Common issues with seed quality:
- Wheat tends to show good germination. Seedling diseases can affect germination test performance, especially in wet years.
- Barley seed tends to show good germination. Seedling diseases can affect germination test performance, especially in wet years. Dormancy in early season seed can make germination assessment more difficult. Mechanical damage can be problematic, especially in dry years. The embryo is exposed and so is more vulnerable to damage
- Field bean seed quality is usually very variable due mainly to problems with seed being mechanically damaged. The problem is often worse in dry years.
- Oilseed rape quality can be problematic for a number of reasons, including heat damage, mechanical damage, chemical damage and occasionally disease.
- Heat damage can be a problem in any type of seed, due to artificial drying or excessively hot weather around harvest.
- The germination quality of over-yeared seed of any species is likely to have deteriorated, especially if storage conditions were not ideal. For over-yeared seed germination should always be re-checked prior to use.
- Chemical damage can be a problem in a number of situations. For example, if seed is stored in facilities used previously for potato storage. Residues of potato sprouting suppressant have the potential to seriously affect seed germination. Or if a crop has been accidentally sprayed with glyphosate the seed may not be killed but it can affect subsequent seed germination, especially in oilseed rape. For both these examples the seeds may be alive but will germinate abnormally and are less likely to produce healthy plants in the field, especially where conditions are less than optimum. Germination tests can identify chemical damage but tetrazolium tests cannot and would over estimate potential field emergence.
Laboratory seed tests aim to provide accurate and reproducible guidance, rather than absolute answers or predictions. Viability, germination and vigour tests all produce results that are usually greater than, and at best equal to, how the seed will actually perform in the field. An appreciation of what viability, germination and vigour measure can help maximise the understanding of the planting value or storage potential of seed.
Given enough time NIAB recommends having both a germination test and a thousand seed rate test to allow calculation of sowing rates. If you have several seed lots of equal germination potential or you want to sow at the wrong time of the year you might consider having a vigour test as well. If you are in a hurry or suspect heat damage use the tetrazolium test and still have a thousand seed weight test to allow the calculation of sowing rate.
Seed germination tests
Seed germination tests measure the number of healthy well-developed seedling under laboratory conditions, not just whether a root has emerged from the seed. Because of this, a germination test will take at least seven days for cereals or for grasses it can take four weeks.
The process of seed germination is complex and can be affected at different stages by many factors and interactions of factors such as temperature, water availability, oxygen, light, substrate, maturity of seed, physiological age of seed. In laboratory germination tests, these factors are optimised in order to measure the maximum number of seeds capable of producing healthy well-developed seedlings. A laboratory germination test does not take into account the effects of non-optimal conditions on the seed. It is therefore useful to view a laboratory germination test result as potential rather than absolute emergence.
This test is a highly standardised measure of seed quality at a relatively low cost.
The standard test is based on 200 seeds. Seeds are germinated under optimum environmental conditions for an optimum period of time according to species. Dormancy breaking measures are used. Distinctions are made between normally and abnormally germinated and dead seed. Can only assess the germination of non-dormant seed. Detailed assessments of damage are not included in the test. Length of test is long in some species.
Results are reported as % germination (% dormant seed where relevant). Major quality problems observed on a sample may be noted but not quantified on the test report. Sowing rate when in combination with a 1,000 seed weight test.
Diagnostic germination test
A more expensive diagnostic germination test, still using 200 seeds, is available for wheat and barley, that provides greater detail on sample’s quality problems than the standard germination test.
Seeds are germinated under optimum environmental conditions for an optimum period of time according to species. Dormancy breaking measures are used. Distinctions are made between normally and abnormally germinated and dead seed. Can only assess the germination of non-dormant seed. Length of test is long in some species.
Results are reported as % germination, % dead seed and % abnormal seedlings, with % given for damage (dormant, heat, chemical, sprouting, disease, mechanical types). Sowing rate table when in combination with a 1,000 seed weight test.
Germination tests on 400 seeds are also available. The larger sample size improves the accuracy of the test result slightly. The test is more expensive.
Thousand seed weight
Used in conjunction with germination % test results allow the calculation of sowing rates tables.
The sample is screened to simulate what would happen when the seed lot is cleaned and processed, and then 1,000 seeds are weighed to produce the weight of 1,000 seeds in grams. This can be used to calculate sowing rates accurately a 1,000-seed weight is needed with a germination test:
Seed rate (kg/ha) = (Target plants/m2 x 1000-seed weight in g x 100) / (Germination % x Potential Field Establishment %)
The % potential field establishment chosen for the calculation depends on the condition of the seedbed. As a guide, rates of 80% are generally used for good seedbeds and 60% for poor expected conditions.
The sample is screened to remove small seeds that would be removed during processing and 1,000 seeds weighed. The result is reported as the weight of 1,000 seeds in grams.
Seed viability (tetrazolium) tests
A viability tests offers a faster germination test than the standard 7-10 days.
Seed viability testing uses a chemical called tetrazolium and aims to determine which seed tissues are alive and have the potential to germinate under optimum conditions. Tetrazolium is a colourless chemical that reacts with living cells and stains them red. In this way, living tissue in seed embryos can be distinguished from non-living tissue. It will not detect seedling abnormalities; it only detects what tissues are alive or viable. Because of this occasionally a tetrazolium test and germination test result for the same sample will not be comparable. This is because any factor that affects the seed as it actually germinates is not detected by tetrazolium tests, i.e. chemical damage, dormancy, disease.
Trained seed analysts can interpret tetrazolium staining patterns in seed embryos and therefore determine germination potential. Tetrazolium tests are particularly good for detecting heat-damaged seed as this kind of damage creates a unique staining pattern. A seed lot may have suffered heat-damage, which is lethal for each individual seed affected, but the deterioration usually occurs over time. The amount of time is dependent on the degree of damage incurred. If a seed sample is tested for germination immediately following the point of damage, the seed may not have fully deteriorated and the germination potential may be over estimated. With a tetrazolium test it is possible to detect the very early stages of heat damage. Mechanical damage to the seed embryo and sprouting can also be assessed with tetrazolium.
A basic tetrazolium test, the dry seeds are cut, stained in tetrazolium and visually assessed. Chemical damage, dormancy, seedling disease are not detected. Detailed assessments of damage are not possible. Cutting dry seed can damage embryo tissues and introduce additional error to test results.
The results are reported as % viable seeds. No additional information is possible.
Rapid and low cost indication of seed viability. Can assess potential germination quality in the presence of dormancy.
Rapid tetrazolium seed viability
A rapid indication of seed viability that can assess potential germination quality in the presence of dormancy.
A tetrazolium test, based on international protocols. Seeds are pre-moistened, embryos removed, stained in tetrazolium and visually assessed. Chemical damage, dormancy, seedling disease are not detected. Detailed assessments of damage are not included in the test.
Results are reported as % viable seeds. Major quality problems observed on a sample, may be noted but not quantified on the test report.
Seed vigour tests
Seed vigour can be considered the closest measure of potential field performance. For seedlings classified as normal by a germination test, there will in fact be differences between the seedlings in their ability to perform well under a wide range of environmental conditions.
Vigour tests aim to measure the ability of the seed to perform well under unfavourable conditions and are used for two main reasons:
- To discriminate between seed lots for suitability for storage.
- To discriminate between seed lots for planting value in relation to optimising establishment (e.g. to promote synchronous emergence or maximal performance under sub-optimal seedbed conditions).
Vigour tests are often species specific and so a large number of tests are in existence. It is important to choose the most appropriate test available and understand its limitations. Tetrazolium vigour tests for example, measure vigour indirectly, and therefore treat vigour as an intrinsic property of the seed. Some, such as physiological stress tests, measure susceptibility to unfavourable conditions directly.
Rapid tetrazolium seed viability and vigour (most species)
A rapid indication of seed viability and potential vigour which can be useful where seedbed conditions are unlikely to be ideal or where there is a choice of lots to be used for seed or storage. The test can assess potential germination quality in the presence of dormancy.
A tetrazolium test, based on international protocol, seeds are pre-moistened, embryos removed, stained in tetrazolium and visually assessed into vigour categories. Chemical damage, dormancy, seedling disease are not detected. Detailed assessments of damage are not included in the test.
Results are reported as % viable seeds, with sub-categories for % high vigour seed, % medium vigour seed and % low vigour seed. Major quality problems observed on a sample, may be noted but not quantified on the test report.
For more information contact Helen Appleyard