Genetic predictions in the form of expected progeny differences (EPDs) represent the beef industry\u27s most powerful source of information for selection and genetic improvement. While EPDs are widely available for traits associated with calving ease, growth, milk and carcass traits, EPDs for reproductive traits are limited. Given the relative economic importance, development of EPDs for reproductive traits should be a priority for the beef industry. Fortunately, recent breakthroughs in analytical procedures have opened the door for potential development of genetic predictions for reproductive traits

Andersen, Kent

English

DigitalCommons@University of Nebraska

University of Nebraska - Lincoln DigitalCommons@University of Nebraska - Lincoln Range Beef Cow Symposium Animal Science Department December 1999 Genetic Predictors of Reproduction Kent Andersen North American Limousin Foundation Follow this and additional works at: https://digitalcommons.unl.edu/rangebeefcowsymp  Part of the Animal Sciences Commons Andersen, Kent, "Genetic Predictors of Reproduction" (1999). Range Beef Cow Symposium. 116. https://digitalcommons.unl.edu/rangebeefcowsymp/116 This Article is brought to you for free and open access by the Animal Science Department at DigitalCommons@University of Nebraska - Lincoln. It has been accepted for inclusion in Range Beef Cow Symposium by an authorized administrator of DigitalCommons@University of Nebraska - Lincoln. Proceedings, The Range Beef Cow Symposium XVIDecember 14, 15 and 16, 1999 Greeley, ColoradoGENETIC PREDICTORS OF REPRODUCTIONKent Andersen, Ph.D.Director of Education and ResearchNorth American Limousin FoundationIntroductionGenetic predictions in the form of expected progeny differences (EPDs) represent the beef industry's most powerful source of information for selection and genetic improvement. While EPDs are widely available for traits associated with calving ease, growth, milk andcarcass traits, EPDs for reproductive traits are limited.  Given the relative economic importance,development of EPDs for reproductive traits should be a priority for the beef industry. Fortunately, recent breakthroughs in analytical procedures have opened the door for potentialdevelopment of genetic predictions for reproductive traits.  Defining ReproductionTo most cattlemen, successful reproduction means pregnancy and a live calf producedevery year.  Reproductive failure is represented by open and late bred cows, as well as by cowsthat abort prior to calving.   More specifically, to most cattlemen, successful reproduction means: • yearling replacement heifers which are cycling and become pregnant early in their firstbreeding season, • wet two year olds which breed back early in their second breeding season, • consistent annual calving after limited fixed breeding seasons until at least break-even age,and • as much longevity as possible to minimize annual replacement rates.   This definition of reproduction is fairly involved, and implies that EPDs for severalreproductive traits might be necessary to adequately describe economically important geneticdifferences.  Specifically, genetic predictions for the fertility of replacement heifers might beneeded in addition to predictions for sustained reproductive performance.  Procedures have beendeveloped to calculate EPDs for two such traits which are direct measures of reproduction, heiferpregnancy and stayability.  A Note about Economically Important Traits Versus Indicator TraitsIn their paper titled "New EPDs: A Rational Vision for the Future," Dr. Bruce Goldenand Dr. Rick Bourdon discuss the concept of economically relevant traits and indicator traits. These authors define economically relevant traits as the traits that directly effect profit by beingdirectly associated with a specific cost of production or an income stream from the sale of aproduct.  Traits which do not directly affect profit, but are used to indicate the merit an animalhas for another trait are called indicator traits. Almost all economically relevant traits havemultiple indicator traits.This concept is applicable to the topic of genetic predictors of reproduction.  The traitswhich are economically relevant, such as heifer pregnancy and stayability, have multipleindicator traits such as scrotal circumference, age at puberty, condition at breeding, etc. Arguably, indicator traits such as age at puberty and condition score are only as important astheir relationship to the economically relevant trait – pregnancy.  These authors go on to pointout that while information from an indicator trait can be used to add accuracy to geneticpredictions for the economically relevant trait, having EPDs for both types of traits may addconfusion to the selection process, and can actually increase the likelihood of a bad selectiondecision because the indicator trait information may be inappropriately "double counted" duringselection (Golden, 1997).  Genetic Predictors of Reproduction - Economically Relevant TraitsHeifer Pregnancy EPDsResearchers at Colorado State University have worked for the past several years in thedevelopment of genetic predictions for heifer pregnancy (Evans, et al. 1996).  Several differentpopulations have been studied, including Hereford, Limousin and Angus cattle.  As well, at leasttwo breeds, Limousin and Red Angus are actively pursuing development of heifer pregnancyEPDs to include to their breed wide performance programs.  Historically, fertility traits have been thought to be lowly heritable, with estimatesprojected at around .10 (Koots et al. 1994).  However, when heifer pregnancy was defined as theobservation of a heifer conceiving and remaining pregnant to palpation, given the heifer had anopportunity to breed, and when using more appropriate analytical procedures, several studieshave indicated that the heritability of heifer pregnancy may be higher (.14 to .30) than previouslythought (Doyle et al. 1996, Evans et al. 1996, Snelling et al. 1996 and Summers et al. 1999).   Asa point of reference, "heritability" is a population statistic which measures the proportion ofobserved differences in a trait that are caused by genes passes from parents to offspring.  Experimental EPDs for heifer pregnancy are expressed in units of  probability, andpredict genetic differences in the likelihood that daughters (retained replacement heifers) willbecome pregnant during their first breeding season. Higher heifer pregnancy EPDs are preferred,and indicate genetics for a better likelihood of pregnancy.  For example, consider the heiferpregnancy EPDs of the following two sires:Heifer Pregnancy EPDSire A +10%Sire B -5%Difference 15%If sires A and B were bred to comparable sets of cows and all heifer offspring wereretained for breeding, 15% more of sire A's daughters are expected to be diagnosed as pregnantfollowing the first breeding season as compared to daughters of sire B.  In other words, heifersfrom sire A have a 15% greater likelihood of pregnancy as compared to sire B's daughters.   In a heifer pregnancy study conducted by Colorado State University for the NorthAmerican Limousin Foundation, records from 2,665 heifers representing 290 sires with at leastone daughter were used to compute a heritability estimate and experimental heifer pregnancyEPDs.  The heritability of heifer pregnancy was estimated at .20, with EPDs ranging from -14%to +11%.  While the standard error associated with the heritability estimate was high because ofthe size of the data set, this study helps to demonstrate the possibility of breed wide heiferpregnancy EPDs.In order for heifer pregnancy EPDs to become widely available, breed associations willhave to develop protocols for data submission and processing.  At present, at least twoassociations are actively engaged in this process.  Additional research using larger data sets willhelp to shed further light on the heritability of heifer pregnancy in different populations of cattle.Stayability EPDSFor several years, EPDs for stayability have been computed for the Red Angus andLimousin breeds.  Stayability EPDs predict genetic differences in the likelihood or probabilitythat daughters will remain in production until six years of age or beyond, given that daughtershad at least one calf reported prior to six years of age.  Since the primary reason cows are culledis because of reproductive failure, EPDs for stayability are primarily thought to indicate geneticdifferences in sustained reproduction.  To a lesser extent, stayability EPDs may also representgenetic differences in a multitude of additional factors which contribute to reasons whydaughters are "liked" by breeders, and as a result have calves reported before and after six yearsof age.  Nonetheless, stayability EPDs help predict sustained reproduction and longevity. The following example illustrates the use of stayability EPDs:Stayability EPDSire A        +15%Sire B        +5%Difference        10%If sires A and B above were bred to comparable groups of cows, 10% more of sire A's daughtersare expected to remain in production until the age of six years as compared to sire B's daughters(15% - 5% = 10%).  Said another way, each daughter of sire A is expected to have a 10% greaterlikelihood of staying in production to the break-even age (defined as six years) as compared todaughters of sire B.  To better understand the tradeoffs between selection for stayability and selection forother traits, the Limousin Foundation has studied the relationships between EPDs for traitsincluded in its performance program.  Favorable, but small relationships exist between EPDs forstayability and EPDs for all other traits, except birth weight.  The magnitude of the relationshipsranged from only .01 between stayability and scrotal circumference EPDs, to .25 for therelationship with yearling weight EPD.  The correlation with scrotal circumference suggests thatpuberty and sustained reproduction are traits controlled by different sets of genes in theLimousin population. The correlation between EPDs for stayability and birth weight was .17. Based on these correlation, it should be possible to simultaneously select for geneticimprovement in stayability and in other production traits. Because of how stayability is defined, it is important to note that compared to other traits,considerably more time is required for animals to achieve high levels of accuracy for stayability. Adoption of inventory based record keeping systems by several breeds should help to insure theintegrity of reproduction and cow disposal data, which in turn will provide opportunities fordevelopment of genetic predictions for "survival" traits.   Genetic Predictors of Reproduction - Indicator Traits Scrotal Circumference EPDs The most widely used indicator trait for reproduction is scrotal circumference.  Severalbreeds have routinely calculated and published scrotal circumference EPDs for both bulls andfemales in their populations.  Scrotal circumference is relatively easy to measure, is highlyheritable, is an excellent indicator of puberty and is normally distributed and easy to analyze.  Interms of genetic predictors for reproduction in bulls, scrotal circumference is favorableassociated with both semen quality and quantity.  For both bulls and females, it goes withoutsaying that puberty is a prerequisite for reproduction.  However, it appears that scrotal circumference primarily  accounts for genetic variationin puberty, and not much variation in observed pregnancy, especially after the first breedingseason.   There appear to be other genetic factors which determine heifer pregnancy andsustained reproductive performance which are not accounted for by genetic differences in scrotalcircumference and age at puberty. Milk, Mature Size and Body ConditionWhile genetic potentials for milk production, mature size and fleshing ability are notnecessarily reproductive traits, depending upon the available nutritional resources, these traitscan significantly influence expressed pregnancy rates.  Conventional wisdom would support thenotion that matching genetic levels for these traits to available feed resources will serve as aninsurance policy for acceptable reproductive performance.  To go one step further, important genetic interactions exist between an animal's inherentgenetics for fertility, the animal's nutritional requirements which are largely determined bygenetic potentials for milk production and mature size, the available feed resources, andultimately the expressed pregnancy rate.  High levels of genetic merit for fertility should providegreater flexibility when matching levels of milk production, mature size and other traits toavailable resources and targeted markets. As well, high levels of inherent fertility might allowfor money to be saved on feed, if cows can exhibit acceptable levels of expressed pregnancydespite lower levels of available nutrition and perhaps body condition.Genetic predictions for maintenance energy requirements, which take into accountgenetic differences in traits such as milk and mature size, would be extremely useful inbalancing selection for reproduction and other economically important traits.  Recognizing thechallenges of developing such predictions, EPDs for mature size and perhaps body conditionscore would help to provide useful information. Calving Ease and ReproductionResearch has also shown that heifers which required assistance at calving had lowerpregnancy rates following the second breeding season by three to five percent, and were about aweek later in terms of day of second calving as compared to heifers not requiring assistance(Andersen, 1991).  Similarly, in a subtle way, gestation length can also impact reproductiveperformance.  Shorter gestation length equates to slightly more time between calving and thestart of fixed breeding seasons, thus allowing more time to recover and begin cycling. Consequently, selection and management for calving ease, especially in replacement heifers,favorably impacts reproductive performance.   Heterosis and ReproductionThe focus of this paper has been on genetic predictors of reproduction.  Geneticpredictions (EPD) estimate additive genetic differences which are passed onto offspring.  Theimportance of heterosis, or non-additive genetic value, and breed complimentarity which isgained through smart crossbreeding should not be underestimated.  The value of heterosis issubstantiated in a recent report from the U.S. Meat Animal Research Center entitled, "CompositeBreeds To Use Heterosis and Breed Differences To Improve Efficiency of Beef Production." The following excerpt summarizes the value of heterosis:  Heterosis (hybrid vigor) for major bioeconomic traits, including reproduction,growth rate, and longevity of beef cattle is important.  Heterosis can be used toincrease weight of calf per cow exposed to breeding by up 20 percent.  Crossbredcows remain in the herd 1.3 years longer and have a 30 percent greater lifetimeproduction than straight bred cows.  Large breed differences exist among breedsof beef cattle for major bioeconomic traits, including growth rate and size,composition of gain, milk production, calving difficulty, age at puberty, andclimatic nutritive adaptability.  These are traits where the optimum is determinedby production environment and by market requirements (Gregory, Cundiff andKoch, 1999).  ConclusionRecent breakthroughs in analytical procedures look promising relative to calculation ofgenetic predictions for economically relevant reproductive traits.  Experimental results suggestthat when modeled appropriately, heifer pregnancy may be more highly heritable than previouslythought.  Preliminary EPDs for heifer pregnancy, and EPDs for the trait "stayability", offerinformation as to the genetic differences in the likelihood that heifers are pregnant after the firstbreeding season, and remain in production until break-even age.  Genetic predictions forindicator traits, such as scrotal circumference, effectively describe genetic differences in age apuberty, but may not adequately describe genetics for heifer pregnancy and sustainedreproductive performance.  As it relates to expressed fertility, interactions exist between geneticpotentials for inherent fertility and traits such as milk production, mature size, and maintenancerequirements.  To help ensure acceptable reproductive performance, the value of heterosis andbreed complimentarity derived through smart crossbreeding should not be overlooked.  ReferencesAndersen, K.J., 1991.  Reproductive, calving ease and growth traits in beef heifers. Ph.D.Dissertation. Colorado State Univ., Fort Collins.Doyle, S.P., R.D. Green, B.L. Golden, G.L. Mathews, C.R. Comstock, and D.G. LeFever. 1996.Genetic parameter estimates for heifer pregnancy rate and subsequent rebreeding rate in Anguscattle. J. Anim. Sci. 74 (Suppl. 1)117 (Abstr.).Evans, J.L. 1996. Genetic parameter estimate of yearling heifer pregnancy and yearling bullscrotal circumference in Hereford cattle.  M.S. Thesis. Colorado State Univ., Fort Collins.Gregory, K.E., L.V. Cundiff, and R.M. Koch. 1999. Composite breeds to use heterosis and breeddifferences to improve efficiency of beef production.  Technical Bulletin Number 1875, October1999.Golden, B.L. and R.M. Bourdon, 1999. New EPDs: A rational vision for the future. Theprinciple of economically relevant traits versus indicators. Dept. of Anim. Sci., Colorado StateUniv., Fort Collins. Golden, B.L., J.L. Evans, R.M. Bourdon, and K.L. Long. 1997. Heifer pregnancy and scrotalcircumference: A model for multiple trait selection using expected progeny differences. Dept. ofAnim. Sci., Colorado State Univ., Fort Collins.Koots, K.R., J.P. Gibson, C. Smith, and J.L. Wilson. 1994. Analysis of published geneticparameter estimates for beef production traits. 1. Heritability. Anim. Br. Abst. 62:309-338. Snelling, W.M., M.D. MacNeil, and B.L. Golden. 1996. Application of continuous and binarytrait methods to reproductive measures of Hereford cattle. J. Anim. Sci. 74 (Suppl. 1):115(Abstr.).Summers, M.D., J.L. Evans, L.R. Hyde, C.R. Comstock, K.J. Andersen, and B.L. Golden. 1999. Estimation of heritability for heifer pregnancy in Limousin cattle. Proc. West. Sec. Amer. Soc.Anim. Sci. Vol. 50

Genetic Predictors of Reproduction

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University of Nebraska - Lincoln 
DigitalCommons@University of Nebraska - Lincoln 
Range Beef Cow Symposium Animal Science Department 
December 1999 
Genetic Predictors of Reproduction 
Kent Andersen 
North American Limousin Foundation 
Follow this and additional works at: https://digitalcommons.unl.edu/rangebeefcowsymp 
 Part of the Animal Sciences Commons 
Andersen, Kent, "Genetic Predictors of Reproduction" (1999). Range Beef Cow Symposium. 116. 
https://digitalcommons.unl.edu/rangebeefcowsymp/116 
This Article is brought to you for free and open access by the Animal Science Department at 
DigitalCommons@University of Nebraska - Lincoln. It has been accepted for inclusion in Range Beef Cow Symposium 
by an authorized administrator of DigitalCommons@University of Nebraska - Lincoln. 
Proceedings, The Range Beef Cow Symposium XVI
December 14, 15 and 16, 1999 Greeley, Colorado
GENETIC PREDICTORS OF REPRODUCTION
Kent Andersen, Ph.D.
Director of Education and Research
North American Limousin Foundation
Introduction
Genetic predictions in the form of expected progeny differences (EPDs) represent the 
beef industry's most powerful source of information for selection and genetic improvement. 
While EPDs are widely available for traits associated with calving ease, growth, milk and
carcass traits, EPDs for reproductive traits are limited.  Given the relative economic importance,
development of EPDs for reproductive traits should be a priority for the beef industry. 
Fortunately, recent breakthroughs in analytical procedures have opened the door for potential
development of genetic predictions for reproductive traits.  
Defining Reproduction
To most cattlemen, successful reproduction means pregnancy and a live calf produced
every year.  Reproductive failure is represented by open and late bred cows, as well as by cows
that abort prior to calving.   More specifically, to most cattlemen, successful reproduction means: 
• yearling replacement heifers which are cycling and become pregnant early in their first
breeding season, 
• wet two year olds which breed back early in their second breeding season, 
• consistent annual calving after limited fixed breeding seasons until at least break-even age,
and 
• as much longevity as possible to minimize annual replacement rates.   
This definition of reproduction is fairly involved, and implies that EPDs for several
reproductive traits might be necessary to adequately describe economically important genetic
differences.  Specifically, genetic predictions for the fertility of replacement heifers might be
needed in addition to predictions for sustained reproductive performance.  Procedures have been
developed to calculate EPDs for two such traits which are direct measures of reproduction, heifer
pregnancy and stayability.  
A Note about Economically Important Traits Versus Indicator Traits
In their paper titled "New EPDs: A Rational Vision for the Future," Dr. Bruce Golden
and Dr. Rick Bourdon discuss the concept of economically relevant traits and indicator traits. 
These authors define economically relevant traits as the traits that directly effect profit by being
directly associated with a specific cost of production or an income stream from the sale of a
product.  Traits which do not directly affect profit, but are used to indicate the merit an animal
has for another trait are called indicator traits. Almost all economically relevant traits have
multiple indicator traits.
This concept is applicable to the topic of genetic predictors of reproduction.  The traits
which are economically relevant, such as heifer pregnancy and stayability, have multiple
indicator traits such as scrotal circumference, age at puberty, condition at breeding, etc. 
Arguably, indicator traits such as age at puberty and condition score are only as important as
their relationship to the economically relevant trait – pregnancy.  These authors go on to point
out that while information from an indicator trait can be used to add accuracy to genetic
predictions for the economically relevant trait, having EPDs for both types of traits may add
confusion to the selection process, and can actually increase the likelihood of a bad selection
decision because the indicator trait information may be inappropriately "double counted" during
selection (Golden, 1997). 
 
Genetic Predictors of Reproduction - Economically Relevant Traits
Heifer Pregnancy EPDs
Researchers at Colorado State University have worked for the past several years in the
development of genetic predictions for heifer pregnancy (Evans, et al. 1996).  Several different
populations have been studied, including Hereford, Limousin and Angus cattle.  As well, at least
two breeds, Limousin and Red Angus are actively pursuing development of heifer pregnancy
EPDs to include to their breed wide performance programs.  
Historically, fertility traits have been thought to be lowly heritable, with estimates
projected at around .10 (Koots et al. 1994).  However, when heifer pregnancy was defined as the
observation of a heifer conceiving and remaining pregnant to palpation, given the heifer had an
opportunity to breed, and when using more appropriate analytical procedures, several studies
have indicated that the heritability of heifer pregnancy may be higher (.14 to .30) than previously
thought (Doyle et al. 1996, Evans et al. 1996, Snelling et al. 1996 and Summers et al. 1999).   As
a point of reference, "heritability" is a population statistic which measures the proportion of
observed differences in a trait that are caused by genes passes from parents to offspring.  
Experimental EPDs for heifer pregnancy are expressed in units of  probability, and
predict genetic differences in the likelihood that daughters (retained replacement heifers) will
become pregnant during their first breeding season. Higher heifer pregnancy EPDs are preferred,
and indicate genetics for a better likelihood of pregnancy.  For example, consider the heifer
pregnancy EPDs of the following two sires:
Heifer Pregnancy EPD
Sire A +10%
Sire B -5%
Difference 15%
If sires A and B were bred to comparable sets of cows and all heifer offspring were
retained for breeding, 15% more of sire A's daughters are expected to be diagnosed as pregnant
following the first breeding season as compared to daughters of sire B.  In other words, heifers
from sire A have a 15% greater likelihood of pregnancy as compared to sire B's daughters.   
In a heifer pregnancy study conducted by Colorado State University for the North
American Limousin Foundation, records from 2,665 heifers representing 290 sires with at least
one daughter were used to compute a heritability estimate and experimental heifer pregnancy
EPDs.  The heritability of heifer pregnancy was estimated at .20, with EPDs ranging from -14%
to +11%.  While the standard error associated with the heritability estimate was high because of
the size of the data set, this study helps to demonstrate the possibility of breed wide heifer
pregnancy EPDs.
In order for heifer pregnancy EPDs to become widely available, breed associations will
have to develop protocols for data submission and processing.  At present, at least two
associations are actively engaged in this process.  Additional research using larger data sets will
help to shed further light on the heritability of heifer pregnancy in different populations of cattle.
Stayability EPDS
For several years, EPDs for stayability have been computed for the Red Angus and
Limousin breeds.  Stayability EPDs predict genetic differences in the likelihood or probability
that daughters will remain in production until six years of age or beyond, given that daughters
had at least one calf reported prior to six years of age.  Since the primary reason cows are culled
is because of reproductive failure, EPDs for stayability are primarily thought to indicate genetic
differences in sustained reproduction.  To a lesser extent, stayability EPDs may also represent
genetic differences in a multitude of additional factors which contribute to reasons why
daughters are "liked" by breeders, and as a result have calves reported before and after six years
of age.  Nonetheless, stayability EPDs help predict sustained reproduction and longevity. 
The following example illustrates the use of stayability EPDs:
Stayability EPD
Sire A        +15%
Sire B        +5%
Difference        10%
If sires A and B above were bred to comparable groups of cows, 10% more of sire A's daughters
are expected to remain in production until the age of six years as compared to sire B's daughters
(15% - 5% = 10%).  Said another way, each daughter of sire A is expected to have a 10% greater
likelihood of staying in production to the break-even age (defined as six years) as compared to
daughters of sire B.  
To better understand the tradeoffs between selection for stayability and selection for
other traits, the Limousin Foundation has studied the relationships between EPDs for traits
included in its performance program.  Favorable, but small relationships exist between EPDs for
stayability and EPDs for all other traits, except birth weight.  The magnitude of the relationships
ranged from only .01 between stayability and scrotal circumference EPDs, to .25 for the
relationship with yearling weight EPD.  The correlation with scrotal circumference suggests that
puberty and sustained reproduction are traits controlled by different sets of genes in the
Limousin population. The correlation between EPDs for stayability and birth weight was .17. 
Based on these correlation, it should be possible to simultaneously select for genetic
improvement in stayability and in other production traits. 
Because of how stayability is defined, it is important to note that compared to other traits,
considerably more time is required for animals to achieve high levels of accuracy for stayability. 
Adoption of inventory based record keeping systems by several breeds should help to insure the
integrity of reproduction and cow disposal data, which in turn will provide opportunities for
development of genetic predictions for "survival" traits.   
Genetic Predictors of Reproduction - Indicator Traits 
Scrotal Circumference EPDs 
The most widely used indicator trait for reproduction is scrotal circumference.  Several
breeds have routinely calculated and published scrotal circumference EPDs for both bulls and
females in their populations.  Scrotal circumference is relatively easy to measure, is highly
heritable, is an excellent indicator of puberty and is normally distributed and easy to analyze.  In
terms of genetic predictors for reproduction in bulls, scrotal circumference is favorable
associated with both semen quality and quantity.  For both bulls and females, it goes without
saying that puberty is a prerequisite for reproduction. 
 
However, it appears that scrotal circumference primarily  accounts for genetic variation
in puberty, and not much variation in observed pregnancy, especially after the first breeding
season.   There appear to be other genetic factors which determine heifer pregnancy and
sustained reproductive performance which are not accounted for by genetic differences in scrotal
circumference and age at puberty. 
Milk, Mature Size and Body Condition
While genetic potentials for milk production, mature size and fleshing ability are not
necessarily reproductive traits, depending upon the available nutritional resources, these traits
can significantly influence expressed pregnancy rates.  Conventional wisdom would support the
notion that matching genetic levels for these traits to available feed resources will serve as an
insurance policy for acceptable reproductive performance.  
To go one step further, important genetic interactions exist between an animal's inherent
genetics for fertility, the animal's nutritional requirements which are largely determined by
genetic potentials for milk production and mature size, the available feed resources, and
ultimately the expressed pregnancy rate.  High levels of genetic merit for fertility should provide
greater flexibility when matching levels of milk production, mature size and other traits to
available resources and targeted markets. As well, high levels of inherent fertility might allow
for money to be saved on feed, if cows can exhibit acceptable levels of expressed pregnancy
despite lower levels of available nutrition and perhaps body condition.
Genetic predictions for maintenance energy requirements, which take into account
genetic differences in traits such as milk and mature size, would be extremely useful in
balancing selection for reproduction and other economically important traits.  Recognizing the
challenges of developing such predictions, EPDs for mature size and perhaps body condition
score would help to provide useful information. 
Calving Ease and Reproduction
Research has also shown that heifers which required assistance at calving had lower
pregnancy rates following the second breeding season by three to five percent, and were about a
week later in terms of day of second calving as compared to heifers not requiring assistance
(Andersen, 1991).  Similarly, in a subtle way, gestation length can also impact reproductive
performance.  Shorter gestation length equates to slightly more time between calving and the
start of fixed breeding seasons, thus allowing more time to recover and begin cycling. 
Consequently, selection and management for calving ease, especially in replacement heifers,
favorably impacts reproductive performance. 
  
Heterosis and Reproduction
The focus of this paper has been on genetic predictors of reproduction.  Genetic
predictions (EPD) estimate additive genetic differences which are passed onto offspring.  The
importance of heterosis, or non-additive genetic value, and breed complimentarity which is
gained through smart crossbreeding should not be underestimated.  The value of heterosis is
substantiated in a recent report from the U.S. Meat Animal Research Center entitled, "Composite
Breeds To Use Heterosis and Breed Differences To Improve Efficiency of Beef Production." 
The following excerpt summarizes the value of heterosis:  
Heterosis (hybrid vigor) for major bioeconomic traits, including reproduction,
growth rate, and longevity of beef cattle is important.  Heterosis can be used to
increase weight of calf per cow exposed to breeding by up 20 percent.  Crossbred
cows remain in the herd 1.3 years longer and have a 30 percent greater lifetime
production than straight bred cows.  Large breed differences exist among breeds
of beef cattle for major bioeconomic traits, including growth rate and size,
composition of gain, milk production, calving difficulty, age at puberty, and
climatic nutritive adaptability.  These are traits where the optimum is determined
by production environment and by market requirements (Gregory, Cundiff and
Koch, 1999).  
Conclusion
Recent breakthroughs in analytical procedures look promising relative to calculation of
genetic predictions for economically relevant reproductive traits.  Experimental results suggest
that when modeled appropriately, heifer pregnancy may be more highly heritable than previously
thought.  Preliminary EPDs for heifer pregnancy, and EPDs for the trait "stayability", offer
information as to the genetic differences in the likelihood that heifers are pregnant after the first
breeding season, and remain in production until break-even age.  Genetic predictions for
indicator traits, such as scrotal circumference, effectively describe genetic differences in age a
puberty, but may not adequately describe genetics for heifer pregnancy and sustained
reproductive performance.  As it relates to expressed fertility, interactions exist between genetic
potentials for inherent fertility and traits such as milk production, mature size, and maintenance
requirements.  To help ensure acceptable reproductive performance, the value of heterosis and
breed complimentarity derived through smart crossbreeding should not be overlooked.
  
References
Andersen, K.J., 1991.  Reproductive, calving ease and growth traits in beef heifers. Ph.D.
Dissertation. Colorado State Univ., Fort Collins.
Doyle, S.P., R.D. Green, B.L. Golden, G.L. Mathews, C.R. Comstock, and D.G. LeFever. 1996.
Genetic parameter estimates for heifer pregnancy rate and subsequent rebreeding rate in Angus
cattle. J. Anim. Sci. 74 (Suppl. 1)117 (Abstr.).
Evans, J.L. 1996. Genetic parameter estimate of yearling heifer pregnancy and yearling bull
scrotal circumference in Hereford cattle.  M.S. Thesis. Colorado State Univ., Fort Collins.
Gregory, K.E., L.V. Cundiff, and R.M. Koch. 1999. Composite breeds to use heterosis and breed
differences to improve efficiency of beef production.  Technical Bulletin Number 1875, October
1999.
Golden, B.L. and R.M. Bourdon, 1999. New EPDs: A rational vision for the future. The
principle of economically relevant traits versus indicators. Dept. of Anim. Sci., Colorado State
Univ., Fort Collins. 
Golden, B.L., J.L. Evans, R.M. Bourdon, and K.L. Long. 1997. Heifer pregnancy and scrotal
circumference: A model for multiple trait selection using expected progeny differences. Dept. of
Anim. Sci., Colorado State Univ., Fort Collins.
Koots, K.R., J.P. Gibson, C. Smith, and J.L. Wilson. 1994. Analysis of published genetic
parameter estimates for beef production traits. 1. Heritability. Anim. Br. Abst. 62:309-338. 
Snelling, W.M., M.D. MacNeil, and B.L. Golden. 1996. Application of continuous and binary
trait methods to reproductive measures of Hereford cattle. J. Anim. Sci. 74 (Suppl. 1):115
(Abstr.).
Summers, M.D., J.L. Evans, L.R. Hyde, C.R. Comstock, K.J. Andersen, and B.L. Golden. 1999. 
Estimation of heritability for heifer pregnancy in Limousin cattle. Proc. West. Sec. Amer. Soc.
Anim. Sci. Vol. 50


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