- Fertility is the biological ability of an individual to reproduce successfully. In animal breeding and quantitative genetics, fertility describes the ability of an animal to produce viable offspring through successful reproductive processes such as mating, fertilization, conception, pregnancy, and successful birth. Fertility is a fundamental component of reproductive performance and has major effects on productivity, lifetime reproductive success, generation interval, herd or flock efficiency, and the economic sustainability of breeding systems.
- Fertility is not a single biological process. It involves a sequence of events that must occur successfully, including reproductive development, puberty, sexual maturity, gamete production, mating behavior, fertilization, embryo development, implantation, pregnancy maintenance, and, depending on the production system, successful parturition and offspring survival. Consequently, fertility is a complex quantitative trait influenced by many biological pathways and by numerous environmental and management factors.
- Important fertility-related traits vary among species and production systems. In cattle, examples include conception rate, pregnancy rate, age at first calving, days to first service, services per conception, and calving interval. In sheep and goats, fertility can include conception rate, pregnancy rate, lambing or kidding rate, age at first reproduction, and reproductive seasonality. In pigs, important measures include farrowing rate, conception, litter size, and reproductive efficiency. In poultry, fertility and hatchability are important measures of successful reproduction. The precise definition of fertility therefore depends on the biological and production system being studied.
- Fertility is influenced by both genetic variation and environmental variation. Genetic differences among animals can affect reproductive physiology, hormone regulation, gamete quality, reproductive anatomy, immune function, disease resistance, behavior, and other characteristics associated with successful reproduction. Environmental influences include nutrition, body condition, health, disease, temperature, season, housing, reproductive management, stress, age, and mating conditions.
- A useful quantitative-genetic framework separates observed fertility into genetic and environmental components. In its simplest form:
- P=G+EP = G + E
- where PP is the observed phenotype, GG represents genetic effects, and EE represents environmental effects. In real breeding populations, the model can be expanded to include additive genetic effects, dominance, epistasis, maternal effects, permanent environmental effects, temporary environmental effects, and other sources of variation.
- The genetic component of fertility is often relatively difficult to measure because fertility outcomes can be strongly affected by environmental conditions and because many reproductive traits are recorded as binary outcomes. For example, an animal may be classified as pregnant or not pregnant, fertile or infertile, or successful or unsuccessful at a particular reproductive stage. These observations provide useful information but may not fully describe the continuous biological variation underlying fertility.
- For binary fertility traits, quantitative genetic analyses often use a threshold model. The basic idea is that an underlying continuous genetic and environmental liability determines whether an animal crosses a threshold and expresses a particular reproductive outcome. This approach allows genetic variation to be estimated even when the observed phenotype is recorded as a categorical trait.
- Heritability describes the proportion of phenotypic variance attributable to genetic variance in a particular population and environment. Fertility traits often have low to moderate heritability, although estimates vary substantially among species, populations, traits, and management systems. Low heritability does not mean that fertility is genetically unimportant. It means that environmental and other non-additive sources of variation may account for a substantial proportion of observed differences.
- The most relevant form for selection is usually narrow-sense heritability, which measures the proportion of phenotypic variance attributable to additive genetic variance. Additive genetic effects are important because they are transmitted predictably from parents to offspring and therefore form the primary genetic basis for response to selection.
- Despite sometimes having relatively low heritability, fertility can respond to genetic selection when sufficient additive genetic variation exists and animals can be evaluated accurately. The expected response depends not only on heritability but also on selection intensity, selection accuracy, and the amount of additive genetic variation. A simplified expression is:
- ΔG≈irσA\Delta G \approx i r \sigma_A
- where ΔG\Delta G is expected genetic change, ii is selection intensity, rr is selection accuracy, and σA\sigma_A is the standard deviation of additive genetic effects.
- Improving selection accuracy is particularly important for fertility because individual reproductive records may provide limited information. Genetic evaluation can combine an animal’s own records with information from relatives, repeated records, pedigree relationships, offspring, and genomic information. This allows breeders to estimate the breeding value of animals more accurately.
- An estimated breeding value (EBV) represents an estimate of an animal’s genetic merit for a particular trait. For fertility, an EBV can indicate whether an animal is expected to transmit genetic effects associated with better or poorer reproductive performance to its offspring. In modern breeding programs, EBVs may be enhanced by genomic information to produce genomic estimated breeding values (GEBVs).
- Genomic selection can be especially useful for fertility because many fertility traits are difficult to measure and may only become observable after animals reach reproductive age. Genomic information can allow young animals to be evaluated before they have their own reproductive records or offspring. This can increase selection accuracy and potentially reduce the generation interval.
- Genomic research can also investigate the genetic architecture of fertility. Genome-wide association studies (GWAS) and QTL mapping can identify genomic regions associated with reproductive performance. However, fertility is generally polygenic, meaning that many genetic variants contribute to differences among animals. Large numbers of genes and genomic regions can influence reproductive physiology, making fertility a classic example of a complex trait.
- Some genetic variants can influence multiple characteristics through pleiotropy. Consequently, fertility can be genetically correlated with traits such as growth, body composition, production, health, longevity, and feed efficiency. Understanding these relationships is important because selection for one trait can produce correlated responses in another trait.
- The relationship between fertility and production is particularly important in livestock. Selection for increased production may sometimes be associated with changes in reproductive performance, depending on the species and population. For example, reproductive performance can be influenced by energy balance, body condition, metabolic status, and physiological demands associated with high production. Genetic correlations between fertility and production traits therefore need to be considered when designing breeding objectives.
- Fertility can also be associated genetically with body weight and body composition. Animals need adequate physiological development and energy reserves to reproduce successfully, but excessive body condition can also create reproductive problems in some systems. Therefore, the relationship between body size, fat reserves, growth, and fertility is not necessarily linear and may vary with species, age, sex, and management.
- Feed intake and feed efficiency can also interact with fertility. Reproduction requires energy and nutrients, and inadequate nutritional status can reduce reproductive performance. From a breeding perspective, selecting for feed efficiency should therefore be evaluated alongside fertility and other fitness-related traits rather than treating production efficiency as an isolated objective.
- Health is another important component of fertility. Disease resistance, immune function, reproductive tract health, metabolic health, and general physiological condition can influence the probability of successful reproduction. Genetic correlations between fertility and health traits may therefore create opportunities for multi-trait selection.
- Fertility is also strongly influenced by environmental conditions. Genotype–environment interaction (G×E) occurs when animals with different genetic backgrounds respond differently to environmental conditions. Heat stress, nutritional limitations, disease pressure, seasonal changes, and management systems can all influence reproductive performance. Genetic evaluations that account for relevant environmental effects can therefore improve the accuracy and usefulness of fertility selection.
- Age is another major factor. Fertility can change throughout an animal’s reproductive life, and young animals may have different reproductive performance from mature animals. Repeated reproductive records can provide additional information about genetic differences, although repeated observations must be analyzed appropriately because they may be affected by common permanent environmental influences.
- Repeatability can be useful when fertility-related performance is measured repeatedly on the same animal. Repeatability describes the correlation between repeated measurements of the same trait and reflects both genetic and permanent environmental sources of variation. It can help determine how much information a previous reproductive record provides about future performance.
- Maternal and offspring effects can also be relevant to reproductive systems. Traits such as litter size, offspring survival, birth weight, and weaning performance can involve both the genetics of the offspring and the genetic ability of the mother to provide a suitable reproductive and postnatal environment. These relationships may be incorporated into breeding models through maternal genetic effects and maternal environmental effects.
- Fertility is closely connected with other reproductive traits. Conception rate, pregnancy rate, age at puberty, age at sexual maturity, calving interval, litter size, offspring survival, and reproductive longevity can all contribute to overall reproductive efficiency. These traits may be genetically correlated but should not necessarily be treated as identical characteristics.
- One important measure is conception rate, which describes the proportion of mating or insemination events that result in conception under a defined system. Pregnancy rate measures successful pregnancies relative to a defined eligible population or breeding opportunity. Although these traits are related, their definitions and biological determinants differ, so genetic evaluations should use carefully defined phenotypes.
- Another important concept is calving interval or the corresponding reproductive interval in other species. Shorter appropriate reproductive intervals can increase lifetime offspring production, but selection for reproductive timing should be balanced against health, longevity, offspring survival, and animal welfare. Optimizing fertility is therefore more appropriate than simply maximizing reproductive frequency.
- Fertility can influence lifetime productivity because animals that reproduce successfully and regularly can remain productive in the breeding population for longer periods. This creates a strong connection between fertility and longevity. Selecting simultaneously for fertility, health, survival, and productive lifespan can help create more sustainable breeding populations.
- Fertility also affects the generation interval. Animals that reproduce at younger ages can contribute genetic material to the next generation sooner. When combined with adequate selection accuracy, reduced generation interval can increase the annual rate of genetic improvement. However, rapid generation turnover should not come at the expense of genetic diversity or long-term population health.
- Because fertility is connected with many biological and economic traits, selection index methods can be useful. A selection index combines information from multiple traits according to their economic importance, genetic relationships, and breeding objectives. Fertility can therefore be balanced with production, growth, feed efficiency, health, longevity, and product quality.
- Maintaining genetic diversity is also important when selecting for fertility. Intensive use of a small number of highly ranked breeding animals can increase the risk of inbreeding and reduce effective population size. Sustainable breeding programs therefore need to balance genetic improvement with the preservation of genetic variation.
- Fertility has an even broader significance in evolutionary biology because successful reproduction directly contributes to biological fitness. Natural selection can favor genetic variants that improve reproductive success under particular environmental conditions. At the same time, reproductive strategies involve trade-offs with growth, survival, parental investment, metabolism, and other life-history characteristics.
- Accurate fertility phenotyping is therefore essential for genetic improvement. Breeding programs should use consistent definitions and carefully record reproductive events, including mating, insemination, conception, pregnancy, birth, reproductive failure, and offspring survival. Relevant environmental information such as nutrition, disease, season, age, parity, management, and reproductive technology can help distinguish genetic effects from environmental effects.
- Modern fertility improvement increasingly combines traditional quantitative genetics with genomic information. Pedigree relationships, BLUP, phenotypic records, reproductive technologies, and genomic selection can be integrated to estimate genetic merit. Improved reproductive phenotyping and genomic prediction can make it possible to identify animals with favorable fertility genetics earlier and more accurately.
- Overall, fertility is a fundamental but complex trait in animal breeding and quantitative genetics. It is influenced by genetic variation, additive genetic effects, physiology, health, nutrition, environment, management, and interactions among these factors. Although fertility traits can have relatively low heritability and can be difficult to measure, they can still respond to well-designed genetic selection. Understanding heritability, additive genetic variation, breeding value, genetic correlation, G×E, selection accuracy, and genomic selection allows fertility to be incorporated effectively into modern breeding programs. Improving fertility while maintaining health, longevity, productivity, welfare, and genetic diversity is essential for sustainable genetic improvement and long-term reproductive success.
