Weed Control in Glyphosate-Tolerant Lettuce

Weed Technology. 2003. Volume 17:738–746

Weed Control in Glyphosate-Tolerant Lettuce (Lactuca sativa)1 STEVEN A. FENNIMORE and KAI UMEDA2 Abstract: Field studies were conducted in Arizona and California to evaluate the performance of glyphosate-tolerant lettuce and to determine the critical time of weed removal. Glyphosate was applied as a single or as a sequential application at 840 g ae/ha. Single glyphosate applications were made to lettuce at the two-, four-, six-, and eight-leaf stages. Sequential applications were made to lettuce at the two- or four-leaf stage followed by (fb) a second application 14 d after the first. Weed control efficacy, weeding times, and lettuce yield were all measured. Overall, glyphosate applied postemergence (POST) provided better weed control than the commercial standards bensulide or pronamide applied preemergence. Single glyphosate applications at the four-leaf stage and sequential applications at the two-leaf stage fb a second application 14 d later provided excellent control of most weeds, including redroot pigweed. Estimates of the critical time of weed removal were 26 to 29 d after emergence. Glyphosate treatments caused no adverse effects on lettuce. Lettuce head fresh weights in the glyphosate treatments were equal to or higher than those in bensulide or pronamide treatments. For crops such as lettuce, with few effective herbicides, the development of glyphosatetolerant lettuce offers the opportunity to develop effective POST weed control programs. Nomenclature: Bensulide; glyphosate; pronamide; redroot pigweed, Amaranthus retroflexus L. #3 AMARE; lettuce, Lactuca sativa L. ‘RR Raider 323’. Additional index words: CAPBP, Capsella bursa-pastoris, CHEAL, CHEMU, Chenopodium album, Chenopodium murale, ECHCO, Echinochloa colonum, glyphosate-tolerant lettuce, herbicide tolerance, iceberg lettuce, LEFUN, Leptochloa uninerva, MALPA, Malva parviflora, POROL, Portulaca oleracea, Solanum sarrachoides, SOLSA, time-of-application. Abbreviations: DAE, days after emergence; fb, followed by; POST, postemergence.

INTRODUCTION

bicide options (Cudney et al. 2002; Fennimore 2001). Pesticide manufacturers are unwilling to invest in registrations for crops with small markets and high liability. The success of glyphosate-tolerance in major crops such as soybean may also contribute to the lack of new vegetable herbicides because fewer herbicides in development for major crops will result in fewer new herbicides available to screen for natural tolerance in vegetable crops. The smaller size of the U.S. herbicide market also reduces the incentives for herbicide manufacturers to seek new herbicides for use in major crops (Shaner 2000). Therefore, development of herbicide-tolerant varieties may offer the most direct path to the creation of a new weed management system for lettuce. In the United States, 121,000 ha of lettuce are grown annually with about 85% grown in California, 12% in Arizona, and the remainder divided among several other states (USDA 2001). Weed management in lettuce is accomplished through an integrated set of practices that include cultivation, hand weeding, and herbicides (Cudney et al. 2002). Three herbicides are used primarily in lettuce: benefin, bensulide, and pronamide (CA DPR

The advantage of glyphosate-tolerant crops is that most weeds can be controlled economically with a single herbicide (Shaner 2000). The introduction of glyphosatetolerant cotton (Gossypium hirsutum L.) and soybean [Glycine max (L.) Merr.] varieties has dramatically altered weed control programs for these crops (Faircloth et al. 2001; Norsworthy and Oliver 2001; Scott et al. 1998). Glyphosate-tolerant field corn (Zea mays L.) and canola (Brassica napus L.) are available, and glyphosatetolerant wheat (Triticum aestivum L.) is in development (Clayton et al. 2002; Endres et al. 2002; Shaner 2000). Biotechnology has the potential to improve weed control for vegetable crops such as lettuce, a crop with few her1 Received for publication August 28, 2002, and in revised form April 14, 2003. 2 Extension Specialist, Department of Vegetable Crops and Weed Science, University of California–Davis, 1636 East Alisal, Salinas, CA 93905; Extension Agent, University of Arizona, Maricopa County, 4341 East Broadway Road, Phoenix, AZ 85040. Corresponding author’s E-mail: [email protected]. 3 Letters following this symbol are a WSSA-approved computer code from Composite List of Weeds, Revised 1989. Available only on computer disk from WSSA, 810 East 10th Street, Lawrence, KS 66044-8897.

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2001). These herbicides are applied preemergence (PRE) for control of broadleaf and grass weeds. Pronamide is the most widely used lettuce herbicide in California (CA DPR 2001). However, the future status of pronamide is unclear because it is a probable human carcinogen (USEPA 1994), and its regulatory status is being reviewed under the provisions of the Food Quality Protection Act of 1996 (Goldman 1997). Currently, there are no postemergence (POST) broadleaf herbicides registered for lettuce (Cudney et al. 2002). Furthermore, benefin, bensulide, and pronamide provide control of only a limited number of broadleaf and grass weeds. Lettuce producers are forced to depend on prophylactic applications of PRE herbicides with a limited weed spectrum. The potential for crop loss due to weeds in lettuce is quite severe if weeds are allowed to compete with lettuce (Lanini and LeStrange 1991; Roberts et al. 1977; Shrefler et al. 1994). Roberts et al. (1977) found that lettuce yields were reduced if weeds were allowed to persist for more than 3 to 4 wk after lettuce emergence. One of the limitations of glyphosate is the lack of soil residual activity (WSSA 2002). To compensate for the lack of soil residual activity, researchers in glyphosatetolerant soybean have examined the use of sequential applications and mixtures with residual herbicides to provide full-season weed control (Scott et al. 1998). Similarly, weed control programs for new glyphosatetolerant crops such as lettuce must be carefully constructed to minimize yield loss. Askew and Wilcutt (1999) reported that glyphosate-tolerant cotton was stunted by weed competition when glyphosate application was delayed until 3 wk after emergence. Therefore, glyphosate applications in glyphosate-tolerant lettuce must be early to avoid the potential for yield loss yet timed late enough to control as many weeds as possible. The development of glyphosate-tolerant lettuce provides a potentially useful weed management strategy for lettuce growers. Glyphosate-tolerant lettuce cultivar ‘RR Raider 323’, an iceberg lettuce type (Ryder 1999), was developed from parents transformed with the glyphosate tolerance gene (S. King, personal communication), as described in Torres et al. (1999). Preliminary field evaluations of glyphosate-tolerant lettuce in Florida indicated that glyphosate applied at the three- to four-leaf stage was safe (Nagata et al. 2000). However, a weed management program for glyphosate-tolerant lettuce has not been developed. We have attempted to develop preliminary information required for a glyphosate-tolerant lettuce weed management program using typical lettuce production practices in two major lettuce production reVolume 17, Issue 4 (October–December) 2003

Table 1. Time after emergence to herbicide applications, lettuce thinning, hand weeding, and harvest. Salinas Event

2000

Yuma 2001

2000

2001

Days after emergence Two-leaf lettuce Four-leaf lettuce Six-leaf lettuce Thinning Eight-leaf lettuce Two-leaf lettuce fb 14 da Four-leaf lettuce fb 14 d Hand weeding Harvest a

7 14 19 19 26 7 and 21 14 and 28 34 55 and 58

8 17 22 20 28 8 and 22 17 and 31 36 57 and 60

8 12 16 8 20 — — 36 65

7 14 21 21 25 — — 32 111

Abbreviation: fb, followed by.

gions: the Salinas Valley of California and Yuma, AZ. The objectives of these studies were to determine the optimal time to apply glyphosate relative to lettuce emergence, to evaluate the effect of glyphosate tolerance on lettuce yield and quality, and to estimate the critical time of weed removal in glyphosate-tolerant lettuce. MATERIALS AND METHODS

Site Information, Trial Establishment, and Plant Material. Local cultural practices were followed for soil fertility, irrigation scheduling, row cultivation, and for scheduling of thinning, weed removal, and harvest (Ryder 1999). A field study was repeated four times during a 2-yr period. Lettuce, RR Raider 323, was direct-seeded on raised beds May 9, 2000, and May 8, 2001, near Salinas, CA, and on September 22, 2000, and October 8, 2001, near Yuma, AZ (Table 1). The Yuma study initiated in October 2001 was not harvested until early 2002 but will be referred to as the ‘‘Yuma 2001’’ study. The soil at Salinas was a Chualar series loamy sand (fine-loamy, mixed, thermic Typic Argixeroll), and the soil at Yuma was an Indio silt loam (coarse-silty, mixed [calcareous], hyperthermic Typic Torrifluvent) in 2000 and a Holtville clay loam (clayey over loamy, monmorillonitic [calcareous], hyperthermic Typic Torrifluvent) in 2001. All studies, except Yuma in 2000, used seeds coated with gypsum. Gypsum coating is a standard industry practice and helps ensure an even plant population (Ryder 1999). A commercial Stanhay4 planter was used to plant the lettuce in all studies except at Yuma in 2000. Because of insufficient time between seed availability and planting time, uncoated lettuce seed was used in the Yuma 2000 study and was planted by hand. The 4 Stanhay 800 series planter. Stanhay LTD., Exning, Newmarket, Suffolk, U.K.

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FENNIMORE AND UMEDA: WEED CONTROL IN GLYPHOSATE-TOLERANT LETTUCE

Table 2. Total monthly precipitation and average monthly maximum and minimum air temperatures at Salinas, CA, and Yuma, AZ, during the study period.

Location

a,b

Air temperatures

Month and year

Precipitation Maximum mm

Salinas Salinas Salinas Yuma Yuma Yuma Yuma Salinas Salinas Salinas Yuma Yuma Yuma Yuma

May 2000 June 2000 July 2000 September 2000 October 2000 November 2000 December 2000 May 2001 June 2001 July 2001 October 2001 November 2001 December 2001 January 2002

18.0 3.6 0 0 22.4 0 0 0 0.5 5.1 2.5 0 0.3 0

Minimum C

21.6 22.7 21.0 39.1 30.0 22.2 22.2 24.2 23.7 20.9 34.4 26.6 19.4 20.5

9.6 11.6 11.8 22.8 15.5 6.7 6.1 9.5 10.1 12.6 17.2 11.7 3.3 4.4

a Salinas weather data were recorded by the CIMIS south Salinas weather station. b Yuma weather data were recorded by an on site weather station.

lettuce was planted at a density of 387,000 seed/ha and about 3 wk after emergence was thinned with a hoe to 48,400 plants/ha as per standard practices (Ryder 1999). Many weeds present in the seed line near lettuce plants are removed during the thinning process, but weed removal is of secondary importance. Hand weeding was conducted about 2 wk after thinning to remove remaining weeds. At Salinas, mechanical cultivation was performed just before thinning and hand weeding (Table 1). With the exception of a 9-cm band over each seed line that was left uncultivated, the entire bed and row middle was cultivated. Mechanical cultivation was not performed at Yuma, with the exception that the row middles were cultivated once in 2001. Weeds escaping herbicide treatments and cultivations were removed by hand 32 to 36 d after crop emergence (DAE). At Yuma in 2001, a field crew weeded the entire study inadvertently at 32 DAE. Overhead sprinkler irrigation was used in Salinas, and furrow irrigation was used in Yuma. Precipitation

and air temperatures for Salinas were recorded by the CIMIS5 south Salinas weather station approximately 4 km from the site (Table 2). Weather data for the Yuma studies were recorded by the AZMET6 weather station on site at the Yuma field station. Treatments. Single PRE treatments of pronamide and bensulide were included in the study. Pronamide was applied at 1.3 kg/ha and bensulide at 5.6 kg/ha in Salinas. At Yuma, 1.1 and 6.7 kg/ha of pronamide and bensulide were applied, respectively. The differences in pronamide and bensulide rates between Salinas and Yuma reflect differences in local use patterns due to soil type and weed spectrum. Herbicides were activated immediately after application with sprinkler irrigation at Salinas and with furrow irrigation at Yuma. Glyphosate treatments were applied POST at four lettuce growth stages: (1) two, (2) four, (3) six, and (4) eight fully expanded leaves. In Salinas, sequential applications of glyphosate were made at the two-leaf stage followed by (fb) a second application 14 d later (two-leaf fb 14 d) and at the four-leaf stage fb a second application 14 d later (four-leaf fb 14 d). Weed sizes at each glyphosate application time are listed in Table 3. All glyphosate applications were made at the rate of 840 g ae/ha. Herbicides were applied with CO2 backpack sprayers equipped with 8002VS flat fan nozzles calibrated to deliver 187 L/ha at 290 kPa. The herbicides were applied as 56-cm bands directly to the top of the raised beds at Salinas and as broadcast applications at Yuma. The hand-weeded check was weeded at least once per week from time of crop emergence to harvest. The treatments were arranged in a randomized complete block design with four replications. Each plot consisted of two raised beds 1 m wide 5 CIMIS weather data for select California locations is available online at http://www.ipm.ucdavis.edu/weather/weather1.html. 6 AZMET weather data for the Yuma, AZ, field station is available online at http://ag.arizona.edu/azmet/02.htm.

Table 3. Weed size, in number of leaves or height, for each application date at Salinas, CA, and Yuma, AZ. Salinas

Yuma

2000

2001

Lettuce stage AMARE

CAPBP

MALPA

Two leaf Four leaf Six leaf Eight leaf

cot.-2 2–6 6–8 10–14

cot. c-3 2–4 7–10

a

SOLSA

2000

AMARE

CAPBP

CHEAL

CHEMU

LEFUN

1–2 2–5 6–8 10–18

2–5 4–7 5–8 10–14

cot.-2 cot.-4 6–11 6–20

2 4–6 6 (7–10)

2–4 3–5 (12–15) (17–20)

leaf no. cot.b 2–3 2–5 8–13

cot.-1 2–3 4–5 4–8

2001 POROL

ECHCO

POROL

leaf no. (height cm) 4 4–6 (10–12) (25)

2 4 4–6 (7–17)

cot. 4–6 (5–12) (20)

a Abbreviations: AMARE, redroot pigweed; CAPBP, shepherd’s-purse; CHEAL, common lambsquarters; CHEMU, nettleleaf goosefoot; ECHCO, junglerice; LEFUN, Mexican sprangletop; MALPA, little mallow; POROL, common purslane; SOLSA, hairy nightshade. b Abbreviation: cot., cotelydon stage.

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by 7.6 m long, and two seed lines 27 cm apart were planted per bed. Data Collection. Treatment effects were measured by three methods: (1) weed density counts, (2) time required to hand weed, and (3) yield. Weed densities were measured before thinning, cultivation, and hand-weeding operations, i.e., two weed counts were made (Table 1). Time from lettuce emergence to thinning varied somewhat between Salinas and Yuma because of constraints on crew availability and irrigation timing. Cultural practices at Salinas differed from normal practices only in that the thinning hoe was used only to thin the lettuce. Normally, thinning crews remove many weeds in the lettuce seed line at the time of thinning. During thinning, some weeds were removed, but fewer weeds were removed than is standard cultural practice. Thinning times were measured to determine if use of glyphosate in glyphosate-tolerant lettuce would increase thinning efficiency. Lettuce thinning times were recorded for each treatment at Salinas only, and hand-weeding times were recorded for each treatment at Salinas in 2000 and 2001 and at Yuma in 2000. These times were measured for an area one bed wide by 7.6 m long at Salinas or two beds wide by 7.6 m long at Yuma. Phytotoxicity was assessed every 2 wk throughout the season at Salinas and periodically after each POST application at Yuma, with zero 5 no crop injury and 100 5 crop death. At harvest, the numbers of marketable lettuce heads were counted and fresh weight determined. Statistical Analysis. Data were subjected to ANOVA using the SAS7 general linear model (GLM) procedure. Data tested were weed densities, labor input requirements, number of marketable lettuce heads, and lettuce head fresh weight. When the ANOVA indicated a nonsignificant experiment-by-treatment effect at P 5 0.05, the data from different sites were pooled for analysis. Single degree of freedom linear contrasts were used to compare treatments at P 5 0.05 (SAS 1991). The untreated and hand-weeded controls were not included in the contrasts, but the data are presented for comparison purposes. The critical period of weed removal was determined by fitting lettuce head fresh weights to Gompertz and logistic equations as described by Halford et al. (2001). The data were fitted to these equations using SigmaPlot8: 7 SAS for Windows, version 8.0. Statistical Analysis Systems Institute. Cary, NC 27511. 8 Sigma Plot for Windows, version 4.0. SPSS Inc. Chicago, IL 60611.


Y 5 d 1 a 3 e{2e[2(X 2 c)/b]}

[1]

Y 5 a/{1 1 e[b 3 (X 2 c)]} 1 d

[2]

where Y is the yield (% hand-weeded control), X the days after lettuce emergence, a the range between highest and lowest yield (%), c the point of greatest inflection (days), b the slope at c, d the lowest yield (%), and e the mathematical constant. The equation with the best fit (r2) to the data was selected after conditions for normality and homogeneity were considered. RESULTS AND DISCUSSION

Weed Control Evaluations. Principal weed species at Salinas in both 2000 and 2001 were redroot pigweed and shepherd’s-purse (Capsella bursa-pastoris) (Tables 3 and 4). Hairy nightshade (Solanum sarrachoides) and little mallow (Malva parviflora) were present in 2000, and common lambsquarters (Chenopodium album) was present in 2001. Weed densities are reported for the period 25 to 35 DAE, timed to occur after all herbicides had been applied but before hand weeding took place. Some weeds were removed by cultivation and thinning operations that were conducted before 25 to 35 DAE. However, with the exception of the eight-leaf glyphosate application, the weed densities at 25 to 35 DAE best characterize the treatment effects on weed populations, before cultivation and hand weeding removed all remaining weeds. Sufficient time after application had not elapsed for weeds in the eight-leaf glyphosate treatments to die by 25 to 35 DAE. The single glyphosate four-leaf application was chosen as the reference treatment against which other glyphosate treatments as well as bensulide and pronamide were compared because it consistently provided good weed control. There were no differences in glyphosate efficacy on any weeds at Salinas, except common lambsquarters, whether applied at the two-, four-, or six-leaf stage. Results suggest that the four-leaf stage was the best time to control common lambsquarters. More effective common lambsquarters control resulted from the single glyphosate four-leaf application than from the single two-leaf application. This difference was likely due to weed emergence after the time of the two-leaf application because the weed density counts taken before thinning indicated good common lambsquarters control with this treatment (data not shown). Common lambsquarters and redroot pigweed control provided by bensulide at 5.6 kg/ha and the single glyphosate four-leaf treatment did not differ. The glyphosate four-leaf application provided better control of hairy nightshade, little mallow, and shepherd’s-purse 741


Table 4. Effect of time of glyphosate application on weed densities at Salinas, CA, in 2000 and 2001. Weed densities were measured after thinning and cultivation, at 28 DAE in 2000 and 35 DAE in 2001, but before hand weeding. Weed densities

Treatment

AMARE 2000 and 2001a,b

Timing

CAPBP 2000 and 2001b

MALPA 2000

SOLSA 2000

CHEAL 2001

29.3 0.5 12.7 24.4 6.3 0.0 4.4 23.4 0.5 0.0

13.7 2.4 14.6 3.9 11.7 3.9 10.3 6.8 8.3 1.5

no./m2 Untreated Hand weeded Pronamidec Bensulided Glyphosatee Glyphosate Glyphosate Glyphosatef Glyphosate Glyphosate ANOVAg Glyphosate Glyphosate Glyphosate Glyphosate Glyphosate Glyphosate

— — Preemergence Preemergence Two leaf Four leaf Six leaf Eight leaf Two leaf fb 14 d Four leaf fb 14 d four leaf vs. glyphosate two leaf four leaf vs. glyphosate six leaf four leaf vs. bensulide four leaf vs. pronamide two leaf vs. glyphosate two leaf fb 14 d four leaf vs. glyphosate four leaf fb 14 d

75.2 2.0 66.4 6.1 23.7 4.2 9.3 19.5 6.6 1.7 0.07 0.63 0.85 ,0.001 0.11 0.82

9.3 1.5 7.3 12.7 2.0 2.0 4.2 11.7 2.0 0.5

17.1 0.5 8.8 14.8 11.2 7.8 3.9 8.3 3.4 8.8

1.0 0.37 ,0.001 0.0297 1.00 0.55

0.30 0.24 0.029 0.77 0.020 0.77

0.37 0.54 0.001 0.08 0.41 1.00

0.036 0.09 1.0 0.005 0.35 0.50

a Abbreviations: fb, followed by; AMARE, redroot pigweed; CAPBP, shepherd’s-purse; CHEAL, common lambsquarters; DAE, days after emergence; MALPA, little mallow; SOLSA, hairy nightshade. b Density counts were pooled for 2000 and 2001. c The pronamide rate was 1.3 kg/ha. d The bensulide rate was 5.6 kg/ha. e The glyphosate rate was 840 g ae/ha. f The weed density counts in 2000 were made 2 d after the eight-leaf stage glyphosate application was made and insufficient time had elapsed for weed death. g Single degree-of-freedom contrasts used to compare treatments at P 5 0.05.

than bensulide and provided better control of all weeds than pronamide, with the exception of little mallow. None of the herbicides completely controlled little mallow. The sequential glyphosate applications did not significantly reduce weed densities compared with the single glyphosate applications, with the exception that the glyphosate two-leaf fb 14 d sequential treatment provided better control of little mallow than the single two-leaf application. The principal weeds at Yuma were nettleleaf goosefoot (Chenopodium murale), common purslane (Portulaca oleracea), junglerice (Echinochloa colonum), and Mexican sprangletop (Leptochloa uninerva) (Table 5). Glyphosate time-of-application had no effect on control of nettleleaf goosefoot, junglerice, or Mexican sprangletop but did affect common purslane control. Glyphosate applied at the six-leaf lettuce stage controlled 5- to 12cm-tall common purslane in 2001 but failed to control 10- to 12-cm-tall common purslane in 2000. The differences between the 2 yr in glyphosate activity at the sixleaf stage was perhaps due to cooler conditions in 2001 at Yuma compared with 2000, which resulted in smaller common purslane plants in 2001 that were more readily 742

killed (Tables 2, 3, and 5). The eight-leaf glyphosate application failed to control common purslane because these weeds were large, 20- to 25-cm tall at the time of application (Table 3), and sufficient time for glyphosate translocation and weed death had not elapsed by the time weed density counts were made (Table 5). It was not possible to postpone weed density counts until later than 25 to 35 DAE because this would have required a delay in hand weeding and normal cultural practices such as fertilizer application. The poor common purslane control in the eight-leaf stage glyphosate application indicates that this application timing is too late to be compatible with cultural practices such as cultivation, hand weeding, and fertilizer application. The glyphosate four-leaf treatment reduced the number of common purslane (2000 only) and nettleleaf goosefoot more effectively than bensulide, but both herbicides provided effective junglerice and Mexican sprangletop control. At Yuma, the glyphosate four-leaf treatment provided more effective control of all weeds than pronamide. Glyphosate application timing had an effect on the time required to thin lettuce at Salinas (Table 6). Thinning took less time with a single two-leaf stage glyVolume 17, Issue 4 (October–December) 2003

WEED TECHNOLOGY

Table 5. Effect of glyphosate timings on weed densities at Yuma, AZ, in 2000 and 2001. Weed densities were measured after thinning, at 35 DAE in 2000 and 26 DAE in 2001, but before hand weeding. Weed densities Treatment

CHEMU 2000a

Timinga

LEFUN 2000

POROL 2000

ECHCO 2001

POROL 2001

no./m2 Untreated Hand weeded Pronamideb Bensulidec Glyphosated Glyphosate Glyphosate Glyphosatee ANOVAf Glyphosate Glyphosate Glyphosate Glyphosate

— — Preemergence Preemergence Two leaf Four leaf Six leaf Eight leaf four four four four

leaf leaf leaf leaf

vs. vs. vs. vs.

glyphosate two leaf glyphosate six leaf bensulide pronamide

32.8 0.0 32.0 18.0 0.0 0.0 0.0 0.0

4.0 0.0 3.2 0.0 0.0 0.0 0.0 0.0

1.0 1.0 ,0.001 ,0.001

30.9 0.0 32.8 14.5 0.0 0.0 14.8 29.6

1.0 1.0 1.0 ,0.001

9.1 0.0 7.5 0.0 1.6 0.0 0.0 4.3

1.0 ,0.001 ,0.001 ,0.001

0.63 1.0 0.57 0.028

14.0 0.0 7.5 2.7 0.5 0.0 0.0 16.1 0.85 1.0 0.34 0.013

a Abbreviations: fb, followed by; CHEMU, nettleleaf goosefoot; DAE, days after emergence; ECHCO, junglerice; LEFUN, Mexican sprangletop; POROL, common purslane. b The pronamide rate was 1.1 kg/ha. c The bensulide rate was 6.7 kg/ha. d The glyphosate rate was 840 g ae/ha. e The weed density counts in 2001 were made 1 d after the eight-leaf stage glyphosate application was made and insufficient time had elapsed for weed death. f Single degree-of-freedom contrasts used to compare two treatments at P 5 0.05.

Table 6. Effect of glyphosate timing on lettuce thinning and hand weeding times at Salinas, CA, and Yuma, AZ. Weeding time

Treatment

Timing

Thinning time Salinas 2000 and 2001a

2000

— — Preemergence Preemergence Two leaf Four leaf Six leaf Eight leaf Two leaf fb 14 d Four leaf fb 14 d

25.2 20.7 21.6 21.1 18.9 20.3 24.8 25.0 19.6 20.4

222.9 39.5 159.6 132.4 83.5 49.0 62.8 91.3 40.1 43.0

0.041 —f —f 0.64 0.31 —f —f

0.036 0.40 0.011 ,0.001 ,0.001 0.009 0.71

Salinas 2001

Yuma 2000

h/ha Untreated Hand weeded Pronamideb Bensulidec Glyphosated Glyphosate Glyphosate Glyphosate Glyphosate Glyphosate ANOVAe Glyphosate Glyphosate Glyphosate Glyphosate Glyphosate Glyphosate Glyphosate

four leaf vs. glyphosate two leaf four leaf vs. glyphosate six leaf four leaf vs. glyphosate eight leaf four leaf vs. bensulide four leaf vs. pronamide two leaf vs. glyphosate two leaf fb 14 d four leaf vs. glyphosate four leaf fb 14 d

151.5 66.1 126.4 53.1 37.7 34.8 39.1 53.5 30.4 17.9 0.91 0.87 0.48 0.49 0.002 0.78 0.53

181.1 22.0 159.2 86.4 15.7 22.0 60.4 86.1 — — 0.58 0.002 ,0.001 ,0.001 ,0.001 — —

Thinning times for 2000 and 2001 from Salinas, CA, were combined. Pronamide rates in kg/ha at Yuma, AZ, were 1.1 and at Salinas were 1.3. c Bensulide rates in kg/ha at Salinas, CA, were 5.6 and at Yuma were 6.7. d The glyphosate rate was 840 g ae/ha. e Single degree-of-freedom contrasts used to compare two treatments at P 5 0.05. f The application had not been applied by time of thinning, so this contrast was not made. a

b


743


Table 7. Effect of glyphosate timing on number of marketable heads at Salinas in 2000 and lettuce fresh weights at Salinas, CA, and Yuma, AZ. Fresh weights

Treatment

Marketable heads

Timing

Salinas 2000

Yuma 2001

1,000/ha Untreated Hand weeded Pronamidea Bensulideb Glyphosatec Glyphosate Glyphosate Glyphosate Glyphosate Glyphosate ANOVAd Glyphosate Glyphosate Glyphosate Glyphosate Glyphosate Glyphosate Glyphosate

— — Preemergence Preemergence Two leaf Four leaf Six leaf Eight leaf Two leaf fb 14 d Four leaf fb 14 d four leaf vs. glyphosate two leaf four leaf vs. glyphosate six leaf four leaf vs. glyphosate eight leaf four leaf vs. bensulide four leaf vs. pronamide two leaf vs. glyphosate two leaf fb 14 d four leaf vs. glyphosate four leaf fb 14 d

2000

2001

1,000 kg/ha

21.0 55.8 31.0 34.4 40.3 52.3 56.5 50.4 57.8 58.4

14.2 40.8 19.9 25.3 27.4 39.2 39.4 34.4 43.3 40.4

20.2 34.4 22.9 27.5 36.1 37.8 34.4 30.9 41.4 34.2

0.11 0.57 0.79 0.019 0.005 0.020 0.41

0.61 0.97 0.44 0.03 0.003 0.023 0.839

0.74 0.52 0.19 0.06 0.008 0.31 0.50

16.5 60.9 17.4 39.8 65.1 59.1 55.2 47.4 — — 0.19 0.39 0.015 ,0.001 ,0.001 — —

39.7 37.8 37.2 40.2 40.7 41.4 40.4 36.2 — — 0.72 0.61 0.018 0.57 0.05 — —

Pronamide rates in kg/ha at Salinas, CA, were 1.3 and at Yuma, AZ, were 1.1. Bensulide rates in kg/ha at Salinas, CA, were 5.6 and at Yuma, AZ, were 6.7. c The glyphosate rate was 840 g ae/ha. d Single degree-of-freedom contrasts used to compare two treatments at P 5 0.05. a

b

phosate application compared with a single four-leaf stage application. Because the six- or eight-leaf treatments had not been applied early enough to affect thinning times, they were not compared with the single fourleaf treatment. There were no differences in thinning time among the single glyphosate four-leaf application, bensulide, and pronamide. The sequential glyphosate treatments did not influence thinning times because none of the second applications had been made at the time of thinning (data not shown). At Salinas in 2000, the single four-leaf glyphosate treatment took less time to weed than both the single two- and eight-leaf applications, but in 2001 there was no effect of glyphosate application stage on weeding time. The two-leaf fb 14 d sequential treatment had lower weeding times at Salinas in 2000 than the two-leaf single application. In Yuma, the glyphosate application timing was significant, and weeding time in the four-leaf treatment was less than in the sixor eight-leaf treatments. The four-leaf stage glyphosate application reduced weeding times at Salinas and Yuma in 2000 compared with bensulide, and it reduced weeding times at Salinas both the years and at Yuma in 2000 compared with pronamide. Because of the inadvertent weeding of the Yuma trial at 32 DAE in 2001, weeding time was not recorded. Crop Evaluations and Yields. None of the glyphosate treatments caused visual injury symptoms on the lettuce 744

in any of the four studies (data not shown). Timing of single glyphosate applications had no effect on the number of marketable heads at Salinas in 2000 (Table 7). Marketable heads were increased with a single glyphosate application at the four-leaf stage when compared with bensulide or pronamide. Where glyphosate was applied sequentially at the two-leaf fb 14 d stages, marketable heads increased compared with a single application at the two-leaf stage. There were no differences in the number of marketable heads at Salinas in 2001 or in either year at Yuma (data not shown). There were no differences in fresh weights between the single glyphosate application timings at Salinas (Table 7). However, at Yuma, fresh weights increased in the single four-leaf stage glyphosate application compared with the eight-leaf stage application. Fresh weights were higher in the single glyphosate four-leaf stage application compared with bensulide and pronamide both years at Salinas and in 2000 at Yuma. Lettuce fresh weights were higher in the sequential glyphosate two-leaf fb 14 d application compared with the single two-leaf application in 2000 but not in 2001. The differences in lettuce fresh weight responses between 2000 and 2001 were likely due to competition from weeds that escaped the single application in 2000 but that were controlled by the single application in 2001. This finding suggests the need for flexibility in lettuce weed control programs to allow for differences Volume 17, Issue 4 (October–December) 2003

WEED TECHNOLOGY

in weed emergence patterns from year to year (Forcella et al. 1992). Critical Time of Weed Removal. Because of the successive nature of weed removal by the treatments in this study, we were able to fit the data to sigmoidal functions to estimate the critical time of weed removal. Lettuce head fresh weights were fitted to logistic and Gompertz equations as described by Halford et al. (2001), and the logistic equation (2) was found to best fit the data. At Salinas, the 2000 and 2001 data were pooled, and the parameters for equation (2) were a 5 54.4, b 5 20.0, c 5 29.2, d 5 45.8, and r2 5 0.88. At Yuma in 2000, the parameters for equation (2) were a 5 95.4, b 5 4.0, c 5 25.7, d 5 7.7, and r2 5 0.99. Parameter c indicates that the critical time of weed removal was 29.2 d at Salinas and 25.7 d at Yuma in 2000. Attempts to fit the Yuma 2001 data to either sigmoidal function were not successful, likely because of slow crop development due to cool weather conditions compared with the other three studies (Table 2). The critical time of weed removal for lettuce was 3 to 4 wk, which was similar to previous research (Roberts et al. 1977). These results suggest that glyphosate should be applied at the two- to six-leaf stage. However, a second application may be necessary to control weed escapes. In canola, early single glyphosate applications generally out-yielded single applications made later (Clayton et al. 2002). Canola is densely planted and competitive, so that when early season weeds are removed, crop competition does not permit later emerging weeds to compete. However, lettuce is grown at relatively lower densities, and weeds that escape early glyphosate applications can still affect lettuce yields. Lettuce is more similar to soybean, where sequential glyphosate applications were found to be more cost-effective than single applications (Swanton et al. 2000). Lettuce Weed Management Systems. Our hypothesis is that glyphosate has the potential to enhance the efficiency of lettuce production by allowing chemical removal of weeds in the seed line before thinning and hand weeding. The two-leaf stage glyphosate application had the lowest thinning time (Table 6), likely because this treatment enhanced lettuce-thinning efficiency through chemical weed removal before the lettuce was thinned. However, at Salinas in 2000, considerable weed emergence occurred after the two-leaf stage, and a sequential application was needed to provide season-long weed control. Therefore, future studies need to be conducted that evaluate this system in commercial production Volume 17, Issue 4 (October–December) 2003

fields. A single eight-leaf application is riskier because this growth stage approaches or exceeds the critical time of weed removal for lettuce (Roberts et al. 1977), and a significant reduction in lettuce yield was observed at Yuma in the eight-leaf treatment (Table 7). There are additional possible changes in lettuce production with the integration of glyphosate-tolerant cultivars, and these benefits include reduction or elimination of mechanical cultivation. This was not the objective of this work and further studies will need to be undertaken before any conclusions can be arrived at. In conclusion, our results suggest that glyphosate-tolerant lettuce is a technically viable product. Effective weed control was provided by glyphosate applications made when lettuce was in the four-leaf stage. Sequential applications may improve weed control compared with single applications. Glyphosate treatments generally provided more effective control than the present commercial standards bensulide or pronamide. ACKNOWLEDGMENTS

We thank Steven King of Seminis Vegetable Seeds, Ken Dubas formerly of Genecorp Seeds, Carlos Reyes, Henry Wu, and Vint Hicks of Monsanto Company for their suggestions, technical advice, and financial support. We also thank Grant Manning, Jose Valdez, and technical personnel at the Yuma Valley Agricultural Center for help with trial maintenance and evaluations. LITERATURE CITED Askew, S. D. and J. W. Wilcut. 1999. Cost and weed management with herbicide programs in glyphosate-resistant cotton (Gossypium hirsutum). Weed Technol. 13:308–313. [CA DPR] California Department of Pesticide Regulation. 2001. 2000 Annual Pesticide Use Report. Department of Pesticide Regulation, Sacramento, CA. Clayton, G. W., K. N. Harker, J. T. O’Donovan, M. N. Baig, and M. J. Kidnie. 2002. Glyphosate timing and tillage system effects on glyphosate-resistant canola (Brassica napus). Weed Technol. 16:124–130. Cudney, D. W., C. E. Bell, R. F. Smith, W. T. Lanini, M. LeStrange, S. A. Fennimore, and W. E. Bendixen. 2002. Lettuce Herbicide Treatment Table. UC DANR Publication 3339. Web page: http://www.ipm.ucdavis. edu. Endres, G. J., P. E. Hendrickson, B. Schatz, and S. A. Valenti. 2002. Weed control and crop response with Roundup-Ready spring wheat. Proc. West. Soc. Weed Sci. 55:68–69. Faircloth, W. H., M. G. Patterson, C. D. Monks, and W. R. Goodman. 2001. Weed management programs for glyphosate-tolerant cotton (Gossypium hirsutum). Weed Technol. 15:544–551. Fennimore, S. A. 2001. Future weed management options for vegetable producers. Weed Sci. Soc. Am. Abstr. 41:140. Forcella, F., R. G. Wilson, K. A. Renner, J. Dekker, R. G. Harvey, D. A. Alm, D. D. Buhler, and J. Cardina. 1992. Weed seedbanks of the U.S. Corn Belt: magnitude, variation, emergence, and application. Weed Sci. 40: 636–644. Goldman, L. R. 1997. Raw and processed food schedule for pesticide tolerance reassessment notice. Fed. Reg. 67:42019–42030.

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Halford, C., A. S. Hamill, J. Zhang, and C. Doucet. 2001. Critical period of weed control in no-till soybean (Glycine max) and corn (Zea mays). Weed Technol. 15:737–744. Lanini W. T. and M. Le Strange. 1991. Low-input management of weeds in vegetable fields. Calif. Agric. 45:11–13. Nagata, R. T., J. A. Dusky, R. J. Ferl, A. C. Torres, and D. J. Cantliffe. 2000. Evaluation of glyphosate resistance in transgenic lettuce. J. Am. Soc. Hort. Sci. 125:669–672. Northsworthy, J. K. and L. R. Oliver. 2001. Competitive potential and economic analysis of a glyphosate-tolerant/conventional soybean (Glycine max) mix. Weed Technol. 15:177–183. Roberts, H. A., R. T. Hewson, and M. A. Ricketts. 1977. Weed competition in drilled summer lettuce. Hortic. Res. 17:39–45. Ryder, E. J. 1999. Crop Production Science in Horticulture 9. Lettuce, Endive and Chicory. Wallingford, UK: CABI Pp. 79–89. [SAS] Statistical Analysis Systems. 1991. SAS System for Linear Models. 3rd ed. Cary, NC: Statistical Analysis Systems Institute. Pp 151–152. Scott, R., D. R. Shaw, and W. L. Barrentine. 1998. Glyphosate tank mixtures with SAN 582 for burndown or postemergence applications in glyphosate-tolerant soybean (Glycine max). Weed Technol. 12:23–26. Shaner, D. L. 2000. The impact of glyphosate-tolerant crops on the use of

other herbicides and on resistance management. Pestic. Manag. Sci. 56: 320–326. Shrefler, J. W., J. A. Dusky, D. G. Shilling, B. J. Brecke, and C. A. Sanchez. 1994. Effects of phosphorous fertility on competition between lettuce (Lactuca sativa L.) and spiny amaranth (Amaranthus spinosus L.). Weed Sci. 42:556–560. Swanton, C. J., A. Shrestha, K. Chandler, and W. Deen. 2000. An economic assessment of weed control strategies in no-till glyphosate-resistant soybean (Glycine max). Weed Technol. 14:755–763. Torres, A. C., R. T. Nagata, R. J. Ferl, T. A. Bewick, and D. J. Cantiliffe. 1999. In vitro assay selection of glyphosate resistance in lettuce. J. Am. Soc. Hort. Sci. 124:86–89. [USDA] United States Department of Agriculture. 2001. Agricultural Statistics 2001. Chapter IV. Statistics of Vegetables and Melons. Washington, DC: Agricultural Statistics Board, NASS USDA. [USEPA] United States Environmental Protection Agency. 1994. R.E.D. Facts: Pronamide. Washington, DC: Environmental Protection Agency EPA-738-F-94-007. 7p. [WSSA] Weed Science Society of America. 2002. Herbicide Handbook. 8th ed. W. K. Vencill, ed. Lawrence, KS: Weed Science Society of America. Pp. 231–234.

Composite List of Weeds—1989 Available on computer disk in ASCII format The names of 2,076 weed species of current or potential importance in the United States and Canada are arranged by scientific name, by WSSA approved common name and by five-letter ‘‘Bayer code.’’ The Composite List of Weeds—1989 is available only on computer disk. Computer disk priced at US $10.00 (remittance to accompany order). (Shipping charge: $3.50 for first copy; $0.75 for each additional copy.) Ship to: Name Address City

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Signature Make checks payable to Weed Science Society of America and mail to P.O. Box 7050, 810 East 10th St., Lawrence, KS 66044-8897. Ph: (800) 627-0629 (U.S. and Canada), (785) 843-1235; Fax: (785) 843-1274; E-mail: [email protected]. WETE/master_fillers filler07

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