Oakes Irrigation Research Site
Carrington Research Extension Center * North Dakota State University
P.O. Box 531, Oakes, ND 58474-0531, Voice: (701) 742-2189, FAX: (701) 742-2700, email: rgreenla@ndsuext.nodak.edu

ONION WEED CONTROL STUDY, 1996

Richard G Greenland

Weed control is difficult in onions because onions do not compete very well with weeds and few herbicides are available for onion production. This is the fourth year of onion weed control studies at the Oakes Irrigation Research Station.

Results summary

Table 59. Herbicide treatments.

Table 60. Yields, bulbs per acre, and size of onions.

Table 61. Weed control and onion injury ratings.


MATERIALS AND METHODS

Soil: Maddock sandy loam; pH=7.5; 2.4% organic matter; soil-P was medium; soil-K was high; soil-S was very low.
Previous crops: 1995 - soybean; 1994 - carrot; 1993 - pumpkin.
Seedbed preparation: Disk twice April 17. Multiweeded (field cultivated) twice on April 18 to incorporate fertilizer and smooth the seedbed.
Planting: Drilled barley cover crop on April 18 (1 bu/acre, 7-inch rows). Direct seeded 'Santos' onions on April 18 with a Stanhay vegetable planter at a population of about 200,000 seeds/acre. Planted in paired rows (3" apart), with the paired rows on 16" centers. Onions were planted perpendicular to the barley cover crop.
Plots: Plots were 17 ft long by 6 ft wide on 8 ft centers (giving a 2-ft check strip between plots). The study had 4 reps.
Fertilizer: On April 17, broadcast 14 lbs N/acre and 70 lbs P2O5/acre as 10-50-0, 16 lbs N/acre and 19 lbs S/acre as 21-0-0-24, and 94 lbs K20/acre as 0-0-60. Sprayed 18 lbs N/acre as 28-0-0 on April 17. Side dressed with urea twice: 70 lbs N/acre on June 19 and 55 lbs N/acre on July 9.
Irrigation: Overhead sprinkler irrigation as needed.
Pest control: Weed control treatments are given in table 59. Sprayed all plots with Fusilade + Penetrate II (16 oz/acre + 1 pt/25 gal on May 21 and 10 oz/acre + 1 pt/25 gal on June 4) to kill barley cover crop. No insect control needed.
Harvest: Pulled Sept 16. Onions were allowed to dry in the field until about the middle of October, when they were bagged. The tops were removed and onions graded on Nov. 21.

RESULTS

Summarizing the last four years of onion weed control studies, we found that Prowl alone or in combination with Buctril applied preemergence did not control redroot pigweed, hairy nightshade, nor common lambsquarters. Dacthal alone or in combination with Prowl controlled hairy nightshade early in the season and common lambsquarters all season, but did not control redroot pigweed nor did it control hairy nightshade late in the season. Weed competition reduced onion yield when herbicides were only applied preemergence and/or early post emergence. Buctril plus Goal applied when onions had two to three true leaves, in combination with any preemergence or early post emergence herbicide treatment, controlled weeds for most of the season. When either Buctril or Goal was applied individually, onion yield and weed control were lower than when they were applied together. Using the Buctril plus Goal post emergence treatment without any preemergence and/or early post emergence herbicide treatment did not control weeds, and onion yield and quality were lower. Buctril plus Goal applied a second time when the onions had five to six true leaves, increased weed control and onion yield compared with all other treatments. A small amount of onion injury occurred with this treatment, but the onions grew out of it and onion yield and quality were not depressed.

Neither Dual nor Frontier is labeled for onions, but they were tested to see if they could be substituted as an early post emergence treatment in place of Prowl or Dacthal. Weed control and onion yield were equal or better with Frontier vs. Prowl or Dacthal. Dual slightly or severely (1X or 2X rate, respectively) reduced onion stand, stunted onion plants, and lowered onion yield.

For season long control of redroot pigweed, common lambsquarters, and hairy nightshade in onions when using a barley cover crop, a preemergence and/or early post emergence herbicide treatment, and two post emergence treatments should be used. The early treatments can be Dacthal applied just after the barley cover crop emerges, or Prowl applied just after the onions emerge. Buctril plus Goal should be applied when onions have two true leaves and again when onions have five true leaves.

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Table 59. Herbicide treatments for the 1996 onion weed control study.
Treatment number Herbicides Rates(per acre) Application timing1
1 Prowl 1.5 pts POST 1
Buctril + Goal 1 pt + 0.6 pts POST 3
2 Dual 2 pts POST 1
Buctril + Goal 1 pt + 0.6 pts POST 3
3 Frontier 1 pt POST 1
Buctril + Goal 1 pt + 0.6 pts POST 3
4 Dacthal 8 lbs PRE
Buctril + Goal 1 pt + 0.6 pts POST 3
5 Dacthal 8 lbs PRE
Buctril 1 pt POST 3
6 Prowl 3 pts POST 1
Buctril + Goal 1 pt + 0.6 pts POST 3
7 Dual 4 pts POST 1
Buctril + Goal 1 pt + 0.6 pts POST 3
8 Frontier 2 pt POST 1
Buctril + Goal 1 pt + 0.6 pts POST 3
9 Dacthal 8 lbs PRE
Buctril + Goal 1 pt + 0.6 pts POST 3
Buctril + Goal 1 pt + 0.6 pts POST 4
10 Prowl 1.5 pts POST 1
Buctril + Goal 1 pt + 0.6 pts POST 3
Buctril + Goal 1 pt + 0.6 pts POST 4
11 Dacthal 8 lbs PRE
Buctril + Goal 0.5 pt + 0.6 pts POST 3
Buctril + Goal 0.5 pt + 0.3 pts POST 4

1PRE - Applied on May 6, which was after barley emerged but before onions emerged. POST 1 - Applied on May 20, which was just after onions emerged.
POST 2 - Fusilade applied to all plots on May 21 and June 4. Barley was 6" tall on May 21.
POST 3 - Applied on June 7 when onions were at 2 true leaf stage.
POST 4 - Applied on July 2 when onions were at 5 to 6 true leaf stage.

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Table 60. Yields, bulbs per acre, and size of onions in the 1996 onion weed control study.
Treatment Number1 Yields classified by size Bulb
>4" 3 to 4" 2¼ to 3" total number size
--------------------- cwt/acre ------------------------ 1000s/acre oz/bulb
1 57 bc2 473 bcd 132 b 727 de 145 ab 8.0 cd
2 69 b 374 d 113 bc 676 e 129 cd 8.4 bc
3 111 ab 536 ab 112 bc 848 abc 149 ab 9.1 ab
4 80 ab 491 bc 89 cd 761 cde 137 bc 8.8 bc
5 0 c 155 e 169 a 378 f 118 d 5.1 e
6 102 ab 551 ab 83 cd 852 abc 147 ab 9.3 ab
7 72 ab 136 e 73 d 381 f 83 e 7.3 d
8 117 ab 430 cd 102 bcd 735 de 137 bc 8.6 bc
9 132 a 530 abc 75 d 879 ab 140 abc 10.1 a
10 109 ab 628 a 76 d 935 a 152 a 9.9 a
11 77 ab 528 abc 89 cd 805 bcd 142 abc 9.1abc
C.V. 50 16 22 11 7 8

1See Table 59 for treatment definitions.
2Values in the same column followed by the same letter are not significantly different at the 0.05 level.

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Table 61. Weed control and onion injury ratings for the 1996 onion weed control study.
Treatment Number1 July 16 ratings September 18 ratings
redroot pigweed hairy nightshade onion injury redroot pigweed hairy nightshade lambs- quarters
1 81 d2 95 a 0 a 50 e 65 c 85 abc
2 85 cd 99 a 5 ab 60 de 88 ab 68 c
3 91 bc 96 a 0 a 73 cd 75 bc 80 bc
4 82 d 95 a 1 a 63 de 79 bc 88 ab
5 43 e 63 b 0 a 58 de 10 d 83 abc
6 94 ab 95 a 3 a 73 cd 75 bc 93 ab
7 93 abc 99 a 38 d 70 cd 83 abc 68 c
8 98 ab 98 a 0 a 80 bc 85 ab 80 bc
9 100 a 100 a 20 c 95 ab 100 a 100 a
10 100 a 100 a 13 bc 100 a 100 a 98 ab
11 100 ab 100 a 18 c 94 ab 95 ab 92 ab
C.V.(%) 7 8 87 18 18 15

1See Table 59 for treatment definitions.
2Values in the same column followed by the same letter are not significantly different at the 0.05 level.

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