Mungbean plants with leaves showing signs of long fallow disorder

On this page:

• Fertiliser
• Irrigation
• Harvesting mungbeans

Fertiliser

Nitrogen

Although the mungbean plant has a relatively high nitrogen requirement, the crop shouldn't generally need nitrogen fertiliser provided plants have effectively nodulated (a 1.5 t/ha crop has a total nitrogen requirement of 100 kg N/ha).

Low rates of nitrogen fertiliser may be justified in situations where:

• double-cropping immediately after winter cereals, where low rates of nitrogen (and sulphur) can get the plants off to a good, quick start. Consider 20-30 kg/ha sulphate of ammonia or Starter 15, depending on soil P levels
• late-plant situations where plants often fail to make sufficient vegetative growth to support a reasonable grain yield, consider using Starter Z.

Phosphorus

In low vesicular-arbuscular mycorrhizae (VAM) situations (i.e. long fallows over 12 months duration or after canola), large responses to applied phosphate fertilisers are likely in situations where soil bicarbonate P levels are below 25 mg/kg.

Where soil VAM levels are high (double-crop situations or short fallows of less than six months) responses to applied phosphate fertiliser are only likely in situations where soil bicarbonate P levels are below 10 mg/kg.

Sulphur

Sulphur deficiency is most likely to occur in double-crop situations where soil available sulphur levels have been depleted. Responses to sulphur are most likely in double-crop situations, particularly after high-yielding cereal crops. Sulphate ammonia at 20-30 kg/ha will rectify a sulphur deficiency.

Zinc

Mungbean is very responsive to zinc. Crop responses are likely where soil test levels are:

• below 0.8 mg/kg on alkaline black earth, grey clays and alluvial soils
• below 0.4 mg/kg on acid soils.

Severe deficiencies stunt plants and produce dead leaf tissue between the veins. Mild deficiency can be diagnosed by the upward 'cupping' of the uppermost leaves.

Severe deficiencies can be corrected for a period of 4-6 years with a soil application of zinc sulphate monohydrate worked into the soil several months before planting.

In the first year after application, the soil applied zinc mono may not be fully effective, and a foliar zinc spray may also be required.

In zero-till or double-crop situations, deficiencies can be corrected by using either:

• phosphate-based fertilisers containing zinc
• foliar sprays
• seed treatment, e.g. Agrichem's Broadacre Zinc or Teprosyn Zinc.

A foliar spray of 1.0 kg zinc sulphate heptahydrate + 1.0 kg urea in at least 50 L of water/ha plus wetting agent will correct a mild deficiency. One to two sprays need to be applied within four weeks of emergence.

Foliar sprays containing zinc sulphate heptahydrate are compatible with dimethoate, but will cause nozzle blockages if mixed with Blazer. Chelated forms of zinc are more expensive, but can be mixed with most herbicides or used in hard water.

Zinc seed treatments may be a cost-effective option in situations where soil P levels are adequate but zinc levels are likely to be deficient.

Agrichem recommend that Broadacre Zinc should be applied at 5 L/t of seed. To minimise any damaging effect on the rhizobia, the Broadacre Zinc treatment needs to be applied first and then allowed to dry before applying the inoculum.

Long fallow disorder

Stunted growth and low yields are often a problem where mungbeans are sown on soil that has been fallowed for 12 months or longer. As VAM levels decline in the soil, deficiencies of phosphate and/or zinc become increasingly common. Crops such as mungbeans are highly dependent on VAM. The need for higher rates of P and Zn should be considered in long fallow (low VAM) situations.

Alternatively, consider planting other crops such as sorghum, which handle long fallow disorder relatively better than mungbeans.

For further information see 'Mungbean and soybean disorders: the ute guide'.

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Irrigation

Mungbeans are highly efficient users of water and usually do not respond to irrigation during podding. Mungbeans are sensitive to excessive waterlogging and the importance of good layout and drainage cannot be over-emphasised. Waterlogging events of greater than five days can cause root nodules to die back, with subsequent nitrogen deficiency problems in the crop.

Furrow irrigation

The following irrigation strategies have been developed to help minimise the impact of waterlogging in pulse crops:

• select fields with reasonably steep grades
• fields should not have any low spots - ideally, they should be laser levelled
• form-up high-volume hills or beds which drain quickly after rain
• select fields with short runs - preferably 200-400 m
• an efficient tailwater system which will rapidly drain water away from the paddock is essential
• irrigate down every second, or alternate furrow
• apply water quickly, preferably in 4-8 hours. Adjust the number of siphons per furrow to suit
• irrigating after light rain can help speed up irrigation time
• the first irrigation should be applied before soil cracks open right up.

Nitrogen fertiliser can be used to offset the effects of water logging. This can be applied as a foliar urea spray prior to irrigation, or as 15-25 kg urea/ha in the irrigation water (water run).

Being relatively quick maturing, one of the major advantages of mungbean is their low water use, and relatively high financial returns per megalitre. A strategy of irrigating to plant and then at budding has been found to be most cost-effective in many situations. Other points to consider:

• Crystal offers greater yield potential than other varieties
• narrow rows maximise yields under favourable conditions
• achieving a plant population of at least 30 m^-2 is critical to maximising yield
• higher yields will be recovered from 2 m beds than 1 m hills under high lodging pressure (up to 300 kg/ha^-1 in trials)
• irrigating too early can delay or inhibit nodulation
• the short duration of current varieties places a ceiling on achievable grain yield
• in trials at Emerald in Central Queensland the most effective strategy was flood irrigating to plant and a single in-crop event to fill the profile at flowering/podding
• coordinate irrigation with insect control strategies.

Irrigating too early in the growth of the crop will encourage excessive vegetative growth. The preferred strategy is to pre-water and then plant on a profile of soil water. The first in-crop irrigation is usually best applied around seven days before the start of flowering, i.e. 30-40 days after planting.

On the heavy black earth (Darling Downs), one in-crop irrigation applied around flowering is usually sufficient to achieve reasonable yields. In the hotter western or northern areas, two irrigations may be required with flood irrigation systems.

Avoid growing hard-seeded lines like Emerald under irrigation as its hard seeds can cause volunteer problems in cotton farming systems.

Spray irrigation

This is an option in some areas, and has the advantage of allowing smaller amounts of water to be applied more frequently. This helps reduce water logging and can lift yields. Avoid heavy spray irrigation of young plants as the caked-on dirt from mud splash can reduce growth and thin the plant stand (especially in crusting soils). Approximately 50 mm of water per week will normally be required during flowering and pod-fill.

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Harvesting

The preferred moisture content for delivery to the packing sheds is in the range of 12-14%. The maximum moisture for beans held in storage 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 usually 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.

Desiccation

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 moisture. Consequently they can have flowers, green pods and black pods present on the plant at the same time. This growth habit makes the harvesting decision difficult. The ideal stage is when a majority of pods are physiologically mature and 90% of the pods have turned either yellow or black.

Growers should ensure timely harvest to maximise quality of the first flush of flowers rather than waiting for late pods to mature. At a percentage of 90 or greater, the crop has reached maximum maturity and is at optimum yield and quality. At this stage the crop should be considered ready for either desiccant application, or direct harvest.

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 and diquat are the registered herbicides for desiccation.

Glyphosate is the preferred treatment. This translocated herbicide when used at the higher rates 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 retreatment.

Plant vigour: Large healthy crops that are actively growing with adequate levels of soil moisture require higher product rates that 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 chanels), 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 applications: 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 moiture 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.

Yield

• dryland double-crop 0.25 - 1.25 t/ha
• dryland winter fallow 0.75 - 2.0 t/ha
• irrigation 1.25 - 2.75 t/ha

Grading losses will usually reduce marketable yields by 5-20%.