Mungbeans' troubled reputation in central Queensland could be turned around by research suggesting that sowing time has the greatest influence on yield potential.

Trials by Queensland Department of Agriculture and Fisheries research agronomist, Doug Sands, have found that different sowing dates - and the associated weather - may explain why increases in mungbean dry matter do not always correlate to increases in yield.

For dryland growers, the answer could be a later sowing date.

Mr Sands says the biggest factor impacting mungbean yields is water, and when it is received.

"Plus, nutrition, weather, soil type, variety, insects and disease ... and pretty much anything else you can think of. But what makes the biggest difference is the weather," he says.

Weather scenarios investigated

Mr Sands says that in the past it has been hard to find trends in weather conditions, but sowing date trials have helped that and allowed him to assess the impact of different weather scenarios on plant physiology.

Mungbeans are planted as a summer crop in central Queensland and he undertook four years of sowing date trials. Most trials consisted of three dates spread across the summer period from early December through to early March.

Theoretically, mungbeans are a vegetatively determinant crop and should stop producing vegetatively when flowering begins so that energy can be utilised for producing grain. However, climatic conditions can unsettle this balance.

This is why harvest index (HI) is important.

Mr Sands says HI helps to explain the proportion of grain to total plant mass and a number of trials have shown best mungbean yields occur with a HI of 0.3 to 0.35.

In theory, a bigger plant should generate more yield. But increased water and dry matter have not always correlated to a higher HI.

Mr Sands instead found that the way vegetative dry matter accumulates can have an impact on HI.

Sowing dates affect harvest index

Different sowing date scenarios can cause large differences in vegetative dry matter accumulation around flowering, regardless of soil water conditions, which then lead to a lower HI.

"This means the full potential of the vegetative mass (photosynthetic base) has not been converted into grain yield," Mr Sands says.

"There are several reasons for this to occur, but assuming the time of sowing (TOS) trial has been done well, then the major variable should be weather.

"Weather situations where there is more evaporative pressure on the plant (an earlier TOS) seem to cause the plant to stress and interrupt normal vegetative growth pattern.

"When evaporative conditions ease, the plant continues to expand its vegetative base even after it has started flowering. This pattern of growth is correlated to a negative impact on HI.

"However, when conditions are cooler, which occurs in a later planting (for example early March), there is a lower rate of water extraction required by the plant and consequently no interruption to the normal accumulation of vegetative dry matter leading up to flowering.

"There is then little vegetative growth occurring after flowering has started, which leads to a much better HI and better grain yield."

Mr Sands says the work has shown two preferred times of sowing:

• For dryland growers, a later sowing date (February to March) produced more reliable yields by avoiding weather conditions that cause high evaporative pressure on the plant. It does better from stored moisture.
• Irrigated growers can also benefit from a late summer planting. However, an earlier sowing date (December-January) can have higher yield potential because it can create more dry matter from the higher radiation levels and longer day lengths. This is a high-risk strategy, however, as conditions around flowering can still be bad enough to reduce flower survival, regardless of soil water conditions.

Mr Sands is now experimenting with nitrogen to see if an economically viable response to additional nitrogen is achievable.

More information: Doug Sands, douglas.sands@daf.qld.gov.au, 07 4991 0811.

Useful resources: Australian Mungbean Association and GRDC Mungbeans GrowNotes.