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PREEMERGENCE
AND POSTEMERGENCE COMBINATIONS FOR WEED CONTROL IN ONION
Corey V. Ransom and Joey K. Ishida
Malheur Experiment Station
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,
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.
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|
|
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,
|
|
|
|
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 |
