Introduction
Florpyrauxifen-benzyl is a POST herbicide for control of broadleaf, grass, and sedge weeds in rice (Perry et al. Reference Perry, Ellis, Langston, Lassiter, Thompson, Viator, Walton and Weimer2015; Teló et al. Reference Teló, Webster, Blouin, McKnight and Rustom2018). Florpyrauxifen herbicide is a synthetic auxin (Group 4) in the same chemical family as clopyralid, fluroxypyr, triclopyr, and quinclorac. Florpyrauxifen is applied at a low rate of 30 g ai ha−1. Rice has excellent tolerance to florpyrauxifen, and the herbicide has little to no residual activity (Miller et al. Reference Miller, Norsworthy, Weimer, Huang, Lancaster and Martin2016, Miller and Norsworthy Reference Miller and Norsworthy2018; Teló et al. Reference Teló, Webster, Blouin, McKnight and Rustom2018). Florpyrauxifen is in a new structural class of synthetic auxins in the arylpicolinate family (Weimer et al. Reference Weimer, Yerkes, Schmitzer and Mann2015) and will provide an alternative mode of action for use in rice. Employing alternative modes of action in an overall weed management program can aid weed resistance management (Norsworthy et al. Reference Norsworthy, Ward, Shaw, Llewellyn, Nichols, Webster, Bradley, Frisvold, Powles, Burgos, Witt and Barrett2012).
Many broadleaf, grass, and sedge weeds can be found in rice production across the United States (Zhang et al. Reference Zhang, Webster, Lanclos and Geaghan2003). Yield losses in rice can exceed 90% due to weed competition (Smith Reference Smith1968). Interspecific competition occurs between weeds and rice for sunlight, nutrients, and water resources; therefore, effective weed control is essential for successful rice production (Smith Reference Smith1988).
In Louisiana, rice is an important crop, with approximately 161,870 ha planted in 2017 (USDA-NASS 2018). Grass weeds are common in Louisiana rice production, and many can be difficult to control, especially in southern Louisiana, where perennial grasses such as brook crowngrass, rice cutgrass, southern watergrass, and water paspalum can be troublesome weeds in drill- and water-seeded rice (Webster Reference Webster and Saichuk2014).
Brook crowngrass is identified by many branched glabrous nodes, with decumbent culms at the base, and is extensively creeping, reaching 30 to 100 cm in length (Hitchcock Reference Hitchcock1950). This grass has a membranous ligule, with lanceolate leaf blades 4 to 12 cm in length. Inflorescence is composed of 3 to 5 erect racemes that are 3.5- to 7-cm long (Shaw Reference Shaw2012). Brook crowngrass reproduction occurs by short rhizomes and/or seed (Kartesz Reference Kartesz1994). This weed grows in poorly drained soils, including the edges of lakes, ponds, rice fields, and wet roadside ditches (Shaw Reference Shaw2012). Brook crowngrass is native to the Americas and can be found in the southern United States; it is well distributed across much of southern Louisiana (Shaw Reference Shaw2012; Webster Reference Webster and Saichuk2014).
Rice cutgrass has slender culms, often decumbent at the base. It can reach 1 to 1.5 m in height and is primarily spread by rhizomes that often form dense colonies (USDA-NRCS 2008). Alternate leaves occur along the entire length of each culm. Leaf blades can be 7- to 30-cm long and 8- to 10-mm wide and have a 0.5- to 1.0-mm-long membranous ligule (Hitchcock Reference Hitchcock1950). Rice cutgrass panicles are normally 10- to 20-cm long. Rice cutgrass reproduction can occur by rhizomes and/or seeds (USDA-NRCS 2008). This grass is found mostly in saturated soils along streams, rivers, ponds, lakes, and swales; however, mature plants can tolerate seasonal to permanent flooding up to 1 m in depth. Rice cutgrass can be found in most of the southern and central United States (USDA-NRCS 2008) and is distributed across all of Louisiana (Webster Reference Webster and Saichuk2014).
Southern watergrass is an aquatic and stoloniferous grass, with prostrate culms 30 to 100 cm in length (Barworth et al. Reference Barworth, Capels and Long2007). The plant can form colonies in shallow water with leaves that can survive underwater or floating on the surface. Leaf blades are 1- to 5-cm long and 1- to 4-mm wide, with ligules of 0.5 to 2 mm. Inflorescence is composed of racemes comprising 1 to 3 infertile spikelets. Southern watergrass can be found from North Carolina to Florida and Texas (Gould Reference Gould1975). In Louisiana, southern watergrass is found growing in rice fields and flooded pastures. Rice cutgrass can propagate vegetatively and can spread as floating mats under flooded field conditions (Braverman Reference Braverman1996).
Water paspalum is frequently found submersed or floating and growing in the mud or shallow water in ponds, streams, and rice fields (Shaw Reference Shaw2012). Water paspalum is found in the southern United States and is well distributed across much of southern Louisiana (Shaw Reference Shaw2012; Webster Reference Webster and Saichuk2014). This weed is an aquatic or subaquatic grass with hollow culms 1 to 2.5 m in length and rooted at the nodes (Hitchcock Reference Hitchcock1950). In water, large numbers of roots form at the nodes (Shaw Reference Shaw2012). Leaf blades are 7 to 15 cm in length, with flat sheaths and a prominent white leaf midvein (Hitchcock Reference Hitchcock1950). The plant has a membranous ligule 1.0 to 2.5 mm in length (Shaw Reference Shaw2012). At maturity, water paspalum produces 2 to 3 racemes that can be 5- to 10-cm long (Hitchcock Reference Hitchcock1950). Water paspalum reproduces by seed or vegetatively through stolons that regrow during the late spring or early summer months.
All four of these perennial grasses can be found in rice fields across southern Louisiana. They are well adapted to saturated and flooded soils and have become a problem for Louisiana rice producers. The adoption of reduced tillage by rice growers, coupled with a rice/crawfish (Procambarus clarkia Girard) rotation, has enabled these perennial grasses to take advantage of these cultural practices and flourish (Webster Reference Webster and Saichuk2014).
The objective of this research was to evaluate the activity of florpyrauxifen-benzyl herbicide on brook crowngrass, rice cutgrass, southern watergrass, and water paspalum in glasshouse trials.
Materials and methods
Four separate glasshouse studies were established in November 2017 and repeated in February 2018 on the Louisiana State University campus in Baton Rouge, LA. The experimental design was a two-factor factorial in a completely randomized design with five replications. Factor A consisted of florpyrauxifen (Loyant™ with Rinskor™ active, Corteva Agriscience, Indianapolis, IN) applied at 0 or 30 g ai ha−1, and factor B was the application timings at two growth stages, 3- to 4-leaf and 1- to 2-stolon. Four perennial grass weeds common in southern Louisiana rice production were evaluated in separate studies: brook crowngrass, rice cutgrass, southern watergrass, and water paspalum (Table 1).
Table 1. Grass weeds collected near rice fields in southern Louisiana, with locations listed by parish in Louisiana and GPS coordinates given for where they were collected.
Mature brook crowngrass, rice cutgrass, southern watergrass, and water paspalum plants were collected from various grower locations in southern Louisiana (Table 1). The plants were transplanted 1-cm deep into commercial potting soil (Potting Mix, Miracle-Gro, Marysville, OH) in plastic 20 by 51 by 82 cm containers to allow plants to acclimate to the glasshouse. At 25 d after planting, a 5-cm stem segment with a viable node in the center of the cutting was planted into 6.9 by 17.8 cm Ray Leach Cone-tainers™ (Stuewe & Sons, 31933 Rolland Drive, Tangent, OR) containing the same commercial potting soil. Cones containing the grasses were placed into racks suspended above a 67-L water reservoir to allow for subsurface irrigation. The water was held during the duration of the study to simulate saturated rice field conditions. Urea fertilizer (46-0-0) was added to the water at a rate of 280 kg ha−1 based on surface area.
Florpyrauxifen was applied with a CO2-pressurized backpack sprayer calibrated at 145 kPa to deliver 140 L ha−1 of solution. The spray boom consisted of four flat-fan 110015 nozzles (Flat Fan AirMix® Venturi Nozzle, Greenleaf Technologies, Covington, LA) with 38-cm spacings. A methylated seed oil adjuvant at 1% v/v (Super Soy-surf Extra, Sanders, Cleveland, MS) was added to the florpyrauxifen spray mixture. Before application, plants were removed from the glasshouse and placed outside for 1 h before and after herbicide application to allow the plants to acclimate to the outside environment and to allow the spray to thoroughly dry after application. Application temperatures on November 2017 and February 2018 were 26 and 25 C at the 3- to 4-leaf stage, respectively, and 24 and 24 C at the 1- to 2-stolon stage, respectively. Following treatment, the cones were placed back into the water-holding containers. Glasshouse average temperature in November was 27 ± 4 C during the day and 20 ± 3 C during the night. In February, the glasshouse average temperature during the day was 25 ± 5 C and 20 ± 4 C during the night, with 60 ± 10% relative humidity for both runs. Day length was extended to 14 h with metal-halide lamps at a minimum intensity of 270 μmol m−2 s−1 photosynthetic photon flux.
Weed control was evaluated at 5, 10, 15, and 21 d after treatment (DAT). Visible control was evaluated on a scale of 0% (no injury or control) to 100% (complete plant death). Leaf number, stolon number, and plant height were evaluated at 0, 5, 10, 15, and 21 DAT. The height of each individual plant was measured from the base of the plant to the tip of the extended tallest leaf.
Immediately following the last evaluation at 21 DAT, plant fresh weight was obtained. The plants were carefully removed from the soil and thoroughly rinsed with fresh water. After being rinsed, the aboveground plant material was separated from the belowground portion and blotted dry. The fresh weight of each species was obtained using a precision balance (PM 460 Delta Range [0.001 g], Metter-Toledo, Im Langacher 44, Greifensee, Switzerland).
Data were analyzed using MIXED procedure (SAS 2013). Runs, replications (nested within treatments), and all interactions containing either of these effects were considered random effects. The days after treatment (DAT) were considered fixed effects. All evaluations were analyzed as repeated measures. Considering run or combination of run as random effects permits inferences about treatments over a range of environments (Carmer et al. Reference Carmer, Nyuist and Walker1989; Hager et al. Reference Hager, Wax, Bollero and Stroller2003). Type III statistics were used to test all possible effects of the fixed factors of application timing by florpyrauxifen rate by evaluation date, and Tukey’s test was used for mean separation at the 5% probability level (P ≤ 0.05). Normality of data was checked with the use of the UNIVARIATE procedure of SAS, and assumptions of normality were met (SAS Institute 2013).
Results and discussion
Brook crowngrass
An application timing by evaluation date interaction occurred for brook crowngrass control; therefore, data were pooled over florpyrauxifen rate (Table 2A). Brook crowngrass control did not exceed 71%, regardless of application timings at 3- to 4-leaf or 1- to 2-stolon stages. Control of brook crowngrass did not reach its greatest level until 15 to 21 DAT, indicating slow action of this herbicide on brook crowngrass, and this is similar to florpyrauxifen activity on fall panicum (Panicum dichotomiflorum Michx.) and Nealley’s sprangletop (Leptochloa nealleyi Vasey) (Teló et al. Reference Teló, Webster, Blouin, McKnight and Rustom2018).
Table 2. Visible control, leaf number, stolon number, plant height, and plant fresh weight of brook crowngrass treated with florpyrauxifen-benzyl.a, b
a Means followed by the same letter within and across columns and within each parameter do not significantly differ at P = 0.05 using Tukey’s test.
b Glasshouse study conducted in November 2017 and February 2018 at Louisiana State University campus in Baton Rouge, LA 70803.
An application timing by florpyrauxifen rate by evaluation date interaction occurred for brook crowngrass leaf number (Table 2B). Brook crowngrass treated with florpyrauxifen at 30 g ha−1 ceased leaf production immediately across both application timings. Brook crowngrass treated at the 3- to 4-leaf stage had 3.3 and 3.2 leaves at 15 and 21 DAT, and this was similar to the leaf number at the day of application. Brook crowngrass treated at the 1- to 2-stolon timing had 11.9 leaves per plant; however, at 21 DAT, the brook crowngrass treated with 30 g ha−1 had 12.6 leaves per plant compared with 51.5 leaves for brook crowngrass not treated with florpyrauxifen.
A florpyrauxifen rate by evaluation date interaction occurred for stolon number (Table 2C) and plant height (Table 2D) for brook crowngrass; therefore, data were pooled over application timing. Stolon production ceased when brook crowngrass was treated with florpyrauxifen up to 10 DAT; however, stolon regrowth occurred between 15 and 21 DAT (Table 2C). At 21 DAT, a stolon number reduction was observed across all evaluation dates when comparing brook crowngrass treated with florpyrauxifen at 30 g ha−1 with brook crowngrass not treated with florpyrauxifen (Table 2C). Brook crowngrass ceased growth, in regard to plant height, following a florpyrauxifen application, regardless of evaluation date (Table 2D). Consequently, brook crowngrass treated with florpyrauxifen resulted in a plant height of 14.9 cm compared with 40.3 cm for plants not treated. These data indicate that a reduction in plant height may translate into a less competitive plant later in the season (Bergeron Reference Bergeron2017; Diarra et al. Reference Diarra, Smith and Talbert1985).
A florpyrauxifen rate by application timing interaction occurred for brook crowngrass fresh weight (Table 2E). A reduction in above- and belowground fresh weight for florpyrauxifen-treated brook crowngrass was recorded across application timings compared with the plants receiving 0 g ha−1 of florpyrauxifen. Total fresh weight of florpyrauxifen-treated brook crowngrass was 1.5 and 2.1 g for the 3- to 4-leaf and the 1- to 2-stolon treated plants, respectively, and treated plants weighed less than plants receiving no florpyrauxifen. Similar results were reported with florpyrauxifen-treated Nealley’s sprangletop (Teló et al. Reference Teló, Webster, Blouin, McKnight and Rustom2018). Overall, these results indicate that florpyrauxifen can aid in the management of brook crowngrass in rice production.
Southern watergrass
An application timing by evaluation date interaction occurred for southern watergrass control; therefore, data were pooled over florpyrauxifen rate (Table 3A). Control of southern watergrass increased at 5 to 15 DAT; however, at 21 DAT, control increased to 56% for southern watergrass treated at the 3- to 4-leaf timing. This level of control did not differ from control at 15 DAT for the earlier timing; however, control increased when southern watergrass was treated at the earlier growth stage compared with control observed with application of florpyrauxifen at the 1- to 2-stolon stage.
Table 3. Visible control, leaf number, stolon number, plant height, and plant fresh weight of southern watergrass treated with florpyrauxifen-benzyl.a, b
a Means followed by the same letter within and across columns and within each parameter do not significantly differ at P = 0.05 using Tukey’s test.
b Glasshouse study conducted in November 2017 and February 2018 at Louisiana State University campus in Baton Rouge, LA 70803.
An application timing by florpyrauxifen rate by evaluation date interaction occurred for southern watergrass leaf number (Table 3B). Southern watergrass treated with florpyrauxifen at the 3- to 4-leaf stage ceased leaf production across all evaluation dates. Southern watergrass treated with florpyrauxifen at the 1- to 2-stolon timing resulted in a leaf number of 14.1 at 21 DAT compared with 23.1 leaves on the southern watergrass plants not treated with florpyrauxifen. However, slight regrowth occurred at 21 DAT compared with leaf number at earlier evaluation dates. These data indicate that florpyrauxifen should be applied to small southern watergrass to achieve suppression.
A florpyrauxifen rate by evaluation date interaction occurred for stolon number; therefore, data were pooled over application timing (Table 3C). Southern watergrass treated with florpyrauxifen ceased stolon production up to 15 DAT. Stolon number was reduced by 50% with 1.5 and 2.2 stolons per plant on southern watergrass treated with florpyrauxifen at 30 g ha−1 at 15 and 21 DAT, respectively, compared with plants receiving no florpyrauxifen treatment.
An application timing by florpyrauxifen rate by evaluation date interaction occurred for southern watergrass plant height (Table 3D). Southern watergrass treated with florpyrauxifen at the 3- to 4-leaf and 1- to 2-stolon stages ceased southern watergrass growth across all evaluation dates with no difference observed across application timings. Consequently, plant height was reduced when southern watergrass was treated with florpyrauxifen compared with the southern watergrass not treated with florpyrauxifen at 10, 15, and 21 DAT.
A florpyrauxifen rate main effect occurred for southern watergrass fresh weight; therefore, data were pooled over application timing (Table 3E). Total fresh weight of southern watergrass was reduced following the florpyrauxifen application, resulting in a total fresh weight of 1.3 g compared with 1.5 g for plants not treated florpyrauxifen. However, belowground fresh weight of southern watergrass did not differ when southern watergrass was treated with florpyrauxifen compared with no florpyrauxifen application. Thus, these data indicate that southern watergrass growth is inherently slow, and when southern watergrass was treated with florpyrauxifen at the 3- to 4-leaf stage and the 1- to 2-stolon stage, control was 56% and 40%, respectively, at 21 DAT (Table 3A). Reductions in leaf number, stolon number, and plant height are similar to overall visible control at 21 DAT. The reductions in leaf number, stolon number, and plant height did not translate into a similar reduction in plant biomass. However, these data indicate some suppression occurs for treated southern watergrass, but the treated plant may have the capacity to maintain fresh weight with the reduction in overall plant development. These data indicate that southern watergrass is more difficult to control with florpyrauxifen than brook crowngrass or the annual grasses reported by Teló et al. (Reference Teló, Webster, Blouin, McKnight and Rustom2018).
Water paspalum
An application timing by evaluation date interaction occurred for water paspalum control; therefore, data were pooled over florpyrauxifen rate (Table 4A). Water paspalum control was 36% when treated with florpyrauxifen at the 3- to 4-leaf stage at 15 DAT, and control decreased as water paspalum began to recover at 21 DAT. Consequently, water paspalum control at 21 DAT was 26%, regardless of application timing.
Table 4. Visible control, leaf number, stolon number, plant height, and plant fresh weight of water paspalum treated with florpyrauxifen-benzyl.a, b
a Means followed by the same letter within and across columns and within each parameter do not significantly differ at P = 0.05 using Tukey’s test.
b Glasshouse study conducted in November 2017 and February 2018 at Louisiana State University campus in Baton Rouge, LA 70803.
A florpyrauxifen rate by evaluation date interaction occurred for water paspalum leaf number; therefore, data were pooled over application timing (Table 4B). Water paspalum treated with florpyrauxifen displayed leaf number reduction compared with the plants not treated with florpyrauxifen at 10, 15, and 21 DAT.
An application timing by florpyrauxifen rate by evaluation date interaction occurred for stolon number (Table 4C). Stolon production ceased from 0 to 15 DAT when water paspalum was treated with florpyrauxifen at the 3- to 4-leaf stage and up to 10 DAT for application at the 1- to 2-stolon timing. Slight regrowth occurred at the 21 DAT evaluations for the earlier timing and at 15 DAT for the later timing. Stolon number was reduced when water paspalum was treated with florpyrauxifen compared with plants not treated with florpyrauxifen at 10, 15, and 21 DAT, regardless of application timing. This reduction in stolon number could help reduce future populations of water paspalum by reducing the potential for vegetative reproduction (Bottoms et al. Reference Bottoms, Webster, Hensley and Blouin2011).
An application timing by florpyrauxifen rate by evaluation date interaction occurred for plant height (Table 4D). Water paspalum height was reduced following the florpyrauxifen application compared with no florpyrauxifen treatment at 10, 15, and 21 DAT, regardless of application timing. Florpyrauxifen applied at the 3- to 4-leaf stage ceased plant growth based on the height evaluation up to 15 DAT, with a slight regrowth at 21 DAT. However, water paspalum growth ceased across all evaluation dates when water paspalum was treated with florpyrauxifen at the 1- to 2-stolon stage.
A florpyrauxifen rate main effect occurred for water paspalum fresh weight (Table 4E). Plant total fresh weight was reduced when water paspalum was treated with florpyrauxifen at 30 g ha−1. Consequently, a reduction was observed for above- and belowground fresh weight of water paspalum following the florpyrauxifen application at 30 g ha−1. Leaf number, stolon number, plant height, and plant fresh weight of treated water paspalum were reduced. However, based on the visible control and the growth parameters evaluated, florpyrauxifen provides some suppression of this weed. If water paspalum is present, other options will be necessary to manage this troublesome weed (Webster Reference Webster and Harrell2017).
Rice cutgrass
An application timing by evaluation date interaction occurred for rice cutgrass control; therefore, data were pooled over florpyrauxifen rate (Table 5A). Control of rice cutgrass did not exceed 12% control across all evaluation dates, regardless of application timing.
a Means followed by the same letter within and across columns and within each parameter do not significantly differ at P = 0.05 using Tukey’s test.
b Glasshouse study conducted in November 2017 and February 2018 at Louisiana State University campus in Baton Rouge, LA 70803.
A florpyrauxifen rate by evaluation date interaction occurred for leaf number; therefore, data were pooled over application timing (Table 5B). Leaf number did not differ when rice cutgrass was treated with florpyrauxifen compared with plants not treated with florpyrauxifen within each evaluation date. Based on these data, florpyrauxifen-treated rice cutgrass grew at the same rate as the plants not treated with florpyrauxifen.
Rice cutgrass treated with florpyrauxifen indicated no difference in stolon number, plant height, and plant fresh weight at all evaluation dates, regardless of florpyrauxifen rate (unpublished data). These data indicate that florpyrauxifen application had little influence on rice cutgrass growth. Norsworthy et al. (Reference Norsworthy, Scott, Smith, Still and Meier2009) reported quinclorac application to rice cutgrass achieved 9% control, with little activity on plant growth. Rice cutgrass is a native, weak perennial, and it thrives in moist soils and slow-moving to stagnant water. Rice cutgrass forms dense stands or colonies, primarily spread by rhizomes, and is difficult to manage and control (USDA-NRCS 2008). Consequently, florpyrauxifen provides little to no activity on rice cutgrass, with control similar to that observed with water paspalum.
In conclusion, the four perennial grasses evaluated in these studies can be difficult to manage (Webster Reference Webster and Saichuk2014, Reference Webster and Harrell2017). These weeds are commonly found in Louisiana’s rice/crawfish rotation, and the continuous aquatic environment used in this rotation provides a niche for these and many other aquatic weed species (Webster Reference Webster and Saichuk2014). The aquatic weeds found in this rotational crop system are often perennials, with large biomass production, with both seed and vegetative reproductive capabilities, and weeds with these characteristics can be difficult and expensive to manage. A reduction in stolon number was observed for florpyrauxifen-treated brook crowngrass, southern watergrass, and water paspalum, and these reductions may affect vegetative reproduction. Producers must alter cultural practices and herbicides to better manage these weeds in rice production.
Author ORCIDs
Eric P. Webster https://orcid.org/0000-0003-4809-6876
Acknowledgments
Published with the approval of the Director of the Louisiana Agricultural Experiment Station, Louisiana State University Agricultural Center, Baton Rouge, LA 70803, under manuscript number 2018-306-33338. Funding for this project was supplied by the Louisiana Rice Research Board, P.O. Box 25100, Baton Rouge, LA 70894. No conflicts of interest have been declared.
