California Pepper Commission


Smith-LeStrange Preemergence Weed Control

California Pepper Commission - Complete Final Report, 2012

Title: Preemergence Weed Control Trials in Peppers

Principle Investigators:
Richard Smith
Farm Advisor, Monterey County 1432 Abbott Street
Salinas, CA 93901
(831) 759-7350; (831) 758-3018 FAX

Michelle LeStrange
Farm Advisor, Tulare County 4437B S. Laspina St.
Tulare, CA, 93274
(559) 684-3320; Fax: (559) 685-3319

Timeline: March 1, 2012 to February 28, 2013


  1. Examine pre-transplant (at planting) and layby preemergence herbicides for weed control.
  2. Conduct the above trials in the Central Coast and Central Valley production districts.

CENTRAL COAST Summary: Field trials were conducted to evaluate potential new herbicides for use on peppers. Zeus, Nortron and Outlook were compared with the standard herbicides, Dual Magnum and Prowl H2O. In addition, a new low VOC formulation of Prefar was compared with the standard EC formulation of Prefar. Two trials were conducted in commercial bell pepper fields with cooperating growers. Trial No. 1 was conducted to evaluate pre-transplant applications and Trial No. 2 was conducted to evaluate layby applications. There was a high population of hairy nightshade at trial No. 1, but the population was spotty which made determining statistical differences among the treatments difficult. However, a trend indicated that Zeus and Outlook had greater weed control. Zeus at 3.0 fl oz was safer than 6.0 fl oz on the soil type at the trial site (silty clay). The Nortron and Prefar treatments had the highest yields in this trial. In the layby trial, the treatments were applied between and to the sides of the seedlines. The sprays were directed, but inevitably some material contacted the foliage of the plants. Where Zeus and Nortron contacted the foliage they caused phytotoxicity on the leaves: burned lesions with the use of Zeus and distortion of the foliage with Nortron. All materials reduced the number of hairy nightshade plants on two evaluation dates and there were no differences in yield among the treatments.

CENTRAL VALLEY Summary: Three field trials evaluated 5 preemergence herbicides: Zeus, Nortron and Outlook were compared with the standard herbicides, Dual Magnum and Prowl H2O. All were applied at 1x and 2x rates, and a 4x rate of Outlook was also applied, and all were compared to an untreated check. The treatment list was the same for each trial. In two trials the herbicides were applied one day before transplanting the peppers and the difference between the trials was that one was completely grown using subsurface drip irrigation and the other was grown using sprinkler irrigation followed by furrow irrigation. In the pre-transplant furrow application all herbicide applications resulted in some crop phytotoxicity, but crop damage by Outlook (4x) and Zeus (2x) was significantly higher than the rest. A third trial applied the herbicides at layby and the trial was grown using furrow irrigation, however sprinkler irrigation was used to incorporate the layby herbicides. As in the Central Coast layby trial, where Zeus and Nortron contacted the foliage they cause initial phytotoxicity on the leaves: burned lesions with the use of Zeus and distortion of the foliage with Nortron, however these symptoms were greatly reduced with time. A 4x application of Outlook resulted in less phytotoxicity to pepper leaves than a 2x rate of Nortron or a 1x rate of Zeus. An application of a 2x rate of Outlook showed the same pepper phytotoxity as a 1x application of Prowl H2O, both of which diminished as the peppers grew. In all trials Dual Magnum, Prowl H2O, and Outlook provided excellent results in broadleaf and grass weed control.

CENTRAL COAST Methods: Trial No. 1: The trial was conducted in a commercial bell pepper field with a cooperating grower east of Gilroy. The soil at the site was Campbell silty clay. Beds were shaped and treatments were applied prior to transplanting on May 15 with two passes of a one-tip boom (8008EVS) at 30 psi applying the equivalent of 70 GPA of water. The bell pepper variety Baron was transplanted in two seedlines on the beds and was watered the same day. Each plot was one 40-inch bed wide by 20 feet long and replicated three times in a randomized complete block design. Treatments listed in Table 1. See tables for evaluations and dates. Twelve plants per plot were harvested and graded as shown in Table 2.

Trial No. 2: The trial was conducted in a commercial bell pepper field with a cooperating grower north of Hollister. The soil at the site was Sorrento silty clay loam. The peppers variety Baron planted on two seedlines on a 40 inch wide bed in May. The treatments were applied at the layby stage when the plants were 7-9 inches tall on June 15 with three passes with a one-tip wand with an 8008EVS nozzle applying the equivalent of 104 GPA; one pass was made between the seedlines and the other two were made to either side of the seedlines. The plots were carefully weeded to remove all weed seedlings prior to the layby applications (see photos). The applications were made as directed a spray, however some contact with foliage was inevitable. Seven plants per plot were harvested and graded as shown in Table 4.

CENTRAL COAST Results: Trial No. 1: Zeus at 6.0 fl oz had significant phytotoxicity ratings on both evaluation dates (Table 1). There was a great deal of variability between plots in the number of weeds and no statistical differences in total weeds was observed; however, there are strong trends indicating that Zeus at 6.0 fl oz and Outlook at 14.0 fl oz greatly reduced weed pressure and hand weeding time. The untreated, both Nortron and Prefar treatments had the highest yields in the trial (Table 2).

Trial No. 2: Both rates of Zeus and Nortron caused phytotoxicity on the leaves of peppers (Table 3). Even though the spray applications were directed, there was some contact with the materials on the foliage. Zeus caused burned lesions and Nortron caused deformity of the foliage. All materials greatly reduce hand weeding time. There were no statistical differences in yield among treatments (Table 4).

CENTRAL VALLEY Methods: Three field trials investigating five preemergence herbicides at 1x and 2x rates were compared to an untreated check in transplanted bell peppers. In two trials the applications were made at planting and in the third trial the application was made at layby where the crop had no other previous at planting herbicides. The herbicide trials were conducted at the UC West Side Research and Extension Center in Five Points in Fresno County where the soil type is a Panoche Clay Loam. On May 8, 2012 the bell pepper variety "Grande Rio" was transplanted in single rows into 40" beds using a commercial transplanter. Within row spacing was 10" between plants, but stand establishment was not as good as in previous years.

Five preemergence herbicides: Zeus, Nortron and Outlook were compared with the standard herbicides, Dual Magnum and Prowl H2O. All were applied at 1x and 2x rates, and a 4x rate of Outlook was also applied, and compared to an untreated check. In all trials the 12 treatments were replicated 4 times for a total of 48 plots. Plot size was one 40-inch bed wide by 70-feet of row length. The sprayer was a commercial sprayer outfitted with 8003 evs Teejet nozzles and a water volume of 40 GPA at the 1x rate. The 2x rate and 4x rate received 2 passes for a total of 80 GPA. At planting applications were broadcast applied uniformly over the beds and furrows. Prior to the layby application on June 7, the trial area was mechanically cultivated and hand weeded (June 5-6) so that the plots were weed free at the time of application. The layby herbicide application was aimed at the base of the plants (not over the top), but drop nozzles were not used for a directed spray. The layby treatments were applied on June 7, 2012 with weather conditions of 85° F average temperature, clear skies, and wind at 3-4 mph. The herbicides were set with sprinklers, but the trial was grown under furrow irrigation.

Initial weed populations were very high from planting to layby and included: pigweeds (prostrate, redroot, and tumble), purslane, lambsquarters, puncturevine, nightshades (hairy, black, and groundcherry), and jungle rice (very similar to barnyardgrass) as the main weeds. Bindweed was sporadic throughout the area. However, these weed populations were not high in the untreated check plots after layby cultivations; so weed pressure was considerably lower in this trial compared to the at planting applications, even after several furrow irrigations. Weed counts and evaluations were conducted May 31 in the at-planting applications and on June 7, June 29 and August 16 in the layby application. Yield data was not collected this year.

CENTRAL VALLEY Results: Trial #1: Pre-transplant application, Drip Irrigation (Table 5)
In this trial overall weed control ratings (where 10 is a perfect score) for all treatments averaged 6.8 for broadleaf weeds and 7.3 for grass weeds three weeks after transplanting. Weed populations of purslane, lambsquarters, and puncturevine were too low and spotty for significant differences, but nightshade and junglerice populations were particularly heavy in this field. Prowl H2O (1x and 2x), Dual Magnum (2x), and Outlook (2x and 4x) had the highest broadleaf weed control ratings (all were 8.6 or higher) and worked well on pigweeds and nightshades. Nortron (1x = 5.1, 2x = 6.8) and Zeus (1x = 3.8, 2x = 4.5) were considerably weaker, but better than the untreated check (2.5). Prowl H2O (1x and 2x) had the best grass control followed by Outlook (2x and 4x) and Dual Magnum (1x and 2x). Zeus (1x) was no better than the untreated check. Results of this trial (grown completely using subsurface drip irrigation) revealed that at the 1x rate Prowl H2O provided the best weed control. Although the data is not shown, all plots, even the untreated, showed a few dead plants or plants with curly leaves, so phytotoxicity was too hard to distinguish from stand establishment problems. However, in general, phytotoxicity as a result of pre-transplant herbicide application was minimal. Even Outlook at the 4x rate showed very little crop damage symptoms. Only weed control data was collected; the trial was not harvested because poor crop stand, poor weed control with no layby cultivation, and bindweed became too much of an issue.

Trial #2: Pre-transplant application, Furrow Irrigation (Table 6) - Weed control ratings were higher in the sprinkler-furrow trial averaging 8.0 for total broadleaf and grass weed control. Weed counts were similar between drip and furrow treatments but the furrow plots had higher broadleaf counts especially in untreated plots (data shown below).

Number of weeds per 12 square feet
Untreated Plots Subsurface Drip Sprinkler to Furrow
Broadleaf weeds (total) 7797
Grass weeds (total) 5856

Dual Magnum (1x, 2x) Outlook (1x, 2x, 4x) and Prowl H2O (2x) had the best overall ratings in both broadleaf and grass control. Zeus weed control was improved under furrow irrigation, but Nortron was still very weak. Nightshade was significantly reduced by Dual (2x) Outlook (1x, 2x, and 4x) and Prowl (2x) and Zeus (2x). Junglerice was controlled by Dual (1x, 2x), Outlook (1x, 2x, and 4x), and Prowl H2O (1x, 2x). There was no difference in grass control between these herbicides at the 1x rate. Crop phytotoxicity was apparent with all herbicide applications. Even plants in the untreated check plots showed curling, cupping, and stunting symptoms, a result of crop stress from winds, warm temperatures, and thrips damage. The 2x rates had higher ratings, but these were not statistically different from the 1x rates. Only Zeus (2x) and Outlook (4x) had statistically higher crop phytotoxicity ratings than the other herbicide treatments. All symptoms decreased over time as the crop grew (data not shown).

Trial #3: Layby application, Furrow Irrigation (Table 7) - Crop stand counts were collected on the day of layby application just to be sure that there was no difference in pepper stand as a result of mechanical and hand weeding cultivations. Average phytotoxicity ratings were higher on June 29 (2.31) than on August 16 (0.58). The 2x herbicide rate typically had more crop phytotoxicity symptoms with the exception of Dual Magnum. However most crop phytotoxicity was not that damaging and difficult to distinguish from normal wind and stress damage. The exceptions are Nortron and Zeus (especially the 2x rate). Weed populations were low, so only herbicide ratings were collected from this trial. Broadleaf weed control was excellent (greater than 8.0) with all herbicide treatments. Grass weed control was higher than 7.0 for all herbicides except Zeus (1x and 2x = 4.3), which was no different from the untreated plot (3.5). Prowl H2O gave the best results closely followed by Dual Magnum and Outlook.

CENTRAL VALLEY Conclusions: Of the three trials crop phytotoxicity was most obvious in the Pre-transplant herbicide trial where the herbicides were sprinkled in immediately after transplanting and then furrow irrigated for the remainder of the crop production season. Weed control was also best achieved when the herbicides were sprinkler incorporated.

Table 1. CENTRAL COAST: Trial 1
Phytotoxicity ratings and weed counts (per 20 ft2) on two dates and time of weeding
Material/AJune 1June 14
Phytotoxicity1NightshadeSow thistleLambsquarterPhytotoxicity1Total weedsWeed time
Dual Magnum 7.621.4324.0 fl oz0.
Nortron 4SC0.7524.0 fl oz0.
Nortron 4SC1.0032.0 fl oz0.
Zeus 4F0.0943.0 fl oz1.
Zeus 4F0.1886.0 fl oz3.
Outlook 6.00.614.0 fl oz0.
Prefar 4EC6.06 qt0.
Prefar (low VOC)6.06 qt0.
 LSD (0.05)1nsnsns1.1nsns

1 - Scale: 0=no crop damage to 10=crop dead

Table 2. CENTRAL COAST: Trial 1.
Harvest evaluations on October 1
TreatmentsLbs a.i./AMaterial/ARedGreenBreakerCullsTotal Fruit
lbsNo.lbsNo.lbsNo.lbsNo.lbsNo.Mean fruit
weight lbs.
Dual Magnum 7.621.4324.0 fl oz25.748.
Nortron 4SC0.7524.0 fl oz30.
Nortron 4SC1.0032.0 fl oz26.450.
Zeus 4F0.0943.0 fl oz29.456.
Zeus 4F0.1886.0 fl oz25.948.
Outlook 6.00.6014.0 fl oz24.845.
Prefar 4EC6.06 qt31.
Prefar (low VOC)6.06 qt31.561.
LSD (0.05)4.911.1nsnsnsnsnsns4.611.7ns
Table 3. CENTRAL COAST: Trial 2.
Phytotoxicity ratings and weed counts (per 15 ft2) on two dates, and time to weed
Layby ApplicationLbs a.i./AMaterial/AJune 27August 27
Phytotoxicity1NightshadePhytotoxicity1NightshadeMalvaWeed time
Dual Magnum 7.621.4324.0 fl oz0.
Prowl H2O 3.8EC1.501.58 qt0.
Dual Magnum 7.62 +
Prowl H2O 3.8EC
24.0 fl oz
Outlook 6.00.6014.0 fl oz0.
Zeus 4F0.0943.0 fl oz2.
Zeus 4F0.1886.0 fl oz3.
Nortron 4SC0.7524.0 fl oz2.
Nortron 4SC1.0032.0 fl oz2.
 LSD (0.05)

1 - Scale: 0=no crop damage to 10=crop dead

Table 4. CENTRAL COAST - Trial 2.
Harvest evaluations on September 20
TreatmentsLbs a.i./ARedGreenBreakerCullsTotal Fruit
LbsNo.LbsNo.LbsNo.LbsNo.LbsNo.Mean Fruit
wt lbs
Dual Magnum 7.621.4317.936.
Prowl H2O 3.8EC1.5016.733.
Dual Magnum 7.62 +
Prowl H2O 3.8EC
Outlook 6.00.6021.543.
Zeus 4F0.09418.937.
Zeus 4F0.18816.833.
Nortron 4SC0.7520.
Nortron 4SC1.0017.
LSD (0.05)Nsnsnsnsnsnsnsnsnsnsns
Table 5. CENTRAL VALLEY: Trial #1 - Pre-transplant Preemergence Herbicides - Subsurface DRIP IRRIGATION
Weed Control Ratings and Counts - May 31, 2012
Weed Control
Counts/12 ft2
BrdlfGrassPigPursLambsPunctNightAll BrdlvsGrassBind
1Dual Magnum 7.631x1.431.5 pts8.
2Dual Magnum 7.632x2.863 pts8.
3Nortron 4sc1x1.753.5 pts5.15.817.
4Nortron 4sc2x3.507.0 pts6.
5Outlook 6.01x0.0510.7 ozs6.
6Outlook 6.02x1.021.4 ozs8.68.612.
7Prowl H2O 3.8 EC1x1.53 pts9.
8Prowl H2O 3.8 EC2x3.06 pts9.
9Zeus 4F1x0.0943 ozs3.
10Zeus 4F2x0.1886 ozs4.54.412.
11Outlook4x2.042.8 ozs8.
LSD (0.05)

*10 = perfect weed control; 1= no weed control
Weeds = pigweeds, purslane, lambsquarters, puncturevine, nightshades, barnyardgrass, Total broadleaves, bindweed

Table 6. CENTRAL VALLEY: Trial #2 - Pre-transplant Preemergence Herbicides - FURROW IRRIGATION
Weed Control Ratings and Counts - May 31, 2012
Weed Control
Weed Counts/12 ft2 - May 31, 2013June
1, 2013
1Dual Magnum 7.631x1.431.5 pts8.
2Dual Magnum 7.632x2.863 pts9.4100.
3Nortron 4SC1x1.753.5 pts5.65.614.
4Nortron 4SC2x3.507.0 pts7.
5Outlook 6.01x0.0510.7 ozs8.
6Outlook 6.02x1.021.4 ozs9.
7Prowl H2O 3.8EC1x1.53 pts8.
8Prowl H2O 3.8EC2x3.06 pts9.
9Zeus 4F1x0.0943 ozs8.
10Zeus 4F2x0.1886 ozs8.
11Outlook4x2.042.8 ozs9.910.
LSD (0.05)
Pr>Trt 0.00000.00000.00000.00950.00050.09460.00000.00000.00000.61910.0009
Pr>Bl 0.31460.46070.81350.21230.18600.03570.50610.56610.06720.05690.5624
CV% 10.414.858.2121.984.8115.880.558.061.5229.843.4

*10 = perfect weed control; 1= no weed control
Weeds = pigweeds, purslane, lambsquarters, puncturevine, nightshades, barnyardgrass, Total broadleaves, bindweed

Table 7. CENTRAL VALLEY: Trial #3 - Layby Application - Preemergence Herbicides - FURROW IRRIGATION
Pepper Stand, Crop Phytotoxicity, Weed Control Ratings
CodeTreatmentsLbs a.i./AMaterial/AJune 7, 2012June 29August 16, 2012
Pepper StandPhyto
W bedE bed
1Dual Magnum 7.631x1.431.5 pts36.033.50.5008.88.8
2Dual Magnum 7.632x2.863 pts37.335.
3Nortron 4SC1x1.753.5 pts35.033.02.0007.07.0
4Nortron 4SC2x3.507.0 pts37.335.34.7507.07.0
5Outlook 6.01x0.0510.7 ozs33.035.00.500.257.37.3
6Outlook 6.02x1.021.4 ozs38.534.01.250.508.38.3
7Prowl H2O 3.8EC1x1.53 pts31.832.81.2509.09.0
8Prowl H2O 3.8EC2x3.06 pts36.834.31.7509.69.6
9Zeus 4F1x0.0943 ozs36.031.83.500.504.34.3
10Zeus 4F2x0.1886 ozs37.332.
11Outlook4x2.042.8 ozs35.331.
LSD (0.05)

*Weed Control Rating: 10 = perfect weed control; 1= no weed control
Phytotoxicity Rating: 10 = crop totally dead; 0 = no crop injury

Trial 1 on June 15

1. untreated


2. Dual Magnum


3. Zeus 3.0 fl oz


4. Outlook 14.0 fl oz

Note: removed weeds in furrow

Trial 2

5. Layby timing


6. Small nightshade removed before treatment


7. Plot at harvest

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