Key points

• Swathing or mechanical desiccation has been investigated as an alternative to chemical desiccation with glyphosate
• Mechanical desiccation of mungbeans produced similar yield, quality and screening results to chemical desiccation in trials
• Commercial-scale results from the 2022 trials are being assessed

As an alternative to chemical desiccation, swathing or mechanical desiccation could start to be more widely used by mungbean growers following positive research results.

Normally mungbeans, an indeterminate crop, are chemically desiccated using glyphosate as a harvest aid.

Accurately timing glyphosate application can be difficult, particularly when a crop has put on several flushes of pods, resulting in a range of maturities on one plant.

These conditions can significantly increase the risk of detectable glyphosate residues exceeding the importing countries’ maximum residue limits (MRL).

With some trading partners showing a preference for chemical-free products, researchers and the mungbean industry were keen to investigate mechanical desiccation.

Trials in central and southern Queensland by the Queensland Department of Agriculture and Fisheries (DAF) have had positive results.

DAF principal extension officer Jayne Gentry led the research. She says the trials showed that, if done when 90 per cent of the seeds are at physiological maturity, mechanical desiccation produced similar yield and quality results to chemical desiccation.

Mechanical desiccation could prove to be an alternative for growers wanting to reduce the use of glyphosate as a harvesting tool. The biggest benefit of mechanical desiccation was the faster dry-down of the crop. However, timing the cutting (swathing) was critical to ensuring minimal harvest losses.

Small plot trials were conducted in 2021 with commercial-scale trials using commercial machinery taking place this year.

Australian Mungbean Association (AMA) president Dale Reeves agrees and has welcomed initial research findings and the more commercial-scale trials.

He says the need for an alternative to chemical desiccation has been targeted for some time. “We’ve been looking for a better solution for two main reasons. Many growers want to move away from glyphosate use, from a marketing perspective. Also, agronomically, many of our newer varieties are too vigorous to desiccate with existing chemicals.”

Mechanical desiccation seems to fit well agronomically and trade-wise. “We had feedback from some of our prominent growers this year who heard there was research going on. They wanted to know all about it and were very keen to get the data and try it for themselves.”

Ms Gentry says mechanical desiccation may help to develop new overseas markets.

Export markets can have differing MRLs and for some markets these limits can be lower than the Australian MRL, making it difficult for Australian exporters to meet these market requirements. For example, if we want to ship to Taiwan, which is lucrative, we essentially must be chemical-free.

Mechanical desiccation can cost more than using glyphosate. “Using glyphosate, you can run a spray rig that is three times wider and faster than a narrower, heavier and slower machine that will mechanically desiccate the mungbeans. But growers are happy to think about it as an option – even if it’s more costly.”

Mr Reeves says there will be a role for contractors travelling into northern mungbean areas to get the job done. “What everyone does will be different on each farm. But removing glyphosate as a harvest aid will improve our farm sustainability and improve the clean and green image of the Australian mungbean industry.”

At the end of the day, Ms Gentry says, it is about having desiccation options. “It is good to start thinking about alternatives to glyphosate desiccation. I’m focusing on helping growers and the industry be aware that spraying glyphosate as a desiccant at the wrong time can impact marketing options and that there is an alternative to consider.”

Rain prompts on-farm trials

An extra 100 millimetres of rain on his mungbean crop helped prompt south-east Queensland grain grower Alex Meyer to take part in mechanical desiccation trials this year.

Alex farms 1800 hectares with his wife Jenna and father Helmuth 10 kilometres north of Warra and has planted 200 to 300 hectares of mungbeans each season for about a decade.

He says the extra rain came just when the crop was ready to be chemically desiccated. “The rain caused the crop to pod again. That always makes it harder to desiccate. So, I bit the bullet and asked to be included in the trial.”

The mechanical desiccation went well, he says. “I was really happy with it. We had such high biomass that it would have been hard to chemically desiccate. It is good to have another option.”

The decision also saved time. “We beat the next rain event because we didn’t have to wait as long to get back out there and pick it up. It was five days compared to 10.

For Alex, mechanical desiccation would require a contractor or the purchase of attachments for the header. Those decisions will depend on the season and costs. “Sometimes glyphosate costs are so high that it might not be more expensive than chemical desiccation.”

More information: Jayne Gentry, jayne.gentry@daf.qld.gov.au; GRDC Update paper.