Introduction
Wheat (Triticum aestivum L.) is the most planted crop by total land area in Australia, where production accounts for 6% of the total crop worldwide (ABARES 2012; FAO 2011). Weed shifts in the northern grain-growing region of Australia (New South Wales and Queensland) have occurred from long-term use of glyphosate in fallow fields (Peltzer et al. Reference Peltzer, Hashem, Osten, Gupta, Diggle, Reithmuller, Douglas, Moore and Koetz2009; Wicks et al. Reference Wicks, Felton, Murison and Martin2000). Weeds that have not historically been present in wheat systems are beginning to pose a threat to yields, especially later in the season. Windmillgrass is a weed that has become more problematic as the use of glyphosate has increased (Cook et al. Reference Cook, Brooke, Street and Widderick2014). Windmillgrass was confirmed to be glyphosate resistant (Cook Reference Cook2014) in Queensland as early as 2010. Southeastern Queensland is a subtropical region with mild winter temperatures that favor early growth and development of summer weeds. Windmillgrass is a commonly occurring native perennial grass with digitate racemes found across temperate and subtropical regions of Australia (Lamp et al. Reference Lamp, Forbes and Cade2001). It is a competitive summer grass that occurs commonly in wheat-growing areas in Australia (Michael et al. Reference Michael, Borger, MacLeod and Payne2010) and has been shown to reduce wheat biomass and yield by 25% due to competition (Borger et al. Reference Borger, Reithmuller and Hashem2009).
Chloris spp. are commonly occurring weeds in wheat in the northern grain-growing region (Felton et al. Reference Felton, Wicks and Welsby1994). Rhodesgrass is a significant forage species in the tropical and subtropical regions of Queensland, noted for its high nutritive value (Stobbs Reference Stobbs1973). Chloris gayana var. Callide is a palatable variety of rhodesgrass, originally introduced to Australia from Tanzania (Milford and Minson Reference Milford and Minson1968). Feather fingergrass (feathertop rhodesgrass in Australia; Chloris virgata Sw.) is another Chloris spp. that has become problematic in the northern grain-growing region (Cook Reference Cook2014). One aspect of Chloris spp. that makes them particularly competitive is their ability to survive and thrive in areas of poor fertility, including alkaline soils (Yang et al. Reference Yang, Zhang, Liu, Shi and Wang2009) and regions with high levels of heavy metals (Patra et al. Reference Patra, Lenka and Panda1994). Windmillgrass usually flowers during the fallow season in wheat paddocks and is a potential forage in a wheat–fallow system (Syme et al. Reference Syme, Botwright Acuña, Abrecht and Wade2007).
Borger and Ferris (Reference Borger and Ferris2013) observed greater than 83% control of rhodesgrass with four Group 1 (Acetyl CoA carboxylase inhibitors) herbicides (fluazifop-p, clethodim, clodinafop, and haloxyfop). Another study showed windmillgrass control was 89% or better with glyphosate (Group 9: EPSP synthase inhibitors) alone, a paraquat (Group 22: photosystem I cell membrane disrupters) plus diquat (Group 22) combination, a glyphosate plus paraquat plus diquat combination, and haloxyfop (Group 1) alone. Greater than 80% control of tumble windmillgrass (Chloris verticillata Nutt.) was observed with glyphosate, three Group 1 herbicides (sethoxydim, clethodim, and quizalofop), mesotrione (Group 27: 4-hydroxphenylpyruvate dioxygenase inhibitors “bleachers”) plus atrazine (Group 5: photosystem II inhibitors), and a rimsulfuron (Group 2: acetolactate synthase inhibitors) plus nicosulfuron (Group 2) plus atrazine (Group 5) combination (Hennigh et al. Reference Hennigh, Al-Khatib, Stahlman and Shoup2005).
With growing concerns surrounding spray drift, growers are adopting drift-reducing nozzle technologies to increase their droplet size and reduce their drift potentials (Ferguson et al. Reference Ferguson, O’Donnell, Chauhan, Adkins, Kruger, Wang, Urach Ferreira and Hewitt2015). The U.S. Environmental Protection Agency defines physical spray drift as the “the physical movement of a pesticide through the air at the time of application or soon thereafter, to any site other than the one intended for application” (EPA 1999). Previous research has shown that the size of droplets that comprise a spray influence the drift potential of that spray (Hewitt Reference Hewitt1997). Sprays with a volumetric majority of droplets smaller than 200 µm have the highest drift potentials (Byass and Lake Reference Byass and Lake1977; Grover et al. Reference Grover, Kerr, Maybank and Yoshida1978). When the drift potentials of a spray are reduced, environmental losses are reduced (Uk Reference Uk1977).
Drift-reducing nozzles have design characteristics to increase droplet size, thereby reducing the drift potential. Some nozzles use the Venturi process, which causes air to mix into the spray solution in the nozzle, creating larger, air-entrained droplets (Dorr et al. Reference Dorr, Hewitt, Adkins, Hanan, Zhang and Noller2013). These drift-reduction technology (DRT) nozzles also often use a pre-orifice chamber designed to increase the droplet size and alter the spray structure, droplet velocity, and droplet trajectory.
Classifications of spray droplet size or spray quality were developed from the British Crop Protection Council standard (Southcombe et al. Reference Southcombe, Miller, Ganzelmeier, Van de Zande, Miralles and Hewitt1997), which was expanded and adopted by the American Society of Agricultural and Biological Engineers (ASABE, formerly ASAE), producing the current version of ASABE S572.1 standard in 2009 (ASAE 2009). Spray droplet–size classes from the ASABE/ANSI standard (in increasing droplet-size order) are: extremely fine, very fine, fine, medium, coarse, very coarse, extremely coarse, and ultracoarse. Each spray quality is determined from measurements of reference nozzles at defined spray pressures, which sets the curves where treatments of interest can be placed and then classified based on this standard (ASAE 2009).
Previous research showed the droplet-size (nozzle type) effects on herbicide efficacy of six herbicides of different modes of action: clodinafop (Group 1), imazamox (Group 2) plus imazapyr (Group 2), metribuzin (Group 5), glyphosate (Group 9), amitrole (Group 11: carotenoid biosynthesis inhibitors), and paraquat (Group 22) on winter grasses (Ferguson et al. Reference Ferguson, Chechetto, Adkins, Hewitt, Chauhan, Kruger and O’Donnell2018). No droplet-size effect was observed for systemic herbicides (glyphosate, imazamox plus imazapyr, and clodinafop) across species, but the TTI nozzle reduced efficacy of paraquat and amitrole. Additional work further identified optimized sprays for improved technology selection (Ferguson et al. Reference Ferguson, O’Donnell, Chauhan, Adkins, Kruger, Wang, Urach Ferreira and Hewitt2015), for increased coverage (Ferguson et al. Reference Ferguson, Chechetto, Hewitt, Chauhan, Adkins, Kruger and O’Donnell2016a), and for spray drift reduction (Ferguson et al. Reference Ferguson, Chechetto, O’Donnell, Dorr, Moore, Baker, Powis and Hewitt2016b).
The objective of this work was to determine whether droplet size (nozzle type) affected efficacy of six POST herbicides on the control of a wild population of Chloris spp., windmillgrass, and a cultivated species, rhodesgrass. The hypothesis was that droplet size (nozzle type) would not affect the efficacy across herbicide mode of action for control of both Chloris spp. The specific objectives of the study were: (1) to determine the effect of droplet size (nozzle type) on the efficacy of six herbicide modes of action for the control of a wild-type and domestic Chloris spp. and (2) to identify which herbicides control Chloris spp. in wheat.
Materials and Methods
A study to compare droplet-size (nozzle type) effects on POST herbicide efficacy for control of Chloris spp. was conducted at the University of Queensland in Gatton, QLD, Australia. The study was arranged in a factorial arrangement of treatments with six different nozzles (five of which were DRTs) by six herbicide treatments with four replications in each of two experimental runs. The nozzles selected produce four spray droplet–size distributions (fine, medium, coarse, and extremely coarse) with water at 350 kPa according to the results from each manufacturer as published in its catalog. Nozzles selected for the study were the XR11002, Turbo TeeJet® TT11002, AIXR11002, TTI11002 (Spraying Systems, Wheaton, IL, USA); Mini Drift MD11002 (Hardi International, Taastrup, Denmark); and the TurboDrop Asymmetric Dual Fan TADF11002 (Agrotop GmbH, Obertraubling, Germany). Treatments in the study were applied at 100 L ha−1 using a 10.4 km h−1 driving speed and 350 kPa operating pressure. Nozzles were selected based on previous experiments that focused on the effect of nozzle droplet size across multiple application scenarios (Ferguson et al. Reference Ferguson, O’Donnell, Chauhan, Adkins, Kruger, Wang, Urach Ferreira and Hewitt2015, Reference Ferguson, Chechetto, Hewitt, Chauhan, Adkins, Kruger and O’Donnell2016a, Reference Ferguson, Chechetto, O’Donnell, Dorr, Moore, Baker, Powis and Hewitt2016b). Herbicide treatments and their respective adjuvant additions are listed in Table 1.
Table 1 Herbicide treatments and their adjuvant additions applied over tillering Chloris spp. in both timings of the study in 2015 and 2016, represented from Ferguson et al. (Reference Ferguson, Chechetto, Adkins, Hewitt, Chauhan, Kruger and O’Donnell2018), as treatments were identical to the previous study.
a Indicates not applicable, as the label does not require the use of an adjuvant at rates used in the study.
b Bolded entries are the brand name for those specific adjuvants.
Chloris spp.
The windmillgrass seeds were collected from populations in a field approximately 4 km northwest of Dalby, QLD, Australia (27.15694444 S, 151.247222 E) in November 2014. Seeds were harvested from an established putative glyphosate-susceptible windmillgrass population collected from a farm that did not have continuous glyphosate use. Rhodesgrass seed was a cultivated variety ‘Callide’ (AusWest Seeds, Forbes, NSW, Australia). Rhodesgrass was included to ensure uniformity of a related cousin that has shown similar responses to herbicide treatments in prior studies (Borger and Ferris Reference Borger and Ferris2013; Borger et al. Reference Borger, Reithmuller and Hashem2009). One seed from each species was planted 0.5 to 1.0 cm deep in 10-cm-deep by 10-cm-diameter pots. Each pot was filled with 0.5 L of a standard University of Queensland, Gatton, nursery potting media (1 m3 of composted pine bark [2 to 10 mm]; 2 kg Osmocote Plus 8–9 month [N-P-K: 15.0+3.9+9.1 g plus 1.5 g Mg and trace elements]; 1 kg Osmocote Exact+3–4 month [N-P-K: 16.0+5+9.2 g plus 1.8 g Mg and trace elements]; 2 kg Nutricote 7 month [N-P-K: 16.0+4.4+8.3 g plus trace elements]; 1.2 kg Saturaid granular wetting agent; 1.2 kg dolomite; and 1.3 kg Osmoform 4-month release [Isobutylidenediurea] [N-P-K: 18.0+2.2+11.0 g plus trace elements]) The study was repeated in time by planting on September 7, 2015, and January 11, 2016 for the two runs, respectively. Plants were grown in an outdoor nursery and irrigated twice daily. Weather conditions during the trial periods for both runs are described in Table 2. Pots were placed into 0.5 by 0.4 m trays (20 pots tray−1) and trays were rearranged every 7 d in a completely randomized design to minimize environmental impact on individual pots.
Table 2 Weather conditions during each Chloris spp. run’s growing period before the spray application.
Details of the Spray Applications for Both Runs
Chloris spp. plants were sprayed at the tillering stage on October 6, 2015 (29-d old), and February 4, 2016 (24-d old), for the two runs, respectively. Warmer conditions in the second run shortened the grass development time, allowing for an earlier spray date compared with the first run. Treatments were applied at a speed of 10.4 km h−1 to achieve the 100 L ha−1 application volume rate. Spray applications were made using a pull-behind sprayer (UA300B/20S/6BX, Croplands Equipment, Adelaide, SA, Australia) with a 6-m spray boom pulled by an all-terrain vehicle (Yamaha Grizzly 350, Yamaha Motor, Wetherill Park, NSW, Australia). Treatments were applied at 350 kPa. Nozzle spacing was 50 cm, and the boom height was 50 cm above rhodesgrass (approximately 70 cm above the ground). For the application, pots were removed from trays and placed on the ground. Four plants of each species were arranged in a line with 50-cm spacing between individual plants and treated with one of each nozzle by herbicide treatment, with n=300 total plants in each run. Following treatment, the pots were returned to their original location and watered daily as described earlier. At 28 d after treatment (DAT), surviving plants were clipped at the soil level, put into paper bags, placed in a dryer at 65 C, and dried for 48 h, after which weights were recorded.
Visible injury estimates (VIEs) were taken at 7, 14, 21, and 28 DAT by estimating the percent injury on individual plants for each treatment. The four individual plants (replications) were assessed to create a composite VIE for each species from each nozzle by herbicide treatment. These ratings were taken at approximately the same time of day for each rating date.
Droplet-Size Analysis
This Chloris control study was included in a larger study that used identical treatments for spray quality and efficacy comparisons for four winter annual grass species. The droplet-size measurements from Ferguson et al. (Reference Ferguson, Chechetto, Adkins, Hewitt, Chauhan, Kruger and O’Donnell2018) are represented here, as the treatments were identical (Table 3). The methods for obtaining droplet size at the University of Queensland Center for Pesticide Application and Safety Wind Tunnel Research Facility are described in Ferguson et al. (Reference Ferguson, Chechetto, Adkins, Hewitt, Chauhan, Kruger and O’Donnell2018).
Table 3 Median droplet size (Dv0.5) at 350 kPa for all nozzle types and herbicide tank-mix combinations used in the study measured using a laser-diffraction instrument as reproduced from Ferguson et al. Reference Ferguson, Chechetto, Adkins, Hewitt, Chauhan, Kruger and O’Donnell2018.
a Letters following droplet sizes (in µm) are the ASABE droplet-size categories measured using the ASABE-defined reference nozzles at their respective pressures to delineate the classification. F, fine; M, medium; C, coarse; XC, extremely coarse; UC, ultracoarse.
Statistical Analyses
Plant dry weight results were analyzed using a generalized linear mixed model (PROC GLIMMIX) in SAS (Statistical Analysis Software v. 9.4, Cary, NC, USA) with means separations made at α=0.05. Before being inserted into the model, dry weights were transformed into dry weight reductions
$${\rm DWR}{\equals} \left[ {1{\minus}\left[ {{{{\rm individual}\,{\rm plant}\,{\rm dry}\,{\rm weight}} \over { \left[ {\left( {\mathop{\sum}{{\rm Cdw}_{1} {\plus}{\rm Cdw}_{{2{\rm }}} {\plus}{\rm Cdw}_{{3{\rm }}} {\plus}{\rm Cdw}_{4} } } \right) \ {\div}\ 4} \right] }}} \right]} \right]{\times}100$$
Plant dry weight was divided by the mean of the sum of the four control plant dry weights (Cdw1 … Cdw4) and subtracted from 1, then multiplied by 100, to obtain a percent DWR. The model was analyzed as: DWR=nozzle type by herbicide treatment by species, and experimental run and plant number were set to random. Fixed effects were nozzle type, herbicide treatment, and species. Experimental run was treated as a random effect. Bias protection for the denominator degrees of freedom (df) was achieved with the inclusion of the Kenward-Roger adjustment for the generalized linear mixed model (Kenward and Roger Reference Kenward and Roger1997). Sidak’s adjustment was included for the variable comparisons to improve the confidence and power for reported differences (Sidak Reference Sidak1967).
Visible injury estimation results for each species were analyzed in a repeated-measures analysis using a generalized linear mixed model (PROC GLIMMIX) in SAS v. 9.4. Means separations were made at α=0.05 using the model VIE=[nozzle type×herbicide treatment×(rating 1, rating 2, rating 3, rating 4)]. As for the DWRs, experimental run was treated as a random effect. The repeated-measures analysis was performed, because the VIE ratings are meant to show the result over time of the plant to the nozzle-by-herbicide treatment combinations, which negates treating each rating of each plant as an individual data point.
Results and Discussion
Dry Weight Reductions
Over both runs for windmillgrass and rhodesgrass, droplet size (nozzle type), herbicide treatment, species, and herbicide treatment by species were significant (P=0.035, P<0.001, P< 0.001, and P< 0.001, respectively). Clodinafop was the most effective herbicide at reducing dry weights for both rhodesgrass and windmillgrass (Table 4). Metribuzin was the least effective across both runs for both species. Clodinafop, glyphosate, and metribuzin were not different by species, but paraquat, imazamox+imazapyr, and paraquat were. There is not a clear reason why imazamox plus imazapyr resulted in such reduced control for rhodesgrass compared with windmillgrass, as previous research found imazamox plus imazapyr at this application rate to result in the greatest DWRs across grass species (Ferguson et al. Reference Ferguson, Chechetto, Adkins, Hewitt, Chauhan, Kruger and O’Donnell2018).
Table 4 Dry weight reductions (DWRs) at 28 d after treatment and visible injury estimations (VIEs) across all four rating timings for both years by herbicide treatment.a
a Mean separations were made with Sidak’s adjustment at α=0.05. Different letters indicate statistical significance
b The DWRs and VIEs were run using two separate statistical models, indicated with italicized letters for the VIE results.
Across herbicide treatments, windmillgrass DWRs were greater than rhodesgrass DWRs (82% and 64%, respectively). The rhodesgrass plants treated with paraquat and the TTI nozzle were observed to regrow, as the paraquat did not kill the rhodesgrass growing points. Unlike rhodesgrass, windmillgrass did not regrow following paraquat treatments, which explains why paraquat efficacy was greater for windmillgrass than rhodesgrass (Table 4). There was no significant effect for droplet size (nozzle type) by herbicide treatment (P=0.145) or for droplet size (nozzle type) by species (P=0.4315), except for the results with the TTI and rhodesgrass, where paraquat DWRs were significantly lower than paraquat treatments applied with the other droplet sizes (nozzle types) (Table 4).
Percent Visible Estimates of Injury
For both windmillgrass and rhodesgrass, the herbicide treatment was significant (P<0.001). No differences based on droplet size (nozzle type) were observed across either species (P=0.074). For rhodesgrass, the clodinafop treatment had the greatest VIE (87% averaged across nozzle type) and metribuzin had the lowest VIE (12% averaged across nozzle type) (Table 4). The paraquat treatment had the greatest VIE for windmillgrass (91% across nozzle type) and metribuzin had the lowest VIE (9% across nozzle type).
As with previous research (Ferguson et al. Reference Ferguson, Chechetto, Adkins, Hewitt, Chauhan, Kruger and O’Donnell2018), the VIE, while useful in adding a layer of observation to explain dry weight data, may not always provide an accurate reflection of the activity of the herbicide. Glyphosate showed only 69% VIE with rhodesgrass, but had a DWR of 89% (Table 4). Similarly, glyphosate showed only 68% VIE with windmillgrass, but had a DWR of 93% (Table 4). Comparisons across species were similar; with every herbicide except paraquat, percent VIE was within 3% for both Chloris spp. The actual activity on Chloris spp. with paraquat may have been similar, but the regrowth from rhodesgrass may have obscured this result.
Results from the study confirmed the findings from Borger and Ferris (Reference Borger and Ferris2013),in which clodinafop provided greater than 75% VIEs of rhodesgrass. The results also confirmed the findings of Borger et al. (Reference Borger, Reithmuller and Hashem2009), in which greater than 89% control of windmillgrass based on plant survival estimates with both glyphosate and paraquat was observed. Research in the United States compared the control of a related cousin, tumble windmillgrass, with glyphosate and multiple acetyl-CoA carboxylase inhibitors (Group 1) herbicides (Hennigh et al. Reference Hennigh, Al-Khatib, Stahlman and Shoup2005). Results showed that glyphosate at a similar rate to the rate used in this study did not effectively control tumble windmillgrass. Quizalofop at 70 g ai ha−1 effectively controlled tumble windmillgrass (Hennigh et al. Reference Hennigh, Al-Khatib, Stahlman and Shoup2005) similarly to clodinafop in this study. The difference in response to glyphosate treatments may be a result of the fact that the species are different and/or response to herbicides is species specific, among other factors. For example, even with glyphosate rates within 10 g ai ha−1, the specific formulations are different, including surfactants and other nonactive ingredients in the formulation.
Droplet-Size (Nozzle Type) Impact
Droplet-size data for treatments used in this study are reported by Ferguson et al. (Reference Ferguson, Chechetto, Adkins, Hewitt, Chauhan, Kruger and O’Donnell2018), as the nozzles and treatments were the same for both studies (Table 3). The fine sprays from the XR11002 produced the smallest droplet sizes, and the extremely coarse sprays from the TTI11002 produced the largest droplets across treatments. The formulation type affected the droplet-size results, in which the soluble concentrate, metribuzin, resulted in the coarsest spray of any treatment with the TTI, but was also the coarsest spray for the TT11002 and the MD11002. Paraquat and glyphosate generally resulted in the finest sprays for each nozzle, with paraquat producing the finest spray of any treatment with the XR11002 and the finest spray for the medium TT11002 (Table 3). Glyphosate resulted in the finest spray for the TADF11002 and the AIXR11002, which is consistent with previous research in the literature (Mueller and Womac Reference Mueller and Womac1997).
Across droplet size (nozzle type), the MD nozzle resulted in the greatest DWRs (78%), and the TTI the lowest (67%) (Table 5). The other nozzles were not different from the MD or the TTI, and provided DWRs of 74% (TT), 74% (AIXR), 72% (XR), and 72% (TADF), respectively. Even with herbicide treatment expressing the strongest difference across treatments, the trend among DWRs and herbicide treatment by droplet size (nozzle type) were consistent for both species, except for the paraquat treatment with the TTI in rhodesgrass (Table 4).
Table 5 Dry weight reductions at 28 d after treatment by droplet size (nozzle type), grouped across both Chloris spp.a
a Mean separations were made with Sidak’s adjustment at α=0.05.
b Different letters indicate statistical significance.
Even with a range of ~550 µm between the volume median diameters of the coarsest spray to the finest spray in this study, droplet size (nozzle type) did not affect the efficacy of multiple herbicide modes of action on Chloris spp. Clodinafop resulted in the greatest control over both trials for both species, and metribuzin resulted in the least control. For both species across multiple herbicides, fine sprays from the XR11002 were no less efficacious than the extremely coarse sprays from the TTI 11002, except for paraquat with the TTI on rhodesgrass. The results also showed the same trend of comparable control from a fine spray (XR 11002) and several coarse sprays (AIXR, TADF, MD 11002). At least for controlling Chloris spp. in winter crops, clodinafop appears to be a useful in-crop herbicide, and if the winter crop is imidazolinone tolerant (Clearfield® variety), then imazamox plus imazapyr would be an additional option for control in season. Using amitrole, paraquat, or glyphosate in a postharvest situation would be another useful treatment for preventing Chloris spp. seedbanks from building up. Based on results from this study, we accept the hypothesis that droplet size (nozzle type) does not affect efficacy across herbicide mode of action for the control of both Chloris spp.
Acknowledgments
The authors acknowledge the Grains Research and Development Corporation of Australia (GRDC) for its support of this work through the project titled “Options for Improved Insecticide and Fungicide Use and Canopy Penetration in Cereals and Canola.” The authors also thank Peter Tame from AusWest Seeds for providing seed for the study; Frank Taylor from NuFarm Pty Ltd, Pat English from Bayer CropSciences Pty Ltd, and Rob Battaglia from Syngenta Pty Ltd for supplying herbicides for the study. No conflicts of interest have been declared.
