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
And an in
finite #
of s
teps
in b
etw
een!
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.
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:
h2 # Estimates< 0.10 1
0.10 to < 0.20 30.40 to < 0.50 4
> 0.60 3
From: Laster et al., 1972
Age at Puberty (days)Purebred H 389.5Purebred A 372.2Purebred mean (A or H) 380.9Crossbred mean (HxA or AxH) 360.9H = Hereford; A = Angus
Age at Puberty
• Correlated Trait - Weight at Puberty– h2: 0.40 to 0.70
Weight at Puberty (lb)Purebred H 593.5Purebred A 603.4Purebred mean (A or H) 598.3Crossbred mean (HxA or AxH) 590.4H = Hereford; A = Angus
From: Laster et al., 1972
Weight of Heifers Reaching Puberty by 15 Months of Age (lb)Yes No Difference
H x H 606.9 543.9 63.1A x A 640.9 552.9 88.0H x A 662.9 567.9 95.0A x H 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 InterestCorrelation
Weight at start of breeding period -0.02Weight change from weaning to breeding 0.12Date of estrus -0.02Number 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 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 No difficulty; no assistance2 Minor difficulty; no assistance3 Major difficulty; usually mechanical assistance4 Caesarian section or other surgery5 Abnormal presentation
BIF Calving Ease Scores
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
Scrotal Circumference• 1. Predict quality and quantity of
spermatozoa.• 2. Predict age at puberty of
daughters.– Indicator trait.–Why? • Easy to measure.• Highly h2!h2 # Estimates
0.20 to < 0.40 60.40 to < 0.50 50.50 to < 0.80 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 non-additive genetic improvements:– Earlier puberty– Increased pregnancy rate– Decreased dystocia– Increased longevity– Decreased calving interval