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Preemergence and Postemergence Combinations for Weed Control in Onion

PREEMERGENCE AND POSTEMERGENCE COMBINATIONS FOR WEED CONTROL IN ONION

 

 

Corey V. Ransom and Joey K. Ishida

Malheur Experiment Station

Oregon State University

Ontario, OR

 

 

Introduction

 

Because postemergence herbicides cannot be applied until onions have two true leaves, preemergence herbicide applications can be critical for limiting weed growth prior to when postemergence herbicides can be applied.  Preemergence herbicides can control weeds as they germinate and reduce the size and number of weeds that are present until onions are large enough to tolerate postemergence herbicide applications.  This research evaluated preemergence and postemergence herbicide combinations for weed control in onion.

 

Materials and Methods

 

A trial was conducted at the Malheur Experiment Station under furrow irrigation.  On March 31, 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 split-plot design with 4 replicates.  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 150 lb nitrogen, 50 lb phosphorus, 30 lb potassium, 30 lb sulfate, 8 lb zinc, 5 lb manganese, 1 lb copper, and 2 lb boron/acre on June 7.  Registered insecticides and fungicides were applied for thrips and downy mildew control.

 

Combinations of preemergence and postemergence herbicide applications were evaluated for weed control.  Preemergence (PRE) treatments included no herbicide, Roundup® (glyphosate), and Roundup plus Prowl® (pendimethalin).  Another treatment applied prior to the sequential postemergence treatments was Roundup applied preemergence followed by Prowl and Outlook® (dimethenamid-P) applied postemergence to one-leaf onions.  Sequential postemergence treatments included Buctril® (bromoxynil) plus Goal® (oxyfluorfen) or Buctril plus Chateau® (flumioxazin) applied to two- or four-leaf onions.  All plots were treated with Goal when onions had six leaves.  Herbicide treatments were applied with a CO2-pressurized backpack sprayer. Preemergence and one-leaf applications were applied at 20 gal/acre at 30 psi.  All other postemergence applications were applied at 40 gal/acre at 30 psi.  Preemergence treatments were applied on April 12.  One-, two-, four-, and six-leaf applications were made on May 10, May 21, June 4, and June 16, respectively.  All plots received Poast® (sethoxydim) at 0.29 lbs ai/acre plus crop oil concentrate (COC) (1 qt/acre) on May 24 to control grasses.  Weed control and onion injury were evaluated throughout the season.  Onions were harvested September 13-14 and graded by size on September 14-15.

 

Treatments were arranged in a factorial design to allow the analysis of preemergence and postemergence treatment effects and interactions.  Data were analyzed to determine significant effects and means were separated using a protected least significant difference (LSD) at the 5 percent level (0.05).  Significance levels of main effects and interactions are included in Table 1.

 

Results and Discussion

 

Rainfall activated preemergence treatments and postemergence treatments were effective because weeds were actively growing at the time herbicides were applied.  Preemergence treatments did not cause onion injury (data not shown).  On May 28, injury was similar among postemergence treatments (data not shown).  On June 13, the treatment that included Buctril plus Chateau applied to four-leaf onions caused more injury (31 percent) than treatments where Buctril plus Chateau was applied to two-leaf onions (20 percent) or where Buctril plus Goal was applied sequentially (19 percent). 

 

On May 21, weed control was greatest with Roundup plus Prowl applied preemergence compared to the other preemergence treatments (Table 2). Roundup preemergence followed by Prowl plus Outlook applied to one-leaf onions had greater pigweed, common lambsquarters, hairy nightshade, and barnyardgrass control than Roundup alone, but the two treatments provided similar kochia control.  Significant preemergence herbicide by postemergence herbicide interactions were present for pigweed, hairy nightshade, and kochia control ratings taken September 8 (Table 3).  Depending on what preemergence treatments were applied, postemergence treatments responded differently in the control of these species.  When no preemergence herbicides were applied, treatments containing Buctril plus Chateau provided greater control of pigweed, nightshade, and kochia compared to Buctril plus Goal.  When Roundup alone was applied preemergence, Buctril plus Chateau combinations provided greater pigweed control compared to Buctril plus Goal.  Also when Roundup alone was applied preemergence, Chateau plus Buctril applied to four-leaf onions controlled hairy nightshade and kochia better than Chateau plus Buctril applied to two-leaf onions and the Buctril plus Goal treatment.  When Roundup plus Prowl were applied preemergence or Roundup was applied preemergence followed by an application of Prowl plus Outlook to one-leaf onions, all postemergence herbicide combinations provided similar control of pigweed, nightshade, and kochia.  Only preemergence herbicide treatments significantly affected common lambsquarters and barnyardgrass control on September 8 (Table 4).  All preemergence treatments increased control of both species compared to no preemergence herbicide.  Roundup alone had less control than Roundup plus Prowl preemergence or Roundup preemergence followed by Prowl plus Outlook applied to one-leaf onions.  Treatments containing Prowl had the highest common lambsquarters and barnyardgrass control.

 

Onion yields were significantly different among preemergence treatments (Table 5).  Roundup preemergence increased medium, jumbo, total, and marketable yields compared to no preemergence treatment, but yields were significantly less than with Roundup plus Prowl preemergence or Roundup preemergence followed by Prowl plus Outlook at one-leaf onions.  Roundup plus Prowl preemergence had fewer medium onions and greater jumbo, colossal, total, and marketable yields compared to Roundup preemergence followed by Prowl plus Outlook applied to one-leaf onions.  It is possible that this yield increase resulted from the prevention of early weed competition. 

 

This research demonstrates that the effectiveness of postemergence herbicide programs is strongly related to early season weed control.  Although in several instances weed control was similar between different treatments at the conclusion of the season, yields were reduced in treatments where weeds were allowed to compete early.  The use of an effective preemergence herbicide may provide the best opportunity for effectively controlling weeds with postemergence herbicides and for maximizing onion yield.  An effective preemergence herbicide could also potentially allow a producer to use lower rates of postemergence herbicides to achieve acceptable weed control.  This research also demonstrated that Buctril plus Chateau applied to four-leaf onions can provide increased weed control under certain conditions, but application of Buctril plus Chateau at that timing also caused greater injury than the other treatments.  The level of injury observed would likely not be acceptable in commercial onion production.

 

 

 

Table 1. Significance of preemergence and postemergence treatment main effects and the preemergence by postemergence interactions for onion injury and yield and weed control evaluation data, Malheur Experiment Station, Oregon State University, Ontario, OR, 2005.

 

Onion

 

Weed control

Effects

Injury 5-28

Injury 6-13

Total yield

 

Pigweed

Common

lambsquarters

Hairy nightshade

Kochia

Barnyard-grass

 

-----------------------------------------------------------------probability (F)*-------------------------------------------------------------------

Preemergence

0.3416

0.0834

0.0001

 

0.0001

0.0001

0.0001

0.0001

0.0001

Postemergence

0.5878

0.0001

0.0616

 

0.0001

0.5035

0.0008

0.0007

0.5170

PRE X POST

0.2756

0.4218

0.1386

 

0.0005

0.3958

0.0108

0.0073

0.9529

*Main effects or interactions with probabilities equal to or less than 0.05 are considered significant. 


Table 2. Weed control in onion provided by preemergence and early postemergence herbicide applications prior to spraying sequential postemergence treatments, Malheur Experiment Station, Oregon State University, Ontario, OR, 2005.

 

 

 

Weed control§

Treatment

Rate*

Timing

Pigweed

Common

lambsquarters

Hairy nightshade

Kochia

Barnyard-grass

 

lb ai or ae/acre

 

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

None

--

--

0

0

0

0

0

Roundup

0.75

PRE

44

63

46

89

0

Roundup + Prowl

0.75 + 1.0

PRE

 

94

94

77

96

76

Roundup

Prowl + Outlook

0.75

1.0 + 0.84

PRE

1-leaf

66

71

66

85

48

LSD (0.05)

--

--

2

5

10

10

3

*Roundup rates are in lb ae/acre and all other herbicides are lb ai/acre. 

Preemergence (PRE) treatments were applied on April 12 and one-leaf (1-leaf) on May 10.

Pigweed is a combination of redroot pigweed and Powell amaranth.

§Weed control was evaluated on May 21, prior to the beginning of sequential postemergence herbicide applications. 


Table 3. Weed control in onion with preemergence and postemergence herbicides, Malheur Experiment Station, Oregon State University, Ontario, OR, 2005.

 

 

 

Weed control§

Treatment

Rate*

Timing

Pigweed

Hairy nightshade

Kochia

 

lb ai or ae/acre

Leaf

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

None

Buctril+ Chateau

Buctril + Goal

Goal

 

0.125  + 0.094

0.25 + 0.125

0.25

PRE

2-leaf

4-leaf

6-leaf

97

85

76

None

Buctril + Goal

Buctril + Chateau

Goal

 

0.125 + 0.125

0.25 + 0.094

0.25

PRE

2-leaf

4-leaf

6-leaf

95

88

89

None

Buctril + Goal

Buctril + Goal

Goal

 

0.125 + 0.125

0.25 + 0.125

0.25

PRE

2-leaf

4-leaf

6-leaf

82

75

66

Roundup

Buctril + Chateau

Buctril + Goal

Goal

0.75

0.125 + 0.094

0.25 + 0.125

0.25

PRE

2-leaf

4-leaf

6-leaf

88

55

85

Roundup

Buctril + Goal

Buctril + Chateau

Goal

0.75

0.125 + 0.125

0.25 + 0.094

0.25

PRE

2-leaf

4-leaf

6-leaf

86

70

95

Roundup

Buctril + Goal

Buctril + Goal

Goal

0.75

0.125 + 0.125

0.25 + 0.125

0.25

PRE

2-leaf

4-leaf

6-leaf

72

49

85

Roundup + Prowl

Buctril + Chateau

Buctril + Goal

Goal

0.75 + 1.0

0.125 + 0.0.094

0.25 + 0.125

0.25

PRE

2-leaf

4-leaf

6-leaf

99

100

100

Roundup + Prowl

Buctril + Goal

Buctril + Chateau

Goal

0.75 + 1.0

0.125 + 0.125

0.25 + 0.094

0.25

PRE

2-leaf

4-leaf

6-leaf

100

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

4-leaf

6-leaf

100

100

100

Roundup

Prowl + Outlook

Buctril + Chateau

Buctril + Goal

Goal

0.75

1.0 + 0.84

0.125 + 0.094

0.25 + 0.125

0.25

PRE

1-leaf

2-leaf

4-leaf

6-leaf

95

99

97

Roundup

Prowl + Outlook

Buctril + Goal

Buctril + Chateau

Goal

0.75

1.0 + 0.84

0.125 + 0.125

0.25 + 0.094

0.25

PRE

1-leaf

2-leaf

4-leaf

6-leaf

97

100

99

Roundup

Prowl + Outlook

Buctril + Goal

Buctril + Goal

Goal

0.75

1.0 + 0.84

0.125 + 0.125

0.25 + 0.125

0.25

PRE

1-leaf

2-leaf

4-leaf

6-leaf

95

100

98

LSD (0.05)

--

--

6

9

9

*Roundup rates are in lb ae/acre and all other herbicides are in lb ai/acre.

Preemergence (PRE) treatments were applied on April 12,  one-leaf (1-leaf) on May 10,  two-leaf (2-leaf) on May 21, four-leaf (4-leaf)  on June 4, and six-leaf (6-leaf) on June 16.

Pigweed is a combination of redroot pigweed and Powell amaranth.

§Weed control was evaluated on September 8.

Table 4. Weed control in onion provided by preemergence herbicide applications averaged over postemergence treatments, Malheur Experiment Station, Oregon State University, Ontario, OR, 2005.

 

 

 

Weed control

Treatment

Rate*

Timing

Common lambsquarters

Barnyardgrass

 

lb ai or ae/acre

 

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

None

--

--

67

75

Roundup

0.75

PRE

85

88

Roundup + Prowl

0.75 + 1.0

PRE

 

99

96

Roundup

Prowl + Outlook

0.75

1.0 + 0.84

PRE

1-leaf

98

97

LSD (0.05)

--

--

6

8

*Roundup rates are in lb ae/acre and all other herbicides are in lb ai/acre.

Preemergence (PRE) treatments were applied on April 12 and one-leaf (1-leaf) on May 10.

Weed control was evaluated on September 8 and are averaged over postemergence herbicide treatments.

 

 

 

 

Table 5. Onion yield as influenced by preemergence herbicide treatments, Malheur Experiment Station, Oregon State University, Ontario, OR, 2005.

 

 

 

Onion yield

Treatment

Rate*

Timing

Small

Medium

Jumbo

Colossal

Super- Colossal

Total

Market-able

 

lb ai or ae/acre

 

------------------------------------------------cwt/acre-----------------------------------------------

None

--

--

98

208

89

0

0

394

297

Roundup

0.75

PRE

26

284

339

0

0

659

624

Roundup + Prowl

0.75 + 1.0

PRE

 

6

87

838

14

0

945

939

Roundup

Prowl + Outlook

0.75

1.0 + 0.84

PRE

1-leaf

12

155

686

5

0

859

847

LSD (0.05)

--

--

12

27

66

6

NS

52

63

*Roundup rates are in lb ae/acre all other herbicides are in lb ai/acre.

Preemergence (PRE) treatments were applied on April 12 and one-leaf (1-leaf) on May 10. 

Onions were harvested September 13 and 14.

 

 


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