New options for southern fenceline weed control

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Photo of glyphosate-resistant ryegrass treated with glyphosate

1. Glyphosate-resistant ryegrass treated with glyphosate.

Photo of paraquat/diquat + bromacil  (Uragan® at 2kg/ha),

2. Glyphosate-resistant ryegrass treated with paraquat/diquat + bromacil
(Uragan® at 2kg/ha).

Photo of glyphosate-resistant ryegrass treated with glufosinate + bromacil (Uragan® at 2 kg/ha)

3. Glyphosate-resistant ryegrass treated with glufosinate + bromacil (Uragan® at 2kg/ha).

PHOTOS (1, 2, 3): Chris Preston

Reliance on glyphosate for fallow management, prior to crop seeding and for inter-row weed management has resulted in the evolution of glyphosate-resistant weeds that are easily spread across the farm.

When glyphosate-resistant weeds survive in fencelines there is usually little competition, allowing these undesirables to set a lot of seed. This makes the task of weed management along the margins of crops even more challenging.

Finding an effective way to reduce weed seed-set outside the crop is essential to limiting weeds’ spread into the crop.

Non-herbicide options

To reduce seed-set of problem weeds the area adjacent to the crop can be cultivated, slashed, rolled for hay, grazed by stock or treated with herbicide.

However, these options can cause other problems. Cultivation can leave the area prone to erosion and reduce trafficability. Slashing and cutting for hay take extra time and may be a problem for large farms. Grazing can be problematic in sourcing stock – if they are not already on the farm – and stopping them straying into the crop.

Other options include minimising bare space by growing crops closer to fences, or removing fences and moving crop margins each year.

For many growers, herbicides will remain the preferred option for fenceline weed control but, clearly, relying on glyphosate alone is a risky strategy. Likewise, mixing with a Group B herbicide is a poor choice as many weeds already have Group B resistance.

The University of Adelaide conducted a series of trials looking at the herbicide options that could be applied in August–September when growers of winter crops have time to manage these areas. This timing requires application of herbicides to larger, well-established plants.

Successful combination

Among the options investigated, bromacil (for example, Uragan®) in combination with alternative knockdowns proved to be effective. Bromacil is a Group C herbicide with grass and broadleaf activity, is not overly mobile and, importantly, belongs to a subgroup not used in the cropping phase and to which we don’t have resistance. This means that if resistance does occur in the fenceline, it is likely to have less impact in-crop.

Trials at Clare and Kapunda (Table 1) found bromacil at the lower rate was effective in mixtures with either paraquat/diquat (for example, Spray.Seed®) or glufosinate (for example, Basta®). However, in these high annual ryegrass populations, the higher rate of bromacil was sometimes better. Glyphosate was relatively ineffective at both sites at the rate used.

What we learned from these trials is that simply mixing another herbicide with glyphosate was not going to be effective, except where populations were small and without high levels of resistance. Even then, this just places the selection pressure on the mixing partner.

Use of a residual herbicide with appropriate activity on the weeds and registration for mixing with an alternative knockdown herbicide was the best approach. A strategy of two applications of paraquat-based products was also effective and could be used in situations where bromacil cannot be used such as near desirable trees or other non-target plants or where their roots may extend.

For growers favouring pre-emergent control there are other broad-spectrum residual herbicides aside from bromacil, but most need to be applied to bare soil for best effect.

 Treatment Rate (g or L/ha)
 Clare  Kapunda
Seed heads* (heads/m2)
Seed head reduction (%)
Seed heads* (heads/m2)
Seed head reduction (%)
 Nil   3553 a
0 4287 a
0
Glyphosate (540 g/L)
2L 1627 b
54 3253 ab
24
Paraquat (135g/L)/diquat (115g/L)
3.2L 1100 b
69 1580 bcd
63
Glufosinate (200g/L)
5L 833 bc
77 1767 bcd
59
Paraquat (135 g/L)/diquat (115 g/L) + bromacil (800 g/kg)
3.2L + 2kg#
860 bc
76 107 f
98
Paraquat (135 g/L)/diquat (155g/L) + bromacil  (800g/kg)
3.2L + 3kg#
7 f
99.8 73 f
98
Glufosinate (200g/L) + bromacil (800g/kg)
5L + 2kg#
47 ef
99 347 ef
92
Glufosinate (200g/L) + bromacil (800g/kg)
5L + 3kg#
113 def
97 67 f
98

 * Treatments in each column followed by different letters are significantly different (P=0.05).

# Bromacil (Uragan®) label rate for non-crop areas is 3.5 to 6.5kg/ha, with the 2.0 to 6.5kg/ha rate on the label for 'retreatment'.

More information:

Dr Chris Preston,
08 8313 7237,
christopher.preston@adelaide.edu.au

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Weed-identifying robots at your command

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Low rainfall grass weeds need whole-of-system strategy

GRDC Project Code UA00124

Region South