ASSESSING the extent of damage to the outermost individual flowers of wheat heads can play a key role in determining grain yield at the end of the season.
Proximal grains (the two outermost florets or G1 and G2) in wheat spikes are key determinants of grain yield when exposed to frost, but distal grains (the inner florets or G3 and G4) can play a minor role in compensation if the season permits.
Accurate assessment of frost damage is critical when making decisions about salvage options for frosted crops.
Previous advice to growers has been to check crops for symptoms of damage five to seven days after a frost event and to open glumes to determine if there has been damage to the anthers, ovules and/or developing grains.
The same inspection is carried out again five to 10 days later to determine if the damage has progressed.
Performance of wheat and barley cultivars under frost has been quantified by measuring damage in the two outermost florets (proximal grain positions) within the spike.
At the virtual Grains Research and Development Corporation (GRDC) Grains Research Updates last month, details were provided on an experiment which was designed to determine the contribution of the two outermost florets to total spike grain weight and establish if this measurement of frost damage can capture frost damage adequately.
Department of Primary Industries and Regional Development research officer Brenton Leske said field experiments were conducted at the Dale research site in 2018 and 2019.
"To expose material to natural frost, each of the trials was established with eight times of sowing blocks based on a predicted equidistant thermal time of about 250 growing degree days from early April to early June," Mr Leske said.
"This was done so that wheat would be at anthesis from July to October; the frost window for the region is from August to September."
In the trials, frost damage was evaluated by assessing sterility in proximal florets, developing grain maps to determine the distribution of grains across the spike, and measuring yield and yield component traits.
Overall, sterility values were much higher for plots sown from mid-April to late May in 2018 and 2019 when a frost event coincided with heading and anthesis of Wyalkatchem, Scout and Cutlass from August to October.
Between July to October in 2018 and 2019, there were 22 and 33 natural frost events respectively and the duration of frost events was shorter in 2018 than 2019.
Frost events in both seasons caused significant floret sterility damage (20 to 60 per cent).
Grain frost damage was also measured in the early April sowing of Scout and Wyalkatchem in 2018 and 2019, but there were no frost events or high temperature stress events at the end of either season.
"The absence of late environmental stress resulted in less floret sterility damage even when susceptible genotypes such as Wyalkatchem were sown later in mid-May and June," Mr Leske said.
"The maximum temperature did not exceed 28°C from July to September 2019 and only six days were above 25°C during the anthesis period.
"Since pollen sterility occurs at temperatures above 30°C, this provides evidence that the floret sterility present in spike samples was due to frost and not another abiotic stress such as heat or terminal drought."
There was a negative relationship between grain yield and floret sterility measured in the proximal florets in both seasons individually and combined.
There was also a negative relationship between grain yield and the floret sterility in distal florets.
The contribution of grain position to total spike grain weight, and ultimately grain yield, was highest for G1 and G2 followed by G3 and then G4.
Contributions of each grain position varied with sowing date and floret sterility level.
Changes in the contributions of each grain position within the spikelet were reflected in average grain weight and grains per spike.
Mr Leske said under low floret sterility (mid-May), the weight of grain in proximal positions G1 and G2 contributed more than 70pc to the total spike grain weight.
"Proximal positions contributed the most to total spike grain weight under low floret sterility," he said.
"The yield contributions of proximal grain positions to total spike weight did not vary between the three cultivars - ranging from 97 milligrams per spike for G2 in Wyalkatchem through to 126mg/spike for G1 in Cutlass.
"The contributions of distal grain positions to total spike grain weight ranged from 35mg/spike for G4 Wyalkatchem to 62.5mg/spike for G3 Cutlass."
Under high floret sterility, the relative contribution of grain positions to total spike grain weight increased slightly by 1-2.7pc at both G1 and G2 for all cultivars compared to those in the mid-May sowing, except for Wyalkatchem.
"Across all sterility scenarios, the proximal grains contributed the most to spike grain weight and therefore grain yield," Mr Leske said.
"Focusing on in-crop assessments of frost damage on these florets is therefore the most time effective and accurate way to determine frost damage at a paddock scale."
As part of the experiment, hay and grain salvage options were compared and the differences between the two options compared in terms of gross income, minus the salvage option cost.
As sowing progressed later into the season at Dale, grain generally became the more profitable option and the differences in gross income between cultivars reduced.
As a result growers need to consider if they have access to hay-making equipment, hay storage and a market for their product in addition to the costs and revenue.
Mr Leske said growers should continue to monitor crops for frost damage, particularly when their local weather station records screen temperature below 2°C.
"The extent of damage can be determined by assessing the number of sterile/damaged anthers and/or ovules within the outermost florets of the spike to calculate a percentage of sterile florets, as well as the number of spikes that have symptoms of frost damage from a transect of frost-prone parts of the landscape," he said.
"The recovery of crops damaged by frost events varies with the amount of soil water available to plants, the potential for spring rains and the amount of growing season left."
According to Mr Leske, compensation via new tillers was possible if frosts was early in August and average or above average rain occurred in spring or if crops have access to sufficient stored soil moisture.
However, late frosts in September and early October with below average rainfall often result in either no new tillers or tillers that do not go on to form grain.
"The likelihood of tiller compensation following frost should be carefully considered in any post-frost management decisions," he said.
"Increasing grain size within the spike is a possibility, but it is a less likely event, and the underlying physiological mechanism are poorly understood and require further investigation."
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