Hybrid maize

Hybrid seed is the seed produced by hybridization i.e. by crossing between two or more homozygous inbred lines to obtain a desirable type having high yield potential. Only the F1 generation of hybrids is recommended for use as seed for commercial production. Hybrid maize seed production involves deliberately crossing a female parent population with a male parent in isolated fields. A maize hybrid results from the fertilization of one maize plant by another genetically unrelated plant. This seed bears a unique genetic make-up from the female and male parents and will produce a plant with particular characteristics.

Hybrid Composition and types of maize hybrids

A hybrid variety is produced by crossing inbred lines that have been developed by inbreeding and selection for at least five successive generations. Inbreeding results in depression of vigor (plant height, yield, etc.), increasing uniformity (homozygosity), and the appearance of undesirable recessive gene effects that can be eliminated from the population. When two unrelated inbred lines are crossed to form a hybrid, the resultant seed produces plants with restored vigor and a significantly higher yield than either of the two parents. This is known as hybrid vigor.

The types of hybrids in maize are single-cross, three-way, and double-cross hybrids. A single-cross hybrid is obtained by crossing two inbred lines; a three-way hybrid is obtained by crossing a single-cross hybrid with an inbred line; while a double-cross hybrid is obtained by crossing two single-cross hybrids. Two other types of hybrids are top-crosses and varietal crosses. A top-cross hybrid is obtained from an open-pollinated variety crossed with an inbred line, while a varietal cross is a hybrid of two unrelated open-pollinated varieties.

Cultivation Practices for Hybrid Maize seed Production

The selected field of maize should be free from volunteer maize plants, well-drained, aerated, fertile, and good tillage with deep plowing, harrowing, and leveling. Maize is a cross-pollinated crop so to prevent foreign pollen contamination a sufficient isolation distance should be maintained according to respective types of seeds. Generally, for seeds with the same kernel color and texture 400 meters and colorsdifferent colours and textures, 600 meters of isolation distance is maintained. Method of sowing includes sowing in rows maintaining a planting ratio of 4:1 i.e. four rows of female parents to one row of the male parents. Time of sowing varies according to location but normally seeds are sown two weeks before the onset of monsoon ( second week of June to mid-July). Spacing of 75-80 cm row to row and 25-30 cm plant to plant is maintained with optimum seed rate of 12-14 Kg/ha for female parent inbred and 4-5 kg/ha for male parent inbred. Recommended fertilizer dose is 120-150:50-60:40-50 Kg NPK/ha with half Nitrogen, full Phosphorus, full potash at a basal dose and remaining Nitrogen split top-dress at knee height and flowering stage. Irrigation is provided on critical stages of water requirement i.e. knee height stage, flowering and silking stage. Intercultural operations such as timely wedding, earthing up, thinning, gap filling should be conducted to assure good crop stand establishment. Maize comes under the physiological maturity stage at about 30-35 % moisture content. If it can be dried efficiently, we can harvest at the physiological maturity stage when the husk turns yellow and the grains are hard enough. Shelling is performed after harvesting and seed pass through many processing plants to retain the quality of seeds.

Field Standard

Field standards need to be maintained to colors to get true to type hybrid seed deprived of any foreign contaminations. It includes isolation, varietal purity, roughing, and Field inspections.

1.Isolation – Isolation distance should be maintained properly since maize is a cross-pollinated crop with a high chance of foreign pollen contamination.

Isolation Distance –


2.Varietal Purity
–  The maximum permissible limit of off-types plants is 0.2 % for foundation seed and 0.5 % for certified seed along with shedding tassel 0.5 % for foundation seed and 1.0 % for certified seed.

Specific field Standards –


3.Field inspection and roguing
– Regular field inspection in different stages are necessary by the seed inspector or seed certification officer to produce high-quality seed. Four field inspections i.e. One before flowering and three during flowering are necessary. During field inspection, roguing is done. Roguing is the removal of plants that are off type that is phenotypically different from the plants of the variety under production. Roguing should be done periodically based on the position of the cob, the color of silk, arrangements of seeds in the cob, leaves, etc. Shedding tassels are to be removed in rogue. It refers to the tassels in female parents’ rows, shedding pollen, or that has shed pollen in hybrid maize plots. During field inspection, a tassel whose main spike or any side branch or both has shed pollen or shedding pollen in more than 5 cm of branch length is counted as a shedding tassel. During an inspection, the shedding tassels are taken into account for acceptance or rejection of the production plot.

Pollen Control

Detasseling is the most commonly used method for pollen control to ensure hybridization by forced cross-pollination between the female and male parents. Detasseling is the process of removal of the tassel from a female plant before the emergence of silk on-ear shoots of the female parent, so as to ensure that the resulting ears on the female row will be a cross between the ear producing parent and desired pollen parent. Detasseling is started when the tassel is well out of leaf sheath, but before the anther has shed pollen. Immature detasseling should be avoided which may cause few spikelets to be left, which may emerge and shed pollen. During detasseling, grasp the entire tassel so that all pollen bearing parts are fully removed and do not hold the tassel too low on the stalk so as to prevent pulling out of plant tops.

Conclusion

Hybrid seeds can enhance farmer productivity and assure the food security and food self-sufficiency of any nation. For a good quality hybrid seed production all the agronomic practices should be performed efficiently and all the field standards should be also managed smoothly.