappropriate
genetics. 3. Limit the number of breeds to those
that have high levels of performance. 4. Hybrid
vigor is maximized when no breeds are in common
in the sire and dam. 5. More hybrid vigor is
obtained when less related breeds are
crossed. 6. An organized crossbreeding system is
needed to take best advantage of hybrid vigor and
breed complementarity.
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Effective Utilization of Genetics in the U.S.
Sheep Industry
breed selection selection in the purebreds
crossbreeding
Where the commercial sheep industry should be!!
breed selection selection in the purebreds
Where the purebred sheep industry should be!!
Performance
breed selection without genetic improvement
over time in the purebreds
Years
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Adjustment of Lamb Weaning Weight for Age at
Weighing
60-d WW (((actual WW actual BW) / actual wean
age, days) x 60) actual BW
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Adjustment of Lamb Weaning Weight for Age of Dam,
Sex of Lamb, and Type of Birth-Rearing of Lamb
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Final Adjustment of Lamb Weaning Weight for Age
at Weighing, Age of Dam, Sex of Lamb, and Type of
Birth-Rearing of Lamb
Adj. 60-d WW 60-d WW x Age of Dam Adj. x Sex of
Lamb Adj. x Type of Birth-Rearing of Lamb Adj.
Lamb 9038 is estimated to be genetically superior
over lamb 9005 for weaning weight and should be
selected.
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Maximize HV
HV is maximized when there are no breeds in
common between the sire and dam. Hampshire ram x
Polypay ewe Hamp x Polypay cross lambs ½ H, ½
P lambs (100 maximum HVI) (Polypay x Hampshire)
ram x (Polypay x Suffolk) ewe (P x H) x (P x
S) lambs ½ P, ¼ H, ¼ S lambs (75 maximum
HVI) Hampshire ram x (Hampshire x Polypay) ewe
H x (H x P) lambs ¾ H, ¼ P lambs (50 maximum
HVI) Hampshire ram x Hampshire ewe Hampshire
lambs (no HVI)
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