How can I best tackle sterile oat in Northern region wheat crops?
with Gulshan Mahajan, Adjunct Associate Professor, Queensland Alliance for Agriculture and Food Innovation
Sterile oat (Avena ludoviciana) is the northern region’s most competitive grass weed in cereal crops, costing growers more than 20 thousand tonnes in yield loss and a revenue loss of $4.5 million.
Now with resistance to Group 1 [A], 2 [B], 9 [M] and 0 [Z] herbicide modes of action, this competitive Avenaspp weed is becoming even more problematic.
Dr Gulshan Mahajan, Adjunct Associate Professor at Queensland Alliance for Agriculture and Food Innovation, says an integrated approach to controlling this weed will save yield and drive down the weed seed bank.
“Extensive weed ecology research has provided much of the information we need to target sterile oats in winter cereals,” he says. “Widespread and increasing herbicide resistance means it is important to use effective herbicides and support them with strong crop competition.”
“We know that controlling the seed bank is very important for sterile oat,” says Gulshan. “Our weed ecology studies at the Gatton research farm of the University of Queensland have shown that it is possible to regain control of herbicide-resistant populations in just two years if we take an integrated approach to suppress seed set and prevent seed bank recruitment.”
Gulshan says that in paddocks with moderate to low numbers of sterile oat, the best strategy is to plant wheat early at a high seeding rate and apply an effective pre-emergent herbicide package, such as pyroxasulfone and triallate, to maximise early weed control and crop competition.
What effect does wheat planting time have on sterile oat control?
In brief: Early planted crops (7 May) produce higher wheat yield and can suppress weed growth and seed production when protected with effective pre-emergent herbicides, compared to later planted crops (7 June).
The details: In the two-year (2020 and 2021) field trial at Gatton, late-planted wheat (planted 7 June) reduced weed numbers by over 70 per cent in 2020 but made no difference in 2021. Also applying pre-emergent herbicides (pyroxasulfone and triallate) reduced weed density at both planting dates (7 May and 7 June) compared to the early-planted non-treated control plots.
In the early-planted non-treated control plots, weed infestation was 31 plants/m2 in 2020 and 29 plants/m2 in 2021. Other studies at Gatton demonstrated that 16 sterile oat plants/m2 will halve crop yield due to reduced wheat spikes. Since early-planted crops are likely to face high weed pressure, increase early crop vigour using high seeding rate and apply effective pre-emergent herbicide to protect crop yield and suppress weed germination.
2021 was a more favourable season than 2020 and resulted in high weed pressure at both planting times. Peak emergence of sterile oat is expected in July if soil moisture is close to field capacity. High rainfall at this time of year is also likely to leach residual herbicides and reduce herbicide efficacy.
How does wheat seeding rate affect sterile oat growth and wheat yield?
In brief: Averaged across all treatments, the higher wheat seeding rate markedly reduced weed seed production at both planting times compared to the recommended seeding rate while preserving yield.
The details: Wheat seeding rate of 200 seeds/m2 kept sterile oat seed production below 1000 seeds per m2 at both planting times. The reduction in seed production was 68% and 48% for the two planting dates (7 May and 7 June) compared to the recommended seeding rate of 100 wheat seeds/m2.
Where sterile oat is well-controlled, wheat grain yield is highest when the crop is sown early (7 May) at the recommended seeding rate of 100 seeds/m2. Pyroxasulfone- and triallate-treated plots, planted on 7 May using the seeding rate of 100 seed/m2, increased yield by 129% and 104%, respectively, compared with the non-treated control. This demonstrates the impact of sterile oat competition on wheat yield.
Increased crop seeding rate protects crop yield and reduces weed seed production, driving down the weed seed bank.
Pyroxasulfone- and triallate-treated plots, planted on 7 May using the seeding rate of 200 seeds/m2, still resulted in yield improvement of 38% and 32%, respectively, compared with the non-treated control, in addition to suppressing weed seed production.
In late sown crops (7 June) seeding rate made no difference to crop yield due to the reduced weed pressure at this time of year.
What should I do if there is a blow-out in sterile oats and the weed seed bank is very high?
In brief: Sow the wheat crop late at a higher seeding rate and implement other herbicide and non-herbicide control tactics.
The details: Early cohorts of sterile oat are known to be more competitive in wheat than late cohorts. Where there is a known high weed seed bank, a later sowing date may have some advantages. The seed production potential of late cohorts (July emergence) is 84 per cent less than that of early cohorts. Adding pre-emergent herbicides such as pyroxasulfone and triallate further reduces weed seed production.
Where early cohorts of sterile oat have germinated, apply effective knock-down herbicides or cultivate prior to planting.
Harvest weed seed control (HWSC) is an effective tactic against sterile oat, which retains 64 to 80 per cent of the seed in the seed head at wheat harvest. This is in contrast to wild oat (A. fatua), which is more common in the southern and western grains regions, and tends to shed its seed very early in the harvest period.
Management strategies that control all emerged sterile oat seedlings over two years and prevent seeds from returning to the seed bank can completely control this competitive weed, even in herbicide-resistant populations.
Related resources
WeedSmart Science Review – Don’t sow wild oats
