Herbicide Options for the Management of Navua Sedge (Cyperus aromaticus) Plants Established through Seeds

Abstract

Navua sedge is a difficult-to-control perennial sedge in north Australian farming systems, including fallow fields, pastures, and along roadsides and fencelines. A set of pot trials were conducted to evaluate the performance of different herbicides when used alone or in mixtures on different sizes of Navua sedge plants, and the performance of different adjuvant treatments on the rainfastness of halosulfuron-methyl. Glyphosate at 1080 g·ha⁻¹, halosulforon-methyl at 49 g·ha⁻¹, and paraquat at 600 g·ha⁻¹ provided complete control of Navua sedge at the 6-leaf stage. Azimsulfuron at 20 g·ha⁻¹ resulted in 18% and 39% survival of the 14 to 18 leaf stage and 60–70 leaf stage plants, respectively. Compared with the nontreated, this herbicide treatment suppressed plant biomass by 99.6% and 93%, respectively, and the surviving plants did not produce seed heads. A sole application of azimsulfuron, glyphosate, or halosulfuron-methyl was not effective on very large plants (140–150 leaf stage; 40–42 cm tall) of Navua sedge. However, a mixture of any two herbicides resulted in the complete control of Navua sedge. Compared with the currently used adjuvants at 0.5 to 1% concentrations, the use of non-ionic adjuvants at high concentrations (i.e., 2% ActivatorTM, HastenTM, and UptakeTM) improved the rainfastness of halosulfuron-methy when applied at 49 g·ha⁻¹ on Navua sedge. This study identified effective herbicide treatments (e.g., azimsulfuron-based combinations) for the management of large plants of Navua sedge.

1. Introduction

Perennial sedges are one of the major concerns in conservation farming systems. Among perennial sedges, Navua sedge [Cyperus aromaticus (Ridl.) Mattl. & Kuek.] is a difficult-to-control weed in north Australian farming systems. It is a native of Africa and has been introduced in different countries, including Australia, Fiji, Solomon Islands, China, India, Pakistan, Sri Lanka, and Vietnam [1,2,3,4]. In Australia, Navua sedge is a problematic weed in pastures, rice (Oryza sativa L.), sugarcane (Saccharum officinarum L.), and along roadsides in Queensland [4,5]; causing losses to the beef, dairy, and cropping industries. This weed has a low nutritional value and therefore, is not readily palatable to cattle. Navua sedge is highly competitive in pastures; for example, it reduced the carrying capacity of pastures by 40% in Fiji [2].

Navua sedge produces both seeds and rhizomes, while rhizomes are spread mainly by tillage equipment, seeds are dispersed by mowing, wind, birds, animals, storms, and flood water [3]. More than 450 million seeds per hectare were reported to be produced by Navua sedge in Fiji [2], and seeds can persist for more than 5 yr in the soil [5]. A recent study reported that over 80% of Navua sedge seeds were present in the top 5 cm soil layer [6]. Navua sedge seeds can germinate at alternate temperatures of 20/10 to 35/25 C under 12 h light/12 h dark conditions, suggesting that germination can occur at any time of the year in north Australia [7]. Seeds of Navua sedge have an absolute requirement of light for germination and seedlings cannot emerge from a seed burial depth of 2 cm. These results support the observation of its high infestations in no or low soil disturbance systems, such as pastures, roadsides, and fencelines.

Mechanical control options, such as mowing or slashing, are less effective in controlling Navua sedge and contribute to the spread of seeds [3,7]. The use of herbicides may provide effective control of this problematic weed species. Currently, only halosulfuron-methyl and glyphosate are recommended to be used for the control of this weed in Australia; however, these herbicides are effective only on small plants [4,5]. Two applications of halosulfuron-methyl are recommended with a grazing restriction of 10 weeks after each application. These requirements make the application of halosulfuron-methyl challenging. Therefore, there is a need to evaluate alternate herbicide options for the control of Navua sedge in different situations, for example, roadsides, fencelines, and pastures. Although glyphosate may be more effective than other nonselective herbicides, such as glufosinate and paraquat, for the control of Navua sedge established through rhizomes, the latter two herbicides may also be equally effective for the control of plants established through seeds. Similarly, herbicide mixtures may provide better control than a single herbicide [8,9]. Herbicide mixtures can also delay the onset of resistance and mitigate the existing resistance levels [8]. However, information on different herbicide options, used alone or in mixtures, is very limited on Navua sedge.

As mentioned previously, halosulfuron-methyl is the only herbicide recommended to be used in pastures. Although farmers are aware of the rainfastness of halosulfuron-methyl, rain may occur within two hours of its application in a tropical environment and reduce its efficiency. The use of an effective adjuvant at high concentrations may improve the rainfastness. In a previous study, for example, the effect of a non-ionic surfactant on tribenuron-methyl’s rainfastness was significantly higher than that of a vegetable oil-based adjuvant with rain 1 h after herbicide application [10]. Similarly, ammonium sulfate (AMS) can also increase herbicide activity through increased absorption and translocation [11]. The addition of AMS can also increase the control level of water-stressed plants by increasing translocation [12]. Improving halosulfuron-methyl rainfastness will enable more effective Navua sedge management programs.

A set of pot trials were conducted in an open environment to (1) evaluate the performance of different herbicides when used alone or in mixtures on different sizes of Navua sedge plants established through seeds, and (2) evaluate the performance of different adjuvant treatments on the rainfastness of halosulfuron-methyl on Navua sedge plants established through seeds.

2. Materials and Methods

2.1. Seed Collection

Plants of Navua sedge were uprooted with their roots intact from a pasture field in Ingham, Queensland. Then, these plants were placed in a plastic container and shipped to the weed science research facility of the University of Queensland, Gatton, Australia. These plants were transplanted in plastic pots (24 cm in diameter) and pots were placed on benches in the open at the Gatton research farm. A commercial potting mix (Centenary Landscape, Brisbane) was used in the pots. Plants were watered daily but no fertilizer was used. Seeds were collected from these plants in November 2020 and used in this study. A set of pot trials were conducted from December 2020 to May 2022 at the Gatton research farm. There were at least three replications in each trial and trials were repeated once. A 20-cm diameter pot, representing a replication, was used and there were six plants in each pot. An automated sprinkler system was used to water plants twice every day.

2.1.1. Experiment 1 Performance of Single Herbicides on 4–5 cm Tall Plants

About 15 seeds were planted on the surface in pots filled with potting mix (Centenary Landscaping, Mount Omanney, Brisbane, QLD, Australia). The potting mix had a pH of 6.3 and an electrical conductivity of 1.42 dS/m. Seeds were covered with a thin layer of potting mix to enhance moisture availability to the seeds. Immediately after emergence, seedlings were thinned to 6 plants per pot. When plants reached 4–5 cm in height (i.e., 6-leaf stage), they were sprayed with different herbicides (

Table 1. Herbicides, their doses, and adjuvants used in Experiment 1, 2, and 4.

Experiment 1. Single Herbicide (4–5 cm Tall Plants)			Experiment 2. Herbicide Mixtures (8–10 cm Tall Plants)			Experiment 4. Herbicide Mixtures (40–42 cm Tall Plants)
Herbicide Treatments	Dose	Adjuvant	Herbicide Treatments	Dose	Adjuvant	Herbicide Treatments	Dose	Adjuvant
	g a.e./a.i. ha−1			g a.e./a.i. ha−1			g a.e./a.i. ha−1
Control	-	-	Control	-	-	Control	-	-
2,4-D	1050	-	* Ametryn + trifloxysulfuron sodium	1500	0.25% Agral®	Azimsulfuron	20	1% HastenTM
Bentazone	1440	1% BS1000	Amicarbazone	560	1% HastenTM	Azimsulfuron + glyphosate	20 + 1080	1% HastenTM
Carfentrazone	24	1% HastenTM	* Amitrole + paraquat	750	-	Azimsulfuron + paraquat	20 + 600	1% HastenTM
Clomazone	480	-	Bensulfuron	48	1% HastenTM	Glyphosate	1080	-
Diuron	900	-	Bensulfuron + bentazone	48 + 1440	1% HastenTM	Glyphosate + halosulfuron	1080 + 49	1% CanDoTM
Glufosinate	750	-	Bensulfuron + diuron	48 + 900	1% HastenTM	Glyphosate + paraquat	1080 + 600	1% HastenTM
Glyphosate	1080	-	Bentazone	1440	-	Halosulfuron	49	1% CanDoTM
Halosulfuron	49	1% CanDoTM	Diuron	900	-	Halosulfuron + azimsulfuron	49 + 20	1% CanDoTM
Imazapic	48	1% HastenTM	Glufosinate	750	-	Halosulfuron + paraquat	49 + 600	1% CanDoTM
Imazethapyr	98	1% HastenTM	Glyphosate	1080	-	Paraquat	600	1% HastenTM
* Imazamox + imazapyr	48	1% HastenTM	Halosulfuron	49	1% CanDoTM
* MCPA + dicamba	420	-	Halosulfuron + amicarbazone	49 + 560	1% CanDoTM
Metribuzin	360	-	Halosulfuron + bentazone	49 + 1440	1% CanDoTM
Pine oil	2835	-	Halosulfuron + diuron	49 + 900	1% CanDoTM
Pine oil	5670	-	Halosulfuron + ioxynil	49 + 525	1% CanDoTM
Paraquat	600	1% HastenTM	Imazapic	48	1% HastenTM
Saflufenacil	24	1% HastenTM	Imazapic + bentazone	48 + 1440	1% HastenTM
			Imazapic + diuron	48 + 900	1% HastenTM
			Imazapic + ioxynil	48 + 525	1% HastenTM
			Imazethapyr	98	1% HastenTM
			Imazethapyr + bentazone	98 + 1440	1% HastenTM
			Imazethapyr + diuron	98 + 900	1% HastenTM
			Ioxynil	525	-
			Paraquat	600	1% HastenTM

* Commercial mixtures.

), using a research track sprayer (Woodlands Road Engineering, Gatton, QLD, Australia) fitted with Teejet XR110015 flat fan nozzles. The sprayer delivered a water solution of 108 L ha⁻¹ at a pressure of 200 kPa. It sprayed herbicides at a speed of 3 km/h. Among the herbicide treatments, there was a nontreated control and a bioherbicide (pine oil) at two rates (2835 and 5670 g ai ha⁻¹). The trial was arranged in a randomized complete block design with three replications. At 5 wk after herbicide treatment, plants were evaluated for survival and harvested at the base to determine their aboveground biomass. The appearance of at least one new leaf was the criterion for survival. Harvested samples were placed in an oven at 70 °C for at least 72 h, after which their weight was determined using an electronic balance.

2.1.2. Experiment 2 Performance of Herbicide Mixtures on 8–10 cm Tall Plants

As described above, seeds were planted in pots and six plants per pot were maintained. At 8–10 cm height (i.e., 10–12 leaf stage), plants were sprayed with different herbicide treatments (Table 1). Herbicide treatments that provided over 60% biomass reductions in Experiment 1 were used in this experiment with additional treatments. The number of seedlings that survived and their biomass were determined as described above. The trial was arranged in a randomized complete block design with three replications.

2.1.3. Experiment 3 Florpyrauxifen-Benzyl and Azimsulfuron Dose Response

In the USA, florpyrauxifen-benzyl, an auxin herbicide (WSSA Group 4), has been reported to provide control of some sedge species, including yellow nutsedge (Cyperus esculentus L.), rice flatsedge (Cyperus iria L.), and smallflower umbrellasedge (Cyperus difformis L.) [13]. Similarly, azimsulfuron, an acetolactate synthase inhibitor (WSSA Group 2), has been reported to control purple nutsedge (Cyperus rotundus L.) in India [14]. Therefore, a dose–response study was conducted to evaluate the effect of these herbicides on Navua sedge. Florpyrauxifen-benzyl was applied at 0, 22.5, 45, 90, and 180 g ha⁻¹, and azimsulfuron was applied at 0, 5, 10, 20, and 40 g ha⁻¹. As mentioned above, seeds were planted and six plants per pot were maintained. Seedlings were sprayed at the two growth stages: 10–12 cm height (i.e., 14–18 leaf stage) and 28–30 cm height (i.e., 60–70 leaf stage). In these trials, plants were allowed to grow for 8 wk after herbicide treatment to capture data on seed heads. In addition to survival and biomass data, the numbers of seed heads were determined. These dose–response trials were conducted in a factorial randomized block design with three replications in each run. The first factor was plant growth stage (small and large), and the second factor was herbicide treatments.

2.1.4. Experiment 4 Performance of Herbicide Mixtures on 40–42 cm Tall Plants

From Experiment 2, which was conducted with 8–10 cm tall plants, the top three herbicides glyphosate, halosulfuron-methyl, and paraquat were selected for this experiment on 40 to 42 cm tall plants with 140 to 150-leaf stage. In addition, azimsulfuron was included and different herbicide combinations (Table 1) were tested for their effects on very large plants of Navua sedge. Plants were sprayed as described above and their survival, aboveground biomass, and seed head numbers were evaluated 8 wk after herbicide treatment. The trial was conducted in a randomized complete block design with three replications in each run.

2.1.5. Experiment 5 Impact of Adjuvants on Rainfastness of Halosulfuron-Methyl

This experiment was conducted to evaluate the impact of different adjuvants on the rainfastness of halosulfuron. Activator^TM, AMS, CanDo^TM, Hasten^TM, and Uptake^TM were used at 1 and 2% with halosulfuron-methyl (49 g ha⁻¹). As described previously, seeds were planted in pots and six plants per pot were maintained. At the 10–12 leaf stage (8–10 cm in height), Navua sedge plants were sprayed with halosulfuron-methyl mixed with adjuvant treatments. Plants were irrigated for 10 min (i.e., simulating about 3 mm rain) with a hand-held sprinkler system at 0.5, 1, 2, 4, and 24 h after herbicide spray. The experiment was conducted in a factorial randomized block design with these replications. There was a nontreated control treatment in each experiment. Plant survival and biomass data were determined 8 wk after herbicide treatment.

3. Statistical Analyses

Each experiment was conducted twice and data from the two runs were subjected to the analysis of variance (ANOVA). ANOVA found no significant differences between the runs; therefore, the data were pooled for further analysis [15]. The transformation was not needed to improve the homogeneity of variance; therefore, nontransformed values were used in the analysis. Means were separated at p ≤ 0.05 using Fisher’s protected least significant difference (LSD) test.

4. Results and Discussion

4.1. Experiment 1 Performance of Single Herbicides on 4–5 cm Tall Plants

Few herbicides were found to be effective on small plants of Navua sedge (6-leaf stage and 4–5 cm tall) when applied alone (

Table 2. The effect of herbicide treatments on the survival and biomass of Navua sedge when sprayed on 4–5 cm tall plants (6-leaf stage).

Herbicide Treatments	Dose	Survival	Biomass	Biomass Reduction of Control
	g a.e./a.i. ha−1	%	g pot−1	%
Control	-	100	3.874	-
2,4-D	1050	100	2.480	36
Bentazone	1440	100	1.044	73
Carfentrazone	24	100	2.486	36
Clomazone	480	100	2.788	28
Diuron	900	88	1.376	64
Glufosinate	750	100	1.260	67
Glyphosate	1080	0	0	100
Halosulfuron	49	0	0	100
Imazapic	48	80	0.670	83
Imazethapyr	98	96	1.128	71
* Imazamox + imazapyr	48	100	1.544	60
* MCPA + dicamba	420	100	2.910	25
Metribuzin	360	92	1.864	52
Pine oil	2835	100	3.234	17
Pine oil	5670	100	3.596	7
Paraquat	600	0	0	100
Saflufenacil	24	100	2.536	35
LSD		12.3	0.611	-

Abbreviations: LSD, least significant difference at 5%. * Commercial mixtures.

). Glyphosate, halosulforon-methyl, and paraquat resulted in the complete control of Navua sedge. Although bentazone, diuron, glufosinate, imazapic, and imazethapyr resulted in 80–100% plant survival, these herbicides provided >60% reductions in plant biomass compared with the nontreated control treatment. Other herbicide treatments, including the bioherbicide (pine oil), provided poor control of small plants of Navua sedge, and therefore, these treatments were excluded in subsequent trials on larger plants.

In previous studies, some of these herbicides were found to be effective on other sedge species. For example, bentazon applied at 1120 g ha⁻¹ provided 95 to 100% control of yellow nutsedge [16]. Similarly, imazapic (an imidazolinone herbicide) has been reported to provide effective control of annual sedge (Cyperus compressus L.), cylindric sedge (Cyperus retrorsus Chapm.), and globe sedge (Cyperus globulosus Aubl.) [17]. In Australia, imazapic is registered for purple nutsedge control but not for Navua sedge. In a previous study in Australia, imazapic provided 100% control of Navua sedge; however, the dose needed to achieve this level of control was 960 g ha⁻¹ [5]; 20 times greater than that used in our study. This high rate of imazapic may not be cost-effective to control small plants of Navua sedge. In addition, higher rates of imazapic may displace other non-target plant species. In the USA, glufosinate applied at 400 g ha⁻¹ has been reported to control >90% deep root sedge (Cyperus entrerianus Boeck.) [18]. However, in the current study, all plants of Navua sedge survived the application of glufosinate at 750 g ha⁻¹. The surviving plants produced 33% biomass of the nontreated treatment. These results suggest that a sole application of glufosinate may not provide effective control of Navua sedge.

4.2. Experiment 2 Performance of Herbicide Mixtures on 8–10 cm Tall Plants

None of the herbicide treatments provided complete control of 8–10 cm tall plants (i.e., 10–12 leaf stage) of Navua sedge (

Table 3. The effect of herbicide treatments on the survival and biomass of Navua sedge when sprayed on 8–10 cm tall plants (10–12 leaf stage).

Herbicide Treatments	Dose	Survival	Biomass	Biomass Reduction of Control
	g a.e./a.i. ha−1	%	g pot−1	%
Control	-	100	10.05
* Ametryn + trifloxysulfuron sodium	1500	100	2.41	76
Amicarbazone	560	92.5	10.46	−4
* Amitrole + paraquat	750	80.6	2.88	71
Bensulfuron	48	97.6	5.41	46
Bensulfuron + bentazone	48 + 1440	94.8	4.43	56
Bensulfuron + diuron	48 + 900	91.7	3.30	67
Bentazone	1440	100	3.98	60
Diuron	900	86.5	4.21	58
Glufosinate	750	100	4.50	55
Glyphosate	1080	21.1	0.35	97
Halosulfuron	49	100	2.14	79
Halosulfuron + amicarbazone	49 + 560	93.3	3.17	68
Halosulfuron + bentazone	49 + 1440	97.2	3.78	62
Halosulfuron + diuron	49 + 900	97.2	3.34	67
Halosulfuron + ioxynil	49 + 525	97.2	3.02	70
Imazapic	48	100	5.40	46
Imazapic + bentazone	48 + 1440	100	3.70	63
Imazapic + diuron	48 + 900	94.4	4.96	51
Imazapic + ioxynil	48 + 525	100	3.71	63
Imazethapyr	98	97.2	5.73	43
Imazethapyr + bentazone	98 + 1440	86.1	4.03	60
Imazethapyr + diuron	98 + 900	89.1	4.94	51
Ioxynil	525	100	6.35	37
Paraquat	600	86.5	1.05	90
LSD	-	13.6	2.139

Abbreviations: LSD, least significant difference at 5%. * Commercial mixtures.

). Glyphosate at 1080 g ha⁻¹ was the most effective herbicide, resulting in 21% of seedling survival and a 97% reduction in biomass compared with the nontreated control. Although applications of halosulfuron-methyl at 49 g ha⁻¹ and paraquat at 600 g ha⁻¹ resulted in 100% and 87% seedling survival, their applications reduced biomass by 79% and 90%, respectively, compared with the control. Only glyphosate, halosulfuron-methyl, and paraquat were included in the subsequent herbicide mixture trial on very large plants (40–42 cm in height) of Navua sedge.

Halosulfuron-methyl, when applied alone, provided about 80% control of biomass, but mixing with amicarbazone, bentazone, diuron, or ioxynil reduced the control level (62 to 70%), suggesting that these herbicide combinations are not compatible mixture partners. Bensulfuron, imazapic, and imazethapyr, as alone or in mixtures with other herbicides, did not provide effective control of Navua sedge. Imazapic (48 g ha⁻¹) and imazethapyr (98 g ha⁻¹) were more effective on small plants (4–5 cm tall) compared with large plants (8–10 cm tall). Larger weeds usually require a larger dose of herbicides for adequate control. In a previous study, imazethapyr at 96 g ha⁻¹ provided only 13% control of Navua sedge [5]; much lower than in the current study. In the same study, bensulfuron at 51 g ha⁻¹ did not provide control of Navua sedge. In the current study, bensulfuron at 48 g ha⁻¹ applied as alone or in a mixture with bentazone or diuron provided a 46 to 67% reduction in biomass of the 8–10 cm tall plants of Navua sedge. This low level of control will not be acceptable to growers. Bensulfuron has been found to be effective for controlling other sedges in rice crops. For example, bensulfuron at 60 g ha⁻¹ provided complete control of rice flatsedge and globe fringerush [Fimbristylis miliacea (L.) Vahl] [19]. Glufosinate provided only 55% control of 8–10 cm tall Navua sedge plants, suggesting that this is not a feasible herbicide option to be included in controlling Navua sedge in non-cropped situations. The two commercial mixtures, i.e., ametryn + trifloxysulfuron sodium and amitrole + paraquat, provided 71 to 76% reductions in plant biomass; however, survival was 81 to 100%. The use of ametryn in combination with tillage has been reported to provide effective control of purple nutsedge [20] but it was not that effective on Navua sedge when applied with trifloxysulfuron sodium.

4.3. Experiment 3 Florpyrauxifen-Benzyl and Azimsulfuron Dose Response

Navua sedge survival, seed head numbers, and biomass were affected by the interaction between growth stage and florpyrauxifen-benzyl dose (

Table 4. The interaction effect of the growth stage (small, 10–12 cm in height and 14–18 leaf stage; and large, 28–30 cm in height and 60–70 leaf stage) of Navua sedge and florpyrauxifen-benzyl dose on the survival, head number, and biomass of the weed.

Dose	Survival		Head		Biomass
	Large	Small	Large	Small	Large	Small
g a.i. ha−1	%		number pot−1		g pot−1
0	100	100	59.3	62.9	18.24	16.84
22.5	100	89.3	24.7	7.0	13.76	3.74
45	100	85.7	24.4	2.6	13.04	3.70
90	100	85.7	23.1	1.6	12.94	2.45
180	100	60.7	20.1	0.1	12.21	1.47
LSD	9.8		5.1		1.69

Abbreviations: LSD, least significant difference at 5%.

). Irrespective of the dose, 100% of the large plants (60–70 leaf stage; 28–30 cm in height) survived the herbicide application. Seedling survival of the small plants (14–18-leaf stage; 10–12 cm in height), however, was affected by herbicide dose. More than 85% of the seedlings survived the application of florpyrauxifen-benzyl up to 90 g ha⁻¹ and about 61% of seedlings survived the rate of 180 g ha⁻¹. The number of seed heads per pot was similar in the control treatment of both growth stages but it was affected by the dose. The numbers of seed heads were similar for large plants at the tested doses of the herbicide, but they were affected for small plants. When sprayed on small plants, florpyrauxifen-benzyl at 45 to 180 g ha⁻¹ provided >95% reductions in seed heads per pot compared with the nontreated control treatment.

A similar trend was observed for aboveground biomass. Biomass was similar between large and small plants in the control treatment but it was affected by the herbicide dose. When sprayed on large plants, florpyrauxifen-benzyl at the highest dose (180 g ha⁻¹) was able to provide only a 33% reduction in biomass compared with the nontreated control. For small plants, however, the herbicide at the lowest dose (22.5 g ha⁻¹) provided a 78% reduction in biomass, which further reduced to a 91% reduction at the highest dose (180 g ha⁻¹).

In Australia, the maximum dose of florpyrauxifen-benzyl recommended for controlling smallflower umbrellasedge in rice is 45 g ha⁻¹. In the current study, the herbicide at 45 g ha⁻¹ reduced seedhead numbers and biomass by 96% and 78%, respectively, when sprayed on small plants, but this rate provided only 58% and 29% suppression of seed heads and biomass, respectively, when sprayed on large plants. Irrespective of the growth stage, >85% of the seedlings survived the application of the herbicide at 45 g ha⁻¹, suggesting that florpyrauxifen-benzyl cannot be registered as a standalone herbicide for the control of Nauva sedge. However, this could be an effective partner when mixed with another effective herbicide. In the USA, florpyrauxifen-benzyl at 40 g ha⁻¹ provided >95% control of rice flatsedge, smallflower umbrellasedge, and yellow sedge [13]. The authors also suggested that it would be beneficial to tank mix florpyrauxifen-benzyl with other herbicides to lessen the rise of the evolution of herbicide resistance. As our objective was to achieve effective control of large plants of Navua sedge, and florpyrauxifen-benzyl did not provide >33% biomass suppression of large plants (28–30 cm in height; 60–70 leaf stage), even at the highest tested rate (180 g ha⁻¹), we did not evaluate this herbicide further.

Interactions between growth stage and azimsulfuron dose were significant for seedling survival, seed heads, and biomass of Navua sedge (

Table 5. The interaction effect of the growth stage (small, 10–12 cm in height and 14–18 leaf stage; and large, 28–30 cm in height and 60–70 leaf stage) of Navua sedge and azimsulfuron dose on the survival, head number, and biomass of the weed.

Dose	Survival		Head		Biomass
	Large	Small	Large	Small	Large	Small
g a.i. ha−1	%		number pot−1		g pot−1
0	100	100	57.0	60.7	18.16	17.26
5	100	60.7	19.0	5.6	13.43	1.43
10	100	35.7	8.9	2.1	13.33	0.61
20	39.3	17.9	0	0	4.54	0.07
40	21.4	0	0	0	1.27	0
LSD	18.7		4.7		2.16

Abbreviations: LSD, least significant difference at 5%.

). For small plants, only 36% of seedlings survived the application of azimsulfuron at 10 g ha⁻¹ but at this herbicide dose, all large plants of Navua sedge survived. Azimsulfuron at 20 and 40 g ha⁻¹ resulted in 39% and 21% survival of large plants but for small plants, these values were 18% and 0%, respectively. Irrespective of the growth stage, Navua sedge plants did not produce seed heads when azimsulfuron was applied at 20 or 40 g ha⁻¹. Compared with their nontreated control treatments, small and large plants of Navua sedge produced only 3% and 16% of seed heads, respectively, after the application of azimsulfuron at 10 g ha⁻¹. Aboveground biomass was similar between the large and small plants in control; however, herbicide application resulted in a more drastic reduction in biomass of small plants than large plants. Azimsulfuron at 10 g ha⁻¹ provided 96% and 27% reductions in biomass of small and large plants, respectively. Azimsulfuron at 20 and 40 g ha⁻¹ suppressed 75% and 93% biomass, respectively, of large plants compared to the nontreated control treatment. These values for small plants were 99.6% and 100%, respectively.

In Australia, azimsulfuron at 20 g ha⁻¹ is recommended for the control of smallflower umbrellasedge at its 3–6 leaf stage in a rice crop; however, Navua sedge is not on its label. Our results suggest that azimsulfuron at 20 g ha⁻¹ can be included in herbicide programs to manage Navua sedge in rice, especially as an early post-emergence application when the weed is small. In a non-cropped situation, azimsulfuron rates can be increased to control large plants of Navua sedge. No information is available on the selectivity of azimsulfuron in different pasture systems. Such information will help to register this herbicide for the management of Navua sedge in pasture fields. Although 40% of large plants (60–70 leaf stage; 28–30 cm tall) of Navua sedge survived the application of azimsulfuron at 20 g ha⁻¹, these plants could not produce seed heads after 8 wk of spray. These results suggest the potential of using this herbicide to manage the seed bank of Navua sedge, even if applied to large plants. In other countries, azimsulfuron has been found to be effective to control other sedges, including purple nutsedge, rice flatsedge, and globe fringerush [21,22], further suggesting its potential to control multiple species of sedges.

4.4. Experiment 4 Performance of Herbicide Mixtures on 40–42 cm Tall Plants

Herbicides found effective in previous trials were evaluated for their performance when used alone or in mixtures on very large plants (140–150-leaf stage; 40–42 cm in height). A single application of halosulfuron-methyl and paraquat resulted in 100% survival of Navua sedge seedlings (

Table 6. The effect of herbicide treatments on the survival, seed head number, and biomass of Navua sedge when sprayed on 40–42 cm tall plants (140–150 leaf stage).

Herbicide Treatments	Dose	Survival	Head	Head Reduction of Control	Biomass	Biomass Reduction of Control
	g a.e./a.i. ha−1	%	number pot−1	%	g pot−1	%
Control	-	100	81.7		30.02
Azimsulfuron	20	43	0	100	12.22	59
Azimsulfuron + glyphosate	20 + 1080	0	0	100	0	100
Azimsulfuron + paraquat	20 + 600	100	51.8	37	18.80	37
Glyphosate	1080	40	0	100	9.77	67
Glyphosate + halosulfuron	1080 + 49	0	0	100	0	100
Glyphosate + paraquat	1080 + 600	100	51.7	37	19.45	35
Halosulfuron	49	100	0	100	12.35	59
Halosulfuron + azimsulfuron	49 + 20	0	0	100	0	100
Halosulfuron + paraquat	49 + 600	100	26.2	68	13.70	54
Paraquat	600	100	49.0	40	16.52	45
LSD		7.67	5.81		3.25

Abbreviations: LSD, least significant difference at 5%.

). About 40% of seedlings survived the single application of azimsulfuron or glyphosate. However, mixing these herbicides with each other or with halosulfuron-methyl resulted in the complete control of Navua sedge. Interestingly, mixing paraquat with azimsulfuron or glyphosate resulted in 100% survival of Navua sedge, suggesting that paraquat antagonized the effect of glyphosate and azimsulfuron. Adding paraquat with halosulfuron-methyl also did not add to the control provided by the sole application of halosulfuron-methyl. Single applications of azimsulfuron, glyphosate, or halosulfuron-methyl resulted in total seed head suppression; however, adding paraquat to these herbicides resulted in the production of a significant number of seed heads per pot. Similar results were observed for the aboveground biomass of Navua sedge.

In Australia, the application of glyphosate followed by paraquat, commonly called the “double knock strategy”, has been a successful technique for the management of glyphosate-resistant weeds in fallow situations [23]. Our results suggest that mixing these two herbicides to achieve faster control of Navua sedge will not work. When paraquat (i.e., a contact herbicide) is mixed with glyphosate, azimsulfuron, or halosulfuron-methyl (i.e., systemic herbicides), paraquat quickly burns the foliage, which potentially restricts the plant’s ability to absorb a sufficient amount of a systemic herbicide, thereby reducing their efficacy [24].

In non-cropped situations, azimsulfuron or halosulfuron-methyl can be used with glyphosate to manage large plants of Navua sedge. These combinations will also reduce selection pressure on glyphosate. Herbicide mixtures have been reported to be more effective than herbicide rotations in mitigating resistance evolution through herbicide selection [25]. Although the mixture of azimsulfuron and halosulfuron-methyl provided complete control of large plants of Navua sedge, both herbicides belong to the sulfonyl-urea group (WSSA Group 2). Repeated applications of herbicides with this site of action can rapidly evolve resistance; therefore, there is a need to closely monitor survival and follow stewardship guidelines.

4.5. Experiment 5 Effect of Adjuvants on the Rainfastness of Halosulfuron-Methyl

Seedling survival (

Table 7. The interaction effect of adjuvant treatments and irrigation time after herbicide application on the survival of Navua sedge when halosulfuron-methyl at 49 g ha⁻¹ (mixed with different adjuvants) sprayed on 8–10 cm tall plants (10–12 leaf stage).

Adjuvant Treatments	Survival
	Irrigation Time (h after Spray)
	0.5	1	2	4	24
	%
Nontreated control	100	100	100	100	100
1% ActivatorTM	38.0	30.6	22.7	10.8	7.3
2% ActivatorTM	32.8	12.0	11.3	10.4	4.2
1% AMS	100	100	100	100	40.0
2% AMS	100	100	100	100	3.3
1% Can DoTM	55.6	41.5	32.6	30.7	8.3
2% Can DoTM	53.7	38.7	34.1	30.4	8.3
1% HastenTM	64.8	59.5	26.9	10.7	0
2% HastenTM	26.3	16.4	7.0	5.6	0
1% UptakeTM	77.1	73.8	49.2	25.0	0
2% UptakeTM	45.1	29.9	17.9	10.9	0
LSD	22.5

Abbreviations: AMS, ammonium sulfate; LSD, least significant difference at 5%.

) and biomass (

Table 8. The interaction effect of adjuvant treatments and irrigation time after herbicide application on the biomass of Navua sedge when halosulfuron-methyl at 49 g ha⁻¹ (mixed with different adjuvants) sprayed on 8–10 cm tall plants (10–12 leaf stage).

Adjuvant Treatments	Aboveground Biomass
	Irrigation Time (h after Spray)
	0.5	1	2	4	24
	g pot−1
Nontreated control	36.45	34.81	36.54	36.75	35.97
1% ActivatorTM	5.04	4.66	3.02	2.12	1.60
2% ActivatorTM	3.72	1.49	1.30	1.17	0.40
1% AMS	21.79	22.03	24.03	28.07	3.63
2% AMS	21.55	20.37	21.75	26.72	1.77
1% Can DoTM	8.64	6.62	6.70	6.25	2.00
2% Can DoTM	8.37	4.94	4.77	2.14	2.03
1% HastenTM	13.79	11.39	6.77	1.76	0
2% HastenTM	3.84	2.42	0.23	0.09	0
1% UptakeTM	15.76	10.82	8.26	5.67	0
2% UptakeTM	6.18	5.38	1.86	1.23	0
LSD	6.034

Abbreviations: AMS, ammonium sulfate; LSD, least significant difference at 5%.

) of Navua sedge were influenced by the interaction between adjuvant treatments and irrigation time after spray. Except for AMS, all adjuvants significantly reduced seedling survival, when irrigation was given 0.5 to 4 h after herbicide application (Table 7). In general, the 2% concentration of each adjuvant provided better control than their 1% concentration. When irrigation was given 1 h after herbicide application, seedling survival was 12, 16, and 30% for adjuvant treatments with 2% Activator^TM, 2% Hasten^TM, and 2% Uptake^TM, respectively. These values were reduced to 11, 7, and 18%, respectively, when irrigation was given 2 h after herbicide application. Except for 1% AMS, seedling survival was similar among adjuvants when irrigation was given 24 h after herbicide application. However, Hasten^TM and Uptake^TM provided a complete kill of Navua sedge in this irrigation treatment.

None of the adjuvant treatments provided >90% suppression in Navua sedge biomass when plants were irrigated 30 min after herbicide application (Table 8). Delaying irrigation improved biomass suppression. Watering 1 h after herbicide application, for example, resulted in 96, 93, and 85% reductions in biomass for 2% Activator^TM, 2% Hasten^TM, and 2% Uptake^TM, respectively, compared with the nontreated control treatment. For these adjuvant treatments, the biomass reduction was 96, 99, and 95%, respectively, when watering was given 2 h after herbicide application. Although none of the adjuvants provided complete control of Navua sedge when watering was done 0.5 to 4 h after herbicide application, some treatments (i.e., 1% and 2% Activator^TM, 2% Hasten^TM, and 2% Uptake^TM) provided control similar to the treatment in which irrigation was given 24-h after herbicide application. AMS and CanDo^TM were the poorest treatments.

Our results suggest that compared with the currently recommended adjuvant (e.g., 1% CanDo™), the use of Activator^TM, Hasten^TM, or Uptake^TM at 2% (all non-ionic based surfactants) will enhance halosulfuron-methyl efficacy if rain occurs around 2 h after herbicide application. These treatments provided >80% reduction in seedling survival and ≥95% reduction in the biomass of Navua sedge. In a previous study, the use of a non-ionic surfactant improved the rainfastness of tribenuron-methyl with rain at 1 h after herbicide application [10]. In general, foliage-based sulfonylurea herbicides, including halosulfuron-methyl, require adjuvants to improve their efficacy and rainfastness [26,27]. AMS at 1 or 2% did not improve the rainfastness of halosulfuron-methyl, which is consistent with a previous study [27]. In the previous study, AMS did not improve the rainfastness of thifensulfuron, while a vegetable oil provided full rainfastness after herbicide application. In another study, the addition of urea ammonium nitrate and a non-ionic surfactant with bispyribac-sodium enhanced the herbicide efficacy and reduced the time needed between herbicide application and washoff during a rainfall event [28]. Previous studies (e.g., [10,27]) and our results emphasize that effective adjuvants at optimum concentrations are of crucial importance in improving the efficacy of sulfonylurea herbicides, including halosulfuron-methyl, under adverse environmental conditions (e.g., if rain occurs shortly after herbicide application).

5. Conclusions

Glyphosate at 1080 g ha⁻¹, paraquat at 600 g ha⁻¹, and halosulfuron-methyl at 49 g ha⁻¹ provided effective control of Navua sedge at the 6-leaf stage, suggesting that these herbicides can be used in rotation to manage the weed in non-cropped situations and halosulfuron-methyl can be used in pastures. The rainfastness of halosulfuron-methyl, the only herbicide recommended to control Navua sedge in different pastures species, can be improved by using non-ionic adjuvants at high concentrations (e.g., Activator^TM, Hasten^TM, or Uptake^TM at 2%). Florpyrauxifen-benzyl was also found to be effective for the control of the intermediate plant size (14–18 leaf stage) of Navua sedge; however, this herbicide was not effective on large plants (60–70 leaf stage). Azimsulfuron was found to be very effective on intermediate as well as large plants; however, its optimum dose needs to be standardized in field conditions. Although azimsulfuron at 20 g ha⁻¹, glyphosate at 1080 g ha⁻¹, and halosulfuron-methyl at 49 g ha⁻¹ did not provide complete control of very large plants (140–150 leaf stage; 40–42 cm tall) when used as alone, these herbicides did not allow the weed to produce seeds. As mentioned above, Navua sedge is a prolific seed producer and its seeds can remain viable in the soil for more than 5 years. Our results suggest that, in the case of limited resources for mixing different herbicides, the application of these three herbicides will help in managing the seed bank of Navua sedge. Mixing glyphosate (1080 g ha⁻¹) with azimsulfuron (20 g ha⁻¹) or halosulfuron-methyl (49 g ha⁻¹) provided complete control of very large plants of Navua sedge. These herbicide combinations can be used to manage Navua sedge in fallow situations and along roadsides and fencelines. In some crop situations, the mixture of azimulsulfuron and halosulfuron can provide complete control of Navua sedge but this combination should not be used in consecutive years to delay the evolution of resistance as both herbicides have the same site of action.

Future trials should evaluate the suitability of azimsulfuron for different pasture species and integrated management options for Navua sedge (e.g., integrated use of herbicides with pasture species with high competitiveness). We evaluated herbicide efficacy on plants established through seeds. Future research should also include plants established through rhizomes, especially to evaluate azimsulfuron-based herbicide combinations.