- Genetic trends describe the systematic changes in the genetic level of a population over time. In animal breeding, genetic trends are used to determine whether a breeding program is achieving its intended genetic improvement and whether the average genetic merit of animals is increasing, decreasing, or remaining relatively stable for particular traits. A genetic trend can be observed for production, growth, fertility, health, disease resistance, feed efficiency, survival, welfare, adaptation, or other traits included in a breeding objective. Unlike changes in observed phenotype, genetic trends specifically describe changes attributable to genetic differences rather than changes caused by management, nutrition, climate, disease exposure, or other environmental factors.
- A genetic trend is commonly expressed as the change in the population’s average breeding value per unit of time. In a simple form, it can be represented as: Genetic trend = Change in mean breeding value / Change in time. When breeding values are plotted against birth year or another time variable, the slope of the regression line can provide an estimate of the genetic trend. A positive slope indicates increasing genetic merit for the trait, while a negative slope indicates declining genetic merit. The magnitude of the trend indicates the approximate rate at which the population’s genetic level is changing.
- Genetic trends should be distinguished from phenotypic trends. Phenotypic performance can improve because of better feeding, housing, disease control, reproductive management, technology, or other environmental improvements even when genetic merit remains unchanged. Conversely, genetic merit may improve while observed performance changes little if environmental conditions deteriorate. Genetic evaluation therefore separates genetic and environmental components so that breeders can determine whether observed progress is actually due to genetic improvement.
- The distinction can be understood using the basic relationship: P = G + E, where P is phenotypic value, G is genetic value, and E represents environmental effects. A phenotypic trend therefore combines genetic and environmental changes, whereas a genetic trend focuses on changes in the genetic component. This distinction is essential when evaluating long-term breeding programs because environmental improvements can otherwise be incorrectly interpreted as evidence of genetic progress.
- The most direct way to evaluate genetic trends is through changes in average estimated breeding values (EBVs) across generations or birth years. Genetic evaluations provide an EBV for each animal, and the average EBV of animals born in different years can be compared. If the average EBV for a trait increases consistently across successive birth years, this provides evidence of a positive genetic trend. In modern breeding programs, EBVs may be generated using BLUP, pedigree information, phenotypic records, progeny information, and genomic information, allowing genetic trends to be monitored over long periods.
- A simple example is a population in which the average breeding value for growth is -20 units in one birth year, -10 units several years later, and +5 units in a more recent birth year. The increasing average breeding value indicates that the genetic level for growth has improved. The actual interpretation depends on the trait definition and genetic base, but the underlying principle is that changes in average genetic merit across time reveal the direction of genetic change.
- Genetic gain and genetic trend are closely related concepts. Genetic gain generally refers to the improvement achieved through selection, while genetic trend describes the pattern or rate of genetic change observed over time. A breeding program can therefore use genetic trends to monitor whether expected genetic gain is actually occurring. Annual genetic gain can be represented approximately as: ΔG/year = i × r × σ_A / L, where i is selection intensity, r is selection accuracy, σ_A is the additive genetic standard deviation, and L is the generation interval.
- This equation shows several factors that influence the expected rate of genetic improvement. Greater selection intensity can increase genetic gain because a stronger selection differential is applied. Higher selection accuracy increases the probability that animals with superior true breeding values are selected. Greater additive genetic variation provides more potential for response to selection, while a shorter generation interval can increase the rate of improvement per year. Genetic trends therefore provide an empirical way of assessing whether these components are producing the expected long-term response.
- The selection differential is another important component of genetic change. When selected parents have higher average genetic merit than the population from which they were chosen, their genes are disproportionately represented in the next generation. Repeated selection across generations can shift the population’s genetic distribution. The cumulative result of these changes appears as a genetic trend. The magnitude and direction of the trend depend on the selection strategy, trait heritability, genetic variation, accuracy of evaluation, reproductive structure, and generation interval.
- Selection accuracy has become particularly important in modern breeding programs. Traditional selection may depend heavily on individual performance and family or progeny information, while modern systems can incorporate genomic information. Genomic selection can increase the accuracy of genetic evaluation at young ages, allowing genetically superior animals to be identified before extensive progeny records become available. This can increase genetic trends by improving selection accuracy and potentially reducing generation interval.
- Estimated Breeding Values (EBVs) are particularly useful for monitoring genetic trends because they attempt to separate genetic merit from systematic environmental differences. However, EBVs must be interpreted relative to the genetic base used in the evaluation. Changes in the genetic base can alter the numerical scale or average level of reported EBVs without representing an equivalent biological change in the population. Therefore, meaningful genetic trend analysis requires a consistent evaluation framework and appropriate treatment of changes in genetic base.
- Genetic trends can be calculated for individual traits or for several traits simultaneously. In a multiple-trait breeding program, trends may be observed for growth, milk production, fertility, feed efficiency, disease resistance, survival, conformation, welfare, and adaptation. A positive trend in one trait may occur alongside a negative trend in another because of genetic correlations and selection priorities. Therefore, evaluating a breeding program requires looking at the complete set of traits rather than focusing on a single production characteristic.
- Genetic correlation can strongly influence genetic trends across traits. If two traits are genetically correlated, selection for one trait can cause a correlated response in another trait even when the second trait is not directly included in the selection criterion. Genetic correlation can be represented as: r_A = Cov_A(X,Y) / (σ_A,X × σ_A,Y). Favorable genetic correlations can help improve several traits simultaneously, whereas unfavorable correlations can create trade-offs. For example, intense selection for production without sufficient emphasis on fertility, health, or survival may result in undesirable correlated changes.
- For this reason, selection objectives and breeding objectives are central to interpreting genetic trends. A breeding program should define the desired direction of genetic change before selection begins. Economic weights, biological priorities, welfare considerations, environmental adaptation, and long-term sustainability may all influence the relative importance assigned to different traits. A genetic trend is therefore not automatically desirable simply because it is positive; the direction of genetic change must be evaluated against the intended breeding objective.
- Selection indexes provide one method for combining multiple traits into a single selection criterion. A simplified selection index can be represented as: I = b₁x₁ + b₂x₂ + … + bₙxₙ, where the x values represent information on different traits and the b values represent appropriate index coefficients. The resulting selection decisions influence genetic trends across all traits included in the index. Monitoring these trends allows breeders to determine whether the actual genetic response matches the intended direction of the breeding objective.
- Genetic trends are particularly valuable for evaluating production traits. Long-term selection for milk yield, growth rate, carcass quality, egg production, wool production, or other output traits can produce substantial positive genetic trends when sufficient additive genetic variation exists. However, increased production may not necessarily represent balanced improvement if it is accompanied by unfavorable trends in fertility, disease resistance, survival, welfare, or feed efficiency. Modern breeding programs therefore increasingly monitor a broad group of traits simultaneously.
- Feed efficiency provides an important example of why genetic trends must be evaluated carefully. Selection for greater production may increase total output while also increasing feed requirements. Selection for traits such as residual feed intake or feed conversion efficiency may produce a favorable genetic trend in resource efficiency. Because feed efficiency is influenced by production, body size, intake, metabolism, and environmental conditions, accurate genetic evaluation is important when assessing long-term trends.
- Genetic trends in fertility and reproductive traits are also important because these traits often have relatively low heritability and can be influenced strongly by management. A population may show major improvements in reproductive performance because of better nutrition or reproductive management without corresponding genetic improvement. Conversely, strong selection for production without adequate attention to reproduction can result in unfavorable genetic trends. Monitoring breeding values for fertility, age at sexual maturity, litter size, calving or lambing performance, semen quality, and related traits can reveal whether genetic selection is maintaining or improving reproductive fitness.
- The same principle applies to health and disease resistance. Improvements in veterinary care, vaccination, housing, hygiene, and disease management can improve observed health without producing a genetic trend. To determine whether animals are becoming genetically more resistant or resilient, breeding values or other genetic indicators must be evaluated. Long-term genetic trends for disease resistance, immune function, survival, and health-related traits can therefore provide valuable evidence about the sustainability of genetic improvement.
- Welfare-related traits and behavioural characteristics can also be incorporated into genetic trend monitoring. Selection for temperament, structural soundness, longevity, resilience, and other welfare-related traits can contribute to animals that perform successfully under commercial conditions while maintaining health and functional ability. Genetic trends for these traits help ensure that breeding programs do not focus narrowly on production at the expense of animal welfare and biological fitness.
- Adaptation traits are increasingly important as production environments change. Heat tolerance, stress resistance, disease resilience, and climate adaptation may become more important under changing climatic conditions. Genetic trends can show whether a population is becoming better adapted to the environments in which it is expected to perform. However, genetic trends for adaptation should be interpreted together with genotype–environment interaction (G×E) because animals may rank differently under different environmental conditions.
- Genetic trends can also reveal undesirable consequences of intensive selection. Strong selection for a small number of highly valuable traits may produce unfavorable correlated responses in other traits. For example, genetic improvement in production may occur alongside deterioration in fertility, longevity, structural soundness, or health if these traits are genetically antagonistic and are not included adequately in the breeding objective. Monitoring trends across a broad trait portfolio provides an early warning system for such problems.
- Genetic diversity must also be considered when evaluating genetic trends. Rapid improvement in a population may be accompanied by increased use of a small number of genetically superior animals. This can increase genetic concentration, popular sire effects, and inbreeding. Inbreeding can increase homozygosity and expose deleterious recessive alleles, potentially reducing fertility, survival, disease resistance, and reproductive performance. Therefore, sustainable breeding programs aim to achieve genetic improvement while maintaining sufficient genetic diversity.
- The expected increase in inbreeding can be approximated in a simplified population-genetic relationship as: ΔF ≈ 1 / (2Ne), where Ne is the effective population size. A smaller effective population size generally results in a faster increase in inbreeding. Genetic trend analysis should therefore be considered alongside monitoring of inbreeding trends, relationship structures, and effective population size. A breeding program that produces rapid genetic gain but excessive loss of diversity may not be sustainable over the long term.
- Optimal contribution selection and mate allocation can help balance genetic gain and genetic diversity. These approaches allow breeders to use genetically superior animals while controlling their contribution to future generations and managing relationships among selected parents. The goal is not simply to maximize short-term genetic trend but to achieve desirable genetic improvement while maintaining the long-term adaptability and health of the breeding population.
- The interpretation of genetic trends also requires attention to changes in management and data collection. New technologies, changes in measurement methods, improved nutrition, altered housing systems, changes in disease pressure, and modifications in recording procedures can influence observed phenotypic performance. Proper genetic evaluation models must account for these systematic effects so that trends in breeding values are not confused with changes in the environment or measurement system.
- Genetic trends can also be evaluated across breeds, lines, farms, regions, or production systems, but comparisons must be made carefully. Differences in genetic bases, evaluation models, trait definitions, environmental conditions, and recording systems can make direct comparisons misleading. A trend should therefore be interpreted within the population and genetic evaluation framework in which it was estimated.
- For genomic selection, genetic trends can potentially increase more rapidly because genomic information enables earlier and more accurate selection. Young animals can receive GEBVs before they have produced offspring, allowing selection decisions to be made earlier. This can increase selection intensity and reduce generation interval. However, rapid genetic change also increases the importance of monitoring genetic diversity, inbreeding, and unfavorable correlated responses.
- Genetic trends are therefore one of the most useful indicators for evaluating the long-term performance of a breeding program. They show whether the genetic level of a population is moving in the intended direction and whether observed genetic improvement is consistent with the breeding objective. Trends in breeding values can reveal successful selection, unexpected responses, unfavorable correlated changes, insufficient genetic variation, or excessive concentration of genetic contributions.
- A well-designed breeding program should monitor genetic trends regularly rather than evaluating success only through short-term phenotypic performance. Genetic merit, breeding values, EBVs, GEBVs, selection accuracy, genetic gain, and generation interval are all connected to the observed genetic trajectory of a population. Monitoring these components together provides a much clearer picture of whether genetic improvement is occurring efficiently and sustainably.
- Ultimately, the purpose of monitoring genetic trends is not simply to demonstrate that animals are changing genetically. The objective is to determine whether those changes are desirable, balanced, and sustainable. A successful animal breeding program should produce favorable genetic trends for economically and biologically important traits while maintaining fertility, health, welfare, adaptation, survival, and genetic diversity. Genetic trends therefore provide a bridge between the theory of quantitative genetics and the practical evaluation of long-term genetic improvement in real breeding populations.
