Bulk Method – Procedure, Applications, Advantages, Disadvantages

What is Bulk Method?

• The Bulk Method, initially developed by Nilsson-Ehle in 1908, represents a significant approach in plant breeding, also referred to as the mass or population method. This technique involves the cultivation of large quantities of plants from the F1 generation and subsequent generations, referred to as “bulks.” Rather than evaluating individual plants throughout these generations, the bulk method allows for the collective growth of these plants.
• The process begins with the harvesting of F1 and later generations as bulk populations. This approach contrasts with the pedigree method, where individual plant progenies are assessed from the F3 generation onward. In the bulk method, plants are maintained in bulk until the end of the bulking period, which can range from six to thirty or more generations.
• During the bulking period, artificial selection may or may not be implemented. The primary distinction between the bulk and pedigree methods lies in the handling of segregating generations. In the pedigree method, individual plants are selected and evaluated, whereas, in the bulk method, selections are based on the bulk populations as a whole.
• At the conclusion of the bulking phase, individual plants are selected and evaluated similarly to the procedures in the pedigree method. This method simplifies the breeding process by reducing the need for frequent individual plant assessments until later stages, thereby focusing on larger populations.

Definition of Bulk Method

The bulk method, developed by Nilsson-Ehle in 1908, is a plant breeding technique where large quantities of plants from the F1 generation and subsequent generations are grown in bulk. Individual plants are only selected and evaluated after a prolonged bulking period, distinguishing it from methods like the pedigree method where individual progenies are assessed earlier.

The Procedure for Bulk Method

The bulk method of plant breeding involves a series of steps designed to isolate homozygous lines, although the specific procedure can vary based on the breeder’s objectives. Below is a detailed, step-by-step outline of the typical process:

1. Hybridization
   • Objective: To create a genetic cross that aligns with the breeding goals.
   • Process: Select appropriate parent plants based on the breeding objective. A cross is made, which may be either simple (involving two parents) or complex (involving multiple parents).
2. F1 Generation
   • Objective: To grow and manage the initial hybrid plants.
   • Process: Space-plant the F1 generation and harvest it in bulk. The population should be as large as feasible, typically involving more than 20 plants.
3. F2 to F4 Generations
   • Objective: To allow for continued growth and bulk harvesting under natural conditions.
   • Process: Plant the F2 to F4 generations at commercial seed rates and spacings, and harvest in bulk. It is crucial to maintain a large population size, ideally between 30,000 and 50,000 plants per generation. During this period, plants are exposed to environmental factors and potential pest and disease outbreaks.
4. F5 Generation
   • Objective: To select plants with desirable traits from a large population.
   • Process: Space-plant around 30,000 to 50,000 plants. Select 1,000 to 5,000 plants with superior phenotypes based on characteristics such as grain quality and disease resistance. Harvest seeds from these selected plants separately.
5. F6 Generation
   • Objective: To evaluate progenies for homozygosity and desirable traits.
   • Process: Grow individual plant progenies in single or multi-row plots. Harvest progenies in bulk after evaluating them visually. Typically, only 100 to 300 progenies with favorable characteristics are retained. Progenies showing segregation may be rejected unless they exhibit exceptional potential.
6. F7 Generation
   • Objective: To conduct preliminary yield trials.
   • Process: Perform yield trials using standard commercial varieties as controls. Evaluate progenies for yield, plant height, lodging resistance, maturity, disease resistance, and other key traits. This stage determines which lines show promise for further development.
7. F8 to F10 Generations
   • Objective: To conduct replicated yield trials and final evaluations.
   • Process: Conduct replicated yield trials across multiple locations using commercial varieties as checks. Assess the lines for yield, disease resistance, and overall quality. Lines that outperform standard varieties in these trials may be selected for release.
8. F11 Generation
   • Objective: To prepare for distribution.
   • Process: Increase the seed of the selected variety to ensure sufficient supply for distribution to cultivators.

Selection Methods During Bulking

The selection methods employed during the bulking period of the bulk method vary based on the specific objectives and duration of bulking. These methods can be broadly categorized into artificial selection and natural selection, each serving distinct purposes in the breeding process.

1. Artificial Selection During Bulking

• General Practice: Artificial selection is typically minimal during the bulking period. However, it can be utilized to remove inferior and undesirable plants, thereby assisting natural selection in shaping the population.
• Short-Term Bulking: In short-term bulks (6-10 generations), the influence of natural selection may be limited. Therefore, artificial selection is often employed to select for specific traits that may not naturally confer a selective advantage. For instance, traits like seed size or color might be targeted because they could otherwise be overlooked by natural selection.
• Long-Term Bulking: In long-term bulks (20-30 generations), natural selection plays a more significant role. Nevertheless, artificial selection is necessary to maintain or enhance specific characteristics that may not otherwise be favored. For example, dwarf plant types in rice, which are poor competitors against taller types, might be maintained through artificial selection to ensure their persistence in the population.
• Techniques: Methods such as creating artificial disease epidemics or insect infestations can be used to select for disease or insect resistance. Similarly, harvesting crops at specific maturity stages or sieving seeds based on size can help promote desirable traits like earliness or larger seed size.

2. Natural Selection During Bulking

• Basic Principle: Natural selection acts on the population during bulking by favoring genotypes that perform better under prevailing environmental conditions. Over time, the best-performing genotypes increase in frequency, while inferior types are gradually eliminated.
• Population Dynamics: The efficiency of natural selection depends on the duration of bulking. In long-term bulking, natural selection can significantly alter the genetic composition of the population by eliminating less favorable genotypes and enhancing the prevalence of superior ones.
• Selection Effects: Natural selection tends to favor higher-yielding and agronomically superior genotypes. Evidence from studies such as those on Composite Cross II indicates that the frequency of superior genotypes increases with successive generations of bulking. For example, later generations often yield more superior progenies compared to earlier ones.
• Variability: The effectiveness of natural selection can vary based on environmental conditions and genetic interactions. Some traits, like flowering time or disease resistance, may not always align with competitive advantage but can still be crucial for overall population performance.

Modification of the bulk method

The modification of the bulk method integrates artificial selection across multiple generations to enhance plant breeding efficiency. This refined approach, while preserving the foundational principles of the bulk method, introduces systematic selection at various stages to improve plant quality and performance.

1. Early Generations (F1 and F2):
   • In the F1 and F2 generations, plants are space-planted to facilitate observation and selection. A substantial number, ranging from 1,000 to 5,000 desirable plants, are selected based on their traits.
   • Seeds from these selected plants are bulked together. This bulk seed is used to propagate the next generation, maintaining genetic diversity while focusing on desirable characteristics.
2. Intermediate Generations (F3):
   • The F3 generation continues with space-planted crops. Here, 1,000 to 5,000 plants are selected based on specific agronomic traits such as disease resistance, maturity date, and other important characteristics.
   • Seeds from these selected plants are harvested separately, while inferior and undesirable progenies are discarded. Typically, only 10-30% of the progenies are preserved for further breeding.
3. Yield Trials (F4):
   • A preliminary yield trial is established using bulk seed from selected individual plants of the F3 generation. This trial evaluates agronomic characteristics and yield performance.
   • Observations and quality tests are conducted to identify superior progenies. These progenies are then used for further selection.
4. Further Purification (F5):
   • In the F5 generation, the superior progenies identified from the yield trial are space-planted for further purification. Individual plants from these progenies are selected based on performance.
   • Seeds from these selected plants are harvested separately, focusing on improving the consistency and quality of the progenies.
5. Final Selection and Testing (F6):
   • Individual plant progenies are grown, with inferior or segregating progenies being rejected. The remaining selected progenies are bulk-harvested.
   • Additional trials and tests are conducted to ensure that undesirable progenies are eliminated, and the best lines are identified.
6. Variety Release and Multiplication (F7-F8):
   • Replicated yield trials are performed across multiple locations, comparing the new lines against standard varieties. Lines that surpass the standards are released as new varieties.
   • In the F8 generation, the seeds of these newly released varieties are multiplied for distribution among farmers, ensuring that the improved varieties are made available.

Key Features and Benefits:

• Early Selection: The modification introduces selection in early segregating generations, improving efficiency and effectiveness compared to traditional methods.
• Reduced Labor: By streamlining the selection process and utilizing bulked seeds at various stages, this method reduces the time and labor required compared to the pedigree method.

Single-seed-descent method

The Single-Seed-Descent Method is a plant breeding technique designed to expedite the process of generating homozygous lines from a cross while minimizing space and labor. This method involves several key steps and features that distinguish it from other breeding approaches.

1. Generation Advancement:
   • The Single-Seed-Descent Method aims to rapidly advance generations. Initially, plants from the F1 generation are bulked to raise the F2 generation.
   • In each subsequent generation (F3, F4, etc.), a single random seed is selected from each plant to create the next generation. This approach ensures that a diverse range of genetic material is carried forward.
2. Homogenization:
   • The process continues until the F4 or F5 generation, at which point the plants are expected to be nearly homozygous. In these later generations, a large number of individual plants (ranging from 100 to 500) are selected for further evaluation.
3. Selection and Evaluation:
   • Once the plants reach the F4 or F5 stage, selection is focused on individual plant progenies. These progenies are then grown in the next generation. Selection is conducted mainly among these progenies, and the number of progenies is reduced to facilitate replicated trials in subsequent generations.
   • Selection primarily targets outstanding families showing significant segregation.
4. Efficiency and Facilities:
   • The Single-Seed-Descent Method is characterized by its high efficiency. It allows for the advancement of 2-3 generations per year using off-season nurseries and greenhouse facilities. This rapid advancement is possible because the method does not prioritize individual plant vigor in early generations and maintains very high plant densities.
5. Key Features:
   • Lack of Early Selection: Selection, either natural or artificial, is generally avoided until the population is reasonably homozygous (usually by the F4 or F5 generation). This approach helps in maintaining a broad genetic base in earlier generations.
   • Space and Labor Efficiency: The method requires minimal space and labor compared to traditional methods, making optimal use of greenhouse and nursery facilities.
6. Advantages:
   • Speed: The method advances generations at a maximum possible rate, which is beneficial for accelerating the development of new plant lines.
   • Economy: It reduces space and labor requirements, making it cost-effective.
7. Disadvantages:
   • No Selection in Early Generations: The lack of selection in the early generations means that only the most promising plants are evaluated in the later stages.
   • Population Size Reduction: Each successive generation may experience a reduction in population size due to poor germination, plant losses from diseases, pests, and other factors. This reduction can be particularly problematic in crops like pulses.

Merits of bulk method

The bulk method in plant breeding offers several significant advantages, making it a valuable technique for developing new varieties. Here are the key merits of the bulk method:

1. Simplicity and Cost-Effectiveness:
   • The bulk method is straightforward and easy to implement. Its simplicity reduces complexity in the breeding process, making it an inexpensive approach compared to more elaborate methods.
2. Natural Selection Benefits:
   • Natural or artificial disease outbreaks, as well as environmental factors like winter killing, can naturally eliminate undesirable types. This process increases the prevalence of desirable types within the population, facilitating the isolation of these preferred traits.
3. Increased Frequency of Superior Types:
   • Long-term bulking allows for the natural increase in frequency of superior types through ongoing selection. Progenies selected from extensive bulks generally exhibit improved traits compared to those selected from smaller, short-term bulks.
4. Reduced Labor and Attention:
   • Minimal effort is required in the F1 and subsequent generations, as the bulk method does not demand intensive management at these stages. This reduction in workload allows breeders to focus on other projects.
5. Efficient Use of Breeding Resources:
   • By minimizing the need for detailed pedigree records, the bulk method saves time and labor. This efficiency enables breeders to allocate resources effectively across various breeding efforts.
6. Potential for Transgressive Segregants:
   • Large populations grown under the bulk method have a higher likelihood of producing transgressive segregants. Natural selection within these populations increases the chances of identifying individuals with traits that exceed the performance of both parent lines.
7. Flexibility in Selection:
   • The method allows for the application of artificial selection to enhance the frequency of desirable types. This flexibility provides breeders with additional tools to refine and improve the population.
8. Suitability for Gene and Genotype Studies:
   • The bulk method is well-suited for studying the survival and dynamics of genes and genotypes within populations. It provides insights into how genetic traits persist and evolve in a breeding context.

Demerits of bulk method

The bulk method, while advantageous in many respects, also presents several limitations that impact its overall effectiveness in plant breeding. Here are the key demerits of the bulk method:

1. Extended Time Requirement:
   • One of the primary drawbacks of the bulk method is the extended time required to develop new varieties. Natural selection only significantly influences the population after the F3 or F4 generations, and bulking may need to continue until the F10 generation or beyond. This prolonged timeline is a significant consideration, leading many breeders to opt for alternative methods.
2. Limited Impact of Natural Selection in Short-Term Bulks:
   • In short-term bulks, natural selection has minimal effect on the genetic composition of the population. Although short-term bulks can be useful for specific purposes such as isolating homozygous lines, they do not fully leverage natural selection to enhance population quality.
3. Minimal Breeder Input:
   • The bulk method provides limited opportunities for breeders to exercise their skill and judgment in selection, particularly in the early generations. This limitation reduces the scope for tailored selection, though the modified bulk method allows for more selection practice.
4. High Number of Progenies:
   • At the end of the bulking period, a large number of progenies must be selected for further evaluation. This requirement can be labor-intensive and may necessitate extensive resources to manage and assess the numerous progenies effectively.
5. Lack of Inheritance Information:
   • The bulk method does not provide detailed information on the inheritance of traits, which is often available through pedigree methods. This lack of genetic insight can be a disadvantage when trying to understand the transmission of specific characteristics.
6. Potential Negative Selection Effects:
   • In some cases, natural selection may act against agronomically desirable types. This unintended consequence can lead to the loss of beneficial traits and reduce the overall effectiveness of the method in developing superior plant varieties.

Applications of the Bulk Method

The bulk method of plant breeding is versatile and finds application in various areas. It is particularly suited for handling segregating generations of cereals, smaller millets, most grain legumes, and oilseeds. The method can be employed for three main purposes:

1. Isolation of Homozygous Lines
   • Purpose: The primary use of the bulk method is to isolate homozygous lines. This involves growing plants in bulk until they become homozygous, then selecting and evaluating individual plants.
   • Process: Once individual plants are selected, they are evaluated similarly to the pedigree method. The aim is to produce uniform progenies where most will be homozygous. A preliminary yield trial is often conducted in the second year following plant selection.
2. Waiting for the Opportunity for Selection
   • Purpose: This application involves delaying selection until favorable environmental conditions occur, which are necessary for evaluating traits such as disease resistance, lodging tolerance, or cold hardiness.
   • Process: The segregating generations are maintained in bulk until the specific environmental conditions arise. Selection of individual plants then occurs under these conditions. The duration of bulking depends on the timing of these environmental factors and may extend to the F3 generation or beyond. This approach is also known as the Mass-Pedigree Method of Harlan.
3. Opportunity for Natural Selection
   • Purpose: The bulk method allows for natural selection to act on the plant population over an extended period.
   • Process: Maintaining bulks is cost-effective and requires minimal effort. Some populations may be kept in bulk up to the F6 or F10 generation, providing a chance for natural selection to influence the population. It is assumed that natural selection will favor higher-yielding genotypes and eliminate less favorable ones. This method, termed the Evolutionary Method of Breeding by Suneson, can lead to the isolation of superior lines more frequently compared to earlier generations.

Achievements of the Bulk Method

Despite its advantages, the bulk method of breeding has seen limited application in crop improvement. The primary reasons for its restricted use include the extended time required for natural selection to manifest—often exceeding ten generations—and the reduced opportunity for breeders to directly exercise their skill in selecting superior plant types during early segregating generations. Nevertheless, the bulk method has achieved notable successes in specific contexts.

1. Study of Gene and Genotype Survival:
   • The bulk method has proven valuable for studying the survival and persistence of genes and genotypes within segregating populations. A prominent example is the Composite Cross II in barley, which demonstrates the method’s utility in understanding genetic behavior over multiple generations.
2. Barley Breeding Applications:
   • Although its use has been somewhat limited, the bulk method has been applied in barley breeding in the United States. Several varieties have originated from bulk populations, illustrating the method’s practical application in crop development.
   • Examples of Successful Varieties:
     • Arivat, Beecher, Glacier, and Gem: These barley varieties were developed from a bulk population derived from the cross Atlas × Vaughn. This bulk was maintained for seven generations, leading to the development of these notable varieties.
     • Composite Cross Varieties: Over fifty varieties have been developed from composite crosses in barley. These varieties either emerged directly from composite crosses or were derived from lines isolated from such crosses, showcasing the bulk method’s impact on barley breeding.

Comparison between bulk and pedigree methods

The bulk and pedigree methods are distinct approaches to plant breeding, each with unique characteristics and applications. Here is a comparison between the two methods:

Pedigree Method

1. Selection Process:
   • Individual plants are selected in the F2, F3, and subsequent generations. Progenies from these selected plants are grown and evaluated individually.
2. Use of Selection and Disease Management:
   • Artificial selection and management of artificial disease epidemics are integral to the pedigree method. These practices are used to enhance the selection process and improve plant qualities.
3. Role of Natural Selection:
   • Natural selection does not play a role in the pedigree method. The method relies heavily on controlled selections and interventions.
4. Pedigree Records:
   • Maintaining detailed pedigree records is essential but can be time-consuming and labor-intensive. These records track the lineage and selection history of individual plants.
5. Time Required for Development:
   • Developing and releasing a new variety using the pedigree method typically takes 14-15 years due to the detailed selection and documentation process.
6. Usage:
   • The pedigree method is widely used in plant breeding due to its systematic approach and thorough selection process.
7. Breeder Involvement:
   • The method requires close attention from the breeder from the F2 generation onward, involving frequent individual plant selections and meticulous record-keeping.
8. Planting and Population Size:
   • The segregating generations are space-planted to allow for individual plant selection, which generally results in a smaller population size.

Bulk Method

1. Selection Process:
   • In the bulk method, the entire bulk of seeds from selected plants is maintained and grown in subsequent generations without individual plant selection until later generations.
2. Use of Selection and Disease Management:
   • While artificial selection and disease management are less emphasized, they may be used to assist natural selection. Artificial selection might be essential in some cases.
3. Role of Natural Selection:
   • Natural selection is a key factor in determining the composition of populations at the end of the bulking period. It helps enhance the frequency of desirable traits.
4. Pedigree Records:
   • No pedigree records are maintained in the bulk method, reducing the labor and time required for record-keeping.
5. Time Required for Development:
   • The development and release of a new variety using the bulk method take much longer, often more than 10 years, as the bulk population must be maintained until natural selection has acted effectively.
6. Usage:
   • The bulk method is used to a limited extent compared to the pedigree method, primarily for specific purposes like studying gene survival and for crops where rapid population development is advantageous.
7. Breeder Involvement:
   • The method requires less day-to-day attention from the breeder during the bulking period. Selection and record-keeping are less intensive compared to the pedigree method.
8. Planting and Population Size:
   • Bulk populations are generally planted at commercial planting rates, resulting in larger populations. This approach increases the likelihood of recovering transgressive segregants due to the broader genetic diversity and natural selection.

Aspect	Pedigree Method	Bulk Method
Selection Process	Individual plants are selected in F2, F3, and subsequent generations. Progenies are grown and evaluated individually.	Entire bulk of seeds from selected plants is maintained and grown in subsequent generations without individual plant selection until later generations.
Use of Selection and Disease Management	Artificial selection and management of artificial disease epidemics are integral to the method.	Artificial selection and disease management may assist natural selection but are less emphasized. Artificial selection might be essential in some cases.
Role of Natural Selection	Natural selection does not play a role. Controlled selection is used.	Natural selection plays a key role in determining the population composition at the end of the bulking period.
Pedigree Records	Detailed pedigree records are maintained, which are time-consuming and labor-intensive.	No pedigree records are maintained, saving time and effort.
Time Required for Development	Typically takes 14-15 years to develop and release a new variety.	Takes longer, often more than 10 years, as bulk populations must be maintained for natural selection to act.
Usage	Widely used in plant breeding due to its systematic approach.	Used to a limited extent, primarily for specific purposes such as studying gene survival.
Breeder Involvement	Requires close attention from the breeder, with frequent individual plant selections and record-keeping.	Requires less day-to-day attention during the bulking period. Selection and record-keeping are less intensive.
Planting and Population Size	Segregating generations are space-planted, resulting in a smaller population size.	Bulk populations are planted at commercial rates, resulting in larger populations. This increases the likelihood of recovering transgressive segregants.