33
Evaluation of Post-Emergence Herbicides for Metribuzin-Sensitive Potato Varieties
K. Locke r K.A. Rykbost 2, and J. Maxwell 2
;
INTRODUCTION
Control of late emerging broadleaf weeds in metribuzin-sensitive potato varieties,
such as Shepody and several red-skinned cultivars, is a problem in commercial and
research situations. If low metribuzin rates are applied, weed control may be
unsatisfactory and yield losses may occur. Weed competition reduces yields and size
when no herbicide is used for late emerging weeds. The objectives of this study were to
evaluate an experimental post-emergence broadleaf herbicide and metribuzin and assess
the effects of several weed suppression options on crop performance for the variety
Shepody.
PROCEDURES
Shepody seed was planted at 8.7-inch spacing in 32-inch rows on May 18. Standard
practices were followed for fertilizer, irrigation, disease, and pest control (see page 19).
Eptam was applied at 3.5 lb ai/A on May 27. Four-row, 20-foot plots were established
to accommodate eight treatments with four replications in a randomized complete block
design. Herbicide treatments were applied with a backpack sprayer on June 25, when
weeds were in the cotyledon to 1-2 true leaf stages. Potato plants were about 6 to 8
inches tall. Treatments included: untreated control; metribuzin (M) alone at 5.3 oz
ai/A; Du Pont's E9636 (E) alone at 0.25, 0.38, and 0.50 oz ai/A; and combinations of
2.0 oz ai/A M plus 0.25 oz ai/A E, 3.0 oz ai/A M plus 0.38 oz ai/A E, and 4.0 oz ai/A
M plus 0.50 oz ai/A E.
Visual ratings of weed control and crop injury were made at 2, 7, and 14 days after
treatment. Crop injury was rated as the percent of plants with visible stunting or leaf
chlorosis symptoms. Weed control ratings represented the percent of weeds of a given
species that were wilted beyond recovery. The predominant weed species present were
redroot pigweed, hairy nightshade, and Indian lovegrass.
Vines were desiccated with Diquat applied at 1.0 pt/A on September 5. Three
plants were removed between plots to eliminate border effects before harvest. Potatoes
were harvested with a one-row digger-bagger on September 24. All tubers from 18 feet
of the center two rows of each plot were stored and graded to USDA standard grades in
late October.
/ Klamath County Cooperative Extension Agent.
2/ Superintendent/Associate Professor and Biological Sciences Research Technician,
respectively, Klamath Experiment Station, Klamath Falls, OR.
1
Acknowledgment: E.I. du Pont de Nemours and Co., Inc. provided the experimental
herbicide E9636. Partial funding was provided by the Oregon Potato Commission.
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RESULTS AND DISCUSSION
Crop injury was just evident two days after treatments were applied (Table 1).
One week after applications, metribuzin injury increased with application rate while the
E9636 product did not produce visible injury symptoms. Two weeks after application,
foliar symptoms were slightly more evident in E9636 plots, and plants were growing out
of metribuzin symptoms. No potato plant death occurred in any of the treatments.
Some concern has been expressed about a synergistic interaction between E9636 and
organophosphate insecticides. There was no evidence of any interaction between E9636
and Di-syston. Di-syston was applied in seed furrows at planting in this study.
Metribuzin produced more rapid desiccation of pigweed and lovegrass than E9636.
Hairy nightshade was controlled about equally well by both products, alone or in
combinations. At low application rates, the combination treatments were slightly less
effective for all three species than E9636 applied alone, over the two-week observation
period. All herbicide treatments provided excellent late season weed control.
Weed competition significantly reduced yield and tuber size in the control plots
compared with all herbicide treatments except metribuzin applied alone (Table 2). Plant
injury for metribuzin alone, and in the high rate combination, reduced yield and tuber
size slightly, but not significantly, compared with other herbicide treatments. The yields
were similar for all rates of E9636 and the two low-rate combination treatments.
In commercial practice, metribuzin is typically applied for late season weed control
at 8 to 12 oz ai/A (0.50 to 0.75 lb ai/A). Locally, concern has been expressed about
development of resistance to metribuzin in hairy nightshade if lower rates are used.
Residual effects of metribuzin on cereal grain and sugarbeet crops have also occurred.
The results of this study indicate that E9636 applied alone at 0.38 oz/A would provide
acceptable late season weed control in potatoes with little or no reduction of crop yields.
This product offers a solution to the problem of late season weed control in research
trials where metribuzin-sensitive varieties are disadvantaged by the use of metribuzin.
Further studies are needed to evaluate E9636 under a range of weed pressure situations.
In particular, the efficacy of this product for control of kochia should be determined as
this species is becoming a common problem in the Klamath Basin.
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Table 1. Effects of postemergence herbicide treatments on percent crop injury and
control of three weed species at 2, 7, and 14 days after treatment, Klamath
Experiment Station, OR. 1992.
Treatment Crop
injuryl
Herbicide
Rate
Pigweed
Weed control 2
Hairy
Indian
Nightshade
Lovegrass
oz ai/A
2 days post treatment
Control
M
E
E
E
M+E
M+E
M+E
Control
M
E'
E
E
M+E
M+E
M+E
Control
M
E
E
E
M+E
M+ E
M+E
0
0
0
0
0
1
0
0
0
70
0
0
0
30
60
50
5.3
0.25
0.38
0.50
2+0.25
3+0.38
4+0.50
7 days post treatment
0
0
60
98
37
3
53
1
1
52
13
80
88
15
90
53
0
53
30
38
45
35
38
60
0
96
33
26
38
55
65
89
5.3
0.25
0.38
0.50
2+0.25
3 + 0.38
4+0.50
14 days post treatment
0
0
91
14
90
3
98
8
100
8
73
8
68
2
95
9
0
89
95
98
100
75
70
95
0
94
75
88
90
64
70
93
5.3
0.25
0.38
0.50
2+0.25
3+0.38
4+0.50
0
3
3
3
3
3
3
3
0
88
0
1
0
68
78
73
/ Crop injury - percent of plants with visible stunting or leaf chlorosis.
- percent of weeds wilted beyond recovery.
2 / Control
1
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Table 2. Effect of postemergence herbicide treatments on yield and grade of
Shepod:y potatoes, Klamath Experiment Station, OR. 1992.
Treatment
Rate
Product
4-6 oz
Yield U.S. No is
>10 oz
6-10 oz
Bs
Total
cwt/A oz ai/A
Control
Total
Yield
No 2s
77
97
53
226
44
14
296
M
5.3
70
131
107
308
29
22
364
E
E
E
0.25
0.38
0.50
98
87
90
151
152
143
130
151
134
379
389
367
36
32
31
26
26
25
456
465
430
2.0+ 0.25
3.0+0.38
4.0+ 0.50
94
87
83
147
153
144
122
129
117
363
369
343
40
32
33
26
36
26
442
443
413
86
27
NS
140
19
NS
118
28
49
343
17
84
34
24
NS
25
45
NS
414
16
94
M+E
M+E
M+E
Average
CV(%)
LSD(.05)