Malheur Experiment Station
Oregon State University
Information for Sustainable Agriculture

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, 2003

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. Valor (flumioxazin) 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 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 28, onions (cv. 'Vaquero', Sunseeds, Parma, ID) were planted at a 3.7-inch spacing in double rows on 22-inch beds. Plots were four 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 row at 3.7 oz/1,000 ft of row. Onions were sidedressed with 117 lb N, 72 lb P, 111 lb sulfate, 114 lb S, 6 lb Zn, and 1 lb 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 Prowl (pendimethalin) at 1.0 lb ai/acre on April 11 and a postemergence application of Poast (sethoxydim) at 0.29 lb ai/acre plus crop oil concentrate (COC) (1.0% v/v) on June 6. Weed control and onion injury were evaluated throughout the season. Onions were harvested September 17 and 18 and graded by size on September 23-25.

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).

Postemergence Valor Combinations

Valor and Goal (oxyflurofen) were applied in combinations with Buctril (bromoxynil) to evaluate weed control and onion tolerance. Buctril, Goal, and Valor were evaluated at two rates. Comparisons of Goal or Valor with Buctril included several combinations of herbicides and rates. Additional treatments included a split application of Valor applied to two-leaf and again to three-leaf onions, and a comparison of Buctril plus Valor treatments following preemergence applications of Roundup, Prowl, and Dacthal (DCPA).
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 without Nortron or with 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) applied in the two-leaf application and Nortron applied in the three-leaf application.

Results and Discussion


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

On June 30, treatments with Buctril alone applied to two-leaf onions had among the least onion injury, while combinations of Buctril with either Goal or Valor had among the greatest injury (Table 1). By July 14, no onion injury was observed. On August 14, pigweed control was improved by the addition of Valor (0.094 lb ai/acre) to Buctril at the low rate (0.125 lb ai/acre) (Table 1). The lower rate of Valor (0.063 lb ai/acre) did not significantly increase pigweed control when added to the low rate of Buctril. Similar trends were apparent for Goal where pigweed control was improved by the high rate (0.25 lb ai/acre) but not the low rate (0.125 lb ai/acre) when added to the low rate of Buctril. Valor (0.094 lb ai/acre) added to Buctril (0.25 lb ai/acre) increased pigweed control, while the addition of Goal at any rate did not significantly improve the control achieved with Buctril at the high rate alone. All treatments provided 88 percent or greater common lambsquarters control. When Buctril was applied at 0.125 lb ai/acre, the addition of Valor significantly increased hairy nightshade control. Hairy nightshade control was increased with the addition of the high rate of Goal, but not the low rate. The addition of Valor or Goal to Buctril (0.25 lb ai/acre) did not provide a significant increase in hairy nightshade control. Buctril (0.125 lb ai/acre) applied alone at the two-leaf onion timing produced more medium onions than any other treatment. Onion yields were related to weed control, with treatments providing less weed control having among the lowest yields and treatments providing the highest weed control producing among the greatest yields. The addition of Valor or Goal to Buctril tended to increase pigweed and hairy nightshade control and resulted in improved yields.

Addition of Nortron to Postemergence Treatments

Nortron or Outlook did not increase onion injury when added to any of the postemergence treatments (Table 2). The addition of Nortron (0.25 lb ai/acre) to Buctril applied to two-leaf onions significantly improved pigweed control. The addition of Nortron at the higher rate (0.5 lb ai/acre) improved both pigweed and hairy nightshade control. The higher Nortron rate improved hairy nightshade control compared to the lower rate. The addition of Goal with Buctril at the two-leaf application improved pigweed control compared to Buctril alone. The addition of Nortron (0.25 and 0.5 lb ai/acre) to Buctril plus Goal applied at the two-leaf application did not affect pigweed control, but significantly improved hairy nightshade control. Common lambsquarters control was 99 percent or greater with all treatments. There were few differences among treatments for onion yield; the treatment with Buctril alone applied at two-leaf onions had among the lowest super colossal yield (Table 3). This research suggests that the addition of Nortron to postemergence herbicide applications may improve weed control without injuring onions. The registration of Nortron for use in onions depends on the residue package submitted by the IR-4 program being reviewed by the U.S. Environmental Protection Agency.

Table 1. Onion injury, weed control, and yield from Goal or Valor combinations with Buctril, Malheur Experiment Station, Oregon State University, Ontario, OR, 2003.




Injury
Weed control†
Onion yield
Treatment Rate Timing* 6-30
Pigweed Commonlambsquarters Hairynightshade
Small Medium Jumbo Colossal S. Colossal Marketable

lb ai/acre Leaf ------------------------------- % --------------------------------
---------------------------------------- cwt/acre -----------------------------------------
Untreated -- -- 0
0 0 0
12 0 0 0 0 0
Hand-Weeded -- -- 0
95 98 96
3 17 610 413 76 1116
Roundup + Prowl
Buctril
Buctril + Goal
Goal 		0.75 + 1.0
0.125
0.25 + 0.125
0.25		 PRE
2-leaf
3-leaf
4-leaf		 1
81 100 47
5 57 657 302 34 1051
Roundup+Prowl
Buctril
Buctril + Goal
Goal 		0.75 + 1.0
0.25
0.25 + 0.125
0.25		 PRE
2-leaf
3-leaf
4-leaf		 4
83 100 79
4 12 665 352 47 1076
Roundup+Prowl
Buctril+Valor
Buctril + Goal
Goal 		0.75 + 1.0
0.125+0.063
0.25+0.125
0.25		 PRE
2-leaf
3-leaf
4-leaf		 5
86 99 90
3 16 683 298 46 1043
Roundup+Prowl
Buctril+Valor
Buctril+Goal
Goal 		0.75 + 1.0
0.125+0.094
0.25+0.125
0.25		 PRE
2-leaf
3-leaf
4-leaf		 11
97 99 91
5 12 639 408 81 1140
Roundup+Prowl
Buctril+Valor
Buctril+Goal
Goal 		0.75 + 1.0
0.25 + 0.063
0.25 + 0.125
0.25		 PRE
2-leaf
3-leaf
4-leaf		 8
92 100 92
4 20 601 443 60 1124
Roundup+Prowl
Buctril+Valor
Buctril+Goal
Goal 		0.75 + 1.0
0.25 + 0.094
0.25 + 0.125
0.25		 PRE
2-leaf
3-leaf
4-leaf		 9
98 100 92
3 13 579 428 81 1100
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
4-leaf		 1
86 100 63
3 20 619 345 51 1035



Injury
Weed control†
Onion yield
Treatment Rate Timing* 5-24
Pigweed Commonlambsquarters Hairynightshade
Small Medium Jumbo Colossal S. Colossal Marketable
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
4-leaf		 6
92 98 91
3 9 648 455 71 1183
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
4-leaf		 8
90 100 91
5 9 619 461 81 1170
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
4-leaf		 11
88 100 88
5 13 621 433 104 1170
Roundup+Prowl
Buctril+Valor
Buctril+Valor
Goal 		0.75 + 1.0
0.125+0.047
0.25 + 0.047
0.25		 PRE
2-leaf
3-leaf
4-leaf		 11
93 100 98
3 13 680 376 70 1139
Roundup+Prowl+Dacthal
Buctril + Valor
Buctril + Goal
Goal 		0.75 + 0.6 +7.5
0.25 + 0.094
0.25 + 0.125
0.25		 PRE
2-leaf
3-leaf
4-leaf		 6
99 99 99
4 9 567 513 105 1194
Roundup+Prowl+Dacthal
Buctril + Valor
Buctril + Goal
Goal 		0.75+0.6+7.5
0.125+0.094
0.25 + 0.125
0.25		 PRE
2-leaf
3-leaf
4-leaf		 6
99 100 97
4 9 605 489 96 1200
Roundup+Prowl+Dacthal
Buctril + Valor
Buctril + Goal
Goal 		0.75+0.6+7.5
0.125+0.094
0.125+0.125
0.25		 PRE
2-leaf
3-leaf
4-leaf		 12
98 97 98
3 11 607 451 94 1163
LSD (0.05)

6
10 3 19
4 24 93 114 41 115
*Preemergence (PRE) treatment applied on April 11, two-leaf (2-leaf) on May 16, three-leaf (3-leaf) on May 27, and four-leaf (4-leaf) on June 9.
†Weed control ratings were taken August 14. Pigweed is a combination of redroot pigweed and Powell amaranth.

Table 2. Onion injury and weed control in response to adding Nortron to postemergence applications of Buctril and Goal, Malheur Experiment Station, Oregon State University, Ontario, OR, 2003.






Weed control



Injury
Pigweed Common lambsquarters Hairy nightshade
Treatment Rate Timing* 6-30
6-30 8-14 8-14 6-30 8-14

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
4-leaf		 10
97 88 100 94 83
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
4-leaf		 11
100 95 99 97 82
Roundup+Prowl
Buctril+Nortron
Buctril+Goal+Nortron
Goal		 0.75 + 1.0
0.125 + 0.25
0.25+0.125+0.25
0.25		 PRE
2-leaf
3-leaf
4-leaf		 10
99 94 100 98 81
Roundup + Prowl
Buctril + Nortron
Buctril+Goal+Nortron
Goal		 0.75 + 1.0
0.125 + 0.5
0.25+0.125+0.5
0.25		 PRE
2-leaf
3-leaf
4-leaf		 11
100 96 100 99 93
Roundup + Prowl
Buctril+Goal+Nortron
Buctril+Goal+Nortron
Goal		 0.75 + 1.0
0.125+0.125+0.25
0.25+0.125+0.25
0.25		 PRE
2-leaf
3-leaf
4-leaf		 10
100 94 100 98 92
Roundup + Prowl
Buctril+Goal+Nortron
Buctril+Goal+Nortron
Goal		 0.75 + 1.0
0.125+0.125+0.5
0.25+0.125+0.5
0.25		 PRE
2-leaf
3-leaf
4-leaf		 10
100 99 100 100 97
Roundup + Prowl
Buctril+Goal+Outlook
Buctril+Goal+Nortron
Goal		 0.75 + 1.0
0.125+0.125+0.84
0.25+0.125+0.25
0.25		 PRE
2-leaf
3-leaf
4-leaf		 10
100 98 100 100 99
LSD (0.05)

NS
2 5 NS 4 9
*Preemergence (PRE) treatment applied on April 11, two-leaf (2-leaf) on May 16, three-leaf (3-leaf) on May 27, and four-leaf (4-leaf) on June 9.

Table 3. Onion yield in response to adding Nortron to postemergence applications of Buctril and Goal, Malheur Experiment Station, Oregon State University, Ontario, OR, 2003.

Onion yield†
Treatment Rate Timing* Small Medium Jumbo Colossal S. Colossal Marketable

lb ai/acre Leaf -------------------------------- cwt/acre --------------------------------
Untreated -- -- 13 0 0 0 0 0
Roundup+Prowl
Buctril
Buctril+Goal
Goal 		0.75 + 1.0
0.125
0.25+0.125
0.25		 PRE
2-leaf
3-leaf
4-leaf		 6 21 715 410 32 1178
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
4-leaf		 4 10 713 419 116 1257
Roundup+Prowl
Buctril+Nortron
Buctril+Goal+Nortron
Goal		 0.75 + 1.0
0.125 + 0.25
0.25+0.125+0.25
0.25		 PRE
2-leaf
3-leaf
4-leaf		 5 15 710 469 61 1255
Roundup + Prowl
Buctril+Nortron
Buctril+Goal+Nortron
Goal		 0.75 + 1.0
0.125 + 0.5
0.25+0.125+0.5
0.25		 PRE
2-leaf
3-leaf
4-leaf		 7 15 655 511 65 1247
Roundup + Prowl
Buctril+Goal+Nortron
Buctril+Goal+Nortron
Goal		 0.75 + 1.0
0.125+0.125+0.25
0.25+0.125+0.25
0.25		 PRE
2-leaf
3-leaf
4-leaf		 5 7 670 526 94 1298
Roundup + Prowl
Buctril+Goal+Nortron
Buctril+Goal+Nortron
Goal		 0.75 + 1.0
0.125+0.125+0.5
0.25+0.125+0.5
0.25		 PRE
2-leaf
3-leaf
4-leaf		 5 11 707 423 79 1221
Roundup + Prowl
Buctril+Goal+Outlook
Buctril+Goal+Nortron
Goal		 0.75 + 1.0
0.125+0.125+0.84
0.25+0.125+0.25
0.25		 PRE
2-leaf
3-leaf
4-leaf		 5 18 665 474 95 1252
LSD (0.05)

7 14 104 147 45 112
*Preemergence (PRE) treatment applied on April 11, two-leaf (2-leaf) on May 16, three-leaf (3-leaf) on May 27, and four-leaf (4-leaf) on June 9.
†Onions were harvested September 17 and 18.

††The information in this report is for the purpose of informing cooperators in industry, colleagues at other universities, and others of the results of research in field crops. Reference to products and companies in this publication is for the specific information only and does not endorse or recommend that product or company to the exclusion of others that may be suitable. Nor should any information and interpretation thereof be considered as recommendations for the application of any pesticide. Pesticide labels should always be consulted before any pesticide use.

 

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