Effect of pre-emergence herbicides on annual ryegrass control and crop safety in wheat
Sam Kleemann, Chris Preston & Peter Boutsalis,
University of Adelaide, School of Agriculture, Food & Wine, Waite Campus
Peter Hooper, Allan Mayfield Consulting
Key findings
 Sakura alone or in combination with Avadex Xtra or Dual Gold provided the best preemergence control of ARG in 2010
 Residual activity of Sakura appears to be important in limiting seed set of ARG
 Although trifluralin in combination with Dual Gold or Dual Gold + Avadex Xtra provided high
levels of ARG control (76-92%), ARG survivors still managed significant seed set to replenish
the weed seedbank
 Boxer Gold or Dual Gold applied PSPE provided no additional control of in furrow ARG
Why do the trial?
Increasing frequency of annual ryegrass (ARG) populations resistant to trifluralin (Group D) is of
growing concern for farmers across the wheat-belt of southern Australia. In addition, growers are
becoming increasingly frustrated with the lack of ARG control within crop rows with trifluralin, a
result of the herbicide being thrown back into the inter-row with the sowing pass. One practice
being used to overcome this limitation, however with limited success, has been the use of snake
chains which try and drag herbicide treated soil back into the furrow. Another approach has been to
apply pre-emergence herbicides directly after sowing and prior to crop emergence, however this can
be difficult during hectic sowing programs where other more pressing duties take precedence.
Given the importance placed on trifluralin for controlling ARG under current farming practices, there
is urgent need to identify alternate pre-emergence herbicide options. Consequently trials have been
undertaken over several seasons at the Hart field site to evaluate the efficacy and crop safety of
alternate pre-emergence herbicides for the control of ARG in wheat. Additional treatments
examining post-sowing pre-emergence applications of these herbicides have also been undertaken
as a tool to improve ARG control.
How was it done?
Plot size
1.4 m × 10 m
Fertiliser
32:10 @ 80 kg/ha
Seeding date
13th of May 2010
Variety
Catalina wheat @ 70 kg/ha
The trial was established as a randomised complete block design with 3 replicates and 17 herbicide
treatments (Table 1). Active ingredients of the herbicides used in the trial are listed in Table 2.
To ensure even ARG establishment across the trial site ARG seed was broadcast at 25 kg/ha ahead of
seeding and tickled in with a shallow pass with the seeder prior to herbicide application. The
ryegrass was a field biotype with approximately 30% resistance to trifluralin.
A standard knife-point press wheel system was used to sow the trial on 22.5 cm (9") row spacings.
Pre-sowing herbicides were applied within an hour of sowing and incorporated by sowing (IBS) the
post-sowing pre-emergence (PSPE) herbicides were applied on 25th May, 12 days after sowing. The
site received 23.8 mm of rainfall on the same day as PSPE application.
Crop emergence was assessed by counting the number of emerged wheat seedlings along both sides
of a 0.5 m rod at 3 random locations within each plot. ARG was counted at 6 & 10 weeks after
sowing (i.e. Jul & Aug) using a 0.1 m2 quadrat from within and between the crop rows from 4
random locations within each plot. ARG spike density was determined in September using 0.16 m2
quadrat placed at 4 random locations within each plot.
Table 1. Pre-emergent herbicides, rates & timings
Treatments
1
Nil (untreated control)
2
Trifluralin 480 1.5 L/ha (IBS)
3
Avadex Xtra 3.0 L/ha (IBS)
4
Boxer Gold 2.5 L/ha (IBS)
5
Sakura 118 g/ha (IBS)
6
Outlook 1.0 L/ha (IBS)
7
Trifluralin 480 1.5 L/ha (IBS) + Avadex Xtra 2.0 L/ha (IBS)
8
Trifluralin 480 1.5 L/ha (IBS) + Avadex Xtra 2.0 L/ha (IBS) + Dual Gold 0.5 L/ha (PSPE)
9
Trifluralin 480 1.5 L/ha (IBS) + Avadex Xtra 2.0 L/ha (IBS) + Sakura 80g/ha (PSPE)
10
Trifluralin 480 1.5 L/ha (IBS) + Avadex Xtra 2.0 L/ha (IBS) + Boxer Gold 1.5 L/ha (PSPE)
11
Boxer Gold 2.5 L/ha (IBS) + Avadex Xtra 2.0 L/ha (IBS)
12
Boxer Gold 1.5 L/ha (IBS) + Boxer Gold 1.0 L/ha (PSPE)
13
Boxer Gold 2.5 L/ha (IBS) + Dual Gold 0.5 L/ha (PSPE)
14
Sakura 80 g/ha (IBS) + Avadex Xtra 2.0 L/ha (IBS)
15
Sakura 80 g/ha (IBS) + Sakura 38 g/ha (PSPE)
16
Sakura 118 g/ha (IBS) + Dual Gold 0.5 L/ha (PSPE)
17
Outlook 0.7 L/ha (IBS) + Outlook 0.3 L/ha (PSPE)
Table 2. Pre-emergent herbicides & their active ingredients
Herbicide
Active ingredients
Trifluralin 480
trifluralin 480 g/L
Avadex Xtra
tri-allate 500 g/L
Boxer Gold
S-metolachlor 120 g/L + prosulfocarb 800 g/L
Sakura (BAY-191 850WG)
pyroxasulfone 850 g/kg
Outlook (Nul-1493)
experimental
Dual Gold
S-metolachlor 960 g/L
Results
All herbicides with the exception of Outlook were safe on wheat with little or no reduction in wheat
establishment under the knife-point press wheel system. Visual assessments based on plant density
and vigour was lowest for Outlook which reduced crop density by 70% of the untreated control (126
plants m-2). Outlook an experimental herbicide developed by Nufarm is highly soluble and will not be
released for use in wheat because of concerns of crop phototoxicity.
All herbicide treatments reduced ARG emergence (Table 3). Trifluralin provided the lowest levels of
ARG control initially (59%) which is to be expected given the ARG sown was 30% resistant to the
herbicide. In contrast, trifluralin in combination with Dual Gold or Avadex Xtra + Dual Gold provided
the greatest initial levels of weed control reducing ARG densities by around 90% of the untreated
control (385 plants m-2). However, these treatments were later shown to be ineffective in limiting
ARG seed set with high spike densities recorded for the few surviving ARG plants (110-244 ARG
spikes m-2). Similarly IBS Boxer Gold, which also provided high levels of initial ARG control (77%), was
unable to maintain enough residual activity to reduce later germinations from setting seed (196 ARG
spikes m-2). This result is not entirely unexpected given the favourable and prolonged moisture
conditions over the growing season and the plastid nature of ARG. In contrast, Sakura alone or in
combination with Avadex Xtra or Dual Gold, which also provided high levels of ARG control (77-94%)
was able to significantly retard ARG seed production and reduced spike densities (31-39 ARG spikes
m-2) in comparison to the untreated control (541 ARG spikes m-2). Previous research (Kleemann &
Boutsalis 2008) showed that residual activity of Sakura was able to reduce growth and root
development in ARG survivors. This would significantly reduce the capacity of the ARG to compete
with the crop and would also reduce seed production by the ARG.
ARG control in the crop furrow was generally poorer than for the inter-row, with control averaging
71% compared to 81%, respectively. Applying split applications of the same herbicide (i.e. IBS &
PSPE) or using combinations of different pre-emergent herbicides provided little or no additional
benefit to in furrow ARG control. Given the risks associated with not being able to return to the
paddock to apply the additional herbicide split (i.e. due to time constraints or paddock conditions too wet) and furthermore increased potential for crop damage with certain herbicides (i.e. Dual
Gold), the small benefits to ARG control with these practices are difficult to justify.
In conclusion, both Boxer Gold (released in 2008) and Sakura which will be made available in 2012
provide effective alternatives for the control of trifluralin resistant ARG. However, use of these
herbicides should be done so in conjunction with robust management strategies that use a diverse
rotation of crops, herbicides and non-chemical strategies (i.e. seed catching) so as to prolong the life
of existing and new chemical groups against ARG.
Acknowledgements
This trial was funded by GRDC & conducted in collaboration with Birchip Cropping Group & the
University of Adelaide
Table 3. Effect of pre-emergent herbicide treatments on ARG emergence (% control) and spike density at Hart in 2010. Density values are expressed as the
number of ARG plants in either a m2 of crop furrow or inter-row (i.e. area between crop rows).
Jul
Aug
Herbicide treatment
Inter-row
Crop furrow
Total
Total
Sep
___________________________
-2
___________________________
_
ARG plants m (% control)
ARG spikes m-2 _
Nil (untreated control)
596
174
385
325
541
Trifluralin 480 1.5 L/ha (IBS)
167 (72)
148 (15)
157 (59)
156 (52)
362
Avadex Xtra 3.0 L/ha (IBS)
170 (71)
93 (47)
132 (66)
119 (63)
235
Boxer Gold 2.5 L/ha (IBS)
115 (81)
63 (64)
89 (77)
136 (58)
196
Sakura 118 g/ha (IBS)
148 (75)
63 (64)
106 (72)
75 (77)
39
Outlook 1.0 L/ha (IBS)
200 (66)
56 (68)
128 (67)
186 (43)
325
Trifluralin 480 1.5 L/ha (IBS) + Avadex Xtra 2.0 L/ha (IBS)
100 (83)
41 (76)
70 (82)
78 (76)
244
Trifluralin 480 1.5 L/ha (IBS) + Avadex Xtra 2.0 L/ha (IBS) +
44 (93)
15 (91)
30 (92)
33 (90)
110
Dual Gold 0.5 L/ha (PSPE)
Trifluralin 480 1.5 L/ha (IBS) + Avadex Xtra 2.0 L/ha (IBS) +
67 (89)
22 (87)
44 (89)
28 (91)
10
Sakura 80g/ha (PSPE)
Trifluralin 480 1.5 L/ha (IBS) + Avadex Xtra 2.0 L/ha (IBS) +
37 (94)
30 (83)
33 (91)
25 (92)
80
Boxer Gold 1.5 L/ha (PSPE)
Boxer Gold 2.5 L/ha (IBS) + Avadex Xtra 2.0 L/ha (IBS)
85 (86)
30 (83)
57 (85)
64 (80)
152
Boxer Gold 1.5 L/ha (IBS) + Boxer Gold 1.0 L/ha (PSPE)
148 (75)
48 (72)
98 (75)
72 (78)
128
Boxer Gold 2.5 L/ha (IBS) + Dual Gold 0.5 L/ha (PSPE)
96 (84)
37 (79)
67 (83)
75 (77)
79
Sakura 80 g/ha (IBS) + Avadex Xtra 2.0 L/ha (IBS)
30 (95)
15 (91)
22 (94)
31 (90)
31
Sakura 80 g/ha (IBS) + Sakura 38 g/ha (PSPE)
145 (76)
48 (72)
96 (75)
50 (85)
40
Sakura 118 g/ha (IBS) + Dual Gold 0.5 L/ha (PSPE)
82 (86)
30 (83)
56 (85)
53 (84)
31
Outlook 0.7 L/ha (IBS) + Outlook 0.3 L/ha (PSPE)
178 (70)
56 (68)
117 (70)
136 (58)
250
LSD (0.05)
80
45
48
60
58
Shaded treatments represent significant reduction in ARG seed production