Integrated Management of Wild Oat (Avena fatua) and Feather Fingergrass (Chloris virgata) Using Simulated Grazing and Herbicides

Abstract

Wild oat (Avena fatua L.) and feather fingergrass (Chloris virgata Sw.) are among the most problematic weed species in Australian winter and summer cropping systems, respectively. Pot trials were conducted in respective seasons to evaluate the integrated effect of simulated grazing and foliar-applied herbicides on the control of these weed species. Different herbicides were applied 1, 5, and 12 d after grazing on A. fatua plants and 1, 3, 7, 10, and 14 d after grazing on C. virgata. In general, the efficacy of herbicides was better on A. fatua when applied 5 or 12 d after grazing (i.e., 7–20 cm tall plants) and 7 to 14 d (i.e., 10–22 cm tall plants) after grazing on C. virgata. Clethodim 90 g a.i.·ha⁻¹, and haloxyfop 52 and 78 g a.i.·ha⁻¹ resulted in 100% mortality of A. fatua seedlings, irrespective of their application timings. Delayed application (5 or 12 d after grazing) of clethodim 60 g a.i.·ha⁻¹ and pinoxaden 20 and 30 g a.i.·ha⁻¹ resulted in 100% mortality of A. fatua. Glyphosate at the field rate (370 g a.e. ha⁻¹) was the least effective herbicide to control A. fatua plants after grazing. Glufosinate application after grazing resulted in the greatest mortality (69 to 81%) of C. virgata. Depending on application timing, only glufosinate was able to provide complete control of C. virgata seed production. Butroxydim, clethodim, and haloxyfop were found to be promising herbicides to manage C. virgata after grazing but their applications had to be delayed by 7 to 14 d after grazing. This study identified several successful herbicide treatments that could be applied after grazing or mowing for integrated control of A. fatua and C. virgata. However, to achieve complete control of C. virgata, the application of these herbicides needs to be followed by other tools, including additional herbicide applications.

1. Introduction

Weeds are the most important biological constraint to agricultural production worldwide. They cost more than AUD 4 billion per year to Australian growers and of this, AUD 3.3 billion per year is incurred to the Australian grain industry [1]. A fallow phase (summer or winter) is common in Australian cropping systems because of limited soil moisture. Approximately AUD 500 million per year is spent on fallow weed control [1], suggesting the importance of controlling weeds during the fallow phase.

Wild oat (Avena fatua L.) is a problematic weed in more than 20 crops across 55 countries [2]. In Australia, it is the second most important grass weed, after rigid ryegrass (Lolium rigidum Gaud.), causing a revenue loss of AUD 28 million per year to grain growers [1]. In the eastern grain region of Australia, A. fatua ranks first in terms of infested area (630,000 ha). In a previous study, A. fatua caused approximately 80% yield loss in a wheat (Triticum aestivum L.) crop [3]. A recent study reported that approximately 15 plants m⁻² of A. fatua reduced wheat grain yield by 50% [4]. In the same study, the weed produced approximately 4800 seeds m⁻² in the wheat crop. In fallow conditions, sterile oat (A. sterilis ssp. ludoviciana) produced approximately 2500 seeds plant⁻¹ [5]. Avena fatua has been reported to produce a greater number of seeds than A. sterilis [6], indicating that A. fatua can produce >2500 seeds plant⁻¹ in fallow conditions. Despite its prolific nature, A. fatua has a short-lived seed bank, especially under no-till situations [7]. Although A. fatua is predominantly a winter species, a recent study in eastern Australia reported the continued emergence of this weed into the spring season [7], suggesting the importance of an extended period of management, especially in no-competition situations (i.e., fallow). Herbicides are widely used to manage weeds during the fallow phase, but several populations of A. fatua have evolved resistance to commonly used herbicides, that is, acetyl-coenzyme-A carboxylase (ACCase) and acetolactate synthase (ALS) inhibitors [8]. Recently, the world’s first glyphosate-resistant A. fatua population was reported from eastern Australia [9]. The overreliance on glyphosate for weed control in fallow situations is placing high selection pressure on the herbicide.

Feather fingergrass (Chloris virgata Sw.) is the most problematic summer grass species in the eastern grain region of Australia, causing infestations in 120,000 ha [1]. In this region, it causes a grain yield loss of 39,000 tonnes per year. A study in this region reported that approximately 35 plants m⁻² of C. virgata reduced mungbean [Vigna radiata (L.) Wilczek] grain yield by 50% [10]. In addition to grain crops, C. virgata is also a problematic weed in cotton (Gossypium hirsutum L.) fields, fallow fields, and along roadsides. This weed was reported in the top 10 most common weed species in the cotton-growing fields of eastern Australia [11]. Although C. virgata has a short-lived seed bank in conservation agriculture systems [12], a single plant can produce more than 140,000 seeds in fallow conditions [13]. Stimulation of seed germination by light and greatest germination of surface seeds indicate C. virgata dominance in no-till systems [14,15], which are the main farming practices in Australia. In no-till farming systems, weeds are managed using herbicides. Overreliance on the nonselective herbicide glyphosate to manage weeds in fallow and glyphosate-resistant cotton crops has resulted in the evolution of glyphosate-resistant C. virgata populations [8,16,17].

In the eastern grain region, wheat is the most common winter crop. In summer, grain sorghum and mungbean are commonly grown in this region. Grazing is practiced in some areas, and cattle and sheep grazing could provide effective weed control [18,19,20,21]. A. fatua has nutritional traits and is considered a palatable forage species for grazing animals [22]. Similarly, C. virgata plants were found to be high in nutritional values, also making them palatable [23]. Grazing is also considered in the restoration of degraded ecosystems [20,24]. Grazing, however, does not provide complete weed control [19,21]. Therefore, grazing needs to be integrated with extra tools, such as the use of herbicides, to increase the level of weed control [20]. Large plants are difficult to control using herbicides, which can be a problem if growers missed the opportunity of spraying young plants in fallow conditions due to environmental constraints [25]. Grazing may make large plants susceptible to herbicides by facilitating the removal of plant biomass. However, such information is not available for A. fatua and C. virgata control. Therefore, studies were conducted to evaluate the performance of commonly used herbicides on A. fatua and C. virgata growth and seed production when applied after grazing.

2. Materials and Methods

2.1. Seed Collection

Seeds of A. fatua were collected from a chickpea (Cicer arietinum L.) field in November 2017 from Moree (29.4455° S; 149.8577° E), New South Wales, Australia. Seeds of C. virgata were collected from a sorghum [Sorghum bicolor (L.) Moench] field in April 2017 from Cecil Plains (27.2935° S; 151.1283° E), Queensland, Australia. Seeds of each species were collected from at least 50 plants and stored at room temperature (25 ± 2 °C) until used.

2.2. Trial Setup

Avena fatua. A pot trial was conducted during the winter season of 2021 in an open environment at the weed science research facility of the University of Queensland, Australia, and the trial was repeated once over time (June to December 2021). Seeds (10) of A. fatua were planted at 1 cm depth in 20 cm-diameter plastic pots filled with a commercial potting mix (Centenary Landscape, Queensland, Australia). Pots were placed on benches and irrigation was applied using an automated-sprinkler system. Immediately after emergence, seedlings were thinned to 6 plants per pot. No fertilizer was added to the plants. Just before panicle emergence, plants were slashed using a scatier 1–2 cm above the soil surface to simulate grazing. Plants were sprayed with different herbicides (

Table 1. Herbicide (systemic) treatments, herbicide doses, and adjuvants used with herbicides in the grazing experiments conducted with Avena fatua and Chloris virgata.

Avena fatua			Chloris virgata
Herbicide	Dose	Adjuvant	Herbicide	Dose	Adjuvant
	(g a.i. or a.e. ha−1)			(g a.i. or a.e. ha−1)
Control	-	-	Control	-
Clethodim	60	1% Supercharge®	Butroxydim	30	1% Supercharge®
Clethodim	90	1% Supercharge®	Butroxydim	45	1% Supercharge®
Glyphosate	370	-	Clethodim	60	1% Supercharge®
Glyphosate	740	-	Clethodim	90	1% Supercharge®
Haloxyfop	52	1% Hasten®	Glufosinate	750	-
Haloxyfop	78	1% Hasten®	Glufosinate	1500	-
Pinoxaden	20	0.5% Adigor®	Haloxyfop	40	1% Hasten®
Pinoxaden	30	0.5% Adigor®	Haloxyfop	80	1% Hasten®

) at 1, 5, and 12 d after simulated grazing (grazing, hereafter) using a research track sprayer. Plant height at these grazing timings was 2.6 ± 0.1, 6.9 ± 0.3, and 19.7 ± 1.1 cm, respectively. Teejet XR110015 flat fan nozzles were used in the sprayer that delivered a spray volume of 108 L ha⁻¹. Plants were allowed to grow for 10 weeks after the last herbicide application (i.e., 12 d after grazing). After that, plant survival and seed production data were measured. Plants were considered to survive if they had at least one new leaf. After measuring survival and seed number, plants were harvested at the base and placed in an oven at 70 °C for 72 h. After this, the biomass of dry samples was measured.

Chloris virgata. Another pot trial was conducted during the summer of 2021–22 using C. virgata as the target weed species. Twelve seeds were planted at 0.2 cm depth in 20 cm diameter pots filled with potting mix. Pots were maintained as described above for A. fatua. The trial was conducted two times (sowing time: 23 August 2021, and 8 December 2021). Plants (6 per pot) were allowed to grow and grazing was simulated (1–2 cm above the soil surface) before panicle emergence. Plants were sprayed with different herbicides (Table 1) at 1, 3, 7, 10, and 14 d after grazing, as described above. Plant height at these grazing timings was 1.8 ± 0.2, 3.6 ± 0.2, 10.4 ± 1.1, 15.8 ± 1.9, and 22.0 ± 2.0 cm, respectively. As summer weed species take a shorter time to produce seeds compared with winter weed species, C. virgata plants were allowed to grow for 5 weeks after the last herbicide application (i.e., 14 d after grazing). At that time, plant survival, panicle production, seed production, and dry biomass data were measured as described above. In addition, leaf greenness was measured using a chlorophyll meter (SPAD-502; soil-plant analyses development; Konica Minolta Sensing, Inc., Japan). The top three leaves from each plant were selected to measure the chlorophyll content (SPAD readings).

2.3. Statistical Analyses

Each experiment was arranged in a factorial complete block design with three replicates. The first factor was herbicide treatments and the second factor was herbicide application timings (after grazing). All treatments within a replication were randomized on benches and pots were randomly rotated every week to avoid the edge effect. Experiments with each weed species were repeated over time. The analysis of variance (ANOVA) showed no difference between the experimental runs and the interaction between the runs and treatments was also nonsignificant; therefore, the data were pooled over the two runs [26]. Data were subjected to transformation, but it did not improve the homogeneity of variance; therefore, original data were subjected to ANOVA. The main effects (herbicide treatments or herbicide application timings) are reported if no interaction was found between the herbicide and application timings; otherwise, interactions are reported. Means were compared using Fisher’s protected least significant difference (LSD) at 5%.

3. Results and Discussion

3.1. Avena fatua

Analysis of variance revealed a significant interaction between herbicide treatment and their application timing for seedling survival (Figure 1), seedling biomass (Figure 2), and seed production (Figure 3) of A. fatua. Grazing alone (control) resulted in 100% survival of seedlings and produced up to 50 g pot⁻¹ of biomass (Figure 2) and 600 to 680 seeds pot⁻¹ (Figure 3). Clethodim at the high rate (90 g a.i.·ha⁻¹) and haloxyfop at both rates (52 and 78 g a.i.·ha⁻¹) resulted in 100% mortality of seedlings (and therefore no biomass and seed production), irrespective of their application timings. Clethodim at the low rate (60 g a.i.·ha⁻¹) and pinoxaden at both rates (20 and 30 g a.i.·ha⁻¹) resulted in 28 to 64% of seedling survival, 20 to 40 g pot⁻¹ of biomass, and 210 to 590 seeds pot⁻¹ when applied 1 d after grazing. However, delayed application of these herbicides to 5 or 12 d after grazing resulted in 100% mortality. Glyphosate application, irrespective of rates, resulted in >70% of seedling survival, 33 to 46 g pot⁻¹ of biomass, and 430 to 600 seeds pot⁻¹ when applied 1 or 5 d after grazing. However, delayed application (12 d after grazing) of glyphosate at the high rate (740 g a.e.·ha⁻¹) achieved complete control of A. fatua.

In general, the efficacy of herbicides was better when applied at 5 (i.e., 7 cm tall plants) or 12 d after grazing (i.e., 20 cm tall plants) compared with the application at 1 d after grazing (i.e., 3 cm tall grazed-plants). Immediately after grazing (i.e., 1 d), the plants may not have enough leaf material to absorb a sufficient amount of herbicides. All the three ACCase-inhibiting herbicide groups (dim, fop, and den) were highly effective on A. fatua when applied 5 or 12 d after grazing, suggesting that the population used in this study was highly susceptible to these herbicides. These results are consistent with the results reported in a recent study [9], in which clethodim 120 g a.i.·ha⁻¹, haloxyfop 78 g a.i.·ha⁻¹, and pinoxaden 20 g a.i.·ha⁻¹ provided complete control of A. fatua when applied at the 3–4 or 6–7 leaf stage. In a similar study, clethodim (280 g a.i.·ha⁻¹) application 7 or 14 d after mowing resulted in 1 to 4 seed heads m⁻² of Vaseygrass (Paspalum urvillei Steud.) [27]. The nontreated control treatment produced 11 to 18 seedheads m⁻².

ACCase inhibitors are commonly used to control A. fatua in a range of crops throughout the world. However, these herbicides are prone to developing resistance and their continued applications have resulted in the evolution of ACCase-inhibiting herbicide-resistant A. fatua populations [8,28,29,30]. A survey in the western Australian grain cropping system reported that 50% of populations of Avena species were resistant to ACCase-inhibiting herbicides [30]. These observations suggest the need to rotate herbicides between ACCase-inhibiting subgroups and other modes of action.

Glyphosate was the least effective in controlling A. fatua plants after grazing. The recommended glyphosate dose to control A. fatua in Australia is 182 to 540 g a.i.·ha⁻¹. In the current study, glyphosate 370 g a.i.·ha⁻¹ did not provide effective control of grazed plants of A. fatua. The herbicide dose was increased above the maximum recommended dose to achieve complete control of A. fatua plants grazed 12 d before herbicide treatment. These results suggest that glyphosate may not be an effective option to control A. fatua plants after grazing in fallow conditions. The glyphosate resistance level in the population used in this study is not known but glyphosate-resistant populations of A. fatua are present in Australia [9]. In a previous study, mowing to a height of 2 to 5 cm followed by an application of glyphosate (3330 g a.e.·ha⁻¹) provided effective control of perennial pepperweed (Lepidium latifolium L.) [31]. In that study, mowing or glyphosate alone were not effective. In a recent study, sequential application of slashing (to a height of 10 cm) and glyphosate 1830 g a.e.·ha⁻¹ (14 d after slashing) provided >70% reduction of node production and viable stolon of drought grass (Ischaemum muticum L.) [32]. In the USA, glyphosate use was recommended as a tool to increase livestock consumption of medusahead [Taeniatherum caput-medusae (L.) Nevski] [20]. However, in that study, glyphosate application was followed by grazing of the weed species.

3.2. Chloris virgata

Seedling survival (

Table 2. The effect of herbicide treatment and their application timing (1, 3, 7, 10, and 14 d after grazing) on the survival (%) of Chloris virgata.

Herbicide and Rate	Survival	Time of Herbicide Application after Grazing	Survival
	(%)	(d)	(%)
Control	100.0	1	77.1
Butroxydim 30	79.8	3	68.8
Butroxydim 45	76.9	7	49.5
Clethodim 60	87.4	10	55.0
Clethodim 90	61.8	14	55.9
Glufosinate 750	31.3
Glufosinate 1500	19.2
Haloxyfop 40	53.2
Haloxyfop 80	41.7
LSD	14.9	LSD	11.1

) and biomass (

Table 3. The effect of herbicide treatment and their application timing (1, 3, 7, 10, and 14 d after grazing) on the aboveground biomass (g pot⁻¹) of Chloris virgata.

Herbicide and Rate	Biomass	Time of Herbicide Application after Grazing	Biomass
	(g pot−1)	(d)	(g pot−1)
Control	10.04	1	5.64
Butroxydim 30	6.87	3	5.20
Butroxydim 45	6.64	7	3.00
Clethodim 60	6.03	10	4.36
Clethodim 90	4.23	14	4.91
Glufosinate 750	1.42
Glufosinate 1500	0.78
Haloxyfop 40	3.56
Haloxyfop 80	2.04
LSD	1.50	LSD	1.12

) of C. virgata were affected by the main effects, i.e., herbicide treatment and time of herbicide application (after grazing). Compared with the grazing-only treatment (control), most herbicide treatments reduced seedling survival of C. virgata (Table 2). Glufosinate application after grazing resulted in the greatest mortality (69 to 81%) of C. virgata. Similarly, glufosinate at 750 and 1500 g a.i.·ha⁻¹ resulted in 86% and 92% reductions in biomass, respectively, compared with the grazing-only treatment (Table 3). The next best herbicide treatments were haloxyfop 40 and 80 g a.i.·ha⁻¹, which reduced biomass by 65 and 80%, respectively, compared with the control treatment.

Across herbicide treatments, a greater number of seedlings survived when herbicides were applied 1 or 3 d after grazing (i.e., 2–4 cm tall plants) (Table 2). The lowest number of C. virgata seedlings survived when herbicides were applied 7 d after grazing (i.e., 10 cm tall plants); which was statistically similar to herbicide treatments applied at 10 or 14 d after grazing (i.e., 16–22 cm tall plants). The biomass data revealed that the best time for herbicide applications was 7 d after grazing (Table 3). The maximum biomass (5.6 g pot⁻¹) was produced when herbicides were applied 1 d after grazing, which was similar to the biomass produced by herbicide treatments at 3 and 14 d after grazing.

Leaf chlorophyll content, as measured by SPAD units, was significantly affected by herbicide treatments (

Table 4. The effect of herbicide treatment on chlorophyll content (Soil Plant Analysis Development; SPAD) of Chloris virgata.

Herbicide and Rate	SPAD
Control	22.9
Butroxydim 30	24.2
Butroxydim 45	21.8
Clethodim 60	24.1
Clethodim 90	23.5
Glufosinate 750	13.8
Glufosinate 1500	9.3
Haloxyfop 40	18.5
Haloxyfop 80	14.4
LSD	4.48

) but it was not affected by application timing. The chlorophyll content was similar among the control, butroxydim, and clethodim treatments. Glufosinate reduced the chlorophyll content by 40 to 60% compared with the nontreated control treatment, suggesting that glufosinate was the best herbicide in causing injury in C. virgata. The next best herbicide was haloxyfop, which reduced the chlorophyll content by 19% to 37% compared with the nontreated control treatment.

Analysis of variance revealed an interaction between herbicide treatment and application timing for panicle (

Table 5. The interaction effect of herbicide treatment and their application timing (1, 3, 7, 10, and 14 d after grazing) on the panicle production (number pot⁻¹) of Chloris virgata.

Herbicide and Rate	Panicle Production
	Herbicide Application Timing (d) after Grazing
	1	3	7	10	14
	(Number Pot−1)
Control	20.3	25.0	22.0	28.7	26.2
Butroxydim 30	27.5	15.3	4.0	9.5	11.7
Butroxydim 45	20.8	18.7	3.0	8.5	15.0
Clethodim 60	16.5	13.5	7.0	3.3	4.5
Clethodim 90	9.3	8.2	4.3	6.2	4.2
Glufosinate 750	5.5	4.0	1.3	0.0	1.0
Glufosinate 1500	6.8	0.8	0.2	0.0	0.0
Haloxyfop 40	17.5	10.0	3.2	2.8	2.0
Haloxyfop 80	6.0	5.3	1.8	3.7	2.0
LSD	7.77

) and seed production (

Table 6. The interaction effect of herbicide treatment and their application timing (1, 3, 7, 10, and 14 d after grazing) on the seed production (number pot⁻¹) of Chloris virgata.

Herbicide and Rate	Seed Production
	Herbicide Application Timing (d) after Grazing
	1	3	7	10	14
	(Number Pot−1)
Control	6527	7058	6479	8672	7909
Butroxydim 30	8261	4631	1013	2148	2911
Butroxydim 45	5959	5842	655	2195	4374
Clethodim 60	4817	3469	1499	796	964
Clethodim 90	2478	1860	951	1601	1292
Glufosinate 750	1356	877	266	0	164
Glufosinate 1500	1694	120	17	0	0
Haloxyfop 40	5580	2935	721	596	439
Haloxyfop 80	1736	1302	413	897	421
LSD	2315

) of C. virgata. The grazing-only treatment had similar numbers of panicles and seeds at different timings. Chloris virgata produced the maximum number of panicles (Table 5) and seeds (Table 6) when herbicides were applied 1 day after grazing; however, the numbers were statistically similar across herbicide application timing for clethodim 90 g a.i.·ha⁻¹, glufosinate at both rates (750 and 1500 g a.i.·ha⁻¹), and haloxyfop 80 g a.i.·ha⁻¹. Glufosinate was the only herbicide that resulted in no seed production (i.e., 750 g a.i.·ha⁻¹ when applied 10 d or 14 d after grazing). Butroxydim, irrespective of rates, provided the best control of seed production when applied at 7 d after grazing; resulting in 84% to 90% reductions compared with the grazing-only treatment. Clethodim- and haloxyfop-treated C. virgata plants, irrespective of their rates, produced a similar number of seeds when herbicides were applied 7, 10, or 14 d after grazing (Table 6), suggesting that these herbicides can be applied at any time between 7 and 14 d after grazing.

The results of this study are similar to results reported in recent studies [25,33]. The previous studies suggested that the ‘fop’ herbicides (e.g., haloxyfop) performed better on C. virgata than ‘dim’ herbicides (e.g., butroxydim and clethodim). Clethodim is recommended to control 5-leaf to fully tillered C. virgata and haloxyfop to control 2-leaf to early tillering C. virgata plants. In the current study, these herbicides were applied after grazing (or mowing) and >40% of seedling survival could be due to the large size of the plant. In a recent study, 97% and 100% plant mortality was achieved when clethodim 180 g a.i.·ha⁻¹ and haloxyfop 40 g a.i.·ha⁻¹, respectively, were applied at the 24–28 leaf stage of C. virgata [25]. Another Australian study reported 96 to 98% control of C. virgata when haloxyfop 156 g a.i.·ha⁻¹ was applied to flowering plants [34]. In the current study, glufosinate provided the best control of C. virgata after grazing, resulting in 19 to 31% seedling survival, 86 to 92% reductions in biomass, and reduction in seed production by up to 100%. A recent study also reported that glufosinate 1500 g a.i.·ha⁻¹ provided promising control of large plants (24–28 stage) of C. virgata, resulting in 27% of seedling survival and the survived plants did not produce any panicles [25]. The efficacy of glufosinate could be further increased by adding an adjuvant, such as Hasten^® [25]. As C. virgata is known to have a high level of natural tolerance to glyphosate [35] and this species is increasingly evolving resistance to glyphosate [8,17], alternate herbicide options need to be used to manage C. virgata. Glufosinate and some ACCase-inhibiting herbicides provided promising results to control C. virgata after grazing. Across herbicide treatments, the least number of seedlings survived and the least biomass was produced when herbicides were applied 7 d after grazing. These results suggest that the best control of C. virgata would be achieved when effective herbicides are applied approximately 7 d after grazing (approximately 10 cm in height).

4. Summary and Conclusions

The present study evaluated the integrated effect of grazing and herbicide application on the control of A. fatua and C. virgata. Some herbicide treatments offered complete control of A. fatua, but their applications had to be made 5 or 12 d after grazing. Depending on application timing (after grazing), only glufosinate was able to provide complete control of C. virgata seed production. Butroxydim, clethodim, and haloxyfop were found to be promising herbicides to manage C. virgata after grazing. However, applications of these herbicides need to be followed by other herbicide applications, e.g., the double-knock technique, in which the second herbicide component is usually paraquat [36]. In general, the efficacy of herbicides was lowest when applied 1 d after grazing. These results suggest that plants may not have enough leaf material to absorb herbicides. Herbicide efficacy was improved with the advancement of the stage (after grazing) and the best results were found when herbicides were applied 5 or 12 d after grazing for A. fatua and 7 to 14 d after grazing for C. virgata. Grazing, slashing, or mowing before applying a herbicide may improve herbicide contact on lower leaves or vegetative parts of weeds [32,37,38]. It can also lead to increased translocation of some herbicides in belowground parts [31].

Chloris virgata, in addition to infesting crop fields, also infests pastures and roadsides and spreads through seeds. Results from this study suggest that C. virgata growth and seed production can be greatly suppressed if mowing along roadsides is followed by effective herbicide applications. However, its eradication from Queensland and New South Wales roadsides may require management interventions over multiple growing seasons [27].

In winter and summer fallows, large plants of A. fatua and C. virgata, respectively, could be grazed first before applying a herbicide to achieve a high level of control. Grazing will also help to reduce seed production. In this study, grazing was performed only once but multiple grazing events may make weeds more susceptible to herbicides [27]. Grazing or mowing may influence physiological, morphological, biological, or anatomical traits of the plant and, therefore, affect herbicide deposition patterns, absorption, or translocation in shoots [31]. Therefore, future field trials should evaluate combinations of grazing (or mowing), conducted multiple times, with different rates of single herbicides and double-knock herbicide treatments.