Interaction between wheat establishment timing and pre-emergent herbicide choice on growth and competition of annual ryegrass

Key messages

  • At Dandaragan early seeding outyielded delayed seeding.
  • Seed production of annual ryegrass at the end of the season correlated with soil persistence of pre-emergent herbicides. Triflur X and Boxer Gold degraded the fastest resulting in the most annual ryegrass seeds at the end of the season. Sakura, Mateno Complete and Overwatch were highly residual resulting in reduced ryegrass seed production.
  • Delayed seeding resulted in reduced annual ryegrass seed production.
  • Increasing crop seeding rates consistently reduced annual ryegrass seed production.


In the southern grainbelt of Australia, dry sowing has become popular as it enables growers to plant larger areas with limited machinery, within or before the optimum planting time to maximise yield potentials. At the same time, there has been an increased prevalence of grass weed populations with increased seed dormancy that emerge later to evade knockdown (glyphosate/paraquat) herbicide applications. To control these late emerging individuals there are several pre-emergent ‘residual' herbicides that can be safely used within no tillage farming systems to provide an extended period of herbicidal activity. These herbicides are often applied directly to the soil prior to planting.

To control these late germinating populations, it has long been advised that growers should delay seeding of weedy paddocks to maximise the weed control effectiveness of knockdown applications. However, any delay in seeding results in a sharp decline in crop yield potential. Previously, dry seeding techniques have relied upon low weed seed banks as they place significant reliance on longevity and efficacy of soil applied herbicides that are often applied a long time before crop and weed germinating rains.

It has, however, been identified that with some pre-emergent residual herbicides, early seeding may now be the optimum weed control strategy as crops sown early into higher soil temperatures grow at a faster rate and have a competitive advantage against later emerging weed cohorts (Gomez-Macpherson and Richards 1995). Crops that are seeded late generally grow more slowly and take longer to close their canopy, giving weeds a greater opportunity to establish and grow. Earlier sowing, when soil temperatures are generally warmer, provides an opportunity to increase the crop’s competitive advantage against weeds while maximising crop yield potentials. However, the early use of pre-emergent herbicides leads to increased rates of herbicide dissipation and microbial degradation. Past research by Minkey (2017) demonstrated that pre-emergent herbicides decayed more rapidly in warm soil conditions; with Sakura® (pyroxasulfone) decaying at the slowest rate and Boxer gold® (prosulfocarb + s-metolachlor) and trifluralin decaying faster. The potential degradation of pre-emergent herbicides may therefore offset the value of increased crop competitiveness from earlier seeding.


Experiments were conducted at Dandaragan in the Western Australian grainbelt.

The first time of sowing (TOS1) took place in the first week of May and the second time of sowing (TOS2) in the first week of June. Each trial was direct seeded into cereal stubble. A factorial combination of wheat seeding rate, pre-emergent herbicide chemistry and time of seeding (TOS1 plus TOS2 (four-week delay)) was randomised in complete blocks with four replicates (Table 1). The wheat variety used was Scepter (Intergrain Australia), which is a high yielding, mid-late maturing variety, seeded at 25cm row spacing, with the seeding rate treatments outlined in Table 2. The site was sown with no tillage tine openers with press wheels to provide sufficient seed–soil packing and promote good weed germination. All plots were planted at one sowing depth (approx. 2cm) to minimise the confounding effects of emergence rate and seeding depth differences on biomass and grain yield. The wheat seed was treated with a fungicide/insecticide seed treatment comprising of 300ml/ha of Uniform [322g/L Azoxystrobin + 124g/L Metalaxyl-M, Syngenta Australia] and 500mL/ha Aviator Xpro [75g/L bixafen + 150g/L prothioconazole, Syngenta Australia], applied to the fertiliser to protect against foliar fungal disease. Immediately prior to seeding, the whole experimental area was treated with 1.5L/ha Roundup Ultramax (Glyphosate 540g/L, Sinochem Australia), 100ml/ha Lontrel (Clopyralid 750g/L, DowAgrosciences Australia), to control all germinated weeds; followed by the application of each individual plot’s pre-emergent herbicide treatment (Table 2).

To control dicotyledonous species such as wild radish (Raphanus raphanistrum L.), all plots had a post-emergent application of 670ml/ha Velocity (210g/L Bromoxynil + 37.5g/L Pyrasulfotole, Bayer Australia). For the duration of this study, no additional annual ryegrass control was applied. All herbicides were applied using a motorised sprayer calibrated to deliver a carrier volume of  120 L water/ha at 275kPa  pressure. Each plot size was 2.2m wide by 10m long.  To ensure optimal wheat growth, 100kg/ha Gusto Gold (Summit Fertilisers Australia) (N – 10.2%, P – 13.1%, K – 12%, S – 7.6%, Cu – 0.07%, Zn – 0.14% and Mn – 0.01%) was drilled 3cm below the seed to minimise contact with the germinating wheat seed. To optimise crop growth, supplementary nitrogen fertiliser in the form of urea (Summit fertilisers Australia) (N – 32%) was applied to all plots.


At 10 weeks after emergence (WAE), wheat establishment was assessed by counting two adjacent 50cm rows over four replicate locations per plot. Annual ryegrass density was assessed at 10 WAE by counting the number of plants present in four replicates a 33 x 33cm quadrants (0.11m2) per plot.

Above-ground biomass samples of annual ryegrass were removed 27 WAE in three 0.25m2 quadrats per plot. Biomass samples were dried at 60oC and weighed. From these samples, the number of ryegrass panicles were counted. To estimate annual ryegrass seed production, a representative sample of 50 panicles were collected from each plot and thrashed to extract seed. The number of seeds extracted was counted using an S-25 optical seed counter (Data Technologies, Kibbutz Tzora, Israel) to calculate the mean number of seeds produced per panicle. Total seed produced per plot was estimated by multiplying the average seed yield per panicle by the number of panicles produced.

At 28 WAS, the whole plot (10m length with 6 by 25cm rows) was machine harvested to determine grain yield. Grain samples (400g) were analysed for moisture and protein using an Infratec™ Sofia Near Infrared Spectroscope (NIR) (FOSS analytics, VIC, Australia).

Herbicide bioassay

Starting at the time of pre-emergent herbicide application (week 0) soil samples at the Dandaragan site were collected from each plot at 14-day intervals. Soil samples were collected by sampling six 30mm diameter cores per plot to a depth of 6cm. Soil samples were immediately transferred into sealed plastic trays of dimensions 340mm by 285mm by 50mm, with no holes and stored at <15°C for no more than 24 hours. Upon receipt at the University of Western Australia, all soil samples were moistened within the sampling trays using 75ml deionised water containing TWEEN 20 ionic surfactant (Polyethylene glycol sorbitan monolaurate, Sigma Adrich Australia). Fifty seeds from the known herbicide susceptible annual ryegrass biotype (VLR1) were seeded at 1cm depth of the moistened soil in each tray before being placed in a temperature-controlled naturally lit glasshouse (15°C night 25°C day). To ensure adequate seed germination, the containers were sealed for 24 hours before lids were removed for the remainder of the growth period. 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., 2018b) using the following formula:

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

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 as per (Khalil et al 2018a; Khalil et al 2019b)


At the Dandaragan site, the first time of seeding (TOS1) was 4 May and the second time of seeding (TOS2) was on 3 June . The soil in the top 10cm was a yellow grey sandy loam with a pH 5.6 Cacl2 and organic total carbon content of 1.56% .

The first TOS was seeded into moist soil. The second TOS was seeded into similar soil moisture with no additional rainfall between TOS1 and TOS2, with both TOS providing excellent and rapid germination. Following seeding, subsequent rainfall in June was average with a very wet July relative to the 19-year average rainfall, although August had an average rainfall providing acceptable soil moisture for the rest of the season.

Pre-emergent herbicide persistence bioassay – Dandaragan

Immediately following seeding and at two weekly intervals thereafter, soil samples were taken from the top 10cm of soil from the inter-row region where herbicide would have concentrated following seeding. Using the herbicide susceptible annual ryegrass population VLR1, it was demonstrated that for both TOS1 and TOS2, Sakura, Overwatch and Mateno Complete were the most persistent herbicides, limiting ryegrass growth to <42% of the untreated control (%UTC) in both TOS1 and TOS2 at 14 WAS. Trifluarlin degraded at a fast rate in TOS1 and TOS2 with ryegrass growth of 86 %UTC for TOS1 and 88% for TOS2 at 14 WAS. Among the herbicides tested Boxer Gold provided the lowest efficacy of control from the first week after application, with ryegrass growth of 17 and 28% of the UTC respectively. In TOS1, Overwatch was the most persistent herbicide, however in TOS2, Mateno Complete maintained its persistence resulting in a shoot length of <12% of the UTC at 14 WAS (Figure 1).


Effect of pre-emergent herbicide efficacy, time of crop seeding and wheat seeding rate on ryegrass seed production

Annual ryegrass seed production

The application of herbicides reduced ryegrass seed production with trifluralin, Boxer Gold and Luximax providing the least ryegrass control. When these herbicides were used, delaying seeding (TOS2) and increasing wheat seeding rates further decreased ryegrass seed production. The use of more residual herbicides (Sakura, Overwatch and Mateno Complete) greatly reduced ryegrass seed production, however in these treatments, delayed TOS (P>0.05) and increased seeding rates (P>0.05) did not further reduce ryegrass seed production.


Wheat yield

At the Dandaragan site in 2021, a significant effect of TOS was found (p<0.001), with early TOS increasing yields (Figure 2). At the Dandaragan site, the choice of pre-emergent herbicide or seeding rate did not have a significant effect on yield owing to the high rainfall throughout 2021 and lower ryegrass densities leading to high yields.



In 2021, wheat yields were higher in the TOS1 treatments at the Dandaragan site, however the number of annual ryegrass seeds at the end of the season was consistently greater in the TOS1 where less residual herbicides (trifluralin, Boxer Gold and Luximax) were used. Bioassay assessments concurred with the ryegrass seed production data demonstrating that herbicides such as trifluralin and Boxer gold were not highly residual, whereas, Sakura, Mateno Complete and Overwatch were more residual resulting in reduced ryegrass seed production and often higher wheat yields. While early seeding with an excellent pre-emergent herbicide is considered important for maximising wheat yield, this study found that wheat densities should be practically increased to maximise wheat competitiveness and further reduce ryegrass seed production.


The research was undertaken with support of the GRDC.

Contact details

Michael Ashworth
Australian Herbicide Resistance Initiative
The University of Western Australia