Breeding of Vegetable, Spice and Tuber crops (2+1)

Natural and artificial selection during bulk period

• A large number of F2 populations of several thousand plants are grown at conventional planting rates which are in contrast to the pedigree method.
• At maturity all the plants are harvested in bulk and total seed is collected out of which a random sample is taken to grow approximately in the same area in next season i.e. in F3 generation.
• Normally the same procedure is followed uninterrupted upto F5 or F6 generation till sufficient level of homozygosity is attained.
• During all these years, natural selection is allowed to exert its influence, if any, to eliminate agronomically undesirable genotypes. As such, there is no attempt for artificial selection or to grow the material in off-season nurseries which may lead to loss of certain genotypes only on account of abnormal environmental conditions in the off-season.
• If, however, the off-season environment is expected to either reduce fitness of agronomically undesirable traits or even leave the characters unaffected, the generation can be advanced by taking two or more crops per year.
• Artificial selection is not conducted but there is no reason to loose sight of opportunity to improve the economic worth of population especially through negative selection for qualitative defects like susceptibility to diseases, late maturity etc.
• Even Nilsson-Ehle, the founding father of this method, helped natural selection in his bulk method, by discarding plants with winter damage which would have continued to produce at least some quantity of seed for several years.
• Florell (1929) also suggested that artificial selection towards a definite type may be practised to eliminate weak and undesirable plants.
• The use of artificially created environmental conditions can be made to exert specific type of selection pressure like that of incidence of diseases and pests.
• The single plant selection may be initiated any time in between F5 - F8 generation but that generation must be grown as space planted for better expression of potential of the individual plants as well as sufficient genetic variation which is usually suppressed under influence of interplant competition in thick planting.
• The plants are selected and evaluated in next generation like those in the pedigree method.
• The seed of individual plant is not sufficient for replicated yield trials so the growing of progeny rows serves the twin purpose of seed increase and also to reject some potentially less productive progenies by visual observation.
• Hence after the procedure for yield testing, multiplication and release of selections as new cultivars is essentially the same as in pedigree method

Applications of Bulk Method

• Homozygous lines can be isolated in less time, automatic increase in homozygosity up to F6 or F8generation, after this individual plant selection can be carried out.
• Natural selection may improve considered character (yield), with minimum expenses.
• Suitable for crops which are generally planted at high planting densities, e.g. small grain crops.