Malheur Experiment Station
Oregon State University
Information for Sustainable Agriculture
Weed Control In Onion With Postemergence Herbicides

WEED CONTROL IN ONION WITH POSTEMERGENCE HERBICIDES††

 

Corey V. Ransom, Charles A. Rice, and Joey K. Ishida

Malheur Experiment Station

Oregon State University

Ontario, OR, 2004

 

 

Introduction

 

Weed control is essential for the production of marketable onions.  Weed control in onions is difficult compared to many crops because of the lack of a complete crop canopy and limited herbicide options.  Chateau® (flumioxazin), formerly called Valor®, and Nortron® (ethofumesate) are two experimental herbicides that have been evaluated for use in onions in past research trials.  Trials were conducted this year to determine the benefits of using these experimental herbicides in postemergence herbicide combinations and compare their performance to registered herbicide combinations. 

 

Methods

 

General Procedures

Trials were conducted at the Malheur Experiment Station to evaluate experimental and registered herbicides for weed control and onion tolerance.  Trials were conducted under furrow irrigation.  On March 25, onions (cv. 'Vaquero', Nunhems, Parma, ID) were planted at 3.7-inch spacing in double rows on 22-inch beds.  Plots were 4 rows wide and 27 ft long and arranged in a randomized complete block design with four replications.  Lorsban® was applied in a 6-inch band over each double row at 3.7 oz/1,000 ft of row.  Onions were sidedressed with 175 lb nitrogen (N), 30 lb phosphorus (P), 35 lb sulfate, 38 lb sulfur (S), 2 lb Zinc (Zn), 3 lb manganese (Mn), and 1 lb boron (B)/acre on June 3.  Registered insecticides and fungicides were applied for thrips and downy mildew control.

 

Herbicide treatments were applied with a CO2-pressurized backpack sprayer.  Preemergence applications and postemergence grass herbicides were applied at 20 gal/acre at 30 psi and postemergence treatments were applied at 40 gal/acre at 30 psi.  All plots were treated with a preemergence application of Roundup® (glyphosate) at 0.75 lb ai/acre plus Prow®l (pendimethalin) at 1.0 lb ai/acre on April 5 and a postemergence application of Poast® (sethoxydim) at 0.19 lb ai/acre plus crop oil concentrate (COC) (1.0% v/v) on June 16.  Postemergence treatments were applied to two-leaf onions on May 6, two- to three-leaf onions on May 14, and to five-leaf onions on June 2.  In the Chateau application timing trial, a separate application of Chateau was made to three-leaf onions on May 18.  Weed control and onion injury were evaluated throughout the season.  Onions were harvested September 16 and 17 and graded by size on October 1-4.

 

Data were analyzed using analysis of variance and means were separated using a protected least significant difference (LSD) at the 5 percent level (0.05).

 

Comparison of Postemergence Chateau or Goal Combinations

Chateau and Goal® (oxyflurofen) were applied in combinations with Buctril® (bromoxynil) to evaluate weed control and onion tolerance.  Buctril, Goal, and Chateau were evaluated at two rates.  Comparisons of Goal or Chateau with Buctril included several combinations of herbicides and rates.  Additional treatments included a split application of Chateau applied to two-leaf and again to three-leaf onions, and a comparison of Buctril plus Chateau treatments following preemergence applications of Roundup, Prowl, and Dacthal® (DCPA).

 

Application Timings for Chateau

Chateau was applied at two rates in combination with Buctril to two-leaf or three-leaf onions.  Chateau treatments were compared to Goal plus Buctril.  Additional treatments included Chateau in a separate application 4 days after the Buctril application at the three-leaf application timing.

 

Addition of Nortron to Postemergence Treatments

This trial was conducted to determine if the addition of Nortron to postemergence herbicide applications would improve weed control.  Each treatment was applied with or without Nortron added to the two-leaf and three-leaf applications at either 0.25 or 0.5 lb ai/acre.  One treatment evaluated Outlook® (dimethenamid-P) added to the two-leaf application and Nortron added to the three-leaf application. 

 

Results and Discussion

 

Preemergence herbicides worked fairly well due to rainfall in April.  Adequate rainfall also ensured that weeds were actively growing when postemergence treatments were applied.

 

Comparison of Postemergence Chateau or Goal Combinations

Because the preemergence application of Prowl was so effective, there were no differences in weed control between any of the postemergence herbicide treatments (Table 1).  Control of all species was 85 percent or greater.  There were also no differences in onion injury among treatments.  This is surprising as there were large differences in the herbicide rates applied for different treatments.  There were a few differences in onion yields, with higher yields resulting from treatments with additional soil-active herbicides applied or with higher rates (Table 2). 

 

Application Timings for Chateau

Treatment with Chateau combined with Buctril when applied either to two-leaf or three-leaf onions did not cause greater injury than combinations of Goal plus Buctril (Table 3).  When Chateau was applied alone 4 days after Buctril was applied to three-leaf onions, injury increased significantly.  By July 21 there were no differences in onion injury among treatments.  The injury caused by the delayed application of Chateau was probably related to wet cool weather that occurred after the Buctril application and prior to the Chateau application.  Pigweed (redroot pigweed and Powell amaranth), common lambsquarters, hairy nightshade, and barnyardgrass control was not different among herbicide treatments and was 88 percent control or greater.  Kochia control was significantly greater with treatments that contained Chateau tank-mixed with Buctril and applied at either the two-leaf or three-leaf timing compared to Buctril plus Goal.  Treatments where Chateau was applied alone following the Buctril application to three-leaf onions did not control weeds better than Buctril plus Goal.

 

There were few significant differences in onion yields between herbicide treatments, with marketable yields ranging from 1,107 to 1,459 cwt/acre.

 

Addition of Nortron to Postemergence Treatments

Onion injury was not different among treatments on either evaluation date (Table 5).  Pigweed, hairy nightshade, and barnyardgrass control was similar among herbicide treatments and was 89 percent or higher.  The addition of Nortron at either rate to Buctril significantly increased common lambsquarters control, but control was similar to that provided by Goal plus Buctril.  There was no improvement in common lambsquarters control when Nortron was added to Buctril plus Goal.  Kochia control was less when Buctril was applied alone compared to treatments containing Buctril plus Nortron or Goal or both.  A few of the treatments containing Nortron produced more supercolossal onions than did Buctril alone, but yields were similar to the other herbicide treatments (Table 6).  Nortron is useful for improving weed control in onion but in this trial did not provide greater benefits than the currently registered herbicides.


Table 1. Onion injury and weed control from Goal® or Chateau® combinations with Buctril®, Malheur Experiment Station, Oregon State University, Ontario, OR, 2004.

 

 

 

Injury

Weed control

Treatment

Rate

Timing*

5-24

6-9

Pigweed

Common

lambsquarters

Hairy

nightshade

Kochia

Barnyard-grass

 

lb ai/acre

Leaf

----------------------------------------------------------------------------%-------------------------------------------------------------------------

Untreated

--

--

--

--

--

--

--

--

--

Roundup + Prowl

Buctril

Buctril + Goal

Goal

0.75 + 1.0

0.125

0.25 + 0.125

0.25

PRE

2-leaf

3-leaf

5-leaf

24

28

86

90

96

95

100

Roundup + Prowl

Buctril

Buctril + Goal

Goal

0.75 + 1.0

0.125

0.25 + 0.125

0.25

PRE

2-leaf

3-leaf

5-leaf

24

20

88

85

97

95

98

Roundup + Prowl

Buctril + Chateau

Buctril + Goal

Goal

0.75 + 1.0

0.125 + 0.063

0.25 + 0.125

0.25

PRE

2-leaf

3-leaf

5-leaf

25

18

96

95

96

100

100

Roundup + Prowl

Buctril + Chateau

Buctril + Goal

Goal

0.75 + 1.0

0.125 + 0.094

0.25 + 0.125

0.25

PRE

2-leaf

3-leaf

5-leaf

29

22

93

98

100

96

98

Roundup + Prowl

Buctril + Chateau

Buctril + Goal

Goal

0.75 + 1.0

0.25 + 0.063

0.25 + 0.125

0.25

PRE

2-leaf

3-leaf

5-leaf

21

16

93

98

96

100

99

Roundup + Prowl

Buctril + Chateau

Buctril + Goal

Goal

0.75 + 1.0

0.25 + 0.094

0.25 + 0.125

0.25

PRE

2-leaf

3-leaf

5-leaf

26

21

94

99

97

100

100

Roundup + Prowl

Buctril + Goal

Buctril + Goal

Goal

0.75 + 1.0

0.125 + 0.125

0.25 + 0.125

0.25

PRE

2-leaf

3-leaf

5-leaf

20

19

96

90

99

94

100

Roundup + Prowl

Buctril + Goal

Buctril + Goal

Goal

0.75 + 1.0

0.125 + 0.25

0.25 + 0.125

0.25

PRE

2-leaf

3-leaf

5-leaf

24

17

94

90

95

96

100

Roundup + Prowl

Buctril + Goal

Buctril + Goal

Goal

0.75 + 1.0

0.25 + 0.125

0.25 + 0.125

0.25

PRE

2-leaf

3-leaf

5-leaf

28

16

91

88

99

98

93













Table 1 (continued). Onion injury and weed control from Goal® or Chateau® combinations with Buctril®, Malheur Experiment Station, Oregon State University, Ontario, OR, 2004.

 

 

 

Injury

Weed control

Treatment

Rate

Timing*

5-24

6-9

Pigweed

Common

lambsquarters

Hairy

nightshade

Kochia

Barnyard-grass

 

lb ai/acre

Leaf

----------------------------------------------------------------------------%-------------------------------------------------------------------------

Roundup + Prowl

Buctril + Goal

Buctril + Goal

Goal

0.75 + 1.0

0.25 + 0.25

0.25 + 0.125

0.25

PRE

2-leaf

3-leaf

5-leaf

28

19

90

93

94

100

95

Roundup + Prowl

Buctril + Chateau

Buctril + Chateau

Goal

0.75 + 1.0

0.125 + 0.047

0.25 + 0.047

0.25

PRE

2-leaf

3-leaf

5-leaf

25

18

95

97

97

88

100

Roundup + Dacthal + Prowl

Buctril + Chateau

Buctril + Goal

Goal

0.75 +

7.5 + 0.6

0.25 + 0.094

0.125 + 0.25

0.25

PRE

2-leaf

3-leaf

5-leaf

27

21

97

92

100

96

100

Roundup + Dacthal + Prowl

Buctril + Chateau

Buctril + Goal

Goal

0.75 + 7.5 + 0.6

0.125 + 0.094

0.125 + 0.25

0.25

PRE

2-leaf

3-leaf

5-leaf

27

18

96

98

100

100

100

Roundup + Dacthal + Prowl

Buctril + Chateau

Buctril + Goal

Goal

0.75 + 7.5 + 0.6

0.25 + 0.094

0.125 + 0.125

0.25

PRE

2-leaf

3-leaf

5-leaf

24

20

96

100

98

100

95

LSD (P = 0.05)

--

--

NS

NS

NS

NS

NS

NS

NS













*Preemergence (PRE) treatments were applied on April 5,  two-leaf (2-leaf) on May 6,  three-leaf (3-leaf) on May 14, and five-leaf (5-leaf)  on June 2.

Weed-control ratings were taken September 2.

Pigweed is a combination of redroot pigweed and Powell amaranth.


Table 2. Onion yield in response to Goal® or Chateau