Farming

Seed classes

Hybrid seed production takes place over a number of sequentially-dependent stages. The first task is to prepare the breeder’s seed of the initial inbred lines. This is done by breeders under controlled hand pollination to ensure genetic purity and identity. The breeder’s seed is then bulked up over successive generations into sufficient quantities to make the final hybrid. The generations of seed bulking are termed “seed classes” and are carried out under national seed regulations. With each generation, there may be a decline in genetic purity, but if strict procedures are followed, this decline will be minimized.

Planning seed production requirements

The quantity of seed to be produced ought to be tied to the marketing plan of the seed company. From the planned future sales, the seed production plans of each seed class are established, based on the seed yields and seeding rates of the seed classes to be produced. It is usual for seed companies to plan to produce 50 to 100% more pre-basic and basic seed, and 10 to 25% more certified seed, than estimated sales requirements. This is to cover for production risks, such as droughts and quality failures, and possible losses due to seed processing or deterioration. Excess production may also serve as a buffer in the event that actual sales exceed plans.

For three-way and double-cross hybrid seed production, the respective single-cross components have to be produced before the final hybrid cross can be made. The number of component isolation productions for the various seed classes for each hybrid type varies from 5 to 7. Seed production planning may be a complex task if a number of hybrid varieties are produced with many different parents. Poorly-made plans or mistakes in production along the seed class sequence will have deleterious consequences on future productions and sales, and hence profitability.

Insufficient foundation seed is a common reason why good hybrids fail to reach the market. For planning hybrid production, assumptions have to be made about seed rates and yields of the female and male components, which will be a function of the ratio and plant densities of females and males in the field. As a general rule of thumb, seed requirements are 20 kg/ha for the female and 7 kg/ha for the male when considering the field as a whole, but these may vary significantly depending on required plant density, seed weight, and germination percentage.

Thus, when establishing the actual seed rate at sowing, consideration should be given to desired plant density, seed weight, germination percentage, and expected field losses at emergence.

Pre-basic, basic and certified seed production

The rules and procedures for certified seed production are laid down in the national seed regulations of the country in which the seed is to be produced. The aim of certification is to produce seed with an acceptable level of genetic purity and a specified seed quality in terms of minimum germination percentage (usually 90% for maize), maximum seed moisture (12.5%), and minimum seed purity (99% pure seed with less than 3% total defects). Genetic purity is assured by using the correct seed parents of the right seed class, growing the seed in isolation from contaminant crops removing off-types, and, in the case of hybrids, controlling the pollination of the female. Certification procedures are based on standards for growing conditions (e.g., field history, isolation, female-male identity preservation, removal of off-types and detasseling of female plants in the case of hybrid production), field inspections, the prevalence of weed seeds, the proportion of defective seeds, germination percentage and seed moisture content. If a seed field or seed lot does not meet the prescribed standards for the intended seed class, it will be rejected for certification. Consequently, seed producers must be familiar with and adhere to the national seed regulations for the seed class that is being grown.

Hybrid seed crop genetic contamination

The main sources of genetic contamination of a hybrid seed crop are.

  • Seed admixture in the parents prior to sowing.
  • Impure seed sources and inadequate removal of off-types.
  • Foreign pollen contamination due to inadequate spatial or temporal isolation.
  • Re-growth plants of the same crop from previous seasons.
  • Poor detasseling of the female, resulting in female selfing.
  • Poor synchronization of male and female plants, exposing the female plants to a greater risk of contamination from foreign pollen.

The standards for pre-basic and basic seed production are higher than those for certified seed production, particularly as regards isolation requirements and the maximum percentage of off-types. The standards are evaluated in every seed field by means of field inspections and laboratory tests. The field inspections are required to verify the origin of source seed, identify the variety, determine the cropping history, check isolation distance (or time) and production practices and ensure that all certification procedures are adhered to. Usually, three-to-five field inspections are required through the season. Following shelling or threshing, samples of seed are taken for laboratory tests to evaluate the purity of the seed, the presence of other seeds, the germination percentage, and the moisture content.

The sampling procedures assure that a sufficiently representative sample is drawn from the seed lot. For seed sampling, the entire seed lot should be accessible to the seed sampler to enable the collection of a representative sample. This is the final stage in the certification process, and if the seed lot passes all the standards, it is granted certified status.

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