Management of Echinochloa polystachya in Drill-seeded Rice

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Management of Echinochloa polystachya in Drill-seeded Rice

Eric P. Webster and R. Matthew Griffin, School of Plant, Environmental, and Soil Sciences, Louisiana State University Agricultural Center, and David C. Blouin, Department of Experimental Statistics, Louisiana State University, Baton Rouge 70803

Abstract

A study was conducted in 2004 and 2005 near Crowley, LA to evaluate soil-applied herbicides labeled for barnyardgrass control in rice for activity on Echinochloa polystachya. The study included: clomazone applied preemergence (PRE), clomazone plus quinclorac applied PRE, pendimethalin plus quinclorac applied delayed PRE, imazethapyr applied PRE, and mesotrione applied PRE. Each soil applied herbicide program was followed by cyhalofop applied postemergence (POST). Averaged over 7 to 49 DAT, all of the herbicide programs controlled barnyardgrass at least 87%. Averaged over herbicide program, barnyardgrass control was 91 to 93% from 28 to 42 DAT; however, control at all evaluations was at least 85%. The herbicide programs with a single application of clomazone or imazethapyr applied PRE, or pendimethalin plus quinclorac applied delayed PRE controlled E. polystachya 78 to 81%. Control was 72% for programs containing mesotrione or clomazone plus quinclorac applied PRE. The combination of clomazone and quinclorac PRE followed by (fb) cyhalofop POST controlled E. polystachya 72% while clomazone PRE fb cyhalofop POST controlled the weed 80%.

Introduction

In Louisiana, rice (Oryza sativa L.) producers integrate weed management programs with cultural, mechanical, and chemical methods to maximize yields (12). Water-seeding, which is direct broadcasting dry or pre-soaked seed into flooded fields, is the predominant method of rice seeding used in Louisiana (14,25). Rice tolerates low oxygen (hypoxic) conditions better than most weeds; thus, flooding has traditionally been used as an effective method of cultural control for many weed species including red rice, an economically important weed genetically identical to rice (10). In addition, flooding is used to aid in land leveling, tillage, and crawfish production (12,14); however, these practices may promote the spread of weeds that thrive in continuously flooded conditions. Since the introduction of imidazolinone-resistant rice more producers are drill-seeding rice in south Louisiana to aid the management of perennial aquatic weeds that thrive in a water-seeded production system.

Echinochloa polystachya (Kunth) A.S. Hitchc. is a C₄ perennial grass found in Florida, Louisiana, Texas, and Puerto Rico (30,31,32). It is a close relative of E. crus-galli (barnyardgrass), which is one of the most widespread and problematic weeds of rice in Louisiana. The subtropical environment in Louisiana not only allows for favorable rice growing conditions, but also provides an acceptable climate to a wide range of grass species, including E. polystachya (1,16). This weed has been found to be quickly spreading through the rice-producing parishes of Louisiana due to its ability to reproduce vegetatively by stolons and rhizomes which are transported by tillage and movement of the contaminated equipment (29). E. polystachya thrives in flooded conditions such as those found in Louisiana rice production, having the ability to remain above fluctuating water levels found in its native terrain of the Amazon basin (23).

Preemergence (PRE) herbicides have been used successfully in rice production for weed control (7,33,38). Clomazone (Command 3ME, FMC, Philadelphia, PA), labeled for use in rice, has been shown to control barnyardgrass 90 and 83% at 7 and 35 days after PRE application in rice, respectively (37). This control is due to lack of accumulation of plastid pigments (6).

Pendimethalin (Prowl 3.3 EC, BASF Corporation, Research Triangle Park, NC) and quinclorac (Facet 75 DF, BASF Corporation), are two PRE herbicides used for barnyardgrass control (35, 36). Pendimethalin and quinclorac can be substituted for multiple applications of propanil (Stam M4, Dow AgroScience, Indianapolis, IN) to control propanil-resistant barnyardgrass, improve overall weed control, and increase rice yield (11,33,34).

The development of herbicide-resistant rice allows the use of imidazolinone herbicides for weed control on resistant cultivars (21,22). Imazethapyr (Newpath, BASF Corporation) and imazamox (Beyond, BASF Corporation) are the imidazolinone herbicides labeled for use with imidazolinone-resistant (IR) rice. Barnyardgrass control of 90% has been reported with imidazolinone herbicides applied PRE and postemergence (POST) in an IR rice production system (17,21).

Mesotrione (Callisto, Syngenta Crop Corporation, Wilmington, DE) is a selective herbicide that can be applied PRE and POST for control of broadleaf and grass weeds in corn (Zea mays L.) (18,19,28). Although not labeled for use in rice, mesotrione is of interest because of activity on barnyardgrass.

Due to the lapse in time before permanent flood establishment in a drill-seeded, delayed-flood production system, residual herbicides may not always provide complete control of weeds and require a POST herbicide application (12). Cyhalofop (Clincher SF, Dow AgroScience, Indianapolis, IN) is a POST herbicide that controls barnyardgrass. Cyhalofop has been reported to control barnyardgrass at least 88% when applied POST at 2.8 oz ai/acre (20). Griffin et al. (8) reported 94% barnyardgrass control 16 DAT with 4.0 followed by (fb) 4.5 oz ai/acre of cyhalofop applied POST. A herbicide program with a PRE herbicide fb a POST application of cyhalofop may be an additional management tool for E. polystachya, and could be implemented in a management program.

The rapid spread and growing characteristics of E. polystachya could potentially reduce rice yield and cause production difficulty where significant populations of the weed exist. The goal of this research is to evaluate current total weed control programs and cultural practices in Louisiana rice for the management and control of E. polystachya.

Field Evaluation of Current Management of E. polystachya

A field study was conducted in 2004 and 2005 near Crowley, LA to determine the most effective barnyardgrass herbicide program for control of E. polystachya. Soil was a Crowley silt loam. Seedbed preparation consisted of fall and spring disking and two passes in opposite directions with a two-way bed conditioner equipped with rolling baskets and S-tine harrows set to operate at a depth of 2.4 inches.

Plots consisted of eight 7.5-inch spaced rows, 16 feet long. 'CL-161' rice was drill-seeded at 100 lb/acre on 16 May and 9 April in 2004 and 2005, respectively. The entire study area was surface irrigated immediately after seeding, at the two- to three-leaf rice stage, and at the three- to four-leaf rice stage. A 2.5-inch permanent flood was established when rice was at four- to five-leaf stage. Soil fertility management consisted of 250 lb/acre of 8-24-24 (N:P₂O₅:K₂O) fertilizer preplant and 250 lb/acre of 46-0-0 (N:₂O₅:K₂O) urea fertilizer immediately before permanent flood establishment.

Segments of E. polystachya stolons were grown in 28- by 55- by 5-inch plastic containers in a Commerce silt loam soil. The soil was thoroughly mixed with 7-21-21 fertilizer to simulate a preplant incorporated application rate of 250 lb/acre used in rice production. These stolon segments were allowed to grow to a length of 45- to 50-inches.

At approximately 24 inches from the rooting sections, stem segments consisting of an individual node were trimmed to a length of 3 inches. After rice seeding and initial surface irrigation, E. polystachya was introduced at a density of three segments/yard² or 14,520 segments/acre. The stem segments were oriented vertically and placed into the soil so that the node was covered by 0.5 to 1.0-inch of soil and the stem was slightly exposed.

The experimental design was a randomized complete block with four replications consisting of six herbicide programs. Preemergence treatments were applied before rice and E. polystachya emerged through the soil. Delayed PRE treatments were applied approximately four days after planting and prior to rice and E. polystachya emergence. POST treatments were applied at the three- to four-leaf rice stage and two- to five-leaf E. polystachya stage. Herbicide programs included: 6.4 oz ai/acre clomazone applied PRE, 6.4 oz ai/acre clomazone plus 6 oz ai/acre quinclorac applied PRE, 16 oz ai/acre pendimethalin plus 6 oz ai/acre quinclorac applied delayed PRE, 1 oz ai/acre imazethapyr applied PRE, and 2.5 oz ai/acre mesotrione applied PRE. Each herbicide application was followed by a POST application of 4.5 oz ai/acre cyhalofop. Herbicide treatments were applied using a CO₂-pressurized backpack sprayer calibrated to deliver a volume of 15 gal/acre at 193 kPa.

E. polystachya and barnyardgrass control were visually estimated 7 DAT and continued weekly until 49 DAT on a scale of 0 to 100%, where 0 = no injury and 100 = plant death. Rice plant height was measured from the soil surface to the tip of the extended panicle immediately prior to harvest. Rough rice grain yield was determined by harvesting the four center rows of each plot with a small-plot combine. Grain yield was corrected to 12% moisture.

Data were subjected to the mixed procedure of SAS with year used as a random factor (SAS Institute Inc., Cary, NC). Years, replication (nested within years), and all interactions containing either of these effects were considered random effects; herbicide program and DAT were considered fixed effects. Considering year or combination of year as random effects permits inferences about herbicide programs over a range of environments (5,9). DAT was considered a repeated measures variable, which allowed comparisons across DAT and the change in control over time (4). The objectives of repeated measures data analysis are to examine and compare response trends over time (15), in this case the control of E. polystachya throughout the growing season. Type III statistics were used to test all possible effects of fixed factors (herbicide program or DAT) and least square means were used for mean separation at a 5% probability level (P ≤ 0.05).

Weed Control and Crop Response

Barnyardgrass. No herbicide program by DAT interaction was observed for barnyardgrass control (Table 1). Herbicide program and DAT main effects were detected; therefore, data are presented separately by herbicide program averaged over DAT (Table 2) and by DAT averaged over herbicide program (Table 3).

Averaged over 7 to 49 DAT, all of the herbicide programs controlled barnyardgrass at least 87% (Table 2). Averaged over herbicide program, barnyardgrass control was 91 to 93% from 28 to 42 DAT; however, control at all DAT evaluations was at least 85% (Table 3). Because of neighboring field trials with late POST applications as well as delaying flood to aid in control of E. polystachya, permanent flood was not established until 14 and 21 DAT in 2004 and 2005, respectively; use of flood waters may not have fully benefited early-season barnyardgrass control which was 85 to 87% from 7 to 21 DAT.

Table 1. Statistical parameters for barnyardgrass and Echinochloa polystachya control with preemergence herbicide programs at different rating dates (DAT) in imidazolinone-resistant rice near Crowley, LA in 2004 and 2005 with data averaged over years.

Effect	barnyardgrass		*E. polystachya*
Effect	F value	Pr>F	F value	Pr>F
herbicide program	3.93	0.012	12.07	<0.001
DAT	21.33	<0.001	16.13	<0.001
herbicide program*DAT	0.50	NS	1.52	NS

Table 2. Evaluation of preemergence herbicide programs for control of barnyardgrass and Echinochloa polystachya, near Crowley, LA in 2004 and 2005.

Herbicide programs^w	Rate (oz ai/acre)	Timing^x	Control^y,z%
Herbicide programs^w	Rate (oz ai/acre)	Timing^x	barnyardgrass	*E. polystachya*
clomazone fb cyhalofop	6.4 4.5	PRE POST	89 a	80 a
clomazone + quinclorac fb cyhalofop	6.4 6.0 4.5	PRE PRE POST	89 a	72 b
imazethapyr fb cyhalofop	1.0 4.5	PRE POST	89 a	81 a
mesotrione fb cyhalofop	2.5 4.5	PRE POST	87 b	72 b
pendimethalin + quinclorac fb cyhalofop	16.0 6.0 4.5	DPRE DPRE POST	88 ab	78 a

^w Crop oil concentrate, Agri-dex, Helena Chemical Co., Collierville, TN at 2.5% (v/v) used with cyhalofop and at 1% (v/v) used with pendimethalin plus quinclorac.

^x Abbreviations: DPRE = applied just before rice and E. polystachya emergence; fb = followed by; POST = postemergence, applied at the three- to four-leaf rice stage and two- to five-leaf E. polystachya stage; PRE = preemergence, applied before rice and E. polystachya emerged through the soil.

^y Data averaged over year and rating date. Means in a column followed by the same letter do not significantly differ at P = 0.05.

^z Visual ratings were taken 7 days after POST herbicide application and continued weekly until 49 days after application.

Table 3. Response of barnyardgrass and Echinochloa polystachya
to preemergence herbicide programs at different rating dates
(DAT) in drill-seeded imidazolinone-resistant rice near Crowley,
LA in 2004 and 2005. ^y

DAT	Control^z (%)
DAT	barnyardgrass	*E. polystachya*
7	85 d	80 a
14	86 cd	81 a
21	87 cd	79 a
28	93 a	82 a
35	91 ab	78 a
42	91 ab	71 b
49	89 bc	67 b

^y Preemergence herbicide programs included: 6.4 oz ai/acre
clomazone PRE, 6.4 oz ai/acre clomazone plus 6.0 oz ai/acre
quinclorac PRE, 1 oz ai/acre imazethapyr PRE, 2.5 oz ai/acre
mesotrione PRE, and 16 oz ai/acre pendimethalin plus 6.0 oz
ai/acre quinclorac DPRE. Each preemergence herbicide
application was followed by a POST application of 4.5 oz ai/acre
cyhalofop.

^z Data averaged over herbicide program and years. Means in a
column followed by the same letter do not significantly differ at
P ≤ 0.05.

Echinochloa polystachya. No herbicide program by DAT interaction was observed for E. polystachya control (Table 1). Herbicide program and DAT main effects were detected; therefore, data are presented separately by herbicide program averaged over DAT (Table 2) and by DAT averaged over herbicide program (Table 3).

The herbicide programs that contained a single application of clomazone or imazethapyr applied PRE, or pendimethalin plus quinclorac applied delayed PRE controlled E. polystachya 78 to 81% (Table 2). Control was 72% for programs containing mesotrione or clomazone plus quinclorac applied PRE. The combination of clomazone and quinclorac PRE fb cyhalofop POST controlled E. polystachya 72% while clomazone PRE fb cyhalofop POST controlled the weed 80%. This control difference may be due to control of other weeds found in the test area which were competing with E. polystachya for light, nutrients, and water and this would allow E. polystachya to thrive under a less competitive environment (24).

Averaged across herbicide programs, E. polystachya control was 78 to 82% at 7 to 35 DAT (Table 3). This control was probably due to increased activity of the herbicides applied to small (two- to five-leaf) E. polystachya plants and the combined effects of PRE applications, which may have provided initial control or suppression followed by the POST application of cyhalofop which provided more activity on E. polystachya. Control of E. polystachya decreased to 71 and 67%; respectively, at 42 and 49 DAT. This reduced control was probably due to E. polystachya regrowth late in the growing season. Early ratings reflected bleaching and leaf chlorosis from herbicides applied PRE or delayed PRE and leaf chlorosis from the cyhalofop application applied POST; however, new shoots emerged late in the season from lower nodes of surviving E. polystachya.

Baruch and Merida (3) stated E. polystachya has the ability to greatly increase stolon biomass and root production once under flooded conditions. This may account for late-season recovery of the weed after permanent flood establishment. Pompeo et al. (23) reported similar growing characteristics of E. polystachya; the ability to grow upwards at a rate which allows it to remain above fluctuating water levels. The establishment of a permanent flood after POST applications may not have affected E. polystachya control at later rating dates.

Rice response. Rice heights at harvest were 35 to 36 inches for all treated rice; nontreated rice height was 32 inches (Table 4).

Table 4. Effect of preemergence herbicide programs on rice yield and height near Crowley, LA in 2004 and 2005.

Herbicide programs^w	Rate (oz ai/acre)	Timing^x	Rice^y
Herbicide programs^w	Rate (oz ai/acre)	Timing^x	Height^z (inches)	Yield (lb/acre)
clomazone fb cyhalofop	6.4 4.5	PRE POST	35 a	3540 b
clomazone + quinclorac fb cyhalofop	6.4 6.0 4.5	PRE PRE POST	35 a	3730 b
imazethapyr fb cyhalofop	1.0 4.5	PRE POST	35 a	4950 a
mesotrione fb cyhalofop	2.5 4.5	PRE POST	35 a	3840 b
pendimethalin + quinclorac fb cyhalofop	16.0 6.0 4.5	DPRE DPRE POST	36 a	3970 b
nontreated			32 b	1740 c

^w Crop oil concentrate, Agri-dex, Helena Chemical Co., Collierville, TN at 2.5% (v/v) used with cyhalofop and at 1% (v/v) used with pendimethalin plus quinclorac.

^y Data averaged over year and rating date. Means in a column followed by the same letter do not significantly differ at P = 0.05.

^z Rice height at harvest.

The location used in this study was infested with red rice. Only one herbicide program evaluated contained an imazethapyr application, which is labeled and used for red rice control (2,22,27). Rice yield has been reduced 86 to 88% by red rice competition (13,26). In this study, rice treated with imazethapyr yielded 4950 lb/acre (Table 4). Rice yields were reduced to 3540 to 3970 lb/acre when no imazethapyr applied PRE and this low yield was probably due to a heavy red rice infestation rather than E. polystachya interference.

Conclusion

Barnyardgrass was controlled in the drill-seeded, delayed-flood production system with the herbicide programs evaluated in this study. Producers have the choice of using conventional or IR rice production systems to manage E. polystachya allowing weed control decisions to be made for other weeds present such as red rice. The herbicide programs in this study were applied to E. polystachya stolons and two- to five-leaf plants, so application timing may be important when attempting to manage this weed. E. polystachya control was higher at 7 to 35 DAT compared with 42 and 49 DAT. This indicates the plant may have recovered from the initial response from the herbicide programs. Management decisions with the herbicide programs evaluated in this study may not provide total control of E. polystachya, but they do allow the weed to be managed at a level that does not negatively impact yield.

E. polystachya control may not have affected rice height or yield, but populations of this weed that are allowed to survive and propagate could impact future rice production. A ratoon rice crop could be impacted through competition for light, water, and nutrients and harvest efficiency may be reduced. Because of the prolific nature of E. polystachya, production decisions in regard to management will have to be made not only to address short-term goals in the current crop cycle but also to address the long-term survivability and spread of this weed.

Acknowledgments

Published with approval of the Director of the Agricultural Experiment Station, Louisiana State University Agricultural Center, Baton Rouge, LA 70803. Research was conducted in partial fulfillment of requirements for the Ph.D. degree in Weed Science at Louisiana State University.

Literature Cited

1. Allen, C. M. 1980. Louisiana as a habitat for grasses. Pages 3-4 in: Grasses of Louisiana. C. M. Allen, ed. Univ. of Sw. Louisiana Press, Lafayette, LA.

2. Avila, L. A., Senseman, S. A., McCauley, G. N., Chandler, J. M., and O’Barr, J. H. 2005. Effect of flood timing on red rice (Oryza spp.) control with imazethapyr applied at different dry-seeded rice growth stages. Weed Technol. 19:476-480.

3. Baruch, Z., and Merida, T. 1995. Effects of drought and flooding on root anatomy in four tropical forage grasses. Int. J. Plant Sci. 156:514-531.

4. Blouin, D. C., Webster, E. P., and Zhang, W. 2004. Analysis of synergistic and antagonistic effects of herbicides using nonlinear mixed-model methodology. Weed Technol. 18:464-472.

5. Carmer, S. G., Nyquist, W. E., and Walker, W. M. 1989. Least significant differences for combined analysis of experiments with two or three-factor treatment designs. Agron. J. 81:665-672.

6. Duke, S. O., Paul, R. N., Becerril, J. M., and Schmidt, J. H. 1991. Clomazone causes accumulation of sesquiterpenoids in cotton (Gossypium hirsutum L.). Weed Sci. 39:339-346.

7. Eleftherohorinos, I. G., and Dhima, K. V. 2002. Red rice (Oryza sativa) control in rice (O. sativa) with preemergence and postemergence herbicides. Weed Technol. 16:537-540.

8. Griffin, R. M., Webster, E. P., Leon, C. T., Zhang, W., and Mudge, C. R. 2004. Comparison of rice graminicides in Louisiana rice. Proc. of the S. Weed Sci. Soc. 57:58.

9. Hager, A. G., Wax, L. M., Bollero, G. A., and Stoller, E. W. 2003. Influence of diphenylether herbicide application rate and timing on common waterhemp (Amaranthus rudis) control in soybean (Glycine max). Weed Technol. 17:14-20.

10. Helms, R. S. 1994. Rice Production Handbook. Pages 1–16 in: Arkansas Coop. Ext. Series MP 192. Little Rock, AR

1. Jordan, D. L., Miller, D. K., and Crawford, S. H. 1998. Barnyardgrass (Echinochloa crus-galli) control in dry-seeded rice (Oryza sativa) with soil-applied and postemergence herbicide programs. Weed Technol. 12:69-73.

12. Jordan, D., and Sanders, D. E. 1999. Pest management. Pages 37-50 in: Louisiana Rice Production Handbook. Pub. No. 2321. Louisiana State Univ. AgCenter, Baton Rouge, LA.

13. Kwon, S. L., Smith, R. J., and Talbert, R. E. 1991. Interference durations of red rice (Oryza sativa) in rice (O. sativa). Weed Sci. 39:363-368.

14. Linscombe, S. D., Saichuk, J. K., Seilhan, K. P., Bollich, P. K., and Funderburg, E. R. 1999. General agronomic guidelines. Pages 5-11 in: Louisiana Rice Production Handbook. Pub. No. 2321. Louisiana State Univ. AgCenter, Baton Rouge, LA.

15. Littell, R. C., Henry, P. R., and Ammerman, C. B. 1998. Statistical analysis of repeated measure data using SAS procedures. J. Anim. Sci. 76:1216-1231.

16. Louisiana Office of State Climatology. Online. Dept. of Geog. and Anthropol., Louisiana State Univ., Baton Rouge, LA.

17. Masson, J. A., Webster, E. P., and Williams, B. J. 2001. Flood depth, application timing, and imazethapyr activity in imidazolinone-tolerant rice (Oryza sativa). Weed Technol. 15:315-319.

18. Mitchell, G., Bartlett, D. W., Fraser, T. E., Hawkes, T. R., Holt, D. C., Townson, J. K., and Wichert, R. A. 2001, Mesotrione: a new selective herbicide for use in maize. Pest Manag. Sci. 57:120–128.

19. Norris, S. R., Shen, X., and DellaPenna, D. 1998. Complementation of the arabidopsis pds1 mutant with the gene encoding p-hydroxyphenylpyruvate dioxygenase. Plant Physiol. 117:1317–1323.

20. Ntanos, D. M., Koutroubas, S. D., and Mavrotas, C. 2000. Barnyardgrass (Echinochloa crus-galli) control in water-seeded rice (Oryza sativa) with cyhalofop-butyl. Weed Technol. 14:383-388.

21. Pellerin, K. J., and Webster, E. P. 2004. Imazethapyr at different rates and timings in drill- and water-seeded imidazolinone-tolerant rice. Weed Technol. 18:223-227.

22. Pellerin, K. J., Webster, E. P., and Blouin, D. C. 2004. Herbicide mixtures in water-seeded imidazolinone-resistant rice (Oryza sativa). Weed Technol. 17:836-841.

23. Pompeo, M. L. M., Henry, R., and Moschini-Carlos, V. 2001. The water level influence on biomass of Echinochloa polystachya (POACEAE) in the Jurumirim Reservoir (Sao Paulo, Brazil). Brazilian J. Bio. 61:19-26.

24. Radosevich, S., Holt, J., and Ghersa, C. 1997. Associations of weeds and crops. Pages 163-216 in: Weed Ecology: Implications for Management, 2nd Ed. S. Radosevich, J. Holt, and C. Ghersa, ed. John Wiley and Sons Inc., New York, NY.

25. Seaman, D. E. 1983. Farmer’s weed control technology for water-seeded rice in North America. Pages 167-177 in: Proc. of the Conf. on Weed Control in Rice. Int. Rice Res. Inst., Los Banos, Laguna, Philippines.

26. Smith, R. J., Jr. 1988. Weed thresholds in southern US rice, Oryza sativa. Weed Technol. 2:232–241.

27. Steele, G. L., Chandler, J. M., and McCauley, G. N. 2002. Control of red rice (Oryza sativa) in imidazolinone-tolerant rice (O. sativa). Weed Technol. 16:627-630.

28. Stephenson, D. O., Bond, J. A., Walker, E. P., Bararpour, M. T., and Oliver, L. R. 2004. Evaluation of mesotrione in Mississippi delta corn production. Weed Technol. 18:1111-1116.

29. Strahan, R. E. 2003. New and potential weed problem in rice. Proc. S. Weed Sci. Soc. 56:361.

30. Stutzenbaker, C. D. 1999. Mudflat-millet, millet, river grass Echinochloa polystachya (Kunth in H.B.K.) A.S. Hitchc. Pages 368-369 in: Aquatic and Wetland Plants of the Western Gulf Coast. C. D. Stutzenbaker, ed. Texas Parks and Wildlife Press, Austin, TX.

31. USDA-NASS. 2005. Statistics by state: Louisiana. Online. United States Dept. of Agric, Natl. Agric. Stat. Serv. (USDA-NASS), Washington, DC.

32. USDA-NRCS. 2007. Plants database. Online. United States Department of Agriculture Natural Resources Conservation Service, Washington, DC.

33. Valverde, B. E., Carmiol, J., Riches, C. R., Caseley, J. C., Vargas, E., Chaves, L., Garita, I., and Ramirez, F. 2001. Modified herbicide regimes for propanil-resistant junglerice control in rain-fed rice. Weed Sci. 49:395-405.

34. Vasilakoglou, I. B., Eleftherohorinos, I. G., and Dhima, K. V. 2000. Propanil-resistant barnyardgrass (Echinochloa crus-galli) biotypes found in Greece. Weed Technol. 14:524-529.

35. Vencill, W. K., ed. 2002. Pages 340-342 in: Herbicide Handbook, 8th Ed. Weed Sci. Soc. of Amer., Lawrence, KS.

36. Vencill, W. K., ed. 2002. Pages 386-387 in: Herbicide Handbook, 8th Ed. Weed Sci. Soc. of Amer., Lawrence, KS.

37. Webster, E. P., Baldwin, F. L., and Dillon, T. L. 1999. The potential for clomazone use in rice (Oryza Sativa). Weed Technol. 13:390-393.

38. Zhang, W., Webster, E. P., and Blouin, D. C. 2005. Response of rice and barnyardgrass (Echinochloa crus-galli) to rates and timings of clomazone. Weed Technol. 19:528-531.