Mass Selection in Plant Breeding

1. Introduction

Plant breeding is the science and art of improving plants for human needs. It manipulates genetic variation to develop crops with higher yield, better quality, and enhanced resistance to stresses. Mass selection is one of the oldest and most intuitive methods of plant improvement. Farmers worldwide historically practiced it by saving seed from the best-performing plants from season to season, which gradually improved landraces and adapted them to local environments.

Although modern genetics, hybrid breeding, and molecular tools have transformed plant breeding, mass selection remains highly relevant—especially for rapid, low-cost population improvement, conservation of landraces, and breeding in resource-limited settings.

2. Definition

Mass selection is a breeding method in which a large number of phenotypically superior plants are selected from a heterogeneous population and the seeds of these selected plants are combined (bulked) together to produce the next generation. The objective is to raise the average performance of the population rather than to develop pure-line uniformity.

3. Historical Background

Prior to Mendel, farmers engaged in unconscious mass selection. With the rediscovery of Mendelian genetics, breeders understood how selection alters gene frequencies. Mass selection became recognized as particularly effective in cross-pollinated crops (which maintain heterozygosity and variability). In modern times, mass selection is used to improve and maintain locally adapted populations, and as a preliminary step where more refined breeding methods are not available.

4. Objectives of Mass Selection

• Improve local landraces while retaining adaptability.
• Increase the population mean for yield, quality, and resistance traits.
• Develop farmer-friendly, resilient varieties for marginal environments.
• Preserve genetic diversity to allow future adaptation.
• Provide a low-cost breeding option for small programs and communities.

5. Genetic and Biological Principles

5.1 Genetic variation

Mass selection requires a genetically variable base population. Sources of variability include landraces, genetically heterogeneous cultivars, composite crosses, or germplasm. Without variation, selection cannot change allele frequencies.

5.2 Phenotype versus genotype

Selection is performed on the phenotype. Phenotype = Genotype + Environment. The efficiency of mass selection depends on the trait's heritability (the proportion of phenotypic variance due to genetic variance). Traits with high heritability (e.g., plant height under stable conditions) respond well to mass selection; low-heritability traits (strongly environment-influenced) are less reliably improved by simple visual selection.

5.3 Mating system matters

Mass selection is particularly suited to cross-pollinated crops (maize, pearl millet, sorghum, cotton) because their natural outcrossing maintains diversity each generation. In self-pollinated crops, repeated selection can rapidly reduce variability and may require different strategies (e.g., pedigree selection or single-seed descent) to develop pure lines.

6. Detailed Year-by-Year Procedure

Year 0 — Preparing the base population

Identify or assemble a diverse starting population. This can be a farmer's landrace, an experimental composite, or a mixture of introductions. Ensure adequate seed quantity to ensure representative sampling of the genetic variation.

Year 1 — Field evaluation and initial selection

1. Sow the base population at a suitably representative location.
2. Observe plants through their growth cycle. Record or note desirable traits: yield components (e.g., cob size, grain number), maturity, plant habit, disease/pest resistance, quality attributes (grain colour, oil content, fibre length), and stress tolerance.
3. Select a large number of the best plants (hundreds to thousands depending on crop and population size). The larger the selected sample, the better the retained genetic diversity.
4. Harvest seeds from selected plants separately and store them with individual labels (if needed) or bulk them if that is the program strategy.

Year 2 — Bulking and repeating selection

1. Mix seeds from selected plants thoroughly to form a bulk seed lot.
2. Sow this bulk as the next generation and repeat the selection process during the season.
3. Continue to select a large number of superior plants, harvest, and bulk seeds again.

Years 3–5 — Continued cycles and gradual improvement

With each cycle, allele frequencies for favourable genes increase and the population mean for selected traits rises. The population gradually becomes more uniform in the selected characteristics while retaining some genetic diversity for adaptability.

Years 6–7 — Preliminary testing

Conduct small-scale yield and agronomic trials. Compare the improved bulk with existing standard varieties or checks. Evaluate across seasons and preferably across more than one environment to assess stability and adaptability.

Years 8+ — Multi-location trials and release

When the improved population consistently outperforms checks, conduct multi-location trials under national or regional testing systems. If it meets the criteria for yield, quality, and stability, follow the formal variety release procedures of the region. Multiply breeder, foundation, and certified seed for distribution.

Practical tip: Maintain detailed selection records (number of plants selected, selection criteria used, locations, and environmental conditions). This improves repeatability and helps diagnose poor response to selection when it occurs.

7. Types of Mass Selection

7.1 Positive mass selection

Directly select superior plants and bulk their seeds. This is the most commonly used approach.

7.2 Negative mass selection

Remove plants that display undesirable traits (e.g., disease symptoms, poor grain filling). The remaining plants form the seed source for the next generation. Negative selection is useful when a clear, identifiable defect can be visually screened out.

8. Traits suitable for mass selection

• High-heritability morphological traits (plant height, spike length).
• Visible disease or pest resistance where symptoms are clear and consistent.
• Maturity and phenology that are apparent in the field.
• Some quality traits visible to the eye (colour, grain shape), though biochemical traits may need lab tests.

9. Advantages and Strengths

• Simplest and least expensive method to improve a heterogeneous population.
• Maintains useful genetic diversity and adaptability.
• Works well in farmers' participatory breeding because farmers can perform selection themselves.
• Useful for initial improvement before more intensive breeding steps.

10. Limitations and Risks

• Selection is phenotypic and can be confounded by environmental effects (G×E). Low heritability traits respond poorly.
• Slow: several years are needed to obtain stable improvement.
• Not ideal for producing highly uniform, pure-line varieties required in some markets.
• Potential loss of rare but valuable alleles if selected population size is too small.

11. Practical examples and case studies

Maize: Early maize improvement programs used mass selection to raise mean grain yield and adapt populations to new agro-climatic zones. Selected cobs with full grain and uniform maturity were preferred.

Sorghum and Pearl Millet: Mass selection of landraces for drought tolerance and disease resistance has been successful in semi-arid regions. Farmers selected plants that matured well under moisture stress.

Vegetables (e.g., chillies, okra, brinjal): Farmers often select fruits and plants with best size, shape, and taste. Mass selection helps maintain local preferences while slowly improving productivity.

12. Enhancing mass selection: Complementary approaches

• Mass selection with progeny testing — Harvest and grow a small sample of progeny from selected plants to verify that selected phenotype transmits to offspring before bulking seed.
• Index selection — Combine several traits into a single selection index so that selection is performed on a weighted combination of multiple important traits.
• Participatory plant breeding — Involve farmers in selection at early generations to ensure local adaptation and farmer-preferred traits are captured.
• Complement with molecular markers — Where available, simple marker-assisted selection can be combined to ensure presence of known, important alleles (e.g., disease-resistance genes).

13. Diagram prompt (Year-by-year illustration)

The diagram below is prepared as a detailed prompt for artists, designers, or figure-generating software to produce an educational visual that illustrates the mass selection process across years.