Assessing soybean maturity

Posted in PulseCheck–Coastal on Apr 06, 2019

Assessing soybean maturity

In the lead up to harvest judging the right time to desiccate and harvest can be difficult. In this video Matthew Leighton, Bundaberg Canegrowers and Grain in Cane Co-op. shares some practical tips to help with these important decisions.

Preparing for harvest

The first indicator that harvest is imminent is the green leaves starting to go yellow and pods start to go from green to yellow and through to a mature brown colour. Spots on the outside of the pods are a natural part of the maturing process.

Once the stem starts to go brown the crop is ready for harvest. Sometimes, if there are not enough pods on the plant, the stems can stay green, or if late rain falls the crop seems to hang on and just doesn't mature down normally. This can only be countered against early on in the year by having good insect control and good plant population as this helps to achieve more even crop maturity.

One of the reasons for unevenness is a change of soil type or as a result of not being able to irrigate the crop evenly. Soybeans are a great indicator of a change of soil type or soil moisture issues as they've been growing. Any areas that are stressed early on in the season tend to mature earlier than less stressed areas. The decison to desiccate and harvest needs to be based on the status of the majority of the block and that sometimes means losing small areas that mature too early. The aim is to get the majority of the block and the majority of the beans.

Soybean seed pods can shatter once they are fully mature, especially the variety A6785. It doesn't happen immediately they mature, it usually takes another two or three weeks before mature pods on a plant are ready to shatter. They normally also need a physical reason to shatter such as a rainfall event or the header front.

In coastal environments it is often necessary to take the beans off before they reach optimal moisture content. In drier production environments, away from the coast, soybeans are usually left in the paddock to until the seeds have dried down to 12 per cent moisture, but waiting for that to happen on the coast is very risky. It is better to harvest at a higher moisture level to ensure the beans get into the header and off the paddock.

There's normally a drying cost associated with meeting the standard of 12 percent moisture for the majority of the market places. Coastal beans are often sent in at 13-14 per cent especially in the late April and early May periods where, relative to inland areas, our humidity is still high. The drying cost varies but is generally around $20–25/t and should be included in the budget.

It is very easy to lose some grain by trying to wait for the whole crop to be dried down to 12% and, more importantly, in the situation where there is limited availablity of harvest equipment, waiting for a crop to go down to 12% may mean that another grower isn't able to harvest their crop because all the pods have shattered and its all on the ground. The Grain in Cane Cooperative shares the cost and risk of grain moisture across all growers. Some crops may incur no drying costs while others need drying but everyone pays an average drying cost.

When to desiccate and arrange for the harvester to come

Desiccation is used to dry out the whole crop and ensure that you get the best quality beans in the header and to market.

This key decision is hard to make. It needs to take into consideration the crop maturity and the desiccant chosen. The majority of coastal crops will need to be desiccated to make it easier for the header to operate. Desiccation caused the weeds to drop their leaves and the stems become brittle. Having less green material going through the harvester reduces the risk of seed staining that can downgrade soybean quality.

Only diquat and glyphosate are registered for pre-harvest desiccation in soybeans.

Diquat (e.g. Reglone) Pre-harvest crop desiccation (all states): Spray as soon as the crop has reached full maturity. Helps overcome slow and uneven ripening and weed problems at harvest. WHP: DO NOT harvest for 4 days after application.

Glyphosate (e.g. Roundup Power Max) (Not all glyphosate formulations are registered for these uses) Pre-harvest application to desiccate a crop as a harvest aid and weed control – annual weeds. Application to crops intended for seed production or for sprouting may reduce germination percentage to commercially unacceptable levels. WHP: DO NOT harvest within 7 days of application. Refer to label for specific timings.

If you choose to use glyphosate to desiccate the crop and kill any weeds, you need to wait until there's better than 90 percent of the pods are yellow across the block before you can spray because any seeds in green pods present on the plants will stay green and there's a limit to how many green seeds will be accepted in the marketplace. Allow at least 7 days before harvest and check the label to ensure it includes soybean.

If using reglone, wait for at least four days before harvesting (as that's the withholding period), and generally harvesting four to six days after spraying is ideal.

Once the desiccant is applied you normally you wait for all the leaves to drop off the crop and all the stems to brown out. If there is late rain, the stems may stay green right through to harvest and growers should rely on their own experience or that of their harvest operator or an experienced agronomist to determine the best time to harvest.

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