Controlling Fleabane using the double knock tactic

In recent years flaxleaf fleabane (Conyza bonariensis) has become one of the most persistent and difficult to control weeds in northern no-till fallow. With individual plants capable of producing over 100,000 seeds, stopping all weeds from setting seed is a very desirable objective.

 

Fleabane is tolerant to glyphosate, and a number of post-emergent herbicide mixes with glyphosate have been developed for the control of fleabane in winter fallow. However few of these were able to deliver effective control of this species. This often results in fleabane continuing to be a problem in subsequent crops. In addition, the prevention of seed set from weeds that have survived herbicide application is a critical factor in the prevention of herbicide resistance and for weed management in general.

 

The double knockdown strategy of using two different herbicide modes of action sequentially – both at an effective dose rate if used alone has been adopted to delay or avoid the onset of glyphosate resistant ryegrass in southern Australia with a great deal of success. A strategy using two knockdown herbicide modes of action has also been trialled for its effectiveness in managing fleabane. However, as the rates of the individual herbicides do not on their own kill the weed population, the benefit of using the double knock strategy is to just kill the weeds rather than to protect one of the herbicide modes of action from developing resistance - as is the strategy for ryegrass in southern Australia. 

In a trial conducted at Dalby (Table 1), the tolerance of fleabane to glyphosate was evident with only 55 % control achieved in the glyphosate only treatment. Glyphosate followed by SpraySeed provided good control of plants.  However as time between treatments increased, the level of control decreased.

 

The addition of 2,4-D 300 ipa^® to the glyphosate spray provided better control, but again increased intervals between sprays reduced the level of control. Glyphosate and 2,4-D 300 ipa followed by the higher rate of SpraySeed provided total control of plants in all cases. SpraySeed proved to be an effective tool in a “double knock” strategy for fleabane control

 

The 2^nd trial near Cecil Plains contained a denser population of fleabane also showed a poor result for control achieved with glyphosate alone(Table 2).

 

For the treatments containing 2,4-D, as the period between initial and follow-up applications increased, the level of control decreased. The combined glyphosate + 2,4-D 300 ipa treatment was marginally better than the split application of glyphosate and 2,4-D 625 (containing the same rate of 2,4-D) on the same day and one day later. At longer intervals between glyphosate and Amicide applications (3 and 5 days) fleabane control was considerably poorer.

 

Double knock using glyphosate followed by paraquat or SpraySeed was also trialled. At the same rate (L/ha), paraquat gave a higher level of control than SpraySeed in this trial. Timing of the follow-up application was also important for these herbicides. Highest control for these herbicides was achieved when they were applied at 5 and 7 days after glyphosate.

 

Glyphosate and glyphosate mixes with 2,4-D were applied at a water rate of  73 L/ha at 200 kPa through TT11001 flat fan nozzles. Follow-up applications of 2,4-D were applied in the same manner. Follow-up applications of paraquat and SpraySeed were applied at 100 L/ha at 200 kPa. Glyphosate was applied with a non-ionic 100% wetting agent at a rate of 100mL/100L.

The costs for each herbicide combination (exclusive of application) are listed in Tables 1 and 2. (Note that glyphosate cost is set at a longer term average cost of $6/L – substantially less than the retail price at the time of writing). The double knock tactic costs more to use, but provides better weed kill and less weed seed set. In low weed pressure situations as in the first experiment, Roundup followed by paraquat at 1.6 L/ha should be effective for control. As the weed pressure increases, the addition of 2,4-D 300 ipa will improve control. Getting good coverage with robust rates of paraquat and SpraySeed are also important to get maximum effectiveness from these chemicals. Herbicides applied at sub-optimal rates or in bad conditions end up being more costly over the long-term. The high seed production of fleabane means that even a small proportion of escapes will result in a large weed problem in future years, so it is better to maximise the control achieved.

Jeff Werth

Leslie Research Centre

Department of Primary Industries and Fisheries

13 Holberton St, Toowoomba, Qld 4350

Ph: 07 4639 8851

Fx: 07 4639 8800

Email: jeff.werth@dpi.qld.gov.au

 

Table 1: Control achieved on fleabane at Dalby in 2006. Plants were sprayed at 6-10 leaves

First knock	Second knock	Days between treatments	Control (%)	Cost ($/ha)
				Gly450 $6/L	Gly450 $10/L
Glyphosate 450 2 L/ha	na	0	54	12	20
Glyphosate 450 2 L/ha	SpraySeed 1.6 L/ha	7	95	32	40
		14	96
		21	87
Glyphosate 450 2 L/ha	SpraySeed 2.4 L/ha	7	97	42	50
		14	87
		21	95
Glyphosate 450 2 L/ha	SpraySeed 1.6 L/ha	7	99	38.5	46.5
+ 2,4-D 300 ipa 1.5 L/ha		14	100
		21	95
Glyphosate 450 2 L/ha	SpraySeed 2.4 L/ha	7	99	48.5	56.5
+ 2,4-D 300 ipa 1.5 L/ha		14	99
		21	99
Glyphosate 450 2 L/ha	SpraySeed 2.4 L/ha	7	100	55	63
+ 2,4-D 300 ipa 3 L/ha		14	99
		21	99
Glyphosate 450 2 L/ha	2,4-D 625 1.5 L/ha	0	93	24	32
		3	85
		7	85
Glyphosate 450 2 L/ha	2,4-D 625 3 L/ha	0	90	36	44
		3	97
		7	88

 

Table 2: Control achieved on fleabane at Cecil Plains in 2007. Plants were sprayed at 6-10 leaves

 

First knock	Second knock	Days between treatments	Control (%)	Cost ($/ha)
				Gly450 $6/L	Gly450 $10/L
Glyphosate 450 2 L/ha	na	na	40	12	20
Glyphosate 450 2 L/ha + 2,4-D 300 ipa 1.5 L/ha	na	na	87	18.5	26.5
Glyphosate 450 2 L/ha	2,4-D 625 0.72 L/ha	0	88	18.5	26.5
		1	91
		3	69
		5	63
Glyphosate 450 2 L/ha	Paraquat 1.6 L/ha	1	83	24.8	32.8
		3	80
		5	92
		7	90
		14	67
Glyphosate 450 2 L/ha	SpraySeed 1.6 L/ha	1	61	32	40
		3	58
		5	86
		7	85
		14	62

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