Comparison of Weed Management Programs for ...

Comparison of Weed Management Programs for Furrow-Irrigated and Flooded Hybrid Rice Production in Arkansas Author(s) :Muthukumar V. Bagavathiannan, Jason K. Norsworthy, and Robert C. Scott Source: Weed Technology, 25(4):556-562. 2011. Published By: Weed Science Society of America DOI: URL: http://www.bioone.org/doi/full/10.1614/WT-D-11-00065.1

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Weed Technology 2011 25:556–562

Comparison of Weed Management Programs for Furrow-Irrigated and Flooded Hybrid Rice Production in Arkansas Muthukumar V. Bagavathiannan, Jason K. Norsworthy, and Robert C. Scott* Whether season-long weed control can be achieved in a furrow-irrigated rice system with similar herbicide inputs to that of a flooded system is not known. Field experiments were conducted in 2007 and 2008 at Pine Tree, AR to evaluate different herbicide programs on the weed control efficacy and rice grain yield in furrow-irrigated and flooded rice production systems. Six herbicide programs were evaluated with and without additional late-season ‘‘as-needed’’ herbicide treatments. Minor injury to rice was noted for quinclorac plus propanil. However, the injury was transient and the plants fully recovered. Overall weed control was greater in the flooded system compared with the furrow-irrigated system (up to 20% greater), because flooding effectively prevented the emergence of most terrestrial weeds. In addition, rice grain yields were 13 to 14% greater in flooded compared with furrow-irrigated plots. Irrespective of the irrigation system, herbicide programs that contained a PREapplied herbicide provided greater weed control and resulted in greater yield compared with those that did not contain PREapplied herbicide, indicative of the importance of early-season weed control in achieving higher grain yields. On the basis of weed control, yield, and weed treatment cost, the herbicide program with clomazone PRE followed by propanil at four- to five-leaf rice was more efficient than other programs evaluated in both irrigation systems. However, furrow-irrigated plots required as-needed herbicide applications, which were applied after the four- to five-leaf rice stage when two or more plots within a program exhibited # 80% control for any of the weed species. This suggests that furrow-irrigated rice production demands additional weed management efforts and thereby increases production costs. There is also a possibility for substantial yield reduction in the furrow-irrigated system compared with the flooded system. Nevertheless, furrow-irrigated rice production can still be a viable option under water-limiting situations and under certain topographic conditions. Nomenclature: Clomazone; propanil; quinclorac; rice, Oryza sativa L. ‘XL723’ ORYZA. Key words: Aerobic rice, flooded rice, rice yield, upland rice, water conservation, weed control. Se desconoce si el control de malezas a lo largo de la temporada se puede lograr en un sistema de arroz irrigado por surcos con aplicaciones de herbicida similares a las del sistema de inundacioń. En 2007 y 2008 se realizaron experimentos de campo en Pine Tree, AR, para evaluar el impacto de diferentes programas de herbicidas sobre la eficacia del control de malezas y el rendimiento de grano en sistemas de produccioń de arroz irrigado por surcos o por inundacioń. Seis programas de herbicida fueron evaluados con y sin aplicaciones adicionales de herbicida tarde en el ciclo de crecimiento, ‘‘seguń fuera necesario’’. Se noto´ un danõ menor en el arroz con quinclorac ma´s propanil. Sin embargo, el danõ fue pasajero y las plantas se recuperaron totalmente. En general, el control de malezas fue mayor en los sistemas de inundacioń en comparacioń con el irrigado por surcos (hasta 20% mayor), porque la inundacioń evito´ efectivamente la emergencia de la mayorıá de las malezas terrestres. Adema´s, los rendimientos del grano fueron 13 a 14% mayores en parcelas inundadas que en las irrigadas por surcos. Independientemente del sistema de irrigacioń, los programas que incluıán una aplicacioń de herbicida PRE proporcionaron mejor control de malezas y resultaron en mayores rendimientos comparados con aquellos que no la incluıán, lo que indica la importancia del control de malezas temprano en el ciclo de crecimiento para obtener mayores rendimientos de grano. En base al control de malezas, el rendimiento y el costo del tratamiento, el programa de herbicidas con clomazone PRE seguido por propanil cuando el arroz presento´ 4 a 5 hojas, fue ma´s eficiente que otros programas evaluados en ambos sistemas de irrigacioń. Sin embargo, las parcelas irrigadas por surcos requirieron aplicaciones de herbicida ‘‘seguń fuera necesario’’, que fueron hechas despue´s de la etapa de 4 a 5 hojas cuando dos o ma´s parcelas en un programa exhibieron # 80% de control para cualquier especie de malezas. Esto sugiere que la produccioń de arroz irrigado por surcos requiere esfuerzos adicionales de manejo de malezas y, por lo tanto, incrementa los costos de produccioń. Existe tambień la posibilidad de una reduccioń substancial en el rendimiento con el sistema de irrigacioń por surcos comparado con el sistema de inundacioń. Sin embargo, la produccioń de arroz irrigado por surcos auń puede ser una opcioń viable bajo situaciones limitadas de agua y bajo ciertas condiciones topogra´ficas.

Arkansas is the top rice-producing state in the United States, comprising about half of the total U.S. rice hectarage and production (NASS 2010). Rice production in Arkansas is mainly concentrated in the Mississippi River Delta region, DOI: 10.1614/WT-D-11-00065.1 * Postdoctoral Research Associate and Associate Professor, Department of Crop, Soil, and Environmental Sciences, University of Arkansas, 1366 West Altheimer Drive, Fayetteville, AR 72704; Professor, Extension Weed Science, University of Arkansas, Lonoke Extension and Applied Research Center, P.O. Box 357, Lonoke, AR 72086. Corresponding author’s E-mail: [email protected]

556

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Weed Technology 25, October–December 2011

wherein traditional rice production involves a dry-seeded, delayed-flood system; most of the hectarage is drill seeded and the rest is broadcast, water seeded. In the delayed-flood system (hereafter flooded system), rice is grown as an upland irrigated crop until the four- to five-leaf stage, after which a continuous flood is maintained until after rice heading (Counce et al. 2000; Slaton 2001). Levees and gates are formed to allow for water retention and drainage in the flooded system. Irrigation water is one of the most limiting factors for flooded rice production in Arkansas. Reports indicate that

about 83% of rice hectarage in Arkansas depends on groundwater resources for irrigation (Wilson et al. 2010). Flooding requires pumping a large volume of water, which in addition to increasing the production cost, leads to a rapid depletion of freshwater aquifers. Concerns of groundwater depletion have prompted many producers to explore watersaving tactics, and shifting to furrow irrigation is one tactic. Furrow irrigation requires about 18 cm less water compared with the flooded rice system over a growing season in Arkansas (Tacker 2007). In addition to water conservation, furrow-irrigated rice production is advantageous over the flooded system in terms of time, land, labor, cost, and energy (Tacker 2007; Tracy et al. 1993), and is more suitable for lands with steep slopes where flooding is not feasible (Vories et al. 2002). Despite the benefits of furrow-irrigated rice production, there exist some disadvantages that hinder widespread adoption. There is a general notion that weed management can be a challenge in the furrow-irrigated system and furrow irrigation usually yields lower than the flooded system (Patel et al. 2010; Vories et al. 2002). Because flooding creates an anaerobic condition, the emergence of most terrestrial weeds is typically prevented after the establishment of permanent flood (Slaton 2001). In the furrow-irrigated system, weed emergence may be season long, facilitated by prolonged moist conditions. The weed spectrum associated with the furrowirrigated system of rice production may be different from that of the flooded system (Norsworthy et al. 2008). Research on weed management for furrow-irrigated rice is a priority, and establishment of weed management practices that are comparable in efficacy with the flooded system is vital for promoting the adoption of furrow-irrigated rice production. Although imidazolinone-resistant rice is gaining importance in the Midsouth in recent years, conventional hybrid rice is still grown on a wide hectarage. Clomazone, quinclorac, and propanil are the most commonly used herbicides for weed control in conventional hybrid rice systems in Arkansas and other parts of the Midsouth. Among these, clomazone is the most widely used PRE-applied herbicide, providing effective control of most annual grass weeds in rice (Norsworthy et al. 2007). Quinclorac provides broad-spectrum control of annual grasses and many broadleaf weeds in rice (Street and Mueller 1993). Quinclorac can be applied as a PRE or as an early POST herbicide in rice. Propanil is another broad-spectrum POST herbicide, providing effective control of grass and broadleaf weeds in rice (Norsworthy et al. 2010). Weed management research in furrow-irrigated rice is limited, and is mostly from Asia (e.g., Mahajan et al. 2010; Singh et al. 2006, 2008). In Arkansas, attempts were made to establish efficient weed management options for furrowirrigated imidazolinone-resistant rice (Norsworthy et al. 2008, 2011) or on rice levees (Norsworthy et al. 2010), an environment similar to the furrow-irrigated system. Such options were not investigated for conventional hybrid rice systems, particularly in comparison with the flooded system. The primary objective of this research was to devise a season-long efficacious and economical weed management program in furrow-irrigated rice, comparable with that of conventional flooded rice.

Materials and Methods

Experimental Site and Design. The experiments were conducted in 2007 and 2008 at the Pine Tree Branch Experimental Station in Colt, St. Francis County, AR ( 35u 79 480 N, 90u 489 360 W), on a Calloway silt loam (fine-silty, mixed active, thermic Glossaquic Fragiudalf ) consisting of 1.0% organic matter and a pH of 6.6. The experiments were comprised of two factors, namely irrigation system and weed management. The irrigation system had two levels (furrow irrigation and flood irrigation), and weed management consisted of 13 different herbicide treatments (six herbicide treatments, each with and without late-season ‘‘as-needed’’ herbicides [totaling 12] and a nontreated check). Because each herbicide treatment (i.e., each subplot level) is a combination of more than one herbicide application, hereafter it will be referred as ‘‘herbicide program.’’ The programs were arranged in a split-plot design with the irrigation systems (furrow/ flood) assigned to the main plots and herbicide programs as the subplots. The experiment had four replications, totaling 104 plots, each measuring 2.5 by 12 m. Treatment Details. In furrow-irrigated plots, the soil was disked in the fall and raised beds 76-cm wide were formed to facilitate furrow irrigation, a system similar to row-planted crops such as corn (Zea mays L.). Rice was drillseeded in 30-cm-wide rows at a planting density of 26 seeds m21 of row. Before planting, the potassium salt of glyphosate was applied at 840 g ae ha21 using a tractor-mounted sprayer to control already-emerged weeds. In flooded plots, rice was drill-seeded at a density similar to that of furrow-irrigated plots, and levees were established around the plots to allow for water retention in the bays. The conventional rice hybrid ‘XL723’ was planted April 23, 2007 and May 13, 2008. This hybrid is known to provide high yields in the Midsouth (Stephenson et al. 2007). The subplot treatments (i.e., herbicide programs) included clomazone applied PRE followed by (fb) propanil at four- to five-leaf rice (program 1), clomazone PRE fb propanil plus quinclorac at four- to five-leaf rice (program 2), quinclorac PRE fb propanil at four- to five-leaf rice (program 3), clomazone plus quinclorac PRE fb propanil at four- to five-leaf rice (program 4), propanil at two-leaf fb propanil plus quinclorac plus clomazone at four- to five-leaf rice (program 5), and propanil at two-leaf fb propanil at four- to five-leaf (program 6). In all these treatments, clomazone, propanil, and quinclorac were applied respectively at 336 g ai ha21, 4,480 g ai ha21, and 560 g ai ha21. As-needed herbicides were applied after the four- to five-leaf rice stage when two or more plots within a program exhibited # 80% control for any of the weed species on the basis of visual estimation. The herbicides for as-needed treatments were chosen on the basis of the dominant weed spectrum, and thus, the selection of herbicides differed between the study years. On June 19, 2007, all six programs in furrow-irrigated plots and programs 5 and 6 in flooded plots received penoxsulam at 49 g ai ha21. Additionally, on June 26, 2007, programs 1 to 4 in furrow-irrigated plots received propanil at 4,480 g ha21 plus acifluorfen at 280 g ai ha21. In 2008, as-needed herbicide application was required only for furrow-irrigated plots. Programs 1, 2, 3, and 6 received propanil at 4,480 g ha21 plus acifluorfen at 280 g ha21 Bagavathiannan et al.: Furrow vs. flooded rice

N

557

Table 1. Significance of different factors and their interactions on weed control at 13 wk and rice grain yield. Palmer amaranth Source Irrigation Program As-needed Irrigation 3 program Irrigation 3 as-needed Program 3 as-needed Irrigation 3 program 3 as-needed

Hemp sesbania

Broadleaf signalgrass Barnyardgrassa Morningglorya

Prickly sidaa

Rice grain yield

2007

2008

2007

2008

2007

2008

2007

2008

2008

2007

2008

*** NS NS NS NS NS NS

* NS NS NS NS NS NS

NS NS NS NS NS NS NS

** * NS NS NS NS NS


** ** NS ** NS NS NS

** *** *** NS NS *** NS



* *** ** NS NS ** NS

NS * NS NS NS NS NS

a Species evaluated 1 year only. *** Significant at , 0.0001, ** , 0.001, and * , 0.05 levels; NS, non-significant.

on July 2, 2008, and programs 1, 3, and 6 received an additional fb application of 2,4-D at 1,600 g ae ha21on July 9, 2008. Penoxsulam and POST applications of quinclorac in the herbicide programs were applied with 1.0% v v21 crop oil concentrate (COC), whereas 2,4-D was applied with a nonionic surfactant (NIS) at 0.25% v v21. All herbicide treatments were applied with a CO2-pressurized backpack sprayer calibrated to deliver 93.5 L ha21. No herbicide was applied to the nontreated check plots. Plot Management. From planting until four- to five-leaf rice, the furrow-irrigated plots were irrigated to field capacity whenever soil moisture reached a low level. Thereafter, the plots were irrigated more frequently at 5-d intervals until panicle initiation and at 2-d intervals from panicle initiation to early grain filling. In the event of rainfall above 2.5 cm, the irrigation schedule was reset to the day of the rainfall. The flood-irrigated plots were watered at the same time as the furrow-irrigated plots until four- to five-leaf rice, after which a permanent flood was established. The plots were supplied with nitrogen in the form of urea (46–0–0) three times during the growing season at approximately 3-wk intervals starting with five- to six-leaf rice, each at 52 kg N ha21. Management practices were similar in both years. Data Collection. Data on herbicide injury to rice and weed control were recorded at weekly intervals for up to 13 wk after planting rice. Crop injury and weed control were evaluated on a scale of 0 to 100%, with 0 being no control or injury and 100 being plant death. Weeds naturally present in the experimental plots were evaluated for control. In 2007, the dominant species at the experimental site included Palmer amaranth (Amaranthus palmeri S. Wats.), hemp sesbania [Sesbania herbacea (P. Mill.) McVaugh], broadleaf signalgrass [Urochloa platyphylla (Nash) R.D. Webster], and barnyardgrass [Echinochloa crus-galli (L.) Beauv.]. In 2008, the weeds evaluated include Palmer amaranth, hemp sesbania, broadleaf signalgrass, pitted morningglory (Ipomoea lacunosa L.), and prickly sida (Sida spinosa L.). Additionally, rice grain yield was determined for each plot upon crop maturity. The crop was harvested using a small-plot harvester, and the middle four rows from each plot were used for yield estimation. Final grain yields were adjusted to 12% moisture. Data Analysis. Data pertaining to crop injury, weed control, and rice grain yield were subjected to ANOVA using SAS1 558

N


and the GLM procedure. Before ANOVA, data on crop injury and weed control were arcsine transformed to conform to the assumption of normality, but the nontransformed data are presented. The dominant weed species in the experimental sites differed between the two study years, and there were significant program-by-year interactions (a 5 0.05). Data were analyzed and presented separately for each year and weed species. Means were separated using Fisher’s Protected LSD method (a 5 0.05). In addition, costs incurred for each weed management program (including the chemical cost, adjuvants, if any, and the application cost) were estimated using the Mississippi State Budget Generator2 and the MP-44 Weed Control Guide.3 The weed treatment costs were the same for 2007 and 2008 and were as follows ($ ha21): clomazone, 29.15; propanil, 51.87; quinclorac, 90.57; penoxsulam, 64.73; 2,4D, 15.56; acifluorfen, 21.3; NIS, 3.50; COC, 2.13; ground application, 14.82; aerial application, 18.53. It was assumed that for the flooded system, all the POST herbicides were applied through air and for the furrow-irrigated system, all herbicides were applied via ground. Results and Discussion

Injury to Rice. All herbicides used in the experiments, including the as-needed treatments, were labeled for application in rice, and therefore, injury to rice was not substantial. Low levels of injury in the form of leaf speckling were noted for quinclorac plus propanil the week after applications (data not shown). Between the two study years, leaf injury was greater in 2007 (10 to 15%) than in 2008 (, 5%). Cool, wet conditions during the early growth stages of rice could have caused the injury in 2007 (the crop was planted earlier that year). Injury to rice was only temporary, and the plants compensated for the injury within a short time. Weed Control. Irrigation system had an effect on the control of most weed species and control was typically greater in the flooded plots compared with the furrow-irrigated plots (Table 1). In the flooded plots, weed control was $ 96%, except for barnyardgrass (87%), whereas in the furrowirrigated plots, weed control ranged from 70 (barnyardgrass) to 95% (Palmer amaranth) (data not shown). In the Mississippi River Delta region, rice fields are typically flooded starting at the four- to five-leaf stage, and a continuous flood

Table 2. Efficacy of herbicide programs on barnyardgrass control at 13 wk.a Barnyardgrass control b

c

Herbicide (timing ) 1. Clomazone (PRE) fb propanil (four- to five-leaf) 2. Clomazone (PRE) fb propanil + quincloracd (four- to five-leaf) 3. Quinclorac (PRE) fb propanil (four- to five-leaf) 4. Clomazone + quinclorac (PRE) fb propanil (four- to five-leaf) 5. Propanil (two-leaf) fb propanil + quinclorac + clomazone (four- to five-leaf) 6. Propanil (two-leaf) fb propanil (four- to five-leaf) LSD (a 5 0.05)

Without as-needed

With as-needede

---------------------------------------------------------------------------- % --------------------------------------------------------------------------86 92 97 94 91 93 93 93 40 93 13 56 ----------------------------------------------------------------------------16 --------------------------------------------------------------------------

a

Abbreviation: fb, followed by. Application rates: clomazone at 336 g ai ha21, propanil at 4,480 g ai ha21, and quinclorac at 560 g ai ha21. c Timing of herbicide application corresponds to growth stage of rice crop. d POST applications of quinclorac were applied with crop oil concentrate at 1.0% v v21. e As-needed herbicides: all programs in furrow-irrigated plots and programs 5 and 6 in flood-irrigated plots received penoxsulam at 49 g ai ha21 plus crop oil concentrate at 1.0% v v21; programs 1 to 4 in furrow-irrigated plots received an additional application of propanil at 4,480 g ha21 plus acifluorfen at 280 g ai ha21. b

is maintained until maturity (Counce et al. 2000). Flooding is regarded as an excellent cultural practice that can prevent the emergence of most terrestrial weeds, providing substantial weed management benefits (Norsworthy et al. 2011; Slaton 2001). In furrow-irrigated rice, on the other hand, the environment is different. The occurrence of prolonged moist conditions in the furrow system extends the emergence of most weeds, causing severe weed pressure. Herbicide programs differed in efficacy for the control of barnyardgrass (2007), hemp sesbania (2008), and broadleaf signalgrass (2008) (Table 1). In particular, barnyardgrass control was lower in programs 5 (propanil at two-leaf rice fb propanil plus quinclorac plus clomazone at four- to five-leaf rice) and 6 (propanil at two-leaf rice fb propanil at four- to five-leaf rice), with 66 and 34% control, respectively (data not shown). The control of hemp sesbania and broadleaf signalgrass was lower in program 6 (91 and 81%, respectively). Barnyardgrass was the most difficult-to-control weed in this study, and programs 5 and 6 did not contain a PREapplied herbicide. Regardless of the irrigation system, excluding the residual PRE-applied herbicide (programs 5 and 6) caused more barnyardgrass problems than the lack of a residual herbicide at the four- to five-leaf rice (programs 1, 3, and 4). This evidence demonstrates the value of PRE-applied herbicides in controlling barnyardgrass in rice. PRE-applied herbicides typically control germinating weeds for prolonged periods, resulting in low weed densities. With the POST application at four- to five-leaf rice, any escapes from the PRE application were controlled because of low density and small weed size. Omitting the PRE application and delaying the first herbicide application until after the two-leaf rice (programs 5 and 6) led to substantial escape of early-emerging weeds, which were not adequately controlled by subsequent applications at four- to five-leaf rice. Two residual PREapplied herbicides (i.e., clomazone and quinclorac, program 4) did not improve barnyardgrass control over either PRE herbicide applied alone. Combination of more than one PREapplied herbicide is valuable in the context of herbicide resistance management; barnyardgrass resistance to clomazone and quinclorac is a growing problem in the Midsouth (Malik et al. 2010; Norsworthy et al. 2009).

As-needed applications of penoxsulam improved control of barnyardgrass (Table 1), particularly in programs 5 and 6 (Table 2). Penoxsulam is an effective late-season option for control of barnyardgrass in rice (Ottis et al. 2003). Yet, barnyardgrass control was not acceptable in program 6 (only 56%), which is attributed to the low susceptibility of large plants that were not controlled by earlier treatments. The efficacy of programs 5 and 6 were similar in both irrigation systems. Barnyardgrass is an early-emerging weed with a prolonged emergence pattern (Bagavathiannan et al. 2011), validating the need for residual herbicides for season-long control. Except for barnyardgrass, as-needed applications were not required in the flooded system because flooding provided weed management benefits. In the furrow-irrigated plots, asneeded herbicide applications were required in most cases (on the basis of # 80% control threshold), but improvement in control was not significant for weeds other than barnyardgrass (Table 1). Improved control usually witnessed with additional as-needed applications (data not shown) is still valuable in view of future weed problems that may result from seedbank replenishment from uncontrolled plants. Reducing viable weed seed return is particularly important in the context of mitigating the evolution and spread of herbicide resistance. Several studies have emphasized the value of minimizing seed production in late-season weeds (Brewer and Oliver 2007; Norris 2003). Lower control of hemp sesbania and broadleaf signalgrass with program 6 (propanil at two-leaf rice fb propanil at fourto five-leaf rice) is likely due to the absence of both PRE- and POST-applied residual herbicides in this program. In addition, the irrigation-by-program interaction was significant for broadleaf signalgrass control in 2008 (Table 1), wherein program 6 provided unsatisfactory control in the furrowirrigated system (63%), whereas control was 98% in the flooded system (Table 3). In 2008, broadleaf signalgrass was the dominant grass species because of the absence of barnyardgrass. Because broadleaf signalgrass is a relatively lateemerging weed in rice, program 6 coupled with flood irrigation was effective. However, in the furrow-irrigated system, the lack of residual herbicides combined with the absence of flooding caused poor broadleaf signalgrass control Bagavathiannan et al.: Furrow vs. flooded rice

N

559

Table 3. Efficacy of herbicide programs on broadleaf signalgrass control at 13 wk as influenced by irrigation system in 2008.a Broadleaf signalgrass control b

c

Herbicide (timing )

Furrow irrigation

Flood irrigation

---------------------------------------------------------------------------- % --------------------------------------------------------------------------94 98 93 98 91 98 94 98 89 98 63 98 ----------------------------------------------------------------------------10 --------------------------------------------------------------------------

1. Clomazone (PRE) fb propanil (four- to five-leaf) 2. Clomazone (PRE) fb propanil + quincloracd (four- to five-leaf) 3. Quinclorac (PRE) fb propanil (four- to five-leaf) 4. Clomazone + quinclorac (PRE) fb propanil (four- to five-leaf) 5. Propanil (two-leaf) fb propanil + quinclorac + clomazone (four- to five-leaf) 6. Propanil (two-leaf) fb propanil (four- to five-leaf) LSD (a 5 0.05) a

Abbreviation: fb, followed by. Application rates: clomazone at 336 g ai ha21, propanil at 4,480 g ai ha21, and quinclorac at 560 g ai ha21. c Timing of herbicide application corresponds to growth stage of rice crop. d POST applications of quinclorac were applied with crop oil concentrate at 1.0% v v21. b

in program 6. With regard to overall weed control, programs 1 to 4 provided greater efficacy and were comparable with each other in both irrigation systems. However, these programs required additional as-needed applications in the furrow-irrigated system. Rice Yield. Rice grain yields were generally greater in the flooded system compared with the furrow-irrigated system. Yield difference between the two irrigation systems were 13 (difference: 1,390 kg ha21) and 14% (1,360 kg ha21), respectively, in 2007 and 2008, although the difference was not significant in 2008 (P 5 0.0771). Rice is well adapted to anaerobic conditions, leading to greater yield potential in the flooded (anaerobic) system compared with the furrowirrigated (aerobic) system (McCauley 1990; Patel et al. 2010; Vories et al. 2002). Greater weed interference observed in the furrow-irrigated system may have contributed to the yield differences noted in the present study. In other studies, yield reductions of 16 (Vories et al. 2002), , 20 (McCauley 1990), or 28% (Patel et al. 2010) were reported for aerobic upland rice. Such reduction in grain yield is most commonly attributed to a reduction in the number of panicles and

spikelets (i.e., sink size) (Patel et al. 2010) or to lower individual grain weights (Vories et al. 2002). Irrespective of the irrigation system, significant yield reductions were noted for programs 5 and 6, with much greater reduction in program 6. In programs 5 and 6, lateseason as-needed applications improved rice grain yields in 2007 (Table 4). This could be linked to the improved barnyardgrass control noted with as-needed herbicide applications in 2007 (Table 2). Yet, the yield level was not acceptable in program 6. The absence of residual PRE herbicides and resulting early-season weed competition contributed to the yield reductions observed in programs 5 and 6. Juraimi et al. (2009) have revealed that the critical weed-free period begins as early as 2 d after planting in the direct-seeded rice production system. As such, PRE herbicides are vital in sustaining yield levels in rice. Grain yields were comparable among programs 1 to 4 in both irrigation systems, but as-needed applications did not improve yields in these programs. Yield levels were very low in the nontreated plots; it was not possible to harvest the nontreated plots in 2007 because of the severe barnyardgrass infestation.

Table 4. Effect of different herbicide programs on rice grain yield.a Rice grain yield 2007 b

c

Herbicide (timing )

Without as-needed

With as-needede 21

-----------------------------------------------------------------------------kg ha

---------------------------------------------------------------------------

1. Clomazone (PRE) fb propanil (four- to five-leaf) 11,460 11,330 11,980 11,950 2. Clomazone (PRE) fb propanil + quincloracd (four- to five-leaf) 3. Quinclorac (PRE) fb propanil (four- to five-leaf) 11,290 11,900 4. Clomazone + quinclorac (PRE) fb propanil (four- to five-leaf) 12,040 11,620 5. Propanil (two-leaf) fb propanil + quinclorac + clomazone (four- to five-leaf) 7,870 10,440 6. Propanil (two-leaf) fb propanil (four- to five-leaf) 2,530 6,480 Nontreated check 80 — LSD (a 5 0.05) --------------------------------------------------- 1,780 -------------------------------------------------a

2008

9,280 9,340 9,640 9,130 8,950 7,850 2,390 550

Abbreviation: fb, followed by. Application rates: clomazone at 336 g ai ha21, propanil at 4,480 g ai ha21, and quinclorac at 560 g ai ha21. c Timing of herbicide application corresponds to growth stage of rice crop. d POST applications of quinclorac were applied with crop oil concentrate at 1.0% v v21. e As-needed herbicides: all programs in furrow-irrigated plots and programs 5, 6 in flood-irrigated plots received penoxsulam at 49 g ai ha21 plus crop oil concentrate at 1.0% v v21; programs 1 to 4 in furrow-irrigated plots received an additional application of propanil at 4,480 g ha21 plus acifluorfen at 280 g ai ha21. b

560

N


Table 5. Weed treatment cost compared between furrow-irrigated and flooded system for each herbicide program.a Weed treatment costd Furrow irrigation Without as-needed Herbicideb (timingc)

—

Flood irrigation e

Without as-needed

2008

—

With as-needed 2007

With as-needed 2007

2008

-----------------------------------------------------------------------------------------------$ ha21 ---------------------------------------------------------------------------------------------1. Clomazone (PRE) fb propanil (four- to five-leaf) 2. Clomazone (PRE) fb propanil + quinclorac (four- to five-leaf) 3. Quinclorac (PRE) fb propanil (four- to five-leaf) 4. Clomazone + quinclorac (PRE) fb propanil (four- to five-leaf) 5. Propanil (two-leaf) fb propanil + quinclorac + clomazone (four- to five-leaf) 6. Propanil (two-leaf) fb propanil (four- to five-leaf)

111 203 172 201

279 371 340 369

233 291 294 —f

115 207 176 205

— — — —

— — — —

255 133

335 213

— 343

263 141

346 276

— —

Abbreviation: fb, followed by. Application rates: clomazone at 336 g ai ha21, propanil at 4,480 g ai ha21, and quinclorac at 560 g ai ha21. c Timing of herbicide application corresponds to growth stage of rice crop. d Cost includes the cost of chemicals, adjuvants (if applicable), and the application cost. e As-needed herbicides: 2007—all programs in furrow-irrigated plots and programs 5, 6 in flood-irrigated plots received penoxsulam at 49 g ai ha21 plus crop oil concentrate at 1.0% v v21; programs 1 to 4 in furrow-irrigated plots received an additional application of propanil at 4,480 g ha21 plus acifluorfen at 280 g ai ha21; 2008—only required for the furrow-irrigated plots; programs 2 and 6 received propanil at 4,480 g ha21 plus acifluorfen at 280 g ha21; programs 1, 3, and 6 received a subsequent treatment of propanil at 4,480 g ha21 plus acifluorfen at 280 g ha21 fb 2,4-D at 1,600 g ae ha21 plus nonionic surfactant at 0.25% v v21. f As-needed herbicides were not required. a

b

Weed Treatment Cost. The costs incurred for each herbicide program under each irrigation system are summarized in Table 5. The herbicide program with clomazone PRE fb propanil at four- to five-leaf rice (program 1) was more economical than other programs evaluated in both irrigation systems. Program 1 costs about $111 in the furrow-irrigated system and $115 ha21 in the flooded system. Furrow-irrigated plots required as-needed herbicide applications, which increased the cost an additional $122 to $168 ha21. Greater weed pressure is a concern in the furrow-irrigated system, and additional herbicide applications are often required. Overall, the program with clomazone PRE fb propanil at four- to five-leaf rice combined with as-needed late-season applications (using penoxsulam, propanil, acifluorfen, 2,4-D on the basis of the weed spectrum) may be a suitable weed control option in the furrow-irrigated system. In addition to weed competition, there was a substantial yield penalty in the furrow-irrigated system. Despite these limitations, furrow irrigation may still be a viable option for some farmers under certain situations. In particular, the shortage of groundwater resources and the rising energy costs involved in pumping water may offset the high weed management costs and reduced grain yields associated with the furrow-irrigated system. Flooding requires the establishment of levees, which can consume a substantial land area, particularly in fields with steep slopes. The costs incurred in the construction and destruction of levees can be substantial to some farmers, and weed management on levees is also an issue in the flooded system (Norsworthy et al. 2010). Quick soil drying enables easy harvesting in furrowirrigated rice, which is often a challenge in flooded rice fields (Stephenson et al. 2007). For many producers, these benefits are sufficient enough to consider furrow-irrigated rice production in forthcoming years. Developing rice varieties adapted to upland growing conditions (Bouman et al. 2002) and developing suitable agronomic practices for narrowing the

yield gaps will be vital for the success of furrow-irrigated rice production in the Midsouth. Sources of Materials 1

SAS software, Version 9.1, SAS Institute Inc., P.O. Box 8000, Cary, NC 27513. 2 Mississippi State Budget Generator, Version 6.0 for Windows, Department of Agricultural Economics, Mississippi State University, Mississippi State, MS 39762. 3 MP-44 Weed Control Guide, Cooperative Extension Service, University of Arkansas, Fayetteville, AR 72704.

Acknowledgment

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Received May 10, 2011, and approved July 14, 2011.