Wild oat management New options for old chemistry

| Date: 14 Jun 2008

• Results encouraging from both Triflur® Xcel and Avadex® Xtra but NOT silver bullets (will nearly always require a post-emergent clean up)
• Similar results from both products where ‘incorporated by sowing’
• No consistent dose response to either product alone or in mixture
• In chickpeas, under extreme wild oats pressure, an average net benefit of $310/ha was obtained (range $242-373)
• May have better fit in zero/minimum tillage systems
• Industry needs to reduce Group A usage in crops where viable options exist eg in wheat with Group B and K
• Focus must be on reducing the wild oat seedbank and include winter/ summer crop rotations in the north

Richard Daniel, Northern Grower Alliance (NGA)

Although these pre emergent chemicals will NOT by themselves be a solution to our problem, they may be a component of an overall package to manage wild oats more effectively.

The weed

Wild oats are the key winter grass weed of the northern cropping region. For much of the last two decades, they have been managed in-crop, predominantly using Group A herbicides. In recent years however it has become clear that herbicide resistant wild oats are becoming a key threat to a sustainable northern farming system.

The issue

Resistant wild oats will pose management problems in any crop where these herbicides have previously been relied upon, but the threat appears greater to chickpea production. Why?
  • General lack of competitiveness
  • Often produced on wide rows
  • Only have Group A herbicides registered for post-emergent control
 
When coupled with the fact that chickpeas are the major northern, non-cereal, winter rotation crop, any threat to chickpea production area could have a major impact on the overall farming system.

What was evaluated

Pre-emergent active ingredients such as trifluralin (Group D) and tri-allate (Group E) have not been widely used in northern NSW for a range of reasons including the widespread adoption of reduced tillage, difficulty of effective incorporation, soil types and previously the range of highly effective post emergent chemicals available.
 
NGA established three small plot trials in 2007 to evaluate the potential of these pre-emergent herbicides when ‘incorporated by sowing’ (IBS) in zero/minimum tillage systems. Triflur® Xcel (trifluralin 500 g/L) and Avadex® Xtra (tri-allate 500 g/L) were both evaluated alone and in mixtures.
 
Two of the trials were conducted in chickpeas with the remaining site in wheat. Application was in a total volume of 70 L/ha, using TT11002 nozzles at 200 kPa at 11 km/hr. Row widths were 50 cm in chickpeas, and 25 cm in the wheat trial. Incorporation by planting at all sites was within 24 hours of application at sowing speeds of 10-11 km/hr. All sites had suspected Group A resistance with subsequent established wild oat populations in the untreated ranging from 11-1,100 wild oats/m2.
 
Table 1: Wild oat control with pre-emergent herbicides in wheat
Trial
NGA0717
Untreated wild oats density
~16 plants/m2
Product
Rate L/ha
% control
DAP 65
Triflur Xcel
1.45
88
 bc
Triflur Xcel
1.9
64
ab
Avadex Xtra
1.6
75
 bc
Avadex Xtra
2.0
90
 bc
(Mean of Pre-emergents ALONE)
(79)
Triflur Xcel + Avadex Xtra
1.45 + 1.6
76
 bc
Triflur Xcel + Avadex Xtra
1.45 + 2.0
86
 bc
Triflur Xcel + Avadex Xtra
1.9 + 1.6
94
    c
Triflur Xcel + Avadex Xtra
1.9 + 2.0
81
 bc
(Mean of Pre-emergent MIXTURES)
(84)
 
P=0.0049
DAP = Days After Planting
 
NB an additional replicated commercial trial was conducted by MCA Goondiwindi in a barley crop near Croppa Creek. Triflur Xcel at 1.45 L/ha was mixed with Avadex Xtra at either 1.6 or 2.0 L/ha. By DAP 45 the wild oat control ranged from 79-80%. This supported the level of control observed in the small plot wheat trial.

Wild oat control in chickpeas

The maximum registered rate of Triflur Xcel in chickpeas is 1.6 L/ha in Qld and 1.7 L/ha in NSW but is dependent on soil type. Triflur Xcel at 1.45 L/ha was used in both chickpea trials to compare to the same rate used in wheat. A higher experimental rate (coded T2) was also applied to investigate whether any rate response was evident.
 
The maximum registered rate of Avadex Xtra in chickpeas is 1.6 L/ha. This rate was used in both chickpea trials to compare to the same rate used in wheat. A higher experimental rate (coded A2) was also applied to investigate whether any rate response was evident. Four rates of mixtures were also applied. These were combinations of the two Triflur Xcel rates and the two Avadex Xtra rates.
 
Table 2: Wild oat control with pre-emergent herbicides in chickpeas
Trial
 
NGA0718
NGA0719
Untreated wild oats density
 
~1,100 plants/m2
~11 plants/m2
Product
Rate L/ha
% control
DAP 45
% control
DAP 49
Triflur Xcel
1.45
49
 bcd
70
abc
Triflur Xcel
T2
75
    cd
85
 bcd
Avadex Xtra
1.6
58
 bcd
85
 bcd
Avadex Xtra
A2
41
 bc
45
ab
(Mean of Pre-emergents ALONE)
(56)
(71)
Triflur Xcel + Avadex Xtra
1.45 + 1.6
62
 bcd
92
    cd
Triflur Xcel + Avadex Xtra
1.45 + A2
83
      d
96
    cd
Triflur Xcel + Avadex Xtra
T2 + 1.6
82
      d
97
      d
Triflur Xcel + Avadex Xtra
T2 + A2
73
 bcd
85
 bcd
(Mean of Pre-emergent MIXTURES)
(75)
(93)
 
P=0.0035
P=0.0093
DAP = Days After Planting, treatments and results in italics are above label registered rates

Pre-emergents alone under IBS approach:

1.      No significant rate response was obtained by increasing either Triflur Xcel or Avadex Xtra alone in any of the three trials
2.      Similar levels of wild oat suppression (~70%) were obtained from both products
3.      Improved results were recorded under lower weed pressure

Pre-emergent mixtures under IBS approach:

1.      No significant rate response was obtained by increasing the rate of Triflur Xcel or Avadex Xtra in the mixture
2.      The mixtures generally provided higher levels of wild oat suppression (~85%) than the pre-emergent products alone
3.      Improved results were recorded under lower weed pressure

Key points:

1.      Neither product alone, or in mixture, consistently provided ‘stand alone’ levels of wild oat control
2.      Good levels of control were obtained ‘between the rows’ but escapes must be expected along the planting line, where soil is moved during the sowing operation
3.      Pre-emergents may provide more consistent efficacy in zero/ minimum tillage systems where larger numbers of seed are located on the soil surface

Final wild oat control

All sites received commercial post-emergent spray programs. Atlantis® (Group B) was used in the wheat trial and provided 100% wild oats control in all treatments including the untreated. The two chickpea sites received Group A (fop and dim) combinations with results below.


 
Table 3: Final wild oat control in chickpeas
Trial
 
NGA0718
NGA0719
Crop
 
Chickpeas
Chickpeas
Row spacing
 
50 cm
50 cm
Untreated wild oats density
 
32 panicles/m2
~4 plants/m2
Product
Rate L/ha
% control
DAP 128
% control
DAP 66
Triflur Xcel
1.45
90
 b
91
 bcd
Triflur Xcel
T2
100
    c
95
    cd
Avadex Xtra
1.6
98
 bc
84
abcd
Avadex Xtra
A2
97
 bc
74
abc
(Mean of Pre-emergents ALONE)
(96)
(86)
Triflur Xcel + Avadex Xtra
1.45 + 1.6
98
 bc
95
    cd
Triflur Xcel + Avadex Xtra
1.45 + A2
100
    c
94
    cd
Triflur Xcel + Avadex Xtra
T2 + 1.6
97
 bc
98
      d
Triflur Xcel + Avadex Xtra
T2 + A2
99
 bc
93
    cd
(Mean of Pre-emergent MIXTURES)
(99)
(95)
 
P=0.0011
P=0.0093
DAP = Days After Planting, treatments and results in italics are above label registered rates

Key points:

1.      No consistent rate response to Avadex Xtra, alone or in mixture
2.      No consistent rate response to Triflur Xcel in mixture, but significant rate response at ‘extreme pressure’ wild oat site when used alone
3.      Highest levels of control obtained from use of pre-emergent mixtures followed by post-emergent strategies
4.      Wild oat escapes from pre-emergent programs were well controlled by post-emergent strategies
5.      The post-emergent strategies alone did not provide effective wild oat control

Yield and economics

There was no significant difference in yield between any treatment and the untreated at the two ‘low pressure’ wild oat sites. In both cases there was no net financial benefit from using pre-emergent herbicides (NB There were significantly reduced final wild oat numbers in trial 0719). Table 4 shows the yields where ‘extreme pressure’ of wild oats occurred in chickpeas. Even with a successful post-emergent program, a reduction in average yield of ~700 kg/ha was caused from the early wild oat competition.
 
Table 4: Grain yield and economics under ‘extreme pressure’ of wild oats, chickpeas NGA0718
Product
Rate L/ha
Yield kg/ha
Net benefit over untreated $/ha
Untreated
-
868
 b
 
Triflur Xcel
1.45
1450
a
266
Triflur Xcel
T2
1671
a
373
Avadex Xtra
1.6
1582
a
320
Avadex Xtra
A2
1439
a
242
(Mean of Pre-emergents ALONE)
(1535)
(300)
Triflur Xcel + Avadex Xtra
1.45 + 1.6
1667
a
352
Triflur Xcel + Avadex Xtra
1.45 + A2
1619
a
322
Triflur Xcel + Avadex Xtra
T2 + 1.6
1512
a
271
Triflur Xcel + Avadex Xtra
T2 + A2
1640
a
330
(Mean of Pre-emergent MIXTURES)
(1609)
(319)
Results in italics are above label registered rates
Assumptions: grain price $500/t, Triflur Xcel $7/L, Avadex Xtra $14/L, application $15/ha

Conclusions

The NGA trial data in 2007, together with recent NSW DPI results from the Coonamble district (GRDC Northern Herbicide Resistance project), has shown there is a useful fit for pre-emergents in our northern wild oat management approach. However they will never be ‘set and forget’. Where they are used in winter crops they will need to be followed with a clean up post-emergent spray, preferably with a Group B or K herbicide. Use of non Group A chemistry in crops such as wheat will help ease the pressure on Group A herbicides in crops such as chickpeas where there are no other post-emergent options.
 
Effective winter/ summer rotations are likely to be an important component of a long term wild oat management approach. Our primary goal is to run down the weed seedbank and the use of knockdown herbicides (or other approaches) in winter fallows will be the most effective long term approach combined with thorough management in winter crop.
 
Finally there may well be a place for the use of glyphosate ‘wick wiping’ in chickpeas. It is nearly an ideal situation where there is a large difference in height between the weed and crop, the crop is inherently more tolerant of glyphosate and wild oat escapes will still generally reach a normal height. This may be a very effective in situations where pre-emergents are used to control the majority of, yield limiting, wild oat pressure with wick wiping helping to ensure thorough final control is achieved.
 
Two cautioning points
1.      This is only data gathered from one season where good rains fell in the period shortly after planting. These conditions may have improved overall control (although actual levels of control were generally higher in the Coonamble trial series)
2.      Trifluralin can cause crop effects in winter cereals. In the wheat trial there was a clear, although acceptable impact on establishment at the higher trifluralin rate. This may have been exaggerated by soil throw across the relatively narrow 25 cm row spacing.

Acknowledgements

Thanks to the growers and consultants involved in this trial work and to NuFarm for the provision of herbicides for trials. Thanks to Paul Moylan and Andrew Storrie NSW DPI for their contribution and input.

Contact details

Richard Daniel
Northern Grower Alliance
Ph: 07 4698 7983
Email: richard.daniel@nga.org.au
 
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