Australian Mungbean Association
Australian-grown mungbeans have quality written all over them!
Desiccation and Harvest
Mungbeans have an indeterminate flowering habit. This means that they do not have a defined flowering period and will continue to flower while there is adequate soil moisture. Consequently, they can have flowers, green pods and black pods present on the plant at the same time. This growth habit can make the harvesting decision difficult.
The ideal stage for harvest to maximise yield and quality is when the majority of pods are physiologically mature and 90% of the pods have turned yellow through to black. At this stage the crop should be considered ready for desiccation and harvest.
'Sap' from the mungbean plant can form a film over the seeds during harvest. This sticky coating on the seed then attracts dust during the harvest operation and the glossy lustre of the mungbean sample is lost.
This staining of the seed coat is regarded as the single most important issue impacting on mungbean quality and net returns to growers (with potential losses of $100–300/t). Lower quality beans are also more difficult and slower to market overseas.
Plant sap and dust also combine to form a build-up of gum that and can cause blockages inside the header.
To minimise the level of seed staining it is essential that desiccation is carried as effectively as possible.
The key point when desiccating mungbeans is the use of a robust rate of glyphosate and allowing sufficient time for the crop to dry down before commencing harvest.
Product choice: Glyphosate (e.g. Roundup PowerMax) and diquat (e.g. Reglone) are the registered herbicides for desiccation. Other products may also be registered under a Minor Use Permit.
Glyphosate is the preferred treatment. When used at the higher rates, this translocated herbicide provides the most consistent dry-down of both leaf and stem moisture.
Diquat is a desiccant that provides rapid dry-down of leaves, but poor dry-down of moist stems. It is used mainly when rainfall is imminent. If rainfall interrupts harvest, regrowth may occur as early as 10 days after application and may require re-treatment.
Plant vigour: Large healthy crops that are actively growing with adequate levels of soil moisture require higher product rates than would normally be the case.
Variety: Crystal and Emerald are more likely to require high product rates because of their ability to remain green and healthy late in the season.
Water quality and pH: Reduced results may occur if water containing suspended clay or organic matter (e.g. from dams, streams and irrigation channels), or high levels of calcium, magnesium or bicarbonate ions is used. Water pH should be neutral to acidic. Consider buffering to optimise spray performance.
Spray application: Refer to the product label for complete application details.
Time to harvest: There is a tendency for growers to harvest too soon after desiccation. The rate of dry-down of the crop will depend on the choice of desiccant product, rate used and temperature and moisture conditions. Wait for maximum dry-down of leaf and stem moisture, which can take 5–6 days with diquat and 7–16 days for glyphosate. Other products may also be registered under a Minor Use Permit.
Harvest losses in commercial mungbean crops can often exceed 30%, and is one of the key management issues impacting on the overall profitability of a mungbean crop.
Header set-up to avoid over-threshing is very important in mungbean crops. This will minimise the impact that any residual moisture within the leaves and stems has on seed staining.
Plants should come off the back of the straw-walkers largely intact with minimal crushing of the stems.
The best guide to how effective these strategies have been in minimising seed staining is to compare a hand-threshed sample, with a sample taken from the header box. Always assess gloss/lustre in full daylight against a white background.
Header fronts need to be set and maintained at a height below 25% of total crop height to minimise stubble losses in mungbeans. Grain losses can be substantially reduced by maintaining harvester ground speed at 6–7 km/hr and using:
Early harvesting reduces losses due pods being less prone to shatter or drop. The crop is also easier to gather because it stands more erect, allowing the harvester front to operate at a greater height, reducing the dirt, rock and sticks entering the harvester.
Early harvesting also means there are fewer summer weeds to clog the harvester and also plays a role in disease control and crop establishment in the following crop. Early harvested grain is of better quality in terms of colour, weathering and less disease.
Delaying harvest until the plants begin to die-off and all pods are mature can lead to high losses at harvest due to:
The ideal grain moisture level for delivery to the grading plants is 13%. Samples over 14% can either be rejected, or accepted at the owner's risk and put into aerators.
Handling the grain during the cleaning and grading process will further assist in the drying process, and should ideally be carried out before the sample drops below 12%. Over-dry beans (below 12%) are very easily cracked and chipped. While hairline cracks in the seed coat are difficult to detect visually, they can result in an unacceptably high level of over-soaks and a down grading in quality.
The maximum moisture content for the long term storage of the finished graded product is 12%.
Browning and discolouration of seed
A high proportion of high moisture beans (above 14%) in the sample can lead to browning and discolouration of those beans when held in storage or during shipment to export destinations.
This browning of the seed coat often occurs after delivery to the packing shed, usually a month or more after harvest. Classification of deliveries may be delayed on suspect lines with a significant proportion of high moisture beans in the sample. These suspect lines may be put into short-term storage until the full extent of the problem becomes evident.
Growers contemplating medium-long term storage (3–6 months) need to be aware that mungbean continue to age, and that quality deteriorates over time.
Mungbean will darken in storage, with the rate of seed coat darkening being accelerated by;
Conditions of high relative humidity and high temperatures result in rapid deterioration in grain colour. To maintain bright green colour and minimise darkening of seed, any grain stored above 12% MC will require cooling.
Growers should avoid even short to medium storage of weather damaged grain.