2008.

Two field experiments were conducted in a citrus grove at Polk City, FL, in 2008 to evaluate the efficacy of saflufenacil applied alone or in a tank mix with glyphosate and pendimethalin for weed control. The soil of the experimental site was Florida Candler fine sand (hyperthermic, uncoated, Typic Quartzipsamment). The experiments were laid-out in a randomized complete block design with four replications. The details of herbicide treatments applied in Expts. 1 and 2 are listed in Tables 1 and 2, respectively. All herbicide treatments were applied using a tractor-mounted computerized boom sprayer fitted with 8002 Teejet nozzles (Spraying System Co., Wheaton, IL) and an off center OC-4 flat spray tip, delivering 20 gal/acre spray volume at 30 psi. The plot size was 3 × 18 m, covering five citrus trees per plot. The trees were 3 years old when herbicides were applied. The herbicide treatments in both the experiments were applied on 25 June 2008 when weeds were about 10 to 15 cm tall. All herbicide treatments included ammonium sulfate applied at 2.03 kg/100 L solution and crop oil concentrate at 1% v/v. Visual percent control of grass and broadleaf weeds was recorded at 7, 15, 30, 60, and 90 DAT on a scale 0% to 100%, where 0% being no control and 100% being complete control of all weeds at the time of observation compared with untreated control. Phytotoxicity of herbicide treatments on citrus trees was also evaluated based on a 0% to 100% scale, where 0% being no injury and 100% being complete death of the citrus tree.

Table 1.Effects of herbicide treatments on control of grass and broadleaf weeds in Expt. 1 conducted at Polk City, FL, in 2008.

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Table 2.Effects of herbicide treatments on control of grass and broadleaf weeds in Expt. 2 conducted at Polk City, FL, in 2008.

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Two field experiments were conducted at Haines City, FL, and Dundee, FL, in 2008 to evaluate phytotoxicity of saflufenacil applied alone or in a tank mix with glyphosate in young citrus trees. The treatments included glyphosate (Roundup Weathermax™; Monsanto, St. Louis, MO) applied alone at 2.25 lb/acre, saflufenacil applied alone at 2 or 4 oz/acre or in a sequential application after 2 months, and saflufenacil at 2 oz/acre tank mixed with glyphosate at 2 lb/acre. The experimental design was a randomized complete block with four replications. The herbicide treatments were manually applied with a knap sack sprayer at 20 gal/acre spray volume at 30 psi. The plot size was 3 × 15 m, covering four trees per plot. The citrus trees were 2 years old when herbicides were sprayed. Treatments were applied at different time intervals on 29 Apr. 2008, 30 June 2008, and 29 Aug. 2008. All treatments included ammonium sulfate applied at 2.03 kg/100 L solution and crop oil concentrate at 1% v/v. Citrus injury was visually estimated at 7, 15, and 30 DAT using the procedure described in the previous experiment.

2009.

A field experiment was conducted in a grapefruit grove at Frostproof, FL, to evaluate efficacy of saflufenacil applied alone or in a tank mix for controlling weeds. Soil type was same as explained in 2008 study. The treatment detail is presented in Tables 3 and 4. The herbicide applications were made on 1 May 2009. The adjuvants methylated seed oil (1% v/v) and ammonium sulfate (2.03 kg/100 L) was added to all herbicide treatments. The grapefruit trees were 3 years old at the time of herbicide application. The applications were made using a tractor mounted sprayer as explained in previous experiment. Visual percent control of weed species and potential injury on citrus was evaluated at 15, 30, 45, 60, and 90 DAT as per the procedure explained in previous experiments.

Table 3.Effects of herbicide treatments on control of texas panicum at 15, 30, 45, 60, and 90 d after treatment (DAT) at Frostproof, FL, in 2009.

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Table 4.Effects of herbicide treatments on control of florida/brazil pusley, spanishneedles, and cutleaf evening primrose at 15, 30, and 90 d after treatment (DAT) at Frostproof, FL, in 2009.

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A field experiment was conducted in a citrus grove at the Citrus Research and Education Center, University of Florida, Lake Alfred. The citrus crop was ‘Valencia’ sweet orange. The treatment detail is presented in Tables 5 and 6. Herbicide applications were made on 28 Apr. 2009 as explained in previous study. The nonionic surfactant (Induce at 0.25% v/v) and ammonium sulfate at 2.03 kg/100 L solution were included in herbicide treatments. The sweet orange trees were 3 years old at the time of herbicide application. Herbicide applications were made using a tractor mounted sprayer as explained in previous experiment. The visual percent control of weed species and potential injury on citrus was evaluated at 14, 28, 42, and 86 DAT on the basis of the procedure explained in previous experiments.

Table 5.Effects of herbicide treatments on control of florida/brazil pusley, common purslane, and dogfennel at 14, 42, and 86 d after treatment (DAT) at Lake Alfred, FL, in 2009.

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Table 6.Effects of herbicide treatments on control of spiny amaranth, spanishneedles, and common ragweed at 14, 42, and 86 d after treatment (DAT) at Lake Alfred, FL, in 2009.

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Statistical analysis.

The data from each field experiment was analyzed separately. All data were subjected to analysis of variance (ANOVA), and means were separated using Fisher's protected least significant difference (LSD) test (P = 0.05) in SAS (version 9.2; SAS Institute, Cary, NC). When the ANOVA revealed no significant treatment-by-location interaction for the Hanes and Dundee experimental sites in 2008, data were pooled over locations for analysis. The data of percent weed control were arcsine transformed before analysis; however, nontransformed percentages are presented with mean separation based on transformed data.

2008.

The predominant grass species at the Polk City, FL, in Expt. 1 included texas panicum (Panicum texanum) and guineagrass (Panicum maximum). The visual control ratings were combined for all the grass species and presented as total grass species controlled. Compared with untreated control, all the treatments were significantly effective for controlling grass weeds at 7, 15, 30, 60, and 90 DAT (Table 1). The application of glyphosate alone provided significantly higher grass weed control compared with saflufenacil applied alone at 15, 60, and 90 DAT. Later in the season, the tank mix treatments were superior compared with glyphosate or saflufenacil applied alone for controlling grass weeds. Among tank mixes, percent control of grass weeds declined after 30 DAT where pendimethalin was not included in the tank mix. For example, saflufenacil applied alone or in a combination with glyphosate was less effective than a tank mix of saflufenacil, glyphosate, and pendimethalin at 60 and 90 DAT (Table 1). Pendimethalin is used as a preemergence herbicide for control of grass and small-seeded dicot weed species; however, it can be tank mixed with other herbicide(s) to improve efficacy (BASF Corp., 2009). In the treatments that included pendimethalin as a tank mix partner, control of grass weeds was comparable with a tank mix of glyphosate, norflurazon, and diuron at 15, 30, 60, and 90 DAT (Table 1). Similar to this result, a tank mix of pendimethalin and glyphosate provided 95% control of witchgrass (Panicum capillare) and yellow foxtail (Setaria pumila) compared with glyphosate applied alone at 45 DAT in glyphosate-resistant alfalfa (Medicago sativa) (Affeldt and Rice, 2008).

The major broadleaf weeds infesting the Expt. 1 at Polk City, FL, were florida/brazil pusley (Richardia spp.), dayflower, and black nightshade (Solanum nigrum). The visual control ratings were combined for all the broadleaf species and presented as total broadleaf species controlled. Compared with untreated control, all the treatments were significantly effective for controlling broadleaf weeds at 15, 30, 60, and 90 DAT (Table 1). Although the difference was nonsignificant with some other treatments, maximum control of broadleaf weeds (82%) was obtained with grower's practice of using a tank mix of norflurazon, diuron, and glyphosate at 7 DAT. On the other hand, a tank mix of saflufenacil, glyphosate, and pendimethalin was as effective as a tank mix of norflurazon, diuron, and glyphosate at 30, 60, and 90 DAT (Table 1). In an experiment conducted for weed control in glyphosate-resistant alfalfa, a tank mix of pendimethalin and glyphosate provided 100% control of horseweed compared with a tank mix of pendimethalin with imazamox (25% control) or 2,4-DB (45% control) at 45 DAT (Affeldt and Rice, 2008). Glyphosate tank mixed with pendimethalin provided long-term control of grass weeds; however, addition of saflufenacil provided better broadleaf weed control. The tank mix of pendimethalin at all the rates was equally effective.

Grass and broadleaf weed species present in Expt. 2 at Polk City, FL, were the same as in Expt. 1 in 2008. All treatments were effective for reducing grass weed populations compared with untreated control (Table 2). A tank mix of saflufenacil with glyphosate and pendimethalin provided 82% control of grass weeds at 7 DAT; however, it was not significantly different from other tank mix treatments. Percent control of grass weeds increased gradually with all herbicide treatments at 30 DAT, and thereafter control was reduced slightly with the treatments where pendimethalin was not added in the tank mix. The application of glyphosate alone provided significantly higher percent control of grass weeds compared with saflufenacil applied alone at 30, 60, and 90 DAT (Table 2). This might be due to the mode of action of saflufenacil as it is primarily a dicot herbicide; therefore, it is not much effective on monocot species (BASF Corp., 2010c). For example, experiments conducted to evaluate winter wheat (Triticum aestivum) response to postemergence application of saflufenacil suggested that wheat foliar necrosis was <15% at 10 to 20 DAT and was not evident at 30 DAT (Frihauf et al., 2010). A tank mix of saflufenacil and glyphosate provided similar control of grass weeds compared with a tank mix of saflufenacil, glyphosate, and pendimethalin at 30 DAT. However, a tank mix of saflufenacil with glyphosate provided inferior grass control compared with a tank mix of saflufenacil, glyphosate, and pendimethalin at 60 and 90 DAT (Table 2). Therefore, addition of pendimethalin was very effective to provide residual grass weed control.

All herbicide treatments were effective for controlling broadleaf weeds in all the visual ratings compared with untreated control (Table 2). Percent control of broadleaf weeds was similar when glyphosate or saflufenacil was applied alone. A tank mix of saflufenacil with glyphosate provided significantly better control of broadleaf weeds compared with saflufenacil applied alone at 15, 30, 60, and 90 DAT. At 15 and 30 DAT, a tank mix of saflufenacil, glyphosate, and pendimethalin and a tank mix of glyphosate and pendimethalin provided similar control of broadleaf weeds compared with a tank mix of saflufenacil and glyphosate. Control of broadleaf weeds reduced gradually after 30 DAT in the treatments where pendimethalin was not included in a tank mix. For example, at 60 and 90 DAT, significantly higher control of broadleaf weeds was obtained in the treatments where pendimethalin was tank mixed with glyphosate or with saflufenacil and glyphosate compared with the tank mix of saflufenacil plus glyphosate (Table 2). This might be because pendimethalin is primarily a residual grass herbicide; however, excellent broadleaf weed control with addition of pendimethalin in the tank mix indicates synergistic effect of other two herbicides on the efficacy of pendimethalin. A previous study suggested that saflufenacil with crop oil concentrate and saflufenacil tank mixed with 2,4-D amine controlled blue mustard (Chorispora tenella) ≥91% at 17 and 21 DAT compared with less than 50% control with 2,4-D amine applied alone (Frihauf et al., 2010).

There was no treatment by location interaction for the Hanes City, FL, and Dundee, FL, experimental sites to evaluate the injury of saflufenacil in citrus; therefore data were pooled over locations for analysis. There was no injury from a single or sequential application of saflufenacil applied at 2 or 4 oz/acre on citrus trees at 7, 15, and 30 DAT (data not shown). The label rate of saflufenacil in citrus is 1 oz/acre, so citrus trees have provided 2- to 4-fold safety. This study demonstrated that saflufenacil with adjuvants used in this study can be a good option for broadleaf weed control in citrus groves without any injury on citrus trees (data not shown). By contrast, experiments conducted to determine the response of maize to postemergence application of saflufenacil suggested that addition of an adjuvant to saflufenacil caused 99% injury at three leaf stage and reduced yield up to 59% compared with saflufenacil applied without adjuvant (Soltani et al., 2009).

Overall results of the experiments conducted in 2008 suggested that application of saflufenacil alone was not effective for controlling grass weeds, but provided similar broadleaf weed control compared with glyphosate applied alone; however, a tank mix of saflufenacil with glyphosate usually increased weed control efficacy. Addition of pendimethalin into the tank mix of saflufenacil and glyphosate improved weed control by providing a longer residual activity similar to a tank mix of norflurazon, diuron, and glyphosate. There was no difference in control of grass and broadleaf weeds with the high rate of pendimethalin as a tank mix partner. Therefore, a lower rate of pendimethalin should be used when applied in a tank mix with saflufenacil and glyphosate to achieve sufficient weed control. There was no injury from any saflufenacil treatment applied alone even at higher rates up to 4 oz/acre or in a tank mix with glyphosate or in a sequential application on citrus trees indicating its level of safety (data not shown).

2009.

The only grass weed present at the Frostproof, FL, experimental site was texas panicum. Visual weed control evaluations conducted at different time intervals suggested that all herbicide treatments were effective for controlling texas panicum compared with untreated control (Table 4). A tank mix of saflufenacil with glyphosate provided a greater control of texas panicum compared with saflufenacil applied alone. For example, control at 45, 60, and 90 DAT was 7%, 3%, and 0%, respectively, whereas a tank mix of glyphosate with saflufenacil provided 75%, 77%, and 28% control, respectively (Table 3). The residual weed control of saflufenacil and glyphosate treatments started declining beyond 45 DAT. Later in the season (>30 DAT), a tank mix of saflufenacil, glyphosate, and pendimethalin provided significantly better control of texas panicum compared with other treatments. Control of texas panicum was >82% at 90 DAT in a tank mix of saflufenacil, glyphosate, and pendimethalin indicated the effectiveness of pendimethalin as a tank mix partner (Table 3). Thus, this tank mix combination can provide a long-term weed control option to citrus growers.

The major broadleaf weeds at the Frostproof, FL, experimental site were florida/brazil pusley, spanishneedles (Bidens bipinnata), and cutleaf evening primrose (Oenothera laciniata). All herbicide treatments were effective for controlling broadleaf weeds compared with untreated control (Table 4). The application of saflufenacil alone at 1 oz/acre provided similar control of florida/brazil pusley compared with a tank mix of saflufenacil and glyphosate at 15 and 30 DAT (Table 4). Knezevic et al. (2009) reported that saflufenacil applied at 1 to 2 oz/acre was sufficient to control broadleaf weeds including field bindweed (Convolvulus arvensis), prickly lettuce (Lactuca serriola), henbit (Lamium amplexicaule), shepherd's-purse, dandelion (Taraxacum officinale), and field pennycress (Thlaspi arvense). Control of spanishneedles and cutleaf evening primrose was significantly better in a tank mix treatments at 15 and 30 DAT compared with saflufenacil applied alone (Table 4). A tank mix of pendimethalin, saflufenacil, and glyphosate provided 81% to 99% control of all three broadleaf weeds at 15 DAT. The tank mix treatment that included pendimethalin controlled 76% to 95% broadleaf weeds at 30 DAT. Control of florida/brazil pusley and spanishneedles started declining at 45 DAT (data not shown) and was minimal at 90 DAT when glyphosate or saflufenacil applied alone or in a tank mix (Table 4). A tank mix of saflufenacil, glyphosate, and pendimethalin provided long-term residual weed control. For example, control of florida/brazil pusley was >72% at 90 DAT when pendimethalin was a tank mix partner. Although there was no significant difference with other tank mix treatments, control of cutleaf evening primrose was >80% at 90 DAT with pendimethalin as a tank mix partner (Table 4). There was no injury on grapefruit from any of the treatments.

The only grass weed infesting the experimental site at Lake Alfred, FL, in 2009 was guineagrass. Saflufenacil applied alone provided only 5% to 40% control of guineagrass (data not shown). A tank mix of saflufenacil with glyphosate provided 88% and 84% control of guineagrass, respectively, at 14 and 28 DAT, whereas a tank mixes of saflufenacil with pendimethalin provided relatively less control of guineagrass at 14 and 28 DAT. Similarly, glufosinate and paraquat applied alone provided 70% and 65% control of guineagrass, respectively, at 14 DAT; however, the control was reduced to 58% at 86 DAT (data not shown). Glyphosate applied alone at higher rate provided 90% guineagrass control at 14 DAT compared with 60% control at 86 DAT (data not shown). Similarly, a tank mix of trifloxysulfuron with glyphosate provided 74% to 90% control of guineagrass compared with 48% to 66% control by trifloxysulfuron alone. A tank mix of glyphosate with simazine provided 60% to 90% control of guineagrass, and there was no additional control provided by addition of mesotrione in this treatment combination. When glyphosate was tank mixed with norflurazone, a consistent control of guineagrass (≥80%) was observed throughout the growing season (data not shown). The residual control of saflufenacil and other herbicides applied alone or in a tank mix with glyphosate usually reduced efficacy beyond 56 DAT. Similarly, guineagrass control with postemergence herbicides such as glufosinate and paraquat started declining beyond 56 DAT (data not shown).

The major broadleaf weeds infesting the experimental site at Lake Alfred, FL, included florida/brazil pusley, spanishneedles, common purslane (Portulaca oleracea), dogfennel (Eupatorium capillifolium), spiny amaranth (Amaranthus spinosus), and common ragweed (Ambrosia artemisiifolia). Results showed that all herbicide treatments provided significant control of broadleaf weeds compared with untreated control (Tables 5 and 6). A tank mix of saflufenacil with pendimethalin provided 59%, 93%, and 65% control of florida/brazil pusley, purslane, and dogfennel, respectively, at 14 DAT. Similarly, saflufenacil alone and saflufenacil plus glyphosate provided similar control of these broadleaf weed species at 28 DAT (data not shown). A previous study reported that saflufenacil applied at 30 g·ha⁻¹ (0.43 oz/acre) reduced the density of blue mustard, flixweed (Descurainia sophia), palmer amaranth (Amaranthus palmeri), and tumble pigweed (Amaranthus albus) by 63% to 93% (Geier et al., 2009). A tank mix of pendimethalin and saflufenacil had 68% to 86% control of broadleaf weeds at 28 DAT. The reduced weed control in this treatment combination might be due to the weeds already present at the experimental site before herbicide applications. Similarly, broadleaf weed control with glufosinate and paraquat varied from 65% to 99% at 14 and 28 DAT (Table 5). The broadleaf weed control by glufosinate and paraquat applied alone was comparable to glyphosate and saflufenacil treatments. Similarly, trifloxysulfuron applied alone or tank mixed with glyphosate provided 43% to 65% control of florida/brazil pusley at 14 DAT; however, florida/brazil pusley control was improved at 28 DAT (60% to 73%) (data not shown) with trifloxysulfuron alone or in a tank mix with glyphosate (Table 5). The percent control of florida/brazil pusley and dogfennel was 71% and 81%, respectively, with trifloxysulfuron applied alone or in a tank mix with glyphosate at 86 DAT. A study conducted to evaluate the effects of trifloxysulfuron on 12 weed species of citrus demonstrated various responses of weeds to this herbicide (Singh and Singh, 2004). The differential control by trifloxysulfuron-sodium could be attributed to differences in uptake, translocation, and metabolism in different species (Askew and Wilcut, 2002).

All herbicide treatments provided good control of spiny amaranth ranging from 79% to 95% compared with the untreated control (Table 6). Similarly, all the herbicide treatments provided >79% control of spanishneedles up to 42 DAT. However, control of spanishneedles treated with saflufenacil alone declined to 69% at 86 DAT compared with other treatments (Table 6). Surprisingly, control of common ragweed was relatively less in a tank mix of saflufenacil with pendimethalin at 14 DAT; however, later in the season, control of common ragweed improved in this treatment and provided >76% control at 42 and 86 DAT (Table 6). The efficacy of saflufenacil applied alone declined by 86 DAT, and common ragweed control declined to 38% (Table 6). Other herbicide treatments provided 65% to 85% common ragweed control at 86 DAT. The highest control of broadleaf species was obtained in a tank mix of saflufenacil with glyphosate and pendimethalin, and it was usually comparable with a tank mix of glyphosate plus simazine, glyphosate plus simazine plus mesotrione, and glyphosate plus norflurazon (Table 6). Weed species that are not effectively controlled by a single herbicide have shown increased occurrence in citrus groves and can be better controlled with a tank mix of herbicides. For example, a tank mix of glyphosate plus carfentrazone provided better control of florida/brazil pusley and dayflower compared with applied alone (Sharma et al., 2008).

With the exception of trifloxysulfuron-sodium, there was no injury of any herbicide treatment on ‘Valencia’ sweet orange trees (data not shown). The application of trifloxysulfuron-sodium at higher rate caused some initial injury to citrus trees (5% to 9%) (data not shown). However, those trees recovered later in the season, and no injury symptom was noticed at 86 DAT. Trifloxysulfuron-sodium is an acetolactate synthase inhibitor and belongs to the sulfonylurea group, which has been evaluated for weed control in cotton (Porterfield et al., 2003) and sugarcane (Saccharum officinarum) (Dally and Richard, 2008). Similar to this result, previous studies reported that cotton recovered from trifloxysulfuron injury within 3 to 6 weeks after treatment and caused no yield penalty (Barber et al., 2002; Porterfield et al., 2003).

Overall results of experiments conducted in 2009 suggested that a tank mix of saflufenacil, glyphosate, and pendimethalin provided similar weed control compared with several other tank mix herbicides including a tank mix of glyphosate and simazine, glyphosate plus simazine plus mesotrione, and glyphosate with norflurazon. Therefore, a tank mix herbicides with different mode of action will control problem weed species including herbicide-resistant weeds. Saflufenacil has been reported to be effective in controlling glyphosate-resistant weeds (Ashigh and Hall, 2010) such as glyphosate-resistant horseweed (Owen et al., 2011). Preliminary experiments conducted in California confirmed that saflufenacil is a strong performer on several winter annual broadleaf weeds, including glyphosate-resistant horseweed and hairy fleabane (Conyza bonariensis) in perennial crops (B. Hanson, personal communication).