Can You Breed a “Good Breeder”
Kristi M. Cammack
Department of Animal Science
University of Wyoming
Question of the Day:
What is Fertility?
• Female:
–
–
–
–
–
Pregnancy rate?
Heifer pregnancy?
Calving rate?
1st service conception rate?
Longevity?
• Male:
– Scrotal circumference?
– Breeding soundness?
– Libido/service capacity?
What is Fertility?
• In short, there is no single trait that
defines fertility!
• Too many inputs!
– “Successful reproduction is dependent on
many factors that require sires and dams
capable of carrying out each critical stage of
reproductive development.”
Mating
Fertilization
Gestation / Fetal Development
Parturition
Postnatal survival / growth
Factors Affecting Reproduction
• Species
– Bos taurus vs. Bos indicus
• Breed
– Purebred
– Crossbred
•
•
•
•
Location
Sex
Animal class
Environment
– Management
– Production setting
– Etc.!
Reproduction Trait Evaluation
• National evaluations historically focuses on
production traits.
– Growth traits.
– Carcass traits.
• Why?
– Limited data available for reproduction traits.
• Lack of total-herd reporting.
– Difficulty in analyses procedures.
• Especially binary traits.
– Ex: Pregnancy (Yes, No)
– Generally lowly h2.
h2 of Common Female Reproduction Measures
Reproduction Trait Evaluation
• Why the low h2?
– A large part of the observed variation is
unexplainable.
• Unknown environmental effects.
• Yet unexplained genetic effects.
– Additive, non-additive
– Reproductive traits largely influenced by
management practices.
Female Reproduction
• Beef cattle not reproductively efficient.
– Per service calving rate ~50-60%.
• AI or natural service.
• Function of underlying endocrine and
physiological factors.
• ↑ Efficiency of cow-calf herd requires:
– Improved cow fertility.
– Improved yearling heifer fertility.
• Replacement heifer development program.
Female Reproduction
– The crux of the situation…
Selection has not been practiced to
improve fertility…but instead to
minimize infertility.
Age at Puberty
• Measure of heifer fertility.
– Subsequent reproductive performance.
• Other predictors of heifer fertility:
– Age at first estrous.
– Age at first breeding.
• In general…
– Reproductively efficient heifers reach puberty
sooner, and therefore conceive earlier.
Age at Puberty
• Measured as first observed standing heat.
• Affected by:
– Body weight.
– Nutrition.
– Hormones.
– Breed!
Age at Puberty
• Variable h2 estimates:
2
h
< 0.10
0.10 to < 0.20
0.40 to < 0.50
> 0.60
# Estimates
1
3
4
3
Purebred H
Purebred A
Purebred mean (A or H)
Crossbred mean (HxA or AxH)
H = Hereford; A = Angus
From: Laster et al., 1972
Age at Puberty (days)
389.5
372.2
380.9
360.9
Age at Puberty
• Correlated Trait - Weight at Puberty
– h2: 0.40 to 0.70
Purebred H
Purebred A
Purebred mean (A or H)
Crossbred mean (HxA or AxH)
H = Hereford; A = Angus
From: Laster et al., 1972
Weight at Puberty (lb)
593.5
603.4
598.3
590.4
Weight of Heifers Reaching Puberty by 15 Months of Age (lb)
Yes
No
Difference
HxH
606.9
543.9
63.1
AxA
640.9
552.9
88.0
HxA
662.9
567.9
95.0
AxH
631.0
586.0
45.0
From: Laster et al., 1972
Age at First Calving
• Routinely recorded.
• h2: 0.01 to 0.37
• Genetically correlated with:
– Age at subsequent calvings.
– Interval between subsequent calvings.
• Used to evaluate heifer fertility.
• Later age at first calving:
– Associated with ↓ lifetime productivity.
Calving Date
• Routinely recorded.
• h2: 0.03 to 0.21
• Reflection of:
– Initiation of calving by calf.
– Initiation of estrous cycles by dam.
– Semen quality of sire.
– Libido / service of sire.
Calving Date
• Generally, earlier is better:
– Calves have ↑weaning weights.
• Predetermined calendar date versus weight- or
age-constant weaning date.
– Dams have ↑ postpartum interval.
• Sufficient time to return to estrus.
First Service Conception Rate
• Economically driven:
– Cost of semen.
– Labor for estrus detection.
– Labor for breeding.
– AI versus Natural Service.
• Calf differences.
– Age.
– Performance.
• Management tool:
– 1st breeders versus multiple breeders.
First Service Conception Rate
• h2: 0.03 to 0.22
• Other traits that take AI versus Natural
Service into account:
– Calving to 1st insemination.
– Conceptions per estrous cycle.
– Conceptions per service.
Pregnancy Rate
• Binary trait.
– 1 = pregnant; 0 = not pregnant.
• h2: 0.14 to 0.21
• Heifers:
– Sexual maturity.
– Probability of exposed heifer becoming pregnant,
and remaining pregnant.
• Become pubertal and pregnant by 12 to 15 months of
age.
• Calve by 24 months of age.
Pregnancy Rate
• For economic viability:
– Replacement heifers must calve by 2 years.
– And must remain in productive herd.
• Lifetime Pregnancy Rate:
– # pregnancies / # mating years
– h2: 0.04 to 0.12
– Affected by number of factors, especially length of
breeding season.
• Longer breeding season = ↑ Pregnancy Rates
• But also ↓ weaning weights and↓ postpartum period
potentially.
Pregnancy Rate
• Not generally affected by breed.
– Typically used breed types.
• ↑ conception rates when inseminations made
prior to end of standing estrus.
Pregnancy Rate
Correlations of Conception Rate with Other Reproductive Traits of Interest
Correlation
Weight at start of breeding period
-0.02
Weight change from weaning to breeding
0.12
Date of estrus
-0.02
Number of estrus events before breeding
-0.12
From: Laster et al., 1972
Net Calf Crop
• % Calves weaned per cow exposed.
• “Gross” measure of herd reproductive
ability.
• h2: assumed low
• < 100% calf crop:
– Non-pregnant females.
– Fetal deaths during gestation.
– Peri-natal deaths.
– Post-natal deaths.
Calving Rate
• # Calves produced by a cow / # of
potential calves.
• h2: 0.02 to 0.17
Calving Interval
• Routinely recorded.
• # days between successive calvings.
• h2: 0.13
• Challenges:
• Selection for ↓ calving interval = Indirect selection for later
age at puberty.
– 1st calf born late.
• Biases.
• How to handle those with no record(s).
Dystocia
• Calving difficulty.
• “Risk” factor.
– Increased in heifers.
• h2: 0.22 to 0.42
• Scaled:
1
2
3
4
5
BIF Calving Ease Scores
No difficulty; no assistance
Minor difficulty; no assistance
Major difficulty; usually mechanical assistance
Caesarian section or other surgery
Abnormal presentation
Dystocia
•
•
•
•
↓ Calf survival at birth.
↓ Subsequent milk production.
↓ Calf survival to weaning.
↑ Risk of culling.
– ↓ subsequent reproductive success.
Dystocia
• Why?
– Feto-pelvic incompatibility.
• Oversized calf.
– Higher BW.
– Longer gestation period.
• Undersized pelvic area.
– Structural.
– More “permanent” cause?
• Both.
Longevity / Stayability
• Longevity:
– Length of time in breeding herd.
– Meaning…
• Fewer replacement heifers.
• ↑ # high producing cows.
• ↓ # culled cows.
– However, not measured until late in life.
• Stayability:
– Probability of cow staying in herd until a given age.
• Predicted earlier in life.
– h2: 0.02 to -0.23
• Dependent upon “given age” selected.
Male Reproduction
• AI versus Natural Service
• Bull “fertility” affected by:
– Number of females expected to service.
– Length of mating period.
– Serving capacity
Male Reproduction
• Other considerations:
– Bull:cow ratio
– Behavior
– Temperment
– Management
Scrotal Circumference
• 1. Predict quality and quantity of
spermatozoa.
• 2. Predict age at puberty of daughters.
– Indicator trait.
– Why?
• Easy to measure.
• Highly h2!
2
h
0.20 to < 0.40
0.40 to < 0.50
0.50 to < 0.80
# Estimates
6
5
3
**Highest estimate: 0.78 (Coulter and Foote, 1979)
Scrotal Circumference
• ↑ SC associated with:
– ↑Sperm production.
– ↓Semen quality.
– ↓Age at puberty.
– Growth traits???
Breeding Soundness
• Most practical means of male “fertility”
assessment.
• Includes:
– Physical examination.
– SC measurement.
– Semen evaluation.
• Not sex drive / mating ability.
Breeding Soundness
• Improved reproductive efficiency:
– Identification of subfertile bulls.
– Recurring assessment of “fertile” bulls.
• Reasons for unsatisfactory scores:
– Inadequate SC.
• ≥30 cm by 1 year of age.
– Inadequate sperm motility.
– Abnormal sperm morphology.
– Many more…
Libido and Serving Capacity
• Libido – Sex drive of a bull.
– Single bull + restrained female.
• # Mating attempts.
• Vigor of mating attempts.
• Subjective assessment of sexual interest.
• Serving Capacity – Number of times a bull
mounts and copulates.
– Steroid-treated or non-estrous females + small
group of bulls.
• # Services within specified time frame.
• Subjective score.
• ↑ Scoring bulls = ↑ Pregnancy rates.
Obstacles - Female Reproduction
• Numerous “fertility” traits recorded.
• Long time required to record many such
traits.
– ↓ data reported.
• Low h2.
• Limited data collection in pasture mating
systems.
Obstacles – Male Reproduction
• Variable assessments.
– Serving capacity versus libido.
• Many bull “fertility” traits recorded in the
female.
– Pregnancy rate, etc.
• Few h2 estimates.
– None available for breeding soundness, serving
capacity, or libido.
• Difficult to identify lowly “fertile” bulls in
natural mating situations.
Obstacles - Genetic Analysis
• Lack of whole-herd reporting.
– Recently implemented in most U.S. breeds.
• Binary nature of reproductive traits.
– Yes, no
– 0, 1
• Time required to collect data necessary for
reproductive traits.
• Uniformity of reproductive traits.
– Many similar traits with slight variations.
– BIF guidelines needed?
But there is hope…
• A number of U.S. breed associations now recording
reproductive performance traits.
– SC, heifer pregnancy, and stayability included in some
evaluations.
• American Angus Association, Red Angus
Association, American Hereford Association.
• Genetic correlations with other traits (e.g.
production traits) that are more highly h2.
• Some reproductive traits are themselves moderately
h2.
But there is hope…
• Some reproductive traits show evidence of
genetic influence.
– a.k.a. “Yet unexplained genetic effects”
• Crossbreeding can be used to make nonadditive genetic improvements:
–
–
–
–
–
Earlier puberty
Increased pregnancy rate
Decreased dystocia
Increased longevity
Decreased calving interval
Acknowledgements
• Milt Thomas (New Mexico State University)
• Mark Enns (Colorado State University)
• WERA-1 (Beef Cattle Breeding Committee)