Pre-harvest sprouting in cereal crops caused by unfavourable weather conditions during the grain-filling period and throughout harvest can significantly downgrade the value of a crop. With increasing climate variability, it is yet another risk that growers need to monitor and manage, together with the impacts of frost and drought, but there are a few tools at hand.

In Western Australia, grain suspected of being affected by pre-harvest sprouting is subject to a falling number test at receival. This is a test for increased alpha-amylase activity, an enzyme involved in the spouting process, which when present can significantly reduce grain quality. The lower the number, the higher the enzyme activity, and thus the lower the quality. A falling number of more than 300 seconds is required for most milling grades.

GRDC, in conjunction with Stirlings to Coast Farmers (SCF), South East Premium Wheat Growers Association (SEPWA), Southern Dirt, Facey Group, Fitzgerald Biosphere Group (FBG) and Ravensthorpe Agricultural Initiative Network (RAIN) in the medium-to-high-rainfall zone, are undertaking a survey project, entitled ‘Understanding trends in falling numbers in the medium-to-high-rainfall zones of WA’.

Led by Nathan Dovey (now ex CEO of SCF), the aim of the survey was to obtain a snapshot of growers’ experience with wheat falling numbers and how possible pre-harvest sprouting risk might affect wheat production decisions. This baseline information will potentially be used by GRDC to direct further investment associated with wheat falling numbers in medium and high-rainfall areas of WA.

Preliminary findings

Led by SCF, the survey collected data from more than 60 growers in southern WA. The extension of the outcomes and learnings of the survey will now be managed by new CEO, Elizabeth von Perger.

“Despite the wet growing season in 2021, harvest was relatively dry, with 64 per cent of the survey participants rating harvest rain ‘lower than average’. Although 52 per cent of growers had loads tested for falling numbers, only 18 per cent had downgraded loads,” Ms von Perger says.

Growers were asked to rate six factors affecting falling number results in wheat on a one to five scale, where one was a minimal influence, and five was highly influential. It was no surprise to see harvest rain (4.4) scoring the highest, followed by variety choice (4.0) and harvest timeliness (4.0).

Based on these ratings, it makes sense that the top three tactics for mitigating the risk of falling numbers were:

1. on-farm grain storage and mixing;
2. prioritising harvest of susceptible varieties; and
3. increasing harvest capacity, for example purchasing a new machine or using a contractor.

“Over our survey sample, only 18 per cent said they grow less wheat because of the risk of sprouting downgrading grain quality and 39 per cent prioritised harvesting wheat in relation to other crops,” Ms von Perger says.

However, 60 per cent of this survey sample stated that they have discarded a desirable wheat variety as it had proven susceptible to sprouting, which has serious implications for breeding companies.

“Growers were asked if they seeded later than the ideal window to counter falling number risk, but the response was low at eight per cent and these were from the SCF members in the high-rainfall zone.”

Preparedness

Exposure to rainfall is the primary driver of pre-harvest sprouting and reduction in falling number. However, the response of a crop to this rainfall is significantly influenced by its maturation stage and the pre-harvest sprouting susceptibility of the variety grown. With this knowledge, growers are able to take preventive measures early in the season to lower the risk of exposure to pre-harvest sprouting.

Jeremy Curry, from the Department of Primary Industries and Regional Development (DPIRD) in Esperance, has been investigating this aspect with support from DPIRD’s agronomy research program.

Since 2013, wheat agronomy researchers at DPIRD have assigned a Falling Number Index that rates varieties for their ability to maintain falling number. These indices are available from the DPIRD ‘Crop Sowing Guide’ for current varieties.

The Falling Number Index uses a combination of data from the field, laboratory and through artificially induced (rainfall simulator) sprouting to determine the risk of a variety exhibiting low falling number. On a one to nine scale, the higher the rating the more likely a variety is to maintain falling number.

“There is a large range in susceptibility to pre-harvest sprouting in current wheat varieties, so clearly variety choice is an important tool to manage sprouting, and the Falling Number Index rating provides a guide to a variety’s susceptibility to sprouting,” Mr Curry says.

However, we are also increasingly seeing the importance of maturation timing and how it coincides with certain environmental conditions having a major influence on sprouting risk in a given environment.

A trial was established at Gibson in 2021 to investigate this further, with 12 wheat varieties of varying maturity types and pre-harvest sprouting susceptibility sown at two sowing dates (21 April and 21 May). Grain sampling was conducted weekly from the time each variety approximately reached dough development and these samples were assessed for falling number. The falling numbers of four of the wheat varieties when sown in April are shown over this period in Figure 1.

Figure 1: Falling number of RockStar  (Red), Scepter (Blue), Devil (Yellow) and DS Pascal (Green) sown on 21 April 2021 at Gibson and sampled weekly during grain filling and maturation. Black bars denote daily rainfall (millimetres).

Source: DPIRD

“Given the large differences in their maturation timing and pre-harvest sprouting susceptibility, the responses of the varieties falling number across the sampling period varied significantly. However, their responses generally aligned with their pre-harvest sprouting susceptibility as determined by the Falling Number Index ratings produced by DPIRD,” Mr Curry says.

“However, what was clear from last year’s data was that the earlier sowing date led to many of the varieties maturing under cooler and wetter conditions and this resulted in sprouting occurring quite early in the harvest period, even before the crop was harvest mature.

“RockStarand Devil dropped below the 300 falling number threshold for milling grades after the rainfall events in late October. This correlates with their lower Falling Number Index ratings compared to a variety like Scepter .

“In contrast, at the May sowing time there were relatively fewer instances of varieties having a falling number below 300 at any point of the harvest period, even when left in the paddock until mid-December.

When managing pre-harvest sprouting, growers should understand that sowing earlier with faster maturity types will move their maturation timing earlier and this is likely to increase their risk of downgrades due to pre-harvest sprouting in wet harvest conditions.

“Further work is required to understand the mechanism behind this increased risk from early maturation, with the increased prevalence of cool and wet conditions during maturation and the influence of this weather on the dry-down and grain dormancy loss of wheat crops likely to play a role.”

Falling Number Index ratings should be considered alongside other factors when selecting suitable varieties for an environment and sowing date (Table 1).

Table 1: Falling Number Index Rating and other characteristics of the four wheat varieties from Figure 1.

Source: DPIRD

“Just as you would select longer-maturing varieties to maximise yield when sowing in April, ensuring your variety has a suitable maturity length for your sowing date will reduce your sprouting risk, while also considering its sprouting susceptibility as indicated by the Falling Number Index ratings.”

More information: Elizabeth von Perger, 08 9842 6653, ceo@scfarmers.org.au; Jeremy Curry, 08 9083 1160, jeremy.curry@dpird.wa.gov.au