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Using soil-applied herbicides in combination with glyphosate in a glyphosate-resistant cotton herbicide program
ARTICLE IN PRESS
Crop Protection 23 (2004) 1007–1010
Using soil-applied herbicides in combination with glyphosate in a glyphosate-resistant cotton herbicide program W. James Grichara,*, Brent A. Beslera, Kevin D. Brewera, Brad W. Mintonb a
Texas Agricultural Experiment Station, 3507 Hwy 59E, Beeville, TX 78102, USA b Syngenta Crop Protection, Cypress, TX 77429, USA
Received 20 January 2004; received in revised form 20 February 2004; accepted 2 March 2004
Abstract Field studies were conducted to evaluate pendimethalin-applied preplant incorporated (PPI), prometryn-applied preemergence (PRE), or metolachlor-applied early postemergence (EPOST) in combination with glyphosate-applied EPOST or post-directed (POSDIR) for Amaranthus palmeri and Brachiaria platyphylla control in glyphosate-resistant cotton. Herbicide systems which included glyphosate-applied EPOST or POSDIR in combination with pendimethalin-applied PPI or prometryn-applied PRE controlled greater than 95% Amaranthus or Brachiaria. Herbicide systems which did not include glyphosate or included glyphosateapplied late post-emergence (LPOST) controlled less than 50% B. platyphylla. Greatest cotton yields were obtained with pendimethalin followed by glyphosate-applied EPOST followed by glyphosate plus prometryn-applied POSDIR. r 2004 Elsevier Ltd. All rights reserved. Keywords: Glyphosate; Metolachlor; Pendimethalin; Prometryn; Brachiaria platyphylla (Griseb.) NASH; Amaranthus palmeri S. Wats.; Cotton; Gossypium hirsutum L.; Preplant incorporated; Preemergence; Postemergence
1. Introduction The use of transgenic cotton (Gossypium hirsutum L.) resistant to glyphosate (Askew and Wilcut, 1999; Culpepper and York, 1998, 1999) provides producers the flexibility to control a broad spectrum of weeds with minimal concern for crop damage (Kalaher et al. 1997; Nida et al., 1996). Glyphosate can be applied postemergence (POST) over the top of glyphosate-resistant cotton from emergence through the four-leaf stage. After the four-leaf stage, glyphosate should be postdirected (POSDIR) to avoid potential fruit abortion (Kalaher et al., 1997). Glyphosate inhibits 5-enolpyruvylshikimate-3-phosphate synthase (EPSPS) (E.C.2.5.1.19), the enzyme involved in the conversion of 5-enolpyruvylshikimate3-phosphate into the aromatic amino acids tyrosine, tryptophan, and phenylalanine (Devine et al., 1993; Franz et al., 1997). Inhibition of EPSPS leads to deregulation of the shikimate pathway and subsequent *Corresponding author. Tel.: +1-361-358-6390; fax: +1-361-2932054. E-mail address: [email protected] (W.J. Grichar). 0261-2194/$ - see front matter r 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.cropro.2004.03.004
limitations on the production of the aromatic amino acids (Devine et al., 1993). Glyphosate tolerance in transgenic cotton is due primarily to expression of glyphosate-tolerant EPSPS (Nida et al., 1996). Glyphosate systems controlled a broad spectrum of weeds and gave growers the option of a total POST program in North Carolina wide-row cotton (Culpepper and York, 1998). However, under an ultra narrow row system, consistent and often greater weed control, cotton yields, and net returns were noted when glyphosate systems included soil-applied herbicides (Culpepper and York, 1999). In soybeans [Glycine max (L.) Merr.], one or two applications of glyphosate can control a broad spectrum of weeds comparable to preemergence (PRE) herbicides followed by glyphosate (Miller et al., 1997; Reddy, 1998). In most instances, PRE herbicides were not necessary to supplement total POST programs in glyphosate-resistant soybean for control of common weeds (Reddy and Whiting, 2000). Sequential applications of glyphosate have provided season-long control of most weeds in transgenic cotton (Goldmon et al., 1996; Welch et al., 1997) and glyphosate-only systems in some studies have provided
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higher yields than traditional herbicide systems (Asher et al., 1997; Welch et al., 1997). However, in other studies, glyphosate-only herbicide systems have resulted in yields comparable to traditional herbicide systems and higher net returns (Mills and Voth, 1997; Vencill, 1998; Webster et al., 1997). Other research suggests that glyphosate-tolerant cotton performs best when soil-applied herbicides are used along with glyphosate to control weeds. Its lack of residual activity means glyphosate may need to be applied more than once to control late-season weeds, whereas residual herbicides help control weeds and allow cotton to form a weed-free canopy that lasts until harvest (Culpepper and York, 1998). Glyphosate used in conjunction with residual herbicides has been shown to provide excellent weed control and high yields (Brecke and Colvin, 1997; Isgett et al., 1997; Keeling and Dotray, 1997). The necessity of PRE herbicides to supplement POSTonly programs in glyphosate-resistant cotton to maximize weed control and crop yield has not been fully investigated in south Texas. The objectives of our study were to evaluate PRE weed management systems with or without glyphosate and the influence on cotton yield utilizing glyphosate-resistant cotton.
2. Material and methods 2.1. Field studies Studies were conducted at the Texas Agricultural Experiment Station near Yoakum during the 1999
through 2000 growing season. These studies were conducted in the same field but at different locations in the field each year. Soil at the study site was a Denhawken–Elmendorf complex (fine, montmorillonitic, hyperthermic Vertic Ustochrepts-Arqiustolls) with pH 5.4–6.2 and 1.0% organic matter. Cultural practices including fertilization, insect management, and water management were standard for Texas cotton production. Glyphosate-tolerant cotton cv. DP436RR was planted in conventionally prepared seedbed in 1999 and 2000. Planting dates were 12 April, 1999, and 30 March, 2000. Plots were two 92-cm rows by 9 m long. The experimental design was a randomized complete block with treatments replicated four times. 2.2. Herbicides and application Herbicide treatments consisted of various herbicide combinations shown in Table 1. An untreated check was included for comparison. Herbicides were applied broadcast and plots were not cultivated. Soil-applied and POST over-the-top herbicides were applied using a CO2-pressurized backpack sprayer equipped with 11002 flat fan spray tips (Spraying Systems Co., Wheaton, IL, USA) delivering 187 l/ha water at 180 kPa. The POSTDIR herbicides were applied using a CO2-pressurized sprayer mounted on a POSTDIR rig equipped with two equally spaced 8002 flat fan spray tips per row and calibrated to deliver 250 l/ha at 180 kPa. Pendimethalin was incorporated 4 cm deep with a tractor-driven, vertical-action tiller while PRE applications were made immediately after cotton was planted.
Table 1 Weed control and cotton yield response in glyphosate-tolerant cotton with various herbicide systems Herbicide applicationsa,b
Control
Yield
PPI
PRE
EPOST
POSTDIR
LPOST
A. palmere (%)
B. platyphylla (%)
Seed cotton (kg/ha)
None Pendimethalin Pendimethalin Pendimethalin Pendimethalin Pendimethalin Pendimethalin Pendimethalin Pendimethalin None None LSD (0.05)
None Pro Pro None None None None None None Pro+Met None
None Met Gly+Met Gly+Met Gly Gly+Met Gly Met None None Pro+Met
None None Gly Gly Gly+Pro Gly+Pro Gly None Pro None None
None None None None None None None None None None Gly
0 97 100 99 100 100 100 92 87 100 97 10
0 47 100 100 99 100 96 33 25 15 45 22
117 752 1034 966 1367 1206 1052 768 530 835 380 511
a
PPI=preplant incorporated; PRE=preemergence; EPOST=early postemergence; POSTDIR=Post-directed; LPOST=late postemergence; Pro=prometryn; Met=metolachlor; Gly=glyphosate. b Herbicide rates; pendimethalin at 1.12 kg/ha; prometryn at 1.12 kg/ha when applied PRE or EPOST at 0.56 kg/ha when applied POSTDIR; metolachlor at 1.07 kg/ha when applied EPOST alone or with glyphosate or 1.42 kg/ha when applied PRE or EPOST with prometryn; and glyphosate at 0.84 kg/ha.
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EPOST over-the-top herbicides were applied 3–4 week after planting when cotton was 6–10 cm tall with two to three leaves. POSTDIR herbicides were applied 8–9 week after planting when cotton was 20–28 cm tall with seven to nine leaves. LPOST herbicide was applied 10 week after planting when cotton was 60 cm tall and beginning to bloom. 2.3. Weed populations The two key weeds were Amaranthus palmeri S. Wats and Brachiaria platyphylla (Griseb.) Nash. Amaranthus densities in each year were 6–8 plants/m2. Weed size at EPOST was 4–6 leaves, POSTDIR was 10–12 leaves, and LPOST was 18–24 leaves. Brachiaria densities in each year were 16–20 plants/m2. Weed size at EPOST was 6–8 leaves, POSTDIR was 12–14 leaves and LPOST was 20–28 leaves. 2.4. Data collection and analysis Weed control was estimated visually 2 week after each herbicide application and at 4 week intervals thereafter. Cotton injury was estimated visually 2 week after each herbicide application. Visual estimates of weed control and cotton injury were based on a scale of 0 (no weed control or cotton injury)–100 (complete weed control or cotton death), relative to the untreated check. Each plot was hand-picked and cleaned of trash prior to weighing. Seed cotton yields were then determined. Data were subjected to analysis of variance. There was no year by treatment interaction for weed control or cotton yields, therefore data are pooled over years. Efficacy data were transformed to the arcsine square root and means of data were separated using Fisher’sprotected LSD test at P ¼ 0:05: Nontransformed data of weed control are presented since arcsine transformation did not affect conclusions.
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York (1998) reported that glyphosate-applied EPOST was 20% more effective on Amaranthus than glyphosate-applied mid-POSTDIR. They attributed this to the weed being larger at mid-POSTDIR and larger weeds are more difficult to control with glyphosate (Jordan et al., 1997). Culpepper and York (2000) reported that the greatest control of Amaranthus was obtained when glyphosate was applied EPOST and LPOST. Glyphosate-applied EPOST only controlled this weed 100% early in the season with reduced control (86%) late season (Culpepper and York, 2000). 3.2. B. platyphylla control Any herbicide system which included glyphosateapplied EPOST or POSTDIR controlled at least 96% Brachiaria (Table 1). Herbicide systems which did not include glyphosate or included glyphosate-applied LPOST controlled less than 50%. Culpepper and York (2000) reported that glyphosate controlled this weed 97% when applied EPOST. Weed control is often excellent (at least 95%) with repeated glyphosate applications or glyphosate with soil-applied and POSTDIR herbicides (Askew et al., 1998; Keeling et al., 1998; Keeton et al., 1998). In many studies, soil-applied herbicides were not necessary for weed control with repeated glyphosate applications (Askew et al., 1998; Bloodworth et al., 1997; Wilcut and Hinton, 1997) but often reduced the number of glyphosate applications needed (Askew et al., 1998; Wilcut and Hinton, 1997; Wilcut et al., 1998). Soil-applied herbicides are routinely used to control annual grasses in cotton (Wilcut et al., 1995). However, these herbicides seldom adequately control annual grasses season-long. Therefore, the use of glyphosate as an EPOST application can reduce grass populations. 3.3. Seed cotton yield
3. Results 3.1. Amaranthus control All herbicide systems which included glyphosatecontrolled A. palmeri at least 97% (Table 1). Prometryn plus metolachlor-applied PRE provided 100% Amaranthus control while pendimethalin-applied PPI fb metolachlor-applied EPOST gave 92% control. Using pendimethalin-applied PPI fb prometryn-applied POSTDIR gave less than 90% control (Table 1). Culpepper and York (1998) reported that trifluralin-applied PPI fb fluometuron-applied PRE gave 81% weed control. Glyphosate applied twice did not improve Amaranthus control over prometryn plus metolachlor-applied EPOST fb glyphosate-applied LPOST. Culpepper and
Trends in cotton yield reflected weed control. Herbicide systems which provided at least 96% control of Amaranthus and Brachiaria produced greater than 950 kg/ha of seed lint cotton (Table 1). Where no herbicides were used, cotton lint yield was less than 125 kg/ha. Herbicide systems which did not include glyphosate or included glyphosate-applied LPOST produced less than 850 kg/ha of cotton lint. Askew and Wilcut (1999) reported when soil-applied herbicides were fb glyphosate, cotton yield increased 210–220 kg/ ha compared to glyphosate alone. Interference from weeds prior to the first glyphosate application may have reduced yield from the glyphosate-only program. Glyphosate-applied EPOST and POSTDIR did contribute to improved season-long weed control and increased cotton yields. Additionally, glyphosate
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systems give the added benefit of crop rotational flexibility in certain cotton growing areas (Batts et al., 1998; York, 1993).
Acknowledgements Appreciation extended to Karen Jamison for help in manuscript preparation; also to Syngenta Crop Protection for funding this project.
References Asher, B.S., Keeling, J.W., Dotray, P.A., 1997. Postemergence weed management in Roundup Readys and BXNt cotton. In: Proceedings of the Southern Weed Science Society, Champaign, IL, Vol. 50, pp. 173–174. Askew, S.D., Wilcut, J.W., 1999. Cost and weed management with herbicide programs in glyphosate-resistant cotton (Gossypium hirsutum). Weed Technol. 13, 308–314. Askew, S.D., Bailey, W.A., Wilcut, J.W., Hinton, J.D., 1998. Weed control in cotton with different tillage systems and herbicide resistance. In: Proceedings of the Beltwide Cotton Conference, National Cotton Council of America, Memphis, TN, Vol. 22, p. 886. Batts, R.B., Bradley Jr., A.L., Yelverton, F.H., York, A.C., 1998. Potential for Cotoran to carry over to flue-cured tobacco. In: Proceedings of the Beltwide Cotton Conference. National Cotton Council of America, Memphis, TN, Vol. 22, p. 873. Bloodworth, K.M., Reynolds, D.B., Shaw, D.R., Elkins, W.C., Serviss, B.E., Snipes, C.E., 1997. Roundup Ready weed control programs in various tillage systems. In: Proceedings of the Beltwide Cotton Conference. National Cotton Council of America, Memphis, TN, Vol. 21, p. 782. Brecke, B.J., Colvin, D.L., 1997. Weed management in glyphosatetolerant crops. In: Proceedings of the Southern Weed Science Society, Champaign, IL, Vol. 50, p. 1. Culpepper, A.S., York, A.C., 1998. Weed management in glyphosatetolerant cotton. J. Cotton Sci. 4, 174–185. Culpepper, A.S., York, A.C., 1999. Weed management and net returns with transgenic, herbicide-resistant, and nontransgenic cotton (Gossypium hirsutum). Weed Technol. 13, 411–420. Culpepper, A.S., York, A.C., 2000. Weed management in ultra narrow row cotton (Gossypium hirsutum). Weed Technol. 14, 19–29. Devine, M.D., Duke, S.O., Fedtke, C., 1993. Inhilution of amino acid biosynthesis. In: Physiology of Herbicide Action. Prentice-Hall, Englewood Cliffs, NJ, pp. 252–263. Franz, J.E., Mao, M.K., Sikorski, J.A., 1997. Toxicology and environmental properties of glyphosate. In: Glyphosate: A Unique Global Herbicide. American Chemical Society Monographs, Vol. 189. American Chemical Society, Washington, DC, pp. 103–137. Goldmon, D.L., Guy, C., McClelland, M., Kendig, A., 1996. Weed control systems in Roundup Readys and BXNt cotton. In: Proceedings of the Southern Weed Science Society, Champaign, IL, Vol. 50, pp. 173–174. Isgett, T.D., Murdock, E.C., Keeton, A., 1997. Weed control in Roundup Readys cotton. In: Proceedings of the Beltwide Cotton Conference. National Cotton Council of America. Memphis, TN, Vol. 21, p. 787.
Jordan, D.L., York, A.C., Griffin, J.L., Clay, P.A., Vidrine, P.R., Reynolds, D.B., 1997. Influence of application variables on efficacy of glyphosate. Weed Technol. 11, 354–362. Kalaher, C.J., Coble, H.D., York, A.C., 1997. Morphological effects of Roundup application timings on Roundup-Readys cotton. In: Proceedings of the Beltwide Cotton Conference. National Cotton Council of America, Memphis, TN, Vol. 21, p. 780. Keeling, J.W., Dotray, P.A., 1997. Weed management in conservation tillage systems using Roundup in Roundup Ready cotton. In: Proceedings of the Beltwide Cotton Conference. National Cotton Council of America, Memphis, TN, Vol. 21, p. 781. Keeling, J.W., Dotray, P.A., Osborne, T.S., Asher, B.S., 1998. Annual and perennial weed management strategies in Roundup Ready cotton with Roundup Ultra. In: Proceedings of the Southern Weed Science Society, Champaign, IL, Vol. 51, p. 49. Keeton, A., Toler, J.E., Murdock, E.C., Curtis Jr., C.E., Smith, J.D., 1998. Weed management in conventional and strip-tillage Roundup Readyt cotton. In: Proceedings of the Southern Weed Science Society, Champaign, IL, Vol. 51, pp. 49–50. Mills, J.A., Voth, R.D., 1997. Weed control systems in Roundup Readys cotton. In: Proceedings of the Beltwide Cotton Conference, Memphis, TN, Vol. 20, p. 787. Miller, D.K., Jordan, D.L., Griffin, J.L., Vidrine, P.R., 1997. Herbicide management systems for sicklepod control in Roundup Ready soybeans. In: Proceedings of the Southern Weed Science Society, Champaign, IL, Vol. 50, p. 38. Nida, D.L., Kolace, K.H., Guehler, R.E., Deaton, W.R., Schuler, W.R., Armstrong, T.A., Taylor, M.L., Ebert, C.C., Rogar, G.J., Padgette, S.R., Fuchs, R.L., 1996. Glyphosate-tolerant cotton: genetic characterization and protein expression. J. Agric. Food Chem. 44, 1960–1966. Reddy, K.N., 1998. Weed management in no-till Roundup Ready soybeans. In: Proceedings of the Southern Weed Science Society, Champaign, IL, Vol. 51, p. 270. Reddy, K.N., Whiting, K., 2000. Weed control and economic comparisons of glyphosate-resistant, sulfonylurea-tolerant, and conventional soybeans (Glycine max) systems. Weed Technol. 14, 204–211. Vencill, W.K., 1998. Weed management systems for Roundup Ready cotton in Georgia. In: Proceedings of the Southern Weed Science Society, Champaign, IL, Vol. 51, pp. 46–47. Webster, E.P., Beaty, J.D., Baldwin, F.L., 1997. Weed control systems for Roundup Readys cotton. In: Proceedings of the Beltwide Cotton Conference. National Cotton Council of America, Memphis, TN, Vol. 20, pp. 785–786. Welch, A.K., Rahn, P.R., Voth, R.D., Mills, J.A., Shumway, C.R., 1997. Evaluation of preplant and preemergence herbicides in Roundup Readys cotton. In: Proceedings of the Beltwide Cotton Conference. National Cotton Council of America, Memphis, TN, Vol. 21, pp. 784–785. Wilcut, J.W., Hinton, J.D., 1997. Weed management in no-till and conventional-tillage Roundup Ready cotton. In: Proceedings of the Beltwide Cotton Conference. National Cotton Council of America, Memphis, TN, Vol. 21, p. 780. Wilcut, J.W., York, A.C., Jordan, D.L., 1995. Weed management systems for oil seed crops. In: Smith, A.E. (Ed.), Handbook of Weed Management Systems. Marcel-Dekker, New York, pp. 343– 400. Wilcut, J.W., Snipes, C.E., Nichols, R.L., Hayes, R.M., Chandler, J.M., Bridges, D.C., Brecke, B.J., 1998. A regional evaluation of new technologies for weed management in conventional-tillage cotton. In: Proceedings of the Southern Weed Science Society, Champaign, IL, Vol. 51, pp. 52–53. York, A.C., 1993. Peanut response to fluometuron applied to a preceding cotton crop. Peanut Sci. 20, 111–114.
Crop Protection 23 (2004) 1007–1010
Using soil-applied herbicides in combination with glyphosate in a glyphosate-resistant cotton herbicide program W. James Grichara,*, Brent A. Beslera, Kevin D. Brewera, Brad W. Mintonb a
Texas Agricultural Experiment Station, 3507 Hwy 59E, Beeville, TX 78102, USA b Syngenta Crop Protection, Cypress, TX 77429, USA
Received 20 January 2004; received in revised form 20 February 2004; accepted 2 March 2004
Abstract Field studies were conducted to evaluate pendimethalin-applied preplant incorporated (PPI), prometryn-applied preemergence (PRE), or metolachlor-applied early postemergence (EPOST) in combination with glyphosate-applied EPOST or post-directed (POSDIR) for Amaranthus palmeri and Brachiaria platyphylla control in glyphosate-resistant cotton. Herbicide systems which included glyphosate-applied EPOST or POSDIR in combination with pendimethalin-applied PPI or prometryn-applied PRE controlled greater than 95% Amaranthus or Brachiaria. Herbicide systems which did not include glyphosate or included glyphosateapplied late post-emergence (LPOST) controlled less than 50% B. platyphylla. Greatest cotton yields were obtained with pendimethalin followed by glyphosate-applied EPOST followed by glyphosate plus prometryn-applied POSDIR. r 2004 Elsevier Ltd. All rights reserved. Keywords: Glyphosate; Metolachlor; Pendimethalin; Prometryn; Brachiaria platyphylla (Griseb.) NASH; Amaranthus palmeri S. Wats.; Cotton; Gossypium hirsutum L.; Preplant incorporated; Preemergence; Postemergence
1. Introduction The use of transgenic cotton (Gossypium hirsutum L.) resistant to glyphosate (Askew and Wilcut, 1999; Culpepper and York, 1998, 1999) provides producers the flexibility to control a broad spectrum of weeds with minimal concern for crop damage (Kalaher et al. 1997; Nida et al., 1996). Glyphosate can be applied postemergence (POST) over the top of glyphosate-resistant cotton from emergence through the four-leaf stage. After the four-leaf stage, glyphosate should be postdirected (POSDIR) to avoid potential fruit abortion (Kalaher et al., 1997). Glyphosate inhibits 5-enolpyruvylshikimate-3-phosphate synthase (EPSPS) (E.C.2.5.1.19), the enzyme involved in the conversion of 5-enolpyruvylshikimate3-phosphate into the aromatic amino acids tyrosine, tryptophan, and phenylalanine (Devine et al., 1993; Franz et al., 1997). Inhibition of EPSPS leads to deregulation of the shikimate pathway and subsequent *Corresponding author. Tel.: +1-361-358-6390; fax: +1-361-2932054. E-mail address: [email protected] (W.J. Grichar). 0261-2194/$ - see front matter r 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.cropro.2004.03.004
limitations on the production of the aromatic amino acids (Devine et al., 1993). Glyphosate tolerance in transgenic cotton is due primarily to expression of glyphosate-tolerant EPSPS (Nida et al., 1996). Glyphosate systems controlled a broad spectrum of weeds and gave growers the option of a total POST program in North Carolina wide-row cotton (Culpepper and York, 1998). However, under an ultra narrow row system, consistent and often greater weed control, cotton yields, and net returns were noted when glyphosate systems included soil-applied herbicides (Culpepper and York, 1999). In soybeans [Glycine max (L.) Merr.], one or two applications of glyphosate can control a broad spectrum of weeds comparable to preemergence (PRE) herbicides followed by glyphosate (Miller et al., 1997; Reddy, 1998). In most instances, PRE herbicides were not necessary to supplement total POST programs in glyphosate-resistant soybean for control of common weeds (Reddy and Whiting, 2000). Sequential applications of glyphosate have provided season-long control of most weeds in transgenic cotton (Goldmon et al., 1996; Welch et al., 1997) and glyphosate-only systems in some studies have provided
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higher yields than traditional herbicide systems (Asher et al., 1997; Welch et al., 1997). However, in other studies, glyphosate-only herbicide systems have resulted in yields comparable to traditional herbicide systems and higher net returns (Mills and Voth, 1997; Vencill, 1998; Webster et al., 1997). Other research suggests that glyphosate-tolerant cotton performs best when soil-applied herbicides are used along with glyphosate to control weeds. Its lack of residual activity means glyphosate may need to be applied more than once to control late-season weeds, whereas residual herbicides help control weeds and allow cotton to form a weed-free canopy that lasts until harvest (Culpepper and York, 1998). Glyphosate used in conjunction with residual herbicides has been shown to provide excellent weed control and high yields (Brecke and Colvin, 1997; Isgett et al., 1997; Keeling and Dotray, 1997). The necessity of PRE herbicides to supplement POSTonly programs in glyphosate-resistant cotton to maximize weed control and crop yield has not been fully investigated in south Texas. The objectives of our study were to evaluate PRE weed management systems with or without glyphosate and the influence on cotton yield utilizing glyphosate-resistant cotton.
2. Material and methods 2.1. Field studies Studies were conducted at the Texas Agricultural Experiment Station near Yoakum during the 1999
through 2000 growing season. These studies were conducted in the same field but at different locations in the field each year. Soil at the study site was a Denhawken–Elmendorf complex (fine, montmorillonitic, hyperthermic Vertic Ustochrepts-Arqiustolls) with pH 5.4–6.2 and 1.0% organic matter. Cultural practices including fertilization, insect management, and water management were standard for Texas cotton production. Glyphosate-tolerant cotton cv. DP436RR was planted in conventionally prepared seedbed in 1999 and 2000. Planting dates were 12 April, 1999, and 30 March, 2000. Plots were two 92-cm rows by 9 m long. The experimental design was a randomized complete block with treatments replicated four times. 2.2. Herbicides and application Herbicide treatments consisted of various herbicide combinations shown in Table 1. An untreated check was included for comparison. Herbicides were applied broadcast and plots were not cultivated. Soil-applied and POST over-the-top herbicides were applied using a CO2-pressurized backpack sprayer equipped with 11002 flat fan spray tips (Spraying Systems Co., Wheaton, IL, USA) delivering 187 l/ha water at 180 kPa. The POSTDIR herbicides were applied using a CO2-pressurized sprayer mounted on a POSTDIR rig equipped with two equally spaced 8002 flat fan spray tips per row and calibrated to deliver 250 l/ha at 180 kPa. Pendimethalin was incorporated 4 cm deep with a tractor-driven, vertical-action tiller while PRE applications were made immediately after cotton was planted.
Table 1 Weed control and cotton yield response in glyphosate-tolerant cotton with various herbicide systems Herbicide applicationsa,b
Control
Yield
PPI
PRE
EPOST
POSTDIR
LPOST
A. palmere (%)
B. platyphylla (%)
Seed cotton (kg/ha)
None Pendimethalin Pendimethalin Pendimethalin Pendimethalin Pendimethalin Pendimethalin Pendimethalin Pendimethalin None None LSD (0.05)
None Pro Pro None None None None None None Pro+Met None
None Met Gly+Met Gly+Met Gly Gly+Met Gly Met None None Pro+Met
None None Gly Gly Gly+Pro Gly+Pro Gly None Pro None None
None None None None None None None None None None Gly
0 97 100 99 100 100 100 92 87 100 97 10
0 47 100 100 99 100 96 33 25 15 45 22
117 752 1034 966 1367 1206 1052 768 530 835 380 511
a
PPI=preplant incorporated; PRE=preemergence; EPOST=early postemergence; POSTDIR=Post-directed; LPOST=late postemergence; Pro=prometryn; Met=metolachlor; Gly=glyphosate. b Herbicide rates; pendimethalin at 1.12 kg/ha; prometryn at 1.12 kg/ha when applied PRE or EPOST at 0.56 kg/ha when applied POSTDIR; metolachlor at 1.07 kg/ha when applied EPOST alone or with glyphosate or 1.42 kg/ha when applied PRE or EPOST with prometryn; and glyphosate at 0.84 kg/ha.
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EPOST over-the-top herbicides were applied 3–4 week after planting when cotton was 6–10 cm tall with two to three leaves. POSTDIR herbicides were applied 8–9 week after planting when cotton was 20–28 cm tall with seven to nine leaves. LPOST herbicide was applied 10 week after planting when cotton was 60 cm tall and beginning to bloom. 2.3. Weed populations The two key weeds were Amaranthus palmeri S. Wats and Brachiaria platyphylla (Griseb.) Nash. Amaranthus densities in each year were 6–8 plants/m2. Weed size at EPOST was 4–6 leaves, POSTDIR was 10–12 leaves, and LPOST was 18–24 leaves. Brachiaria densities in each year were 16–20 plants/m2. Weed size at EPOST was 6–8 leaves, POSTDIR was 12–14 leaves and LPOST was 20–28 leaves. 2.4. Data collection and analysis Weed control was estimated visually 2 week after each herbicide application and at 4 week intervals thereafter. Cotton injury was estimated visually 2 week after each herbicide application. Visual estimates of weed control and cotton injury were based on a scale of 0 (no weed control or cotton injury)–100 (complete weed control or cotton death), relative to the untreated check. Each plot was hand-picked and cleaned of trash prior to weighing. Seed cotton yields were then determined. Data were subjected to analysis of variance. There was no year by treatment interaction for weed control or cotton yields, therefore data are pooled over years. Efficacy data were transformed to the arcsine square root and means of data were separated using Fisher’sprotected LSD test at P ¼ 0:05: Nontransformed data of weed control are presented since arcsine transformation did not affect conclusions.
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York (1998) reported that glyphosate-applied EPOST was 20% more effective on Amaranthus than glyphosate-applied mid-POSTDIR. They attributed this to the weed being larger at mid-POSTDIR and larger weeds are more difficult to control with glyphosate (Jordan et al., 1997). Culpepper and York (2000) reported that the greatest control of Amaranthus was obtained when glyphosate was applied EPOST and LPOST. Glyphosate-applied EPOST only controlled this weed 100% early in the season with reduced control (86%) late season (Culpepper and York, 2000). 3.2. B. platyphylla control Any herbicide system which included glyphosateapplied EPOST or POSTDIR controlled at least 96% Brachiaria (Table 1). Herbicide systems which did not include glyphosate or included glyphosate-applied LPOST controlled less than 50%. Culpepper and York (2000) reported that glyphosate controlled this weed 97% when applied EPOST. Weed control is often excellent (at least 95%) with repeated glyphosate applications or glyphosate with soil-applied and POSTDIR herbicides (Askew et al., 1998; Keeling et al., 1998; Keeton et al., 1998). In many studies, soil-applied herbicides were not necessary for weed control with repeated glyphosate applications (Askew et al., 1998; Bloodworth et al., 1997; Wilcut and Hinton, 1997) but often reduced the number of glyphosate applications needed (Askew et al., 1998; Wilcut and Hinton, 1997; Wilcut et al., 1998). Soil-applied herbicides are routinely used to control annual grasses in cotton (Wilcut et al., 1995). However, these herbicides seldom adequately control annual grasses season-long. Therefore, the use of glyphosate as an EPOST application can reduce grass populations. 3.3. Seed cotton yield
3. Results 3.1. Amaranthus control All herbicide systems which included glyphosatecontrolled A. palmeri at least 97% (Table 1). Prometryn plus metolachlor-applied PRE provided 100% Amaranthus control while pendimethalin-applied PPI fb metolachlor-applied EPOST gave 92% control. Using pendimethalin-applied PPI fb prometryn-applied POSTDIR gave less than 90% control (Table 1). Culpepper and York (1998) reported that trifluralin-applied PPI fb fluometuron-applied PRE gave 81% weed control. Glyphosate applied twice did not improve Amaranthus control over prometryn plus metolachlor-applied EPOST fb glyphosate-applied LPOST. Culpepper and
Trends in cotton yield reflected weed control. Herbicide systems which provided at least 96% control of Amaranthus and Brachiaria produced greater than 950 kg/ha of seed lint cotton (Table 1). Where no herbicides were used, cotton lint yield was less than 125 kg/ha. Herbicide systems which did not include glyphosate or included glyphosate-applied LPOST produced less than 850 kg/ha of cotton lint. Askew and Wilcut (1999) reported when soil-applied herbicides were fb glyphosate, cotton yield increased 210–220 kg/ ha compared to glyphosate alone. Interference from weeds prior to the first glyphosate application may have reduced yield from the glyphosate-only program. Glyphosate-applied EPOST and POSTDIR did contribute to improved season-long weed control and increased cotton yields. Additionally, glyphosate
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systems give the added benefit of crop rotational flexibility in certain cotton growing areas (Batts et al., 1998; York, 1993).
Acknowledgements Appreciation extended to Karen Jamison for help in manuscript preparation; also to Syngenta Crop Protection for funding this project.
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