III: Crop Resources Management
Management of Seed Crops
ASHOK K. THAKUR* and SUJATA KUMARI
Department of Seed Science and Technology, University of Horticulture and Forestry, Nauni 173230, Solan, Himachal Pradesh, India
Seed is a vital input in agriculture. The quality and genetic production capacity of seed detennines the quantity as well as quality of produce, and also determines economics of production. In a crop production system, seed quality is the basic input to which other inputs are added to have desired output. It ensures other agroproduction and protection inputs to be productive and cost-effective. The management of mother crop plays pivotal role in quality of produced seed. Seed production requires specialized skills and knowhow, unlike general crop production. It involves crop and region-specific genetic as well as agronomic principles that have to be precisely practiced to achieve desired quality of seed. Seed having good quality is the basic input for enhancement of crop as well as other inputs productivity. Well planned evaluation and release of culti- vars, systematic production, and supply of seed, seed testing, certification and enforcement of seed legislation, in toto, is essential to exploit the technological breakthroughs in crop production system. This chapter describes seed quality, its components, and factors governing seed quality. The genetic, agronomic, legislative mechanisms for controlling seed quality are described to have better understanding of seed production. However, the main emphasis remains on management of seed quality at preharvest as well as postharvest level.
CONCEPT OF SEED QUALITY
Seed as input is crucial, critical, vital, and basic in crop production. Seed quality is directly and positively correlated to crop productivity and quality.
Hence, seed quality plays a pivotal role in improving agricultural economy is one of the most essential and effective inputs. Seed quality, according to Thompson (1979) is a complex of several components and their relative importance in different sets of production systems. These components have been categorized as (1) analytical quality, (2) species purity, (3) freedom from weeds, (4) cultivar purity, (5) germination capacity, (6) vigor, (7) size, (8) uniformity, (9) health, and (10) moisture content. These components were categorized in more systematic manner by Nema (1985) into four major components: (1) Genetic quality, (2) physical quality, (3) physiological quality, and (4) health quality.
The importance of quality seed is well understood around the globe. In India, seed quality attained the central stage after introduction of high yielding cultivars of field crops like Bajra, sorghum, and wheat during 6th decade of 20th century. Histoiy of grain production in developing countries clearly reveals that quality seed was one of the keys to the Green Revolution. The introduction of dwarf and high yielding varieties of wheat resulted in 3-4 tunes increase in wheat productivity. The quick and intense enhancement of wheat productivity founded the Green Revolution. Production and availability of quality seeds of these novel cultivars played an equally important role in the Green Revolution.
In crop production, seed quality is basic input to which other inputs are applied to have desired output. The productivity and cost-effectiveness of other production inputs are also dependent on the quality of seed sown. It is the combination of seed quality and other agroinputs that results in productivity enhancement (Fieistrizer, 1975). Well-planned evaluation and release of improved varieties, systematic seed production, and supply, seed testing, certification and enforcement of seed legislation, in toto, are essential to exploit the technological breakthroughs in crop production.
COMPONENTS OF SEED QUALITY
The seed quality components can be categorized into four major groups:
i) genetic quality,
ii) physical quality,
iii) physiological quality, and
iv) seed health.
These components, independently and their interaction at all levels, sum up to constitute the overall seed quality.
11.2.1 GENETIC QUALITY
Genetic quality or purity of a cultivar means all the seeds of a sample (variety) are genetically pure, that is, uniformly possess the essential characteristics of the cultivar in question, without any contamination horn seeds of other variety of the same crop. The genuineness of variety attains the prime position among the most important characteristics of seed quality. In scientific seed production, every care is taken at every stage to maintain the genuineness of variety. In spite of this, varieties may become impure. To verify the genuineness of variety before selling seed is, therefore, an insurance against the supply of impure seed. Genetic purity is an important requirement to maximize the potential of improved varieties.
11.2.2 PHYSICAL QUALITY
Physical seed quality is the proportion of pure seed in the seed lot. It also explains the other constituents of seed lot, namely, inter matter, broken seeds, other crop(s) seeds, seeds of other distinguishable varieties of the same crop, weed seeds, and inert matter, etc. Inert matter includes soil, dust, chaffs, broken seeds, having less than half size, and any material other than seeds. Proportion or percentage of pure seed forms the basis of quality. Higher proportion of pure seed is desired. Other parameters, such as germination and vigor, are determined on the pure seeds after removing all other physical impurities.
11.2.3 PHYSIOLOGICAL QUALITY
The degree of excellence of seeds with respect to all physiological processes that enable a seed to germinated and develops into seedling or plant is collectively is the physiological quality. The main components of physiological quality are viability, germination, and vigor. The speed of germination or germination index is also measured to have a detailed description of germination and vigor. A pure live seed is another parameter that evaluates the physical as well as the physiological quality of seed lot. It is a function of seed purity and germination of seeds. It explains the real worth of seed to have proper crop stand. It is also termed as planting value of seed lot. Planting value is the product of pure seed and germination and expressed in percentage.
Seed is prone to many diseases and pests and many of these are carried by seed from one generation of the crop to others. The freedom from seed- borne biotic stress-causing agents is often termed as seed health. Seed health plays an important role not just in seed quality but other issues related to seed trade like quarantine, etc.
The quality of seed means that seed is superior and pure in terms of its genetic constitution, physiologically strong enough to produce a plant of the desired kind in a wide range of environmental conditions, free from physical impurities and biotic stress-causing agents.
The seed quality is, therefore, the degree of excellence of seed or seed lot in terms of these parameters. There are minimum limits of different seed quality parameters that are imposed legally are Indian Minimum Seed Certification Standards (IMSCS, 2013). Hence, the seed lot possessing the values of parameters more than minimum limits is the quality seed.
FACTORS AFFECTING SEED QUALITY
There are major three groups of the factor affecting seed quality as listed below. The constituents of the individual group are detailed below.
11.3.1 ENVIRONMENTAL FACTORS
The environmental factors include adaptability, light, temperature, sun, rain, wind, soil, and pollinators, etc.
11.3.2 PREHARVEST TECHNOLOGY
The preharvest factors affecting seed quality are cropping history, seed source, isolation, rouging, weeding, plant protection, irrigation, nutrition, and other intercultural operations.
11.3.3 HARVEST AND POSTHARVEST TECHNOLOGY
The harvest and post-harvest operations affecting seed quality are harvesting, curing, drying, grading, seed treatment, bagging, packing, labeling, sealing, storage and transit.
Both quality assurance (QA) and quality control (QC) mechanisms are deployed to manage quality seed production and distribution as well. There are minimum standards for seed quality to regulate seed quality. In India, IMSCS (2013) are in place. The standards are mandatory to produce certified seeds. Seed certification system involving inspections of fields, processing units and issuing of certificate by designated authority provides a mechanism of QA in production chains. This mechanism is applicable only to the foundation and certified classes of seeds of notified varieties of crops. However, certification is not compulsory. Thus, certification is having limited applicability restricted to certified seeds of notified varieties only. Hence, truthfully labeled seeds do not fall in the regulation of certification.
To overcome this limitation of seed certification, another mechanism of compulsory truthfully labeling has been devised. The labeling is compulsory irrespective of seed class and notification status of variety. There are different labels, also called as tags, are listed below:
- • Golden yellow tag (breeder seed)
- • White tag (foundation seed)
- • Azure blue tag (certified seed)
- • Opel green tag (truthfully labeled seed)
These tags are inscribed with the information about the seed in the packs to which these tags are attached or sealed with. Seed quality information on tag includes:
- • germination,
- • genetic purity, and
- • physical purity.
The truthful labeling is essential for QC of seeds. The seed quality information must have compliance to IMSCS and must be tme to seed lot when seed samples are tested for various seed quality parameters. Seed sampling and testing is QC mechanism applicable to all seeds produced and sold. It includes seed testing for physical and physiological parameters and grow-out test and laboratory teclmiques to assess genetic purity.
In addition, there are laws and legislations for international trade of seeds that take care of the quality of seed produced outside the countiy and are not covered under the above mechanisms.
Quality-seed production is specialized production system. It involves high scientific and technical skills and involves high investments. Strict compliance of genetic and agronomic principles and legislative standards is needed to fulfill the minimum prescribed quality of particular crop and class of seed.
11.5.1 GENETIC PRINCIPLES
Seed production, unlike food production, needs to follow certain genetic principles as the genetic purity is of utmost importance among components of seed quality. These principles are described below.
126.96.36.199 DETERIORATION OF VARIETIES
Genetic purity of a variety is determined by many factors during seed production cycles. These are enumerated by Kadam (1942) as listed below:
188.8.131.52 DEVELOPMENTAL VARIATION
The unfavorable environment causing stress to the plants leads to developmental variation. These are the result of differential growth response. The seed crops should, therefore, be cultivated in area and season of adoption to avoid or minimize such variations in varietal characters.
184.108.40.206 MECHANICAL MIXTURES
Mixing of other variety’s seeds at various stages of production and processing leads to varietal deterioration. The deterioration is further aggravated if such plants are not rogued out in cross-pollinated crops.
Mutations are spontaneous and have minor often nondetectable changes that having no remarkable deterioration on varietal characters. However, the detectable mutations need to be rogued out in seed-production fields.
220.127.116.11 NATURAL CROSSING
Natural cross pollination with rogues and off-types deteriorates the varietal purity in seed production cycles. Bateman (1947) suggested factors that determine natural crossing are the breeding system of species, isolation, varietal mass, and pollinating agent.
18.104.22.168 MINOR GENETIC VARIATIONS
Minor genetic variations are often not phenotypically visible. They may even exist in variety at the tune of their release. These variations may get eliminated or may appear with larger magnitude in production cycles. Yield trials of lines propagated from nucleus or breeder seeds have been suggested to eliminate or minimize these variations (Harm, 1953).
22.214.171.124 SELECTIVE INFLUENCE OF DISEASES
High yielding varieties with no resistance or tolerance often show susceptibility under fields because the seed production is generally done under disease-free environment. This may affect the types of varieties having considerable genetic variation.
126.96.36.199 TECHNIQUES OF PLANT BREEDERS
The variety when release must not have genetic variation such as segregation that is not phenotypically visible. The genetic instabilities need to be check or eliminated to avoid any cytogenetic irregularities prior to release.
188.8.131.52 MAINTENANCE OF GENETIC PURITY DURING THE SEED PRODUCTION
Varietal purity is maintained by following the steps enlisted below (Haan, 1953).
- • Seed multiplication using only approved seed.
- • Field inspections.
- • Roguing
- • Seed sampling and prevention of mechanical mixtures.
- • Comparison of crops grown from produced stocks and authentic stocks.
Hartman and Kester (1968) also suggested steps for maintenance of genetic purity of seeds as listed below.
- • Adequate isolation to avoid natural crossing and mechanical mixtures.
- • Rouging of seed production fields before contamination stage such as flowering, harvesting, etc.
- • Periodic testing of genetic purity.
- • Growing crops in well-suited areas to avoid genetic shifts.
- • Seed certification.
- • Adoption of the generation system of seed production.
- • Grow out tests.
The important factors responsible for genetic purity maintenance during seed production enlisted by Agrawal (1995) are
- • control of seed source,
- • preceding crop or cropping history of the land,
- • isolation,
- • rouging,
- • seed certification, and
- • grow-out testing (GOT).
In recent tunes, many other alternate laboratory tests for GOT using biochemical and molecular techniques are devised. These are fast and less expensive.
11.5.2 AGRONOMIC PRINCIPLES
The agronomic principles followed for general crop production are followed for seed production along with certain specific additional principles. These principles mainly focus on the maintenance of seed quality at various stages of seed production and postharvest stages. These are enlisted below.
- • Selection of an agroclimatic region
- • Selection of seed plot
- • Isolation of seed crops
- • Preparation of land
- • Selection of variety and seed
- • Presowing seed treatment
- • Sowing or planting time
- • Seed rate
- • Sowing methods
- • Depth of sowing
- • Rouging
- • Supplementary pollination
- • Weed control
- • Disease and insect control
- • Nutrition of seed crop
- • Irrigation
- • Harvesting of seed crops
- • Drying of seeds
- • Storage of raw seeds