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ISSUE 14   August 6, 1998

 

CANADA THISTLE CONTROL

    Dr Rod Lym, Perennial Weed Research, conducted research in 1996 and 1997 to evaluated Canada thistle control from several herbicides. Treatments were applied in_crop to wheat and post_harvest. In-crop treatments were applied June 27, 1996 when wheat was in the 6_leaf stage and thistle was 4_6 inches tall, on August 2, 1996 at the hard dough stage of wheat and thistle was 8 to 12 inches tall and blooming, and post-harvest (PH) on September 13, 1996 to new thistle regrowth when thistle was in the ROSETTE stage. Treatments with ( / ) means that sequential applications were made. For example: 2,4-D ester/Curtail means 2,4-D was applied first followed by Curtail at a later application date.

Herbicide

 

Canada thistle

Timing

Rate

Sep 96

Jun 97

Jul
97

   

% control

Express + 2,4-D
     Jointing           0.33 oz+0.75 pt

16

60

25

2,4-D ester/Curtail
     Jointing/PH        1.25 pt/2 pt

10

99

94

Curtail
     Jointing                              2 pt

46

94

81

Roundup Ultra
     Pre-harvest                      2 pt

69

79

76

Roundup Ultra +2,4D
     Pre-harvest             2 pt+1 pt

50

71

58

Roundup Ultra
     Post-harvest                      2 pt

--

92

79

Roundup Ultra + 2,4-D
     Post-harvest             2 pt+1 pt

--

94

56

Curtail
     Post-harvest                      2 pt

--

75

61

Curtail
     Post-harvest                      4 pt

--

91

69

Banvel
     Post-harvest                      2 pt

--

91

65

    In general, any Curtail treatment provided the best long_term Canada thistle control. Roundup Ultra alone or with 2,4-D gave better control applied post-harvest than pre-harvest. However, IF DRY WEATHER CONDITIONS EXIST - USE CURTAIL!!! Banvel gave good in_season control but dropped to 65% the following year. Express + 2,4_D did not control Canada thistle.

    2,4-D at jointing followed by Curtail applied post-harvest to ROSETTE thistle provided then best long-term control. In general, post-harvest treatments gave better thistle control than pre-harvest applications.

    It has been thought that Roundup does not give as good thistle control as Curtail one year after application. However, this data shows similar control as Curtail.

    It has been thought that applying 2 herbicides together (i.e. Roundup + 2,4-D) gives greater perennial weed control than high rates of one herbicide alone. That may be true with some combinations BUT APPEARS NOT TRUE WITH ROUNDUP ULTRA + 2,4-D.

   Roundup Ultra gave 80% compared to 56% control from Roundup Ultra + 2,4_D when evaluated in July the year following application.

    The following is data for perennial sowthistle control that was taken at the same time as Canada thistle experiment listed above.

Herbicide

 

Perennial sowthistle

Timing

Rate

Sep 96

Jun 97

Jul
97

   

% control

Express + 2,4-D
     Jointing           0.33 oz+0.75 pt

9

29

18

2,4-D ester/Curtail
     Joint/PH                 1.25 pt/2 pt

23

97

86

Curtail
     Jointing                              2 pt

39

48

58

Roundup Ultra
     Pre-harvest                      2 pt

51

79

63

Roundup Ultra + 2,4-D
     Pre-harvest             2 pt+1 pt

48

85

45

Roundup Ultra
     Post-harvest                      2 pt

--

56

18

Roundup Ultra + 2,4-D
     Post-harvest             2 pt+1 pt

--

54

13

Curtail
     Post-harvest                      2 pt

--

38

33

Curtail
     Post-harvest                     4 pt

--

58

35

Banvel
     Post-harvest                     2 pt

--

61

20

 

FACTORS TO CONSIDER FOR CANADA THISTLE CONTROL:

1.  Banvel, Roundup, Tordon, 2,4-D, Curtail, and Stinger are herbicides that have the greatest activity on Canada thistle.

2.  Tankmixes of some herbicides may provide greater control than one herbicide applied alone (except glyphosate).

3.  Highest rates should be used without interfering with next years cropping pattern.

4.  Situations may occur where spot treatment with herbicides at high rates may be viewed as the best Canada thistle management program even if the carryover may injure next year's crop.

5.  Research has shown at least 90% stand reduction from Banvel, Roundup, Landmaster BW, or Tordon plus 2,4-D.

6.  Timing is a critical factor. Herbicide application after a frost at 32 degrees F may enhance control but application in late October when leaf tissue was destroyed by frost may result in less control due to lack of herbicide uptake.

7.  Tillage can be a critical factor. Tillage in late fall, after spraying increases control and may add an additional 30 to 40% control for treatments that were 30 to 50 percent effective. If lower herbicide rates or less effective herbicides are used, tillage is very important. If tillage is not planned then use the most effective herbicide or tank-mix at higher rates.

8.  Spray rosettes of actively growing plants. See The Hunter Method, explained below.

 

KILL CANADA THISTLE IN SEPTEMBER USING THE HUNTER METHOD

    A key in the successful control of Canada thistle is to understand how Canada thistle grows then use that knowledge to your advantage. Using the right combination of tillage and herbicide improves control and may, in some cases, reduce the rate of herbicide required to kill Canada thistle.

    Over 90% of the root system of Canada thistle is below the depth of cultivation. The only way to effectively control the entire root system is to make applications when maximum translocation to the underground roots will occur. This happens in late summer or fall after day-length is less than 14 to 16 hours.

    Day length of mid-summer days is over 16 hours of daylight. The long photoperiod (long days) induces the Canada thistle plants in the rosette stage to bolt and produce flowers. By preventing thistle plants from bolting when day-length is greater than 16 hours will set the thistle plants up for more effective late summer/fall herbicide applications. During summer months, prevent emerged thistle plants from bolting through cultivation, or in_crop herbicide applications with 2,4_D or other labeled products until day length (photoperiod) is less than 14 to 16 hours. Thistle plants that emerge after day-length is less than 14 hours will not bolt. Shorter day-length will not trigger thistle plants to bolt and they will remain in the rosette form. In these conditions, thistle plants pump more food reserves downward to the root system. Application of phloem mobile herbicides like 2,4_D, Banvel, Tordon, Roundup, and Curtail/Stinger at this time also results in greater translocation to roots and subsequently, greater root kill. Herbicide application to new thistle growth in the rosette form after day-length is less than 14 hours is an optimum time for application. Control of Canada thistle was tested by Dr. Jim Hunter in field plots at the Regina Research Station of Agriculture Canada.

    Half of the plots were summer fallowed in spring, then the thistle plants were allowed to grow to the bud stage and were treated with herbicide the last week of July. The other half of the plots were received normal summer fallow tillage until the end of July (days become shorter earlier in Canada than North Dakota). All Canada thistle regrowth remained as rosettes. The herbicide was applied at the end of August. One year after treatment the number of shoots were counted.

    Banvel at approximately 1 pt/A applied in August when plants were in the rosette stage had 88% fewer shoots than twice as much herbicide applied at the bud stage. Similar results were obtained with Roundup. Both products are registered in Canada for use at the rosette stage.

    Control of Canada thistle with the "Hunter Method" in Canada has been very successful. Translocation studies have documented a four fold increase in translocation of herbicides when applied in the fall in the rosette stage as compared to bolted plants. Roundup applied once at half the recommended rate for application in the bud stage, applied to Canada thistle in the rosette stage resulted in 98% control for two years after treatment.

Canada Thistle Control - The Hunter Method

 

Canada thistle shoots*

 

Bud stage

Rosette stage

 

(no./m2)

Cultivated fallow
Fallow + Banvel (1.05 pt/A )
Fallow + Banvel (1.1 qt/A)
*1 year after treatment

25
16
14

25
2
3

    Herbicide pricing may affect herbicide selection. The question if often asked of which herbicide gives the most control for the dollar. Last year Monsanto introduced a $10/gallon rebate for Roundup Ultra RT. Depending on availability of last years product at the reduced price or continuing the rebate program again this year may affect herbicide selection. See below.

Landmaster BW (glyphosate + 2,4-D) at 54 fl oz/A =
    1 pt of Roundup Ultra +
    1.25 pt/A of 2,4-D
    $9.00/A

However, 1 pt of Roundup Ultra RT at $30/gal = $3.75 + 1.25 of 2,4-D amine at $13.00/gal = $2.00

TOTAL: $5.75 compared to $9.00 for 54 fl oz of Landmaster BW

Fallow Master (glyphosate + dicamba) at 44 fl oz/A =
    1 pt of Roundup Ultra +
    0.33 pt/A Banvel
    $6.00/A

However, 1 pt of Roundup Ultra RT at $30/gal = $3.75 + 0.33 pt/A of Banvel at $82.00/gal = $3.40

TOTAL: $7.15 compared to $6.00 for 44 fl oz/A of Fallow Master.

Conclusion: APPLY AS MUCH AS YOU CAN AFFORD!!!!!

Richard Zollinger
NDSU Extension Weed Scientist


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