Falling numbers vary with each variety

By Shannon Beattie
December 29 2021 - 11:00pm
DPIRD research scientist Jeremy Curry said varieties differed in their susceptibility to low falling numbers.

WHEN it comes to the management of pre-harvest sprouting and falling numbers, growers should ensure they're making an appropriate variety choice and matching it with an appropriate sowing time.

Department of Primary Industries and Regional Development's (DPIRD) research scientist Jeremy Curry made this recommendation at a recent Grain Industry Day in Perth.

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The falling numbers test provides an indicative measure of alpha-amylase, the enzyme that breaks down starch into sugars, in the grain and is conducted at receival sites where suspected pre-harvest sprouting damage has occurred, determined by detection of visually sprouted grains.

With alpha-amylase being negatively attributed with baking quality, excessive levels in grain results in downgrades and a minimum falling number of 300 is required for delivery into most receival grades.

The likelihood of a crop exhibiting low falling numbers is the result of complex interactions between the genetics of the variety, the environmental conditions it is exposed to, and at what growth stages these environmental conditions occur.

Mr Curry said varieties differed in their susceptibility.

"Varieties differ in their levels of grain dormancy, grains of a highly dormant variety will forgo germination even under favourable conditions until this dormancy wears off days to months later," Mr Curry said.

"The physical attributes of the head and grain, as well as physiological factors may increase or decrease pre-harvest sprouting susceptibility, particularly in varieties with minimal grain dormancy."

Environmental conditions can directly induce low falling numbers or increase susceptibility to future weather events.

Rainfall is the key contributor to increased pre-harvest sprouting incidence, with the nature, frequency and duration of rainfall events and the associated weather conditions all affecting the level of pre-harvest sprouting that could occur in a mature or maturing crop.

Conditions that promote long durations of high grain moisture generally have the greatest impact, however certain grain characteristics can be influenced by environmental conditions during important stages in grain filling.

Temperature and drought stress during grain filling can alter the expression of grain dormancy, while rainfall in the lead-up to maturation that does not directly induce sprouting can increase the susceptibility of the grain to sprouting during later rainfall events.

Mr Curry said the susceptibility of any given wheat variety to environmental conditions that would cause low falling numbers was not fixed, but rather changed with maturation stage.

"Grain dormancy is a trait that wears off over time, so a crop exposed to rainfall after a significantly prolonged period after maturation is more likely to exhibit pre-harvest sprouting," he said.

"Grain dormancy is influenced by growth conditions and so a different season or location can induce increased or reduced grain dormancy.

"Rainfall in the lead-up to physiological maturity has been shown to predispose crops to future pre-harvest sprouting damage, or can directly cause pre-harvest sprouting even before the crop has reached maturity."

Since 2013, DPIRD wheat agronomy researchers have assigned a falling number index (FNI) that rates varieties for their ability to maintain falling numbers.

The falling number of a grain sample is influenced by the variety being grown and how it responds to environmental conditions during growth and maturation, as well as environmental conditions it is exposed to following maturation.

The FNI uses a combination of data from the field, laboratory and artificially induced sprouting to determine the risk of a variety exhibiting a low falling number, so a 1-9 scale the higher the rating and the more likely a variety is to maintain falling numbers.

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As an example, Scepter being rated 5 is seen as an OK variety for most of WA and, while growers might get caught in risky environments during a wet harvest, it is generally safe.

There are then varieties such as DS Pascal which has a rating of 7 which is better than Scepter, whereas the likes of Devil and Magenta have a rating of 3, meaning they're higher risk.

Mr Curry said over four years of trials, from 2015 to 2018, they witnessed early maturation of varieties coinciding with low falling numbers.

"Just as we recommend sowing longer maturity varieties at earlier sowing times to maximise yield, this strategy is also likely to reduce risk of quality downgrades like pre-harvest sprouting and blackpoint," he said.

"Early sown situations, that result in earlier maturation, are likely to require varieties that have lower pre-harvest sprouting susceptibility, while later sowing situations are likely to be lower risk and variety pre-harvest sprouting susceptibility may be less crucial.

"With that in mind growers should review the falling number index ratings to be aware of a variety's risk."

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