Weed Control and Crop Tolerance of Onions to Herbicides Applied Postplant Preemergence and as Multiple Postemergence Applications to Seedling Onions††

Charles E. Stanger and Joey Ishida

Malheur Experiment Station
Oregon State University
Ontario, Oregon, 1995
 

Purpose

Several different herbicides were applied singly and in tank-mix combinations at various rates and timing of applications to identify herbicide treatments which were most effective for weed control and crop safety.


Procedures

Seed of Vision variety of yellow sweet Spanish onions was planted on April 16 in silt loam textured soil. The soil was mold-board plowed and bedded in October of 1994. Stephens wheat was the crop before planting onions. The soil pH was 7.3 and had 1.2 percent organic matter. On April 5 the beds were harrowed with a spike-tooth bed-harrow and the onions planted in double rows 2 1/4 inches apart on single beds spaced 22 inches apart. Individual plots for all treatments were four rows wide and 25 feet long. All treatments were replicated three times in blocks using a randomized complete block experimental design.

After planting, the trial area was watered by furrow irrigation to furnish soil moisture for seed germination and plant emergence. Prowl preemergence herbicide treatments were applied at rates of 1.0, 1.5, and 2.0 lb ai/ac on May 2, when the onion seedlings were about 0.5 inch below the soil surface. On May 2 the air temperature was 58°F when spraying, and the wind was blowing from the west at 2 to 3 mph. The skies were overcast, and the soil was moist on the surface.

On May 18 the early postemergence applications were applied to flag leaf onions in this trial to compare preemergence treatments with early postemergence treatments for weed control and crop tolerance. The early postemergence treatments consisted of Prowl, Dual, or Frontier herbicides tank-mixed with Buctril, Nortron, and Prism herbicides and applied to onions with a fully developed flag leaf.

Weed species in the plots treated with postemergence applied herbicides were redroot pigweed, lambsquarters, kochia, hairy nightshade, and barnyardgrass. Broadleaf weed species ranged in size from cotyledon leaf weeds to weeds with 4-true leaves. Barnyardgrass had one to three leaves. Buctril, Nortron, and Prism rates were 0.15, 0.25, and 0.05 lb ai/ac, respectively. Prowl rates were 1.5 and 2.0 lb; Dual rate was 2.0 lb; and Frontier was 1.17 and 2.34 lb ai/ac. Prowl, Dual, and Frontier herbicides were soil activated with 1.41 inches of rain water which fell on six different days from May 3 to May 16. The most rainfall occurring in a single 24-hour period was 0.48 inches which fell on May 2 and again on May 6. On June 1 the onions previously treated with preemergence and early postemergence were sprayed with a final postemergence treatment containing Buctril, Goal, Nortron, and Prism at rates of 0.15, 0.05, 0.25, and 0.05 lb ai/ac, respectively.

In a second trial, herbicide treatments were begun as repeat applications to seedling onions in the flag leaf and 1-true leaf stage of growth. The treatments applied to onions with flag leaves were applied on May 18. The flag leaves were fully developed on all plants, and about 20 percent of the onions had a true leaf about 1 inch long. On May 18, when these treatments were applied, the air temperature was 74°F; soil temperature at 4 inches was 68°F. Wind was blowing at 2 to 3 mph from the west, and the skies were mostly clear. The first herbicide treatments applied to onion plants with 1 true leaf were made on May 24. On May 24 the air temperature was 76°F; the soil temperature at 4 inches was 73°F. The skies were partly cloudy, and wind was 2 mph from the northwest.

The weed species in the postemergence trials included kochia, redroot pigweed, hairy nightshade, lambsquarters, annual sowthistle, and barnyardgrass. When the onions were flag leaf, the weeds were small, ranging in size from cotyledon to four true leaves. The plants with four true leaves had rosettes 1-inch across. When the onions had one true leaf, the weeds had grown to a height of 2 to 4-inches with pigweed, nightshade, and annual sowthistle measuring 2-inches across rosettes. The larger barnyardgrass was three leaves and one tiller. Each trial received a total of 3 applications. The second and third applications were applied 10 and 24 days after the initial application.

All herbicides were applied with a single bicycle wheel plot sprayer. The spray boom had four nozzles with a nozzle centered over each row of the plot. Spray nozzles were Teejet fan size 6502. Spray pressure was 42 psi, and water as the herbicide carrier was applied at 19.5 gal/ac. Each plot was 4 rows wide and 25 feet long. Treatments were replicated three times using a randomized strip-type experimental design.


Results and Discussion

Postplant preemergence applications of Prowl gave good weed control without causing injury to the emerging onion plants. Weed control improved as the rate of Prowl was increased from 1.0 lb to 1.5 lb to 2.0 lb ai/ac. Prowl at 1.5 lb ai/ac controlled about 90 percent pigweed, lambsquarters, kochia, and barnyardgrass, 75 percent control of hairy nightshade, and 15 to 20 percent control of sow thistle. Prowl tank-mixed with Buctril/Nortron/Prism and applied postemergence to flag leaf onions controlled all weeds that emerged with the onions (Table 1). Two repeat applications of Buctril/Nortron/Prism applied 10 and 24 days after the initial application and the soil residual from Prowl kept the onions free of weeds until harvest. Seedling onions were tolerant of Frontier applied with Buctril/Nortron/Prism to flag leaf onions. Frontier did give residual weed control similar to Prowl. Dual applied to flag leaf onions caused injury to the onion plants. Injury symptoms began to show three to four weeks after Dual was applied. Onions with three or more leaves are tolerant to Dual applied at 2 lb ai/ac.

The better treatments in the postemergence trials included tank-mixes containing Prowl/Buctril/Nortron and Poast applied to the flag leaf onions at the respective rates of 1.5, 0.15, 0.25, and 0.1 lb ai/ac. Herbicides applied at the flag leaf gave better weed control than did herbicide applications delayed until onions had one true leaf. Repeat applications of these herbicides without Prowl as new weeds emerged gave season long weed control. Goal tank-mixed with Buctril/Nortron and applied to onions with true leaves can improve control of pigweed too large for control with Buctril. Prism herbicide gave excellent control of barnyardgrass applied alone or in tank-mix combination with Buctril/Nortron. Upbeet caused severe onion injury and did not control weeds effectively. Onion tolerance was not adequate with the emulsifiable formulation of Tough. Onions were more tolerant to the wettable powder formulation of Tough, but Tough treatments were consistently inferior to the Buctril/Nortron tank mixes (Tables 2 and 3).

Table 1. Percent crop injury and weed control from herbicides applied as postplant preemergence and postemergence applications to onions with a flag leaf. Malheur Experiment Station, Oregon State University, Ontario, Oregon, 1995.
  Percent weed control
  Crop injury Pigweed Lambsquarters H. Nightshade Kochia A. Sowthistle Barnyardgrass
Herbicides Rate Applied 1 2 3 1 2 3 1 2 3 1 2 3 1 2 3 1 2 3 1 2 3
  lb ai/ac    %   % 
Prowl 1. 0 Pre 0 0 0 85 85 90 85 90 85 85 65 70 70 85 85 85 0 0 0 85 95 90
Prowl 1. 5 Pre 5 5 5 90 90 90 90 90 90 70 80 80 90 95 90 15 20 15 90  100 100
Prowl 2. 0 Pre 10 10 5 90  95 95 85 90 90 70 85 85 90 95 95 20 20 15 95 100 95
Prowl1 1. 5 Post 0 5 0 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100
Prowl1 2. 0 Post 0 0 0 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 
Dual1 2. 0 Post 40 60 30 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100
Frontier1 1. 17 Post 5 10 5 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100
Frontier1 2. 34 Post 10 15 5 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100
Untreated check -- -- 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
1 Herbicide tank-mixed with Buctril, Nortron, and Prism. Evaluated June 12.            

Table 2. Crop tolerance and weed control ratings for herbicides applied as repeat applications beginning when onion plants were flag leaf. Malheur Experiment Station, Oregon State University, Ontario, Oregon, 1995.
  Percent weed control
  Crop injury Pigweed Lambsquarters H. Nightshade Kochia Sowthistle Barnyardgrass
Herbicides Rate 1 2 3 1 2 3 1 2 3 1 2 3 1 2 3 1 2 3 1 2 3
  lb ai/ac  %  % 
Buctril + Prowl + Poast 0.1 + 1.5 + 0.1 0 0 0 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100
Buctril + Prowl + Poast 0.15 + 1.5 + 0.1 5 0 0 100 100 100 100 100 100 100 100 100 99 100 100 100 100 100 100 100 100
Buctril + Prowl + Nortron + Poast 0.1 + 1.5 + 0.25 + 0.1 0 0 0 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100
Buctril + Prowl + Nortron + Poast 0.15 + 1.5 + 0.25 + 0.1 0 0 0 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100
Buctril + Goal + Prowl + Poast 0.1 + 0.03 + 1.5 + 0.1 0 5 5 100 100 99 100 100 100 100 100 100 100 100 99 100 100 100 100 100 100
Buctril + Goal + Nortron + Prowl + Poast 0.1 + 0.03 + 0.25 + 1.5 + 0.1 10 0 5 100 100 100 100 100 100 99 100 100 99 100 100 100 100 100 100 100 100
Tough ec + Poast 0.5 + 0.1 20 40 65 100 100 100 100 100 100 96 95 95 85 85  85 100 100 100 100 100 100
Tough ec + Poast 1. 0 + 0.1 80 80 80 99 90 90 99 100 99 99 100 99 99 95 95 99 99 99 99 99 99
Tough wp + Poast 0.5 + 0.1 10 10 10 100 100 100 100 100 100 100 90 90 98 85  85 100 100 100 100 100 100
Tough wp + Poast 1. 0 + 0.1 10 15 35 99 95 95 99 95 99 85 90 90 90 85 85 99 95 90 99 95 90
Tough ec + Buctril + Poast 0.25 + 0.15 + 0.1 30 40 60 100 100 100 100 100 100 100 100 100 98 98 98 98 99 99 99 99 99
Tough wp + Buctril + Poast  0.25 + 0.15 + 0.1 5 5 100 95 95 100 100 100 80 95 95 90 85 85 95 98 95 95 98 98
Upbeet 0. 0156 10 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
Upbeet + X-77 0.0156 + 0.25% 35 35 35 40 10 40 30 20 20 30 30 30 60 50 50 35 40 30 0 0 0
Upbeet + Buctril 0.0156 + 0.10 15 15 15 40 40 60 60 60 70 40 40 50 60 50 60 75 80 85 0 0 0
Upbeet + Buctril 0.0156 + 0.15 40 40 40 80 60 60 80 70 80 75 75 60 80 80 80 85 90 85 0 0 0
Prism + COC 0.045 + 1% 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 100 100 100
Prism + COC 0.094 + 1% 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 100 100 100
Buctril + Prism 0.15 + 0.045 0 0 0 85 85 90 100 100 100 100 100 100 90 95 100 98 98 100 100 100 100
Buctril + Prism 0.15 + 0.094 5 0 0 80 85 85 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100
Untreated check -- 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
COC = Crop oil concentrate = MorAct X-77 = non-ionic surfactant Evaluated June 12.            
 
 

Table 3. Crop tolerance and weed control ratings for herbicides applied as repeat applications beginning when onion plants had 1-true leaf. Malheur Experiment Station, Oregon State University, Ontario, Oregon, 1995.
  Percent weed control
  Crop injury Pigweed Lambsquarters H. Nightshade Kochia Sowthistle Barnyardgrass
Herbicides Rate 1 2 3 1 2 3 1 2 3 1 2 3 1 2 3 1 2 3 1 2 3
  lb ai/ac  %   % 
Buctril + Prowl + Poast 0.1 + 1.5 + 0.1 0 0 0 98 98 98 99 99 99 99 99 99 99 99 99 100 100 100 100 100 100
Buctril + Prowl + Poast 0.15 + 1.5 + 0.1 5 0 0 98 98 99 99 99 99 99 100  99 99 100 98 100 100 100 100 100 100
Buctril + Prowl + Nortron + Poast 0.1 + 1.5 + 0.25 + 0.1 0 0 0 98 100  98 99 100 100 99 100  100 98 98 100 100 100 100 100 100 100
Buctril + Prowl + Nortron + Poast 0.15 + 1.5 + 0.25 + 0.1 5 5 5 98 98 99 99 99 99 99 99 99 99 99 98 100 100 100 100 100 100
Buctril + Goal + Prowl + Poast 0.1 + 0.03 + 1.5 + 0.1 0 0 0 98 98 96 99 99 96 98 99 99 98 99 95 100 100 100 100 100 100
Buctril + Goal + Nortron + Prowl + Poast 0.1 + 0.03 + 0.25 + 1.5 + 0.1 5 5 0 98 98 98 99 99 99 99 98 98 98 98 98 100 100 100 100 100 100
Tough ec + Poast 0.5 + 0.1 30 15 15 99 99 96 99 99 99 99 99 96 90 95 95  100 100 100 100 100 100
Tough ec + Poast 1. 0 + 0.1 15 15 70 95 100 98 95 100 99 90 90 98 95 99 95 100 100 100 100 100 100
Tough wp + Poast 0.5 + 0.1 0 0 0 90 90 95 90 95 98 90 90 95 80 85 85 95 98 98 100 100 100
Tough wp + Poast 1. 0 + 0.1 5 5 5 99 99 99 99 99 99 95 98 98 90 95 90 99 100 100 100 100 100
Tough ec + Buctril + Poast 0.25 + 0.15 + 0.1 5 5 10  95 98 99 98 98 99 9 98 99 95 95 99 100 100 100 100 100 100
Tough wp + Buctril + Poast  0.25 + 0.15 + 0.1 5 5 5 60 80 85 75 85 85 80 85 75 70 90 80 95 99 99 100 100 100
Upbeet 0. 0156 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
Upbeet + X-77 0.0156 + 0.25% 60 60 40 20 10 10 30 20 30 10 10 10 65 60 60 40 45 50 0 0
Upbeet + Buctril 0.0156 + 0.10 30 30 30 20 20 25 30 20 25 10 10 15 35 25 30 95 95 95 0 0 0
Upbeet + Buctril 0.0156 + 0.15 70 65 40 30 30 30 40 45 45 20 30 30 40 50 50 60 60 60 0 0 0
Prism + COC 0.045 + 1% 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 100 100 100  100
Prism + COC 0.094 + 1% 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 100 100 100
Buctril + Prism 0.15 + 0.045 0 0 0 80 85 85 95 95 95 90 95 95 90 90 85 90 95 100 100 100 100
Buctril + Prism 0.15 + 0.094 0 0 0 85 85 90 90 90 90 95 95 95 90 90 90 95 100 100 100 100 100
Untreated check -- 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
COC = Crop oil concentrate = MorAct
X-77 = non-ionic surfactant
Evaluated June 12.

††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.