In this article we will discuss about:- 1. Meaning and Features of Bulk Breeding Method 2. Breeding Procedure of Bulk Breeding Method 3. Merits and Demerits.
Meaning and Features of Bulk Breeding Method:
Bulk breeding refers to a selection procedure in which the segregating population of self-pollinated species is grown in bulk plot (From F1 to F5) with or without selection, a part of the bulk seed is used to grow the next generation and individual plant selection is practised in F6 or later generations.
This method is also termed as mass or population method or evolutionary method of crop improvement. The concept of bulk breeding method was developed by Nilsson Ehle in 1908.
Main features of this method are presented below:
1. Application:
This method is used for the genetic improvement of self-pollinated crop plants. It is used when both the parents are adapted or have good agronomic characters and the character for which improvement is to be made is governed by polygenes.
2. Handling of Material:
The material is handled by bulk method from F2 to F5 and by individual plant selection as in pedigree method from F6 onwards. The bulk period varies from 6-10 years in short-term bulks and from 20-30 years in long-term bulks. In crop breeding, a 5-6 year period of bulking is usually adopted.
3. Selection:
In this method both natural as well as human selection operates. Natural selection operates during bulk period and human selection operates in the later generations when individual plant selection is practised.
4. Adaptation:
This method leads to significant evolutionary changes in the gene frequencies in a population. Hence, it is also referred to as evolutionary method of crop improvement. Natural selection favours those genotypes which have better survival capacity and eliminates genotypes with poor survival capacity.
Thus, proportion of the best fit genotypes will increase and of poorly adapted genotypes decrease. The best surviving genotypes sometimes may not be agronomically good. Hence artificial selection should be practised during bulk period to avoid drastic changes in genes and genotypes through natural selection.
Varieties developed by bulk method are more stable against environmental changes than those evolved by pedigree breeding method, because the period of bulking improves the adaptation of population.
5. Genetic Constitution:
The end product of bulk breeding method is a homozygous and homogeneous population, because it is the progeny of a single homozygote.
Breeding Procedure of Bulk Breeding Method:
The bulk breeding method consists of four important steps, viz.:
(1) Bulking period,
(2) Progeny selection and isolation of superior progeny,
(3) Multi location trials of superior progeny, and
(4) Release of the best progeny as a variety.
These steps are briefly discussed below:
1. Bulk Period:
The F1 plants are grown and their F2 seeds are harvested in bulk. The F2 plants are raised from a sample of F2 seeds and F3 seeds are harvested in bulk. This process is repeated until the desired level of homozygosity is achieved. In general, bulk period is allowed upto F5 generation (Fig. 19.1). The material is subjected to biotic and abiotic stresses during bulking period to eliminate undesirable genotypes.
2. Progeny Selection:
In F6 the material is space planted and individual plant selection is practised. The progeny of each selected plant is grown separately in F7 and superior progeny are selected and isolated in F7 and F8.
3. Multi Location Testing:
The selected progeny constitute strains. In eighth year, preliminary yield trial is conducted. From 10th to 14th year multi location testing is carried out and the best performing strain is identified on the basis of 3-4 years performance in the multi-location trials.
4. Release:
The best strain is released and multiplied for seed distribution in the 15th year. Thus bulk method takes 15-16 years for release of new variety.
The bulk material should be tested under such environment which is expected to favour desirable genotypes. For example, screening for disease resistance should be carried out in the disease prone area. Moreover, the screening should be done under natural field condition rather than in green houses.
Merits and Demerits of Bulk Breeding Method:
Merits:
1. Bulk breeding is a simple, convenient and less expensive method of crop improvement.
2. In this method natural selection operates which results in elimination of undesirable genotypes from the bulk population and increases the frequency of desirable plants.
3. The chances of obtaining transgressive segregants are more in this method than pedigree method, because the material is grown in large plots in this method.
4. This method is useful to study the competitive ability of various genotypes in the population.
5. This method leads to improvement in adaptation of genotypes due to bulk period. Thus varieties developed by this method are more adaptable than those evolved by pedigree method.
6. There is no need to maintain pedigree records in this method.
This method has been used only in few crops due to time lag. It has been used in the improvement of barely in USA and more than 50 varieties have been developed from composite crosses by this method. In India, only one variety “Narendra Rai” has been developed in Brown Mustard by this method. Thus bulk method has limited application in practical plant breeding.
Demerits:
1. This method does not provide information about the mode of inheritance of various oligogenic characters which is obtained in pedigree method.
2. The long-term bulking requires 20-30 generations and short-term bulking requires 6-10 generations. Thus this method takes more time than pedigree method in release of new variety. For this reason this method is not preferred by the plant breeders.
3. It is difficult to assess the variability in the population and genotypic frequencies in this method, because they change in each generation of bulking.
4. Sometimes, natural selection may favour undesirable than desirable genotypes.
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