Sheep Breeding and
Reproduction
Dr. Randy Harp
Sheep Genetics
• 27 pairs of Genes
• Two genes that form a gene pair may be the same
(Homozygous)
• If they are different = Heterozygous
• If Heterozygous, then one allele of the gene pair
may express itself over another = dominance
• This dominance can vary from complete to codominance
• A hidden gene expression is called recessive
Sheep Genetics
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Some are sex linked
Examples
Genotype vs Phenotype
Estimated phenotypic variance of the flock
(pg BRD 19)
• Heritability
Heritability
• Proportion of the total phenotypic variation that is
due to the variation in additive gene effects
• In other words, the proportion of differences due
to genetic effects and is important in the prediction
of response rates from selection.
• The square root of the variance is the standard
deviation, which is the ave. deviation of each
indiv. in the pop. from the pop. Average
• Heritability est. – handout BRD 33
Correlation
• Quantifies a relationship between two
variables
• Measured between zero and one
• Positive versus negative correlations
• Multiple effects (Regression models)
Animal Identification
• Accurate identification
• Written records
• Accurate measurements of economic
importance
• ??? Registered or not
• Two ID nos. , one for flock or indiv. ID and
the other for registration ID (if reg.)
Record Keeping
• Determine clear goals
• Allow for records such as pedigree, birth
weight, singles or twins, weaning weight,
etc.
• Five categories of records:
– REPRODUCTION, MATERNAL , GROWTH,
WOOL AND CARCASS
Estimated Breeding Value (EBV)
• EBV = b (P- ave.P), where b is the
heritability of a particular trait
• Example; Ram A has a grease fleece weight
of 15 # and the ave. flock grease fleece
weight is 11 #, then 15-11 = selection
differential [4]
• 4 x .4 (h) = 1.6 # of grease fleece advantage
for breeding Ram A
EPD’s (Expected Progeny
Differences)
• Simply ½ of the EBV
• The ave. EPD in a population is + or – from
the average of the population. A comparison
• Ex. If a ewe has a +.3 for no. of lambs born,
then one would expect the progeny to
produce .3 more lambs per lambing than the
progeny of average ewes.
EPD’s
• Another ex. is two rams A and B
• +.5 and +1.1 for fleece weight, respectively
• Ram B progeny would be expected to have
.6 pounds heavier fleeces than Ram A
• Example 3; Ram has +1.0 and Ewe has +.5
for weaning weight, then the progeny will
be 1.5 # more than the pop. ave. for WW
EPD’s
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Accuracy
Gives an idea of reliability of the estimat
Measured from 0-1
An accuracy of .45 is not very reliable,
whereas > .9 is considered reliable
• Low accuracies are a result of limited
information known about the parents or low
numbers of progeny occur
Generation Interval
• The time lapse between birth of an animal
and the birth of its replacement – usually 34 years for sheep
• A measurement of progress or rate of
improvement
• Genetic Improvement per year = heritability
x selection differential divided by the
generation interval
Methods of Selection for Single
Traits
• Individual selection-selection on their own
performance
• Family selection-selection based on
bloodlines; useful when (h) is low
• Pedigree selection-similar to family and is
dependent upon how closely related the
ancestors. Useful when considering same
sex
Methods of Selection for Single
Traits
• Progeny Test- observing the performance of
the offspring. Must be mated to several
ewes and then look at the offspring. Best
when looking at carcass traits
• Combined Selection- uses more than one of
the above mentioned methods
Methods of Selection for
Multiple Traits
• Tandem Selection- focuses on multiple
traits, yet one at a time. After the
performance of one is achieved, then move
to the next trait
• Independent Culling- Set minimum
standards for more than one trait at a time
for the indiv. Cull any that does not meet
the min. stds for any trait
Methods of Selection for
Multiple Traits
• Selection Index- rank indiv. animals for two
or more traits based on a combination effect
• One the farm testing:
• Primary focus is on the ewe flock
• Therefore, traits such as prolificacy,
weaning weight and fleece weight
National Sheep Improvement
Program
• Oversees the promotion, funding, development
and implementation of a national genetic
evaluation program for sheep
• Minimum criteria for record keeping
• Lamb records: ID #, sire and dam ID, date of
birth, sex, type of birth, & type of rearing.
• Additional records are reproductive, growth and
fleece traits
NSIP
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Focuses on:
Number of lambs born per ewe lambing
Body weight at 30,60,90,120 & 240 days
Grease &/or clean fleece wt.
Staple length
Fleece grade
Pounds of lamb weaned per ewe exposed per year
Adj. Factors for No. of lambs born per
lambing to a common ewe
Age of Dam
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5, 6, & 7
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9+
Adj. Factor
1.45
1.15
1.05
1.00
.96, .96 & .95
.98
1.00
Major Economically Important
Traits
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Reproductive Efficiency
Carcass Merit
Milk Production
Total Ewe Production- # lamb/ewe exposed
Mature Size
Hardiness & Adaptability
Wool Production
Growth Rate
Selection for Growth
• Growth is a very important trait, esp. for
meat breeds
• Adjusted 90 day weaning weight
• If birth weight is known:
• [Actual wt. - birth wt. x adj. Days (90)/
actual age ]+ birth wt.
• If birth wt. is not known:
• Actual wt. X adj. Age in days/age
Selection for Fleece Traits
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Grease and Clean Fleece Wts
Yield
Staple length- at least 3”/yr.
Fiber diameter
Crimp
Color
Density
Belly Wool-undesirable on sides
Importance of Genetic
Improvement in Seedstock
Flocks
• Most produce their own replacements
• Therefore, genetic improvement from outside is
by the Ram purchased
• Theoretically, Genetic merit increases at the same
rate as the genetic merit of the rams, yet because
of generation intervals and replacement
production usually improvement lags two
generations behind
Mating Systems
• Purebreeding- common genetic group
• Outbreeding- unrelated within a breed
• Inbreeding-closely related with one or more
common ancestors (more than 50% related)
• Linebreeding-common ancesters, but not 50%
related
• Crossbreeding- mating of different breeds
• Grading Up-enhanced concentration of
crossbreeding to rams of a single breed
Inbreeding Coefficients
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Full brother mated to sister = .25
Sire on daughter = .25
Half brother to half sister = .125
Sire on Mother = .5
Therefore; > .5 has to be compounded over
time and added generation to generation
Crossbreeding
• Heterosis or Hybrid Vigor
• Superiority of the crossbred indiv. Relative
to the average performance of the parents
• Maximized when parents have no breed
ancestry in common (3 breed terminal X)
Crossbreeding Systems
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Two Breed Terminal(50% Heterosis)
Three Breed Terminal (100%)
Three Breed Rotational (86 %)
Four Breed Rotational (93 %)
Two Breed Rotational
Roto-Terminal- combination of rotational and
terminal systems. The poor ewes would still be
used in a terminal crossing manner
Inherited Defects
• Many defects are recdessive in nature
• Selection pressure is exercised against certain
traits, esp. lethals and fleece defects
• Dwarfism, Spider Syndrome, Jaw Defects,
Rectal Prolapse, Inverted Eyelids,
Cryptorchidism, Horns or Scurs, Face covering,
Color, Skin Folds, Silky, Britch fibers, grey color,
paralyzed limbs, earless, etc.
Sheep
Breeding and Reproduction
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Improved lamb production
More lambs per lambing
More frequent lambing
Increased percent of total sheep nos.
Reducing death losses
Sheep
Breeding and Reproduction
• breed dependent - seasonal breeding
• Puberty-weight and age (70-100 # or 5-9 months)
• Breed to lamb as 2 year olds, yet can breed as a
yearling, but with less efficiency
• Actually, greater production per lifetime from
yearlings; Ramb. is later maturing and less effic.
• gestation - 144-152 days (med wool less)
• heat periods - 20 to 42 hours (no signs)
Sheep
Breeding and Reproduction
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ovulation occurs late in heat cycle
14- 20 day cycle ave. 16-17 days
Prolifacy - > 100% lamb crop
flushing ewes works in sheep, too
Factors affecting reprod.
– daylight (<14 hrs/da), temp. ( <74 F or > 100),
nutrition
Hormonal Control
• Progesterone (Progestins)- produced by the corpus
luteum
• Prostaglandins- induces the regression of the
corpus luteum (ineffective in normally cycling
ewes); lutalyse
• Estrogen
• Gonadotropins- GnRH, HCG, FSH & LH
• Melatonin- regulatory of the seasonal breeder;
produced by the pineal gland
• ACTH
Sheep
Breeding and Reproduction
• Lambing
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> 4 sq. ft. pen for ewes, clean & dry
presentation of front legs
orphan lambs is not uncommon
25% death loss is common
Feed ewes small amounts of water at first with
oats, wheat bran and hay (small amounts)
Sheep
Breeding and Reproduction
• Ram management
• mating guide
– ram lamb
hand (20-25)
pasture (25-35)
– yearling or > hand (50-75)
pasture (35-60)
– use marker rams to detect bred ewes both at
breeding and 60 days after breeding
Replacement Ewe Selection
Item
140 % Prolif 170 % Prolif
No. ewes
100
100
Ewe lambs
70
85
Twin lambs
40
70
Tw lambs > 20
ave. in WW
Ewe lambs 20
exposed
Preg. Ewes
35
30
20
Physiological changes of
reproduction in the ewes
• REPRO – 4 chart
• Ewes in the south have longer breeding
patterns
• Sheep near the equator are less likely to
have seasonal breeding
Factors affecting reproduction in
the ewe
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Heredity
Age
Photoperiod (seasonal)
Temperature and humidity
Nutrition and Exercise
Parturition and lactation
Disease and parasites
Fertility of & assoc. with the ram
Pregnancy Testing
• Breeding Marks
• Ultrasonic Scanning- best detected between
70 & 120 days
• Bagging or Udder Palpation
• Blood Progesterone- at the time of the next
expected heat
• PSPB- a protein called pregnancy-specific
protein B after day 21 of breeding.
Factors affecting the reproduction
in the Ram
• Breeding soundness exam
• Palpation of the testicles, epididymis, and
penis and visual appraisal of feet, legs, eyes
and jaws.
• Semen evaluation
• Disease prevention
• Heat stress
Semen Collection
• Components- seminal fluid and sperm
• Quality of sperm – morphology and
viability (percent live)
• Methods- artificial vagina and
electroejaculation
• Semen handling-dilution characteristics:
glucose or fructose, egg yolk, citrate or
phosphate, antibitotics, glycerol
Insemination
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Natural- 3-5 billion sperm inseminated
AI – vaginal approach – 200 million “
AI – cervical approach- 100 million “
AI - Intruterine insemination via lapraroscopic
surgery- 20 million
• Time of insemination – vaginal or cervical = 12 to
18 hrs after onset of estrus
• Synchronizing with progesterone sponge should
be 48-58 hrs after removal
Accelerated and Out-of-Season
Lambing
• Day length control (natural or artificial)
• Considerations
• Produce lambs when feeding conditions are
favorable (growth rate and cost of feed)
• Market lambs when lamb supply is low
• Fertility and prolificacy
Desirable traits for accelerated
lambing
• Ewes can breed year round
• Ewes that can mate while lactacting
• Ewes that have a good lambing rate (ie
twinning)
• Sires that produce a desirable market lamb
and have the libido and fertility for
conception year round