Seed holds the origin of life — the promise of growth. It is nature’s miracle in preserving the identity of varieties, ensuring that traits are passed on consistently from one generation to the next. Yet, the appearance of hybrid vigour through crossbreeding remains a remarkable wonder, offering enormous economic benefits to farmers.
The practice of keeping seed from open-pollinated crops of the previous harvest for the new planting season is short-sighted and strongly discouraged. Crop results are often inconsistent and unpredictable, and can be costly for the farmer.
Using high-quality seed is universally recognised as a fundamental requirement for economically viable crop production. That’s why it’s important to consider the factors that negatively affect seed quality, as well as the precautions taken to provide producers with seed of the highest standard.
Genetic quality
Genetic quality means that seed is genetically pure and that a seed lot contains only seed with the true and known characteristics of the variety.
Plant breeders manipulate genes to ultimately introduce exceptional cultivars into a region’s planting mix. If such a cultivar’s seed is not pure, the farmer cannot fully realise its potential.
Seed that doesn’t match the description of a specific cultivar can result from:
Outcrossing
Although small grains like wheat and soya beans are primarily self-pollinating, outcrossing can still occasionally occur. Depending on environmental conditions, such as high or low temperatures or drought stress during flowering, and inherent genetic tendencies, up to 5% cross-pollination can occur, resulting in off-type plants.
Physical mixing
Physical seed contamination can occur at any stage of production or processing. Good management begins well before planting, ensuring that the field is free of volunteer plants or crop residues whose seed cannot be mechanically separated.
All equipment used in the planting process must be clean and free from foreign seed. Effective control is crucial when transferring seed between bags or containers, especially when switching between cultivars.
All off-type plants must be removed during the growing season — ideally, immediately upon identification, as they become harder to spot later in the growth cycle.
During harvesting, transport, and processing, care must be taken to ensure all bags, containers, and equipment are free from contamination.
The most significant mixing risk comes from commercial harvesters, which are nearly impossible to clean completely.
For this reason, breeders must regularly provide pure seed on a single-plant basis, handled under strictly controlled conditions. That’s why farmers are strongly encouraged to purchase certified, pure seed in order to reap the full benefits of a cultivar.
Physical quality
Foreign material
Physical purity means seed is free from any visible impurities — including stones, plant debris, animal matter, grain pests, and seeds from other crops or weeds. Harvest machines can be calibrated to minimise foreign material. In some cases, contaminants may be as large as the grain itself and cannot be removed through standard cleaning processes.
Damaged seed
Controllable factors
Proper calibration of harvesting and processing machines prevents mechanical damage to grain, which can lower germination rates. Excessive drum speed or incorrect concave settings lead to broken kernels.
Pest control during the growing phase is essential, as bollworm-damaged seeds are difficult to remove.
Storage pests like weevils and grain moths cause serious damage and must be controlled.
Uncontrollable factors
Environmental conditions during the growth and seed development phases largely determine seed quality.
Excessive heat or extreme cold during the milk or dough stage can lead to thin, shrivelled seed. Cold damage in the late dough stage results in wrinkled and loose seed coats. Prolonged wet and cloudy weather can cause pre-harvest sprouting and dramatically reduce test weight. Planting weathered seed under poor conditions can result in poor germination and seedling vigour.
Diseases
When epidemic outbreaks of stem and leaf rust occur, seed quality can deteriorate significantly due to weathered kernels.
Certain diseases also cause shrivelled, dull, and discoloured seed, which can usually be removed during processing.
Pathogens from seedborne diseases can only be controlled with high-quality chemical seed treatments.
Germination vigour
Germination indicates the percentage of seeds that can produce healthy plants and achieve optimal plant populations. High germination rates are essential for economic planting. Environmental factors like extreme temperatures and excessive rainfall — which can cause pre-harvest sprouting — contribute to poor germination.
Controllable factors affecting germination include:
- Harvesting seed at ideal moisture levels
- Proper machinery calibration
- Controlled storage temperature and humidity
- Ageing: seed loses germination vigour over time, a process accelerated by high storage temperatures (above 15 °C)
Seedling vigour
Vigour refers to a seed’s ability to germinate quickly and uniformly under a range of environmental conditions, producing strong, healthy seedlings.
Seedling vigour is affected by:
- The inherent genetic vigour of each cultivar
- Stress conditions experienced by the mother plant
- Harvesting immature seed, which leads to reduced vigour
- Maximum vigour occurs at physiological maturity, when maximum dry mass is achieved
- After maturity, vigour declines at a rate unique to each species
- Mishandling during drying, cleaning, and grading can reduce vigour
- Poor storage conditions (high temperature and humidity) quickly degrade seed quality
Using seed with proven genetic and physical quality is critical for all crops. Properly produced and technologically processed seed is the farmer’s best insurance for successful crop production.
For more information or enquiries, farmers can contact Pioneer via e-mail at info.rsa@pioneer.com, or visit their website at www.pioneer.com/za.
