Postemergence Herbicide Weed Control In Onions

K. Umeda, C. Fredman

Abstract

Oxyfluorfen (Goal ®) herbicide at 0.125 to 0.25 lb. a.i. /A applied postemergence

(POST) to onions at the 3 -leaf stage of growth effectively controlled London rocket,
yellow sweetclover, and prostrate pigweed with marginal injury to onions. Earlier
POST applications on 1- and 2 -leaf onions caused injury and some stand
reduction. Bromoxynil (Buctril ®) herbicide at 0.25 lb. a.i. /A applied early POST
gave generally good weed control but did not adequately control sweetclover.
Buctril® applied in clear weather did not injure 1 -leaf onions but caused severe
injury on 2- and 3 -leaf onions when applied during cloudy weather conditions.
Buctril® plus pendimethalin (Prowl ®) tank -mix combination applied POST
provided good control of London rocket, sweetclover, and prostrate pigweed;
however, onion injury was severe due to applying Buctril® in cloudy weather.
Buctril® and Goal® effectively controlled weeds present in the onions but timing
of POST applications was critical with respect to onion size and weather conditions
to minimize injury.

Introduction

Season -long onion weed control is difficult because the length of the cropping season is especially long and soil -applied
preemergence (PE) herbicides at planting and subsequent cultivations are not adequate. Postemergence (POST)
herbicides are necessary to supplement early season weed control and to reduce weed competition for the remainder of
the onion growing season. POST herbicides are available for use in onions but the timing of applications are critical for
optimum weed control efficacy and crop safety. Oxyfluorfen (Goal ®) herbicide controls emerged weeds and also
provides PE control of weeds that may emerge later. Bromoxynil (Buctril ®) is an effective contact herbicide on many
emerged broadleaved weeds. Pendimethalin (Prowl ®) is a soil -applied herbicide for grass weeds and some broadleaved
weeds that should be applied when the onions are emerged and actively growing. All of these herbicides are
recommended to be used when onions are at least at the 2 -leaf stage of growth or larger or severe injury could result.
The objectives of this field study were to evaluate and determine the optimum rates of use and timing of applications of
POST herbicides for dry bulb onion weed control.

Materials and Methods

A small plot field study was conducted within a commercially grown onion field near Surprise, AZ. The field was seeded
to dry bulb onions on 40 -inch beds with 6 seedlines per bed on about November 2, 1994. PE herbicide, DCPA
(Dacthal ®) was applied for general weed control before the first irrigation. POST herbicide treatments were replicated
three times in a randomized complete block design. Each plot consisted of two beds measuring 25 feet in length. All
treatments were applied with a hand -held boom having four flat fan 8002 nozzles spaced 20- inches apart and delivered
in 30 gallons per acre of water pressurized with a CO, backpack sprayer at 45 psi.
Goal® 1.6E, Buctril ®, and Buctril® plus Prowl® 3.3EC treatments were applied when onions were at the 1 -leaf stage
of growth on December 13, 1994, 2 -leaf stage on December 22, 1994, and 3 -leaf on January 23, 1995. On December

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 13, onions were at the 1 -leaf stage with few initiating the second leaf, very few Melilotus sp. (yellow sweetclover) at the
unifoliate (1 -leaf) stage and predominately Verbascum sp. (common mullein) were present at the 2 to 4 -leaf stage.
Weather was clear skies with little detectable wind and 66 °F temperature. On December 22, sweetclover was at the
first trifoliate (T)(1 -3 leaflet) stage and common mullein at 5 to 7 -leaf stage. There were high overcast skies, with
temperature at 72 °F and no wind. On January 23, onions at the 3 -leaf stage measured about 6- inches tall, common
mullein had 8- leaves, and sweetclover had 3 to 4T. Temperature was 66 °F, cloudy, and no wind present.
Visual weed control, onion stand reduction, and injury evaluations were made on February 1 and March 8, 1995 after
all treatment applications were made. Ratings were subjected to statistical analysis and means were separated by
Duncan's Multiple Range Test.

Results and Discussion

Buctril® at 0.25 lb. a.i. /A applied POST at the 1 -leaf stage of onions caused the least onion injury and gave good control
of common mullein and Amaranthus sp. (prostrate pigweed), excellent control of Sisymbrium irio (London rocket), and
unacceptable control of sweetclover (Table). Later applications of Buctril® gave acceptable control of most weeds,
however, marginal to severe onion injury occcurred. POST herbicide applications on December 22 and January 23 were
during cloudy, overcast days that prevented the onion foliage from hardening and to sufficiently form a cuticle layer.
Greater injury was observed with the higher rate of Buctril® at 0.38 lb. a.i. /A.
Buctril® + Prowl® combination treatments POST gave good weed control of most weeds and appeared to be improved
compared to Buctril® alone treatments for pigweed and sweetclover control. Onion injury was marginally unacceptable
due to onion susceptibility to Buctril® during cloudy weather.
Goal® treatments POST at the 3 -leaf stage was less injurious than earlier applications and effectively controlled London
rocket, sweetclover, and pigweed. Goal® was only marginally effective against common mullein. Early applications
of Goal® caused injury as well as reduced the onion crop stand relative to the untreated check. An early timing of Goal®
applied twice on very young onions severely injured and reduced the crop stand.
Buctril® and Goal® were effective in providing acceptable weed control but onions were subject to injury during adverse
weather conditions or when the crop was not at the proper label recommended size of growth.

Acknowledgements

I would like to express gratitude to D. Tolmachoff for the providing the area in his commercial onion field to conduct
this field study.

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Table. Postemergence herbicide weed control in onions near Surprise, AZ. (Umeda and Fredman).
% Weed Control
Treatment Rate Timing % CSR % CSI % CSI VERTH SSYIR MEUOF AMABL
(lb a.i. /A) 02/01/95 02/01/95 03/08/95 02/01/95 03/08/95 03/08/95 03/08/95 03/08/95

Untreated check 0e 0h 0g 0d 0d 0b 0d 0d
Goal® + Goal® 0.125 + 0.125 1 -lf + 2-If 13 bcd 17 c -g 25 cd 92 ab 83 abc 98 a 93 a 96 a
Goal® + Goal® 0.25 + 0.25 1 -lf + 2 -1f 43 a 43 a 60 a 96 a 93 a 99 a 96 a 99 a

Buctril® 0.25 1-1f 2e 8 gh 8 efg 93 a 90 ab 99 a 72 c 87 ab

Buctril® 0.38 1-1f 5 de 18 c-g 22 de 95 a 90 ab 96 a 78 bc 70 c
Buctril® + Prowl® 0.25 + 1.0 1-If 8 de 15 d-g 17 def 93 a 88 ab 99 a 98 a 98 a

Goal® 0.125 2 -1f 5 de 12 fgh 18 def 85 bc 80 bc 98 a 83 abc 96 a

Goal® 0.25 2 -1f 20 bc 22 b -f 37 bc 90 ab 73 c 96 a 87 ab 85 a
Buctril® 0.25 2 -1f 12 cd 25 b -e 37 bc 95 a 82 bc 99 a 83 abc 77 bc
Buctril® 0.38 2 -1f 22 b 32 b 50 ab 96 a 85 ab 99 a 77 bc 70 c
Buctril® + Prowl® 0.25 + 1.0 2 -1f 20 be 18 c -g 43 b 96 a 87 ab 99 a 96 a 95 a

Goal® 0.125 3-1f 0e 10 fgh 0g 85 bc 80 bc 99 a 95 a 95 a

Goal® 0.25 3-1f 0e 15 d-g 5 fg 85 bc 80 bc 99 a 90 ab 95 a
Buctril® 0.25 3-1f 0e 13 efg 5 fg 80 c 90 ab 99 a 86 ab 85 ab
Buctril® 0.38 3-1f 0e 28 bc 10 efg 78 c 90 ab 98 a 93 a 75 be
Buctril® + Prowl® 0.25 + 1.0 3-1f 0e 27 bcd 8 efg 78 c 90 ab 99 a 95 a 96 a

Application timings: 1 -leaf (If) on 12/13/94; 2 -leaf (1f) on 12/22/94; 3 -leaf (1f) on 01/23/95.
CSR = crop stand reduction, CSI = crop stand injury
VERTH = common mullein (Verbascum sp.), SSYIR = London rocket (Sisymbrium irio),
MEUOF = yellow sweetclover (Melilotus sp.), AMABL = prostrate pigweed (Amaranthus sp.)
Means followed by the same letter in a column are not significantly different by Duncan's Multiple Range Test at the 5%
level.