PEST MANAGEMENT: WEEDS. Early-Season Palmer Amaranth Interference in Rice, Potential Yield Losses, and Control Options

Transcription

1 PEST MANAGEMENT: WEEDS Early-Season Palmer Amaranth Interference in Rice, Potential Yield Losses, and Control Options J.B. Brennan, J.K. Norsworthy, C.J. Meyer, R.C. Scott, J. Bond, and T.L. Barber ABSTRACT Herbicide-resistant Palmer amaranth (Amaranthus palmeri) is one of the most problematic weeds facing producers in the southern United States today. In a survey conducted by Norsworthy et al. (2013) in 2011, crop consultants listed Palmer amaranth as one of the five most important weeds in rice production. Improved control options for Palmer amaranth and the effect of Palmer amaranth on early-season interference are two of the most important areas of research and educational needs in rice as identified by consultants. Three field studies were conducted in 2013 at University of Arkansas System Division of Agriculture Pine Tree Research Station near Colt, Ark., to evaluate herbicide programs for Palmer amaranth control and potential yield loss associated with early-season Palmer amaranth interference in rice. The first trial was designed to evaluate the efficacy of various pre-emergence (PRE) herbicide programs for controlling Palmer amaranth. Clomazone, pendimethalin, imazosulfuron, thiobencarb, and quinclorac were applied PRE or delayed PRE (DPRE) and evaluated for control of Palmer amaranth. Clomazone (0.3 lb ai/acre) plus quinclorac (0.5 lb ai/acre) applied PRE provided the best control of Palmer amaranth with greater than 92% control 6 weeks after treatment (WAT) compared to 80% control with clomazone alone. In the second trial, application timing was evaluated for effectiveness of early post-emergence (EPOST) Palmer amaranth control options. Five herbicide programs were applied at two different timings, EPOST and preflood (PREFLD). Propanil (4 lb ai/acre) plus acifluorfen (0.25 lb ai/acre) provided the highest level of control for Palmer amaranth with greater than 94% control 2 WAT EPOST, which was significantly better than propanil alone. Rice yields were significantly higher for PREFLD applications than for EPOST treatments, with the exception of propanil (4 lb/acre) applied with carfentrazone (0.016 lb ai/acre) 194

2 B.R. Wells Arkansas Rice Research Studies 2013 and quinclorac (0.5 lb/acre). Higher yields with PREFLD applications can mostly be attributed to better control of hemp sesbania (Sesbania herbacea) and barnyardgrass (Echinochloa crus-galli) post-flood. For the third study, Palmer amaranth was spread at 0, 12, 60, 300, and 1500 seeds/m 2 at planting in both hybrid rice, seeded at 6 seed/ft of row, and conventional rice, seeded at 24 seed/ft of row to determine if yield losses could result from early-season competition. On average, yield for hybrid rice was higher than conventional rice by 27 bu/acre, although weed competition reduced yields for both rice types similarly. Palmer amaranth stand counts were determined in each plot prior to flooding and as Palmer amaranth density increased by 10 plants/m 2, yield decreased by an estimated 1.3 bu/acre. The results from these three experiments shows that there is potential for yield loss due to early-season Palmer amaranth interference in rice, and that season-long herbicide programs are essential to maximize yields in rice. INTRODUCTION Weed management is essential for successful production of rice. Arkansas ranks first among U.S. rice-producing states comprising more than 1.2 million acres of cropland grown every year. Herbicide-resistant Palmer amaranth has become one of the most problematic weeds in agronomic crops in the southern United States (Jha and Norsworthy, 2012). Early-season Palmer amaranth interference is becoming a major concern for rice producers in Arkansas. A survey of rice consultants in 2011 ranked Palmer amaranth as one of the top five most important weed species in rice, and one of the most important needs for research along with barnyardgrass (Norsworthy et al., 2013). Herbicide-resistant Palmer amaranth is prevalent in soybean fields across Arkansas, the crop most commonly rotated with rice. In addition to the prevalence of Palmer amaranth in rice fields, it is difficult to control on rice levees because of 2,4-D restrictions in the state (Norsworthy et al., 2013). Direct dry-seeding is the most commonly used method for growing rice in Arkansas, and subsequently a 2- to 6-inch permanent flood is established 4 to 5 weeks after planting (Norsworthy et al., 2008). Continuous flooding reduces emergence and growth of most semi-terrestrial broadleaf and grass weeds in rice; however, controlling weeds such as Palmer amaranth between planting and flood is critical for maximizing yields in dry-seeded rice (Riar and Norsworthy, 2011). Numerous residual herbicides are available for pre-emergence (PRE) and delayed pre-emergence (DPRE) weed control including pendimethalin, clomazone, quinclorac, and thiobencarb (Riar and Norsworthy, 2011). Adoption of imidazolinone-resistant rice has increased tremendously as well as the increased use of imazethapyr and other acetolactate synthase (ALS)-inhibiting herbicides (Riar et al., 2013). In most cases, ALS inhibiting herbicides provide poor control of Palmer amaranth due to widespread resistance to the mechanism of action (Norsworthy et al., 2008). The objectives of this research were to 1) evaluate Palmer amaranth control with various pre-emergenceapplied herbicide programs in rice, 2) determine efficacy of different post-emergence options for the control of Palmer amaranth, and 3) determine the potential yield loss associated with early-season interference in Palmer amaranth. 195

3 AAES Research Series 617 PROCEDURES Three field experiments were set up at the University of Arkansas System Division of Agriculture Pine Tree Research Station near Colt, Ark., in 2013 on a Calloway silt loam soil. To determine the effects of various PRE-applied herbicide programs on Palmer amaranth control in rice, a study was conducted as a randomized complete block design with each treatment replicated four times. Rice cultivar CL142 was drilled seeded at a rate of 24 seed/ft of row in plots 6-ft wide by 25-ft long. Treatments evaluated included 1) clomazone (0.3 lb ai/acre) as Command applied PRE, 2) clomazone (0.3 lb/acre) PRE followed by (fb) pendimethalin (1.0 lb ai/acre) as Prowl H 2 O applied DPRE, 3) clomazone (0.3 lb/acre) PRE tankmixed with imazosulfuron (0.3 lb ai/acre) as League applied PRE, 4) clomazone (0.3 lb ai/acre) PRE fb thiobencarb (4.0 lb ai/acre) as Bolero applied DPRE, 5) pendimethalin (1.0 lb/acre) DPRE tankmixed with thiobencarb (4.0 lb/acre) DPRE, 6) clomazone (0.3 lb/acre) PRE tankmixed with quinclorac (0.5 lb ai/ acre) as Facet applied PRE, and a nontreated check. Each treatment received propanil (4.0 lb ai/acre) early post-emergence (EPOST) and preflood (PREFLD) to control lateseason weeds such as hemp sesbania and barnyardgrass. Treatments were visually rated for Palmer amaranth, barnyardgrass, broadleaf signalgrass, and hemp sesbania at 2, 4, and 6 WAT on a scale of 0 to 100 where 0 is no control and 100 is complete control. Rice was harvested using a small-plot combine, grain moisture was determined, and yields were converted to bu/acre. Rating and yield data were subjected to analysis of variance, and means were separated using the Fisher s Protected Least Significant Difference test at the 0.05 level. A second study was conducted at the same site to evaluate the influence of application timing on Palmer amaranth control. Clearfield 142 was drill seeded at 24 seeds/ ft of row in 6-ft wide by 25-ft long plots. The experiment was set up in a randomized complete block design with a two by five factorial arrangement of treatments with four replications. The first factor consisted of two application timings EPOST and PRE- FLD. The second factor consisted of five herbicide applications including 1) propanil (4.0 lb/acre), 2) propanil (4.0 lb/acre) plus triclopyr (0.25 lb ae/acre) as Grandstand, 3) propanil (4.0 lb/acre) plus acifluorfen (0.25 lb ai/acre) as Ultra Blazer, 4) propanil (4.0 lb/acre) plus carfentrazone (0.016 lb ai/acre) as Aim, and 5) propanil (4.0 lb/acre) plus carfentrazone (0.016 lb/acre) plus quinclorac (0.5 lb/acre). A nontreated control was included for comparison. Treatments were visually rated for control of Palmer amaranth, barnyardgrass, broadleaf signalgrass, and hemp sesbania at 1, 2, and 4 weeks after the EPOST treatment. Subsequent weed control ratings were recorded post-flood. Rice was harvested using a small-plot combine. Ratings and yield data were subjected to analysis of variance, and means were separated using the Fisher s protected LSD test at the 0.05 level. A third study conducted at the same site aimed to quantify the amount yield loss resulting from early-season Palmer amaranth interference in rice. The experiment was conducted in a randomized complete block design with a two by five factorial arrangement of treatments with four replications. The first factor consisted of two cultivars of rice, hybrid rice seeded at 6 seed/ft of row and conventional rice seeded at 24 seed/ 196

4 B.R. Wells Arkansas Rice Research Studies 2013 ft of row. The second factor consisted of different seeding rates of Palmer amaranth spread at 0, 12, 60, 300, and 1500 seeds/m 2. A herbicide program consisting of a recommended rate of clomazone PRE fb fenoxaprop-p-ethyl plus halosulfuron post-flood was applied to all plots to control typical weeds in rice such as barnyardgrass and hemp sesbania. Plots were visually rated for Palmer amaranth control 4 and 5 WAT, and Palmer amaranth counts were taken PREFLD to estimate the number of plants/m 2. Rice was harvested using a small-plot combine. Yield data was subjected to least squares regression analysis. Yield estimates were then determined as a function of cultivar type and Palmer amaranth plants/m 2. RESULTS AND DISCUSSION Results from the PRE-applied herbicide programs show that clomazone plus quinclorac PRE provided the best control for Palmer amaranth at 96% and 97% at 2 and 4 WAT, respectively (Table 1). Overall, herbicide programs that included a PRE application controlled Palmer amaranth better than DPRE-only programs. Pendimethalin plus thiobencarb applied DPRE controlled Palmer amaranth 72% at 6 WAT, significantly less than clomazone PRE plus pendimethalin DPRE (86% control 6 WAT) and clomazone PRE plus quinclorac PRE (92% control 6 WAT), suggesting the importance of PREapplied herbicides for early-season control of Palmer amaranth. Although pendimethalin plus thiobencarb DPRE resulted in rice yields greater than the nontreated check (183 bu/acre compared to 153 bu/acre), it had the lowest yield out of all the herbicide treatments. Clomazone plus imazosulfuron applied PRE resulted in the highest yield of 202 bu/acre. For POST Palmer amaranth control options, propanil plus acifluorfen applied EPOST provided the best control with 98%, 94%, and 85% control at 1, 2, and 4 WAT, respectively (Table 2). These levels of control were significantly greater than for propanil alone applied EPOST. Propanil plus carfentrazone and quinclorac EPOST also provided adequate control of Palmer amaranth at 95% and 94% control at 1 and 2 WAT. Treatments that received the PREFLD application had significantly higher yields than EPOST treatments, with the exception of propanil plus carfentrazone and quinclorac. This is most likely the result of better post-flood control of late emerging barnyardgrass and hemp sesbania with PREFLD treatments versus EPOST treatments. Results from potential yield losses associated with early-season Palmer amaranth interference show that there is a negative relationship between Palmer amaranth density and rice yield. Yields for hybrid rice (247 bu/acre) were on average 18 bu/acrecre higher than conventional rice (220 bu/acre). Regression analysis show that as Palmer amaranth density increased by 10 plants/m 2, yield decreased by 1.3 bu/acre (Fig. 1). In all trials, Palmer amaranth control reached 100% for all treatments once a permanent flood was established, confirming its inability to survive a flooded environment. SIGNIFICANCE OF FINDINGS While Palmer amaranth is unable to survive a permanent flood, it has the potential to reduce rice yields through early-season competitive interference. Pre-emergence ap- 197

5 AAES Research Series 617 plied herbicide applications and those in combination with DPRE provided better earlyseason control than DPRE applications alone stressing the importance of controlling Palmer amaranth before it has a chance to establish. Various EPOST options exist to control Palmer amaranth escapes and should be used in combination with PRE applications. Yield reductions from post-flood interference of barnyardgrass and hemp sesbania show the importance of PREFLD applications and need for season-long weed control programs in rice. In general, herbicide treatments with multiple mechanisms of action performed better at controlling Palmer amaranth than those with a single mechanism of action. To maximize rice yields and reduce herbicide resistance, producers should utilize herbicide programs consisting of multiple mechanisms of action with season-long application timings to control a broad spectrum of weeds including Palmer amaranth. ACKNOWLEDGMENTS We would like to thank the Arkansas Rice Research and Promotion Board for funding this research. LITERATURE CITED Jha, P. and J.K. Norsworthy Influence of late-season herbicide applications on control, fecundity, and progeny fitness of glyphosate resistant Palmer amaranth (Amaranthus palmeri) Biotypes from Arkansas. Weed Technol. 26: Norsworthy, J.K., J. Bond, and R.C. Scott Weed management practices and needs in Arkansas and Mississippi rice. Weed Technol. 27: Norsworthy, J.K., G.M. Griffith, and R.C. Scott Imazethapyr use with and without Clomazone for weed control in furrow-irrigated, imidazolinone-tolerant rice. Weed Technol. 22: Riar, D.S. and J.K. Norsworthy Use of imazosulfuron in drill-seeded rice (Oryza sativa) in the mid-south United States. Weed Technol. 25: Riar, D.S., J.K. Norsworthy, L.E. Steckel, and D.O. Stephenson IV Adoption of best management practices for herbicide-resistant weeds in midsouthern United States cotton, rice, and soybean. Weed Technol. 27:

6 B.R. Wells Arkansas Rice Research Studies 2013 Table 1. Palmer amaranth control from pre-applied rice herbicide programs at Pine Tree Research station near Colt, Ark. Application Palmer amaranth control Treatment Rate timing 2 WAT 4 WAT 6 WAT (lb ai/acre) (%) Clomazone 0.3 PRE 91 a 90 b 79 ab Clomazone 0.3 PRE 95 a 95 ab 86 a Pendimethalin 1.0 DPRE Clomazone 0.3 PRE 95 a 92 ab 82 ab Imazosulfuron 0.3 PRE Clomazone 0.3 PRE 90 a 97 a 80 ab Thiobencarb 4.0 DPRE Pendimethalin 1.0 DPRE 92 a 88 b 72 b Thiobencarb 4.0 DPRE Clomazone 0.3 PRE 96 a 97 a 92 a Quinclorac 0.5 PRE PRE = pre-emergence and DPRE = delayed pre-emergence. WAT = weeks after treatment. Means followed by the same letter are not significantly different according to Fisher s Protected Least Significant Difference test at the 0.05 level. Table 2. Palmer amaranth control from various early post-emergence herbicide programs in rice at Pine Tree Research Station near Colt, Ark. Palmer amaranth control Treatment Rate 1 WAT 2 WAT 4 WAT (lb ai/acre) (%) Propanil c 78 b 45 b Propanil ab 90 ab 62 ab Triclopyr 0.25 Propanil a 94 a 85 a Acifluorfen 0.25 Propanil bc 82 ab 68 ab Carfentrazone Propanil ab 94 a 81 a Carfentrazone Quinclorac 0.5 WAT = weeks after treatment. Means followed by the same letter are not significantly different according to Fisher s Protected Least Significant Difference test at the 0.05 level. 199

7 AAES Research Series 617 Fig. 1. Early-season Palmer amaranth interference effect on rice yield. Regression lines show the estimated yield reduction by rice type (hybrid and conventional) as Palmer amaranth density increases. 200