The interaction between wheat (Triticum aestivum), time-of-seeding and choice of pre-emergent herbicide on annual ryegrass (Lolium rigidum) growth and competition.

Key messages

  • It is important to know the persistence of pre-emergent herbicides when making time-of-seeding decisions.
  • At all sites, Sakura was the most persistent herbicide and provided lower ryegrass emergence and seed production and higher wheat yield.
  • At the moist Kojonup site, trifluralin was also very effective.
  • High crop competition consistently reduced ryegrass emergence, ryegrass biomass and ryegrass seed production for all herbicide treatments studied.

Aims

To inform growers about the persistence and effectiveness of pre-emergence herbicides in dry-seeding conditions.

Introduction

It has long been advised that delaying seeding of weedy paddocks to control weeds using knockdown (glyphosate/paraquat) applications will result in optimal weed control. However, with the development of pre-emergent ‘residual’ herbicides for use in no-tillage farming systems, early seeding could now be the optimum weed control strategy. Crops sown early into higher soil temperatures, have higher growth rates and gain an earlier and competitive advantage against weeds (Gomez-Macpherson and Richards 1995). However, seeding crops earlier into warm soils can also make weed control more problematic as residual pre-emergent herbicides degrade faster, reducing their effectiveness. Within no-tillage farming systems, there are several pre-emergent ‘residual' herbicides that provide an extended period of herbicide activity to control emerging weeds. Past research by Minkey (2017) demonstrated in bare soil plots that the decay of pre-emergent herbicides was more rapid in warm conditions (similar to those experienced during early seeding). Of all herbicides tested, Minkey (2017) found that Sakura® (Pyroxasulfone) had the slowest rate of decay followed by Boxer Gold® (prosulfocarb + s-metolachlor). In the same study, TrilfurX® (trifluralin) had the fastest rate of decay. The research outlined in this paper aimed to investigate the effect of seeding rate and time-of-sowing of wheat on the effectiveness and degradation of pre-emergent herbicides commonly used to control annual ryegrass in no tillage farming systems.

Methods

Field trials were carried out in 2019 at Kojonup, York and Miling in the Western Australian wheatbelt. Each trial was direct seeded into cereal stubble. A factorial combination of wheat seeding rate, pre-emergent herbicide chemistry and time-of-seeding (four-week delay) was randomised in complete blocks with four replicates (Table1). The wheat variety used was the high-yielding, mid-late maturing variety Magenta (Intergrain Australia), which was seeded at 25cm row spacing. The site was sown with no-tillage tine openers with press wheels to provide sufficient seed soil packing. All plots were planted at only one sowing depth (approx. 2cm) to minimise the confounding effects of emergence rate and seeding depth differences on biomass and grain yield.

Table 1. Factorial combinations of wheat density, pre-emergent herbicide treatment and time-of-sowing at Miling, York and Kojonup sites in Western Australia in 2019.

image of Treatments/comments

Herbicide bioassays

At each site, starting at four weeks after seeding (WAS), soil samples were collected from each plot at 14-day intervals by sampling six 30mm diameter cores per plot to a depth of 6cm. Soil samples were immediately transferred into sealed plastic trays and moistened using 75ml deionised water containing TWEEN 20 ionic surfactant (polyethylene glycol sorbitan monolaurate). Fifty seeds from the known herbicide-susceptible annual ryegrass biotype (VLR1) were seeded 1cm deep into the moistened soil in each tray before being placed in a temperature-controlled naturally lit glasshouse (15°C night, 25°C day). All trays were watered daily to maintain field capacity (Figure 1). The above-ground shoot length was measured 21 days after sowing, with the percentage shoot length inhibition calculated as per Khalil et al (2018) using the formula:

Inhibition (%) = 1-(Lt/Lo) x 100%

where: Lt is the shoot or root length measured in the herbicide-treated soil or crop residue and L0 is the shoot or root length in the untreated soil or crop residue.

image of root length

Figure 1. Pre-emergent herbicide bioassay for herbicide bioactivity. Left to right: 1. Nil herbicide, 2. Trifluralin 480 gai/L, 3. Boxer Gold (S-metolachlor 120 gai/L + Prosulfocarb 800 gai/L) and 4. Sakura (Pyroxasulfone 850 gai/kg) (n=4).

Results

Kojonup

At the Kojonup site, the first time-of-seeding (TOS 1) was 18 April 2019 into moist soil and the second time-of-seeding (TOS 2) was on 15 May 2019 into similarly moist soil. Bioassay assessments using the herbicide-susceptible annual ryegrass population VLR1 demonstrated that for both TOS1 and TOS 2, Sakura was the most persistent herbicide, limiting ryegrass growth to 16% of the untreated control (%UTC) in TOS 1 and 14% of the %UTC in TOS 2 at 6WAS. Conversely, trifluralin was the most rapidly degraded herbicide in TOS 1 increasing the %UTC to 34% at 6WAS. For TOS 2, trifluralin and Boxer Gold degraded growth by similar amounts; 41% and 43% %UTC (Figure 2). Time-of-seeding had a statistically significant effect on ryegrass biomass and seed production with TOS 1 increasing ryegrass seed production compared to TOS 2 (P<0.001). Pre-emergent herbicide choice was also significant (P<0.001) between time-of-seeding treatments with trifluralin treatments reducing ryegrass seed production (Figure 2).

image of Kojonup

Figure 2. A: 2019 rainfall data at Kojonup. B:2019 annual ryegrass seed production at Kojonup. C and D: Root length inhibition of annual ryegrass (VLR1) grown for 14 days in soil sampled at 14-day intervals from plots treated with trifluralin 480 gai/L, Boxer Gold (S-metolachlor 120 gai/L + Prosulfocarb 800 gai/L) or Sakura (Pyroxasulfone 850 gai/kg). Bars are standard error of the mean (n = 4).

Miling

At the Miling site, the first time-of-seeding (TOS 1) was 16 April 2019 while TOS 2)was on 15 May 2019. The site was dry seeded with limited rainfall (<5mm) in March (two weeks before seeding). As a result, the TOS1 treatment did not germinate before the seeding of the TOS 2 treatment. Wheat germination and emergence at the Miling site did not occur until June when 87mm fell in the month. Bioassay assessments demonstrated that for both TOS1 and TOS 2, Sakura was the most persistent herbicide, limiting ryegrass growth to 21% of the untreated control (%UTC) in TOS 1 and 22% of the %UTC in TOS 2 at 6WAS. Conversely, trifluralin was the most rapidly degraded herbicide in TOS 1 increasing the %UTC to 69% and 54% %UTC in TOS2 at 6WAS (Figure 3). Time-of-seeding had a statistically significant effect on ryegrass biomass and seed production with the TOS 1 treatments found to have less ryegrass seed production than TOS 2 (P<0.001). Interestingly, despite wheat germinating at a similar time in both TOS treatments, the germination and seed production of ryegrass in TOS 1 was less than that of TOS 2. Pre-emergent herbicide choice was also significant (P<0.001) in reducing ryegrass seed production with all herbicides reducing ryegrass seed production compared to the nil control. Of all herbicide treatments, Sakura was consistently the most effective at reducing ryegrass seed production (Figure 3).

image of miling

Figure 3. A: 2019 rainfall data at York.  B:2019 annual ryegrass seed production at York.  C and D: Root length inhibition of annual ryegrass (VLR1) grown for 14 days in soil sampled at 14-day intervals from plots treated with trifluralin 480 gai/L, Boxer Gold (S-metolachlor 120 gai/L + Prosulfocarb 800 gai/L) or Sakura (Pyroxasulfone 850 gai/kg). Bars are standard error of the mean (n = 4).

York

At the York site, the first time-of-seeding (TOS 1) was 30 April while TOS 2 was on 27 May. The site was dry seeded with limited rainfall (<15mm) in April (two weeks before seeding). As a result, the TOS1 treatment did not emerge from the soil before the seeding of the TOS 2 treatment. However, there was concern regarding the vigour of this TOS 1 crop as the seed remained in the soil in a germinated state (with an emerged radicle) for two weeks before the second TOS. The second TOS treatment was similarly dry seeded following <10mm in May. Wheat germination and emergence did not occur until June when 132mm fell in the month (Figure 4). Bioassay assessments using the herbicide-susceptible annual ryegrass population VLR1 demonstrated that for both TOS1 and TOS 2, Sakura was the most persistent herbicide limiting ryegrass growth to 9% of the untreated control (%UTC) in TOS 1 and 18% of the %UTC in TOS 2 when assessed at 6WAS. Conversely, Boxer Gold was the least effective herbicide in TOS 1 increasing the %UTC to 37% at 6WAS and 62% in TOS 2 (Figure 4). Time-of-seeding did not significantly affect ryegrass biomass and seed production at the York site (P>0.05) as both TOS treatments germinated at a similar time. Wheat seeding rate, however, reduced the mean ryegrass seed production by 34% when wheat seeding rates were increased from 100 to 200 plants/m2 and no pre-emergent herbicide was applied in TOS1. However, in the TOS 2 treatment ryegrass seed production was only reducedby 1.7% when the wheat seeding rate was increased from 100 to 200 plants/m2 (Figure 4). All pre-emergent herbicides significantly (P<0.001) reduced ryegrass seed production compared to the nil control. Of all herbicide treatments, Sakura was consistently the most effective at reducing ryegrass seed production (Figure 4).

image of york

Conclusion

The bioassay studies at all sites showed that Sakura was the most persistent herbicide, as evidenced through lower ryegrass emergence, lower ryegrass seed production and higher wheat yields. At the dryer sites (Miling and York), Sakura was the most effective herbicide when seeded early. The Kojonup trial, which was sown into excellent soil moisture with a timely germination, had better ryegrass control with trifluralin than the other herbicide treatments. The time-of-seeding response was variable with the first time-of-seeding delivering better weed control and wheat yields at Miling and York, while TOS 2 was most effective at the moister Kojonup site. Increasing wheat density in the TOS1 treatment allowed for increased crop competition, which reduced ryegrass emergence, ryegrass biomass and ryegrass seed production in all herbicide treatments.

Acknowledgements

This research is funded by the Grains Research and Development Corporation.

Reviewed by: Prof Hugh Beckie, The University of Western Australia.

Contact details

Dr Mike Ashworth
Australian Herbicide Resistance Initiative
(08) 6488 7872
mike.ashworth@uwa.edu.au