Coupling pre emergent herbicides and crop competition for big reductions in weed escapes

​​Take home message

  • Extensive resistance to post-emergent herbicide in grass weeds means greater reliance on pre-emergent herbicides for weed control
  • Early sowing of wheat coupled with effective pre-emergent herbicide packages can provide greater competition against grass weeds, increase wheat yields and limit grass weed seed set
  • The competition provided by hybrid canola cultivars provides an opportunity to reduce annual ryegrass seed set by about 50% compared to open-pollinated cultivars.

Why crop competition again?

The increasing occurrence of herbicide resistance in grass weeds in particular is creating new challenges for weed management in crops. Over recent decades there has been the introduction of a suite of herbicides that could control grass weeds post-emergent in crops. This made grass weed management relatively simple. As resistance to these post-emergent herbicides has increased to the point where few of them are effective any more there has been greater reliance on pre-emergent herbicides.

Pre-emergent herbicides are more difficult to use than post-emergent herbicides for effective weed control. They need to be applied before the weeds emerge, need to be close to the weed seed in the soil, and need the right moisture conditions to activate. These herbicides also rely on residual activity to control weeds after the crop has emerged. This leads to situations where weed escapes occur early in the season; and these escapes, if not managed, set a large amount of seed.

Crop competition has long been known to be a useful tool in weed management. Practices such as decreasing row spacing, increasing seeding rates, and growing more competitive cultivars have all been demonstrated in research settings as practices that can reduce grass weed impacts. However, adoption of more competitive crops has been mixed. There are numerous reasons for this including seeder design: the need to manage stubble at sowing; other priorities in crop management; and reducing costs and risk management.  Where post-emergent herbicides still work, the value of crop competition is less obvious and other management desires can take precedence. As a result of the escapes from pre-emergent herbicides and the lack of later management options, crop competition becomes much more important in the absence of effective post-emergent herbicides. The question is not so much “do I need to employ crop competition?,” but “how do I improve crop competition in a practical way?”

Over the past few years, we have been looking at how to improve weed control where pre-emergent herbicides are the only grass weed options. This work has highlighted a couple of opportunities for reducing weed seed set through improving the competitive nature of the crop in combination with pre-emergent herbicides.

The interaction of sowing date and pre-emergent herbicides for grass weed control in wheat

It has been a long held mantra that weedy paddocks should be sown last in the cropping rotation. This is so an extra knockdown application, or a double knock, can be employed to reduce the number of weeds emerging in the crop. This is a perfectly valid practice when post-emergent herbicides can be employed to clean up the remainder of the weed problem and has the added benefit of taking pressure of post-emergent herbicides.

However, one consequence of later sowing is that wheat grows more slowly as the soil temperature decreases going into winter, taking more time for canopy closure and giving weeds a greater opportunity to use resources. The question is whether in the absence of post-emergent herbicides sowing the worst paddocks last is still the best strategy.

In collaboration with the Hart Fieldsite Group we conducted a trial in 2014 with two times of sowing. Scout wheat was sown on 4 May and 2 June 2014. There was a knockdown herbicide treatment between the two sowing times. Pre-emergent herbicide treatments used at each time of sowing were: nil, Sakura® (118 g ha-1) and Sakura (118 g ha-1) + Avadex® Xtra (2 L ha-1) .

In this trial, the delay in sowing did not lead to a reduction in the number of ryegrass plants present in crop (Table 1). Where no pre-emergent herbicide was used, there were more head counts from the early time of sowing. In contrast, where effective pre-emergent herbicides were used there was no difference in the number of seed heads produced. Wheat yield from TOS1 was 4.15 T ha-1 and from TOS 2 2.93 T ha-1.

Table 1. Annual ryegrass present in crop and seed heads at maturity for two times of sowing (TOS) at Hart in 2014. For each measurement, different letters indicate significant differences in means.

Pre-emergent herbicide*

Plant counts (8 Aug)

(m-2)

Head counts (10 Oct)

(m-2)

TOS1

TOS2

TOS1

TOS2

Nil

59 a

77 a

350 a

164 b

Sakura (118 g ha-1)

8 b

8 b

39 c

41 c

Sakura (118 g ha-1) + Avadex Xtra (2 L ha-1)

3 b

3 b

32 c

9 c

*Rates listed in this table are for trial purposes. If using these products commercially ensure you follow the rates listed on the registered labels.

The 2014 season in SA ended with an exceptionally dry spring period, although there was enough stored early moisture to allow most crops to finish well. There had also been abundant winter rainfall that helped pre-emergent herbicides to work well.

In 2015 we conducted a trial at Roseworthy (SA) where we used three times of sowing. Mace wheat was sown on 8 May, 27 May or 9 Jun 2015. There was a knockdown herbicide treatment prior to each sowing time. Pre-emergent herbicide treatments used at each time of sowing were: nil, Sakura (118 g ha-1), Boxer Gold® (2.5 L ha-1), and Sakura (118 g ha-1) + Avadex Xtra (2 L ha-1).  

In this trial (Table 2), the numbers of annual ryegrass plants remaining in the crop were high for all treatments. Sakura + Avadex Xtra had the lowest numbers. Seasonal conditions during 2015 in SA were difficult. Rainfall around sowing was patchy, but there were good falls of rain through July and August before rainfall fell away again. The early patchy rainfall meant that pre-emergent herbicides struggled and the good rains in July and August germinated a late cohort of ryegrass.

Despite this, the herbicides were effective at reducing annual ryegrass seed heads. Sakura + Avadex Xtra was the most effective pre-emergent herbicide option. The early applications of Sakura and Boxer Gold struggled; Sakura as a result of the low rainfall for 2 months after sowing and Boxer Gold due to its shorter persistence. The trial was not harvested as it was destroyed by the Pinery fire.

Table 2. Annual ryegrass present in crop and seed heads at maturity for three times of sowing (TOS) at Roseworthy in 2015. For each measurement, different letters indicate significant differences in means.

Pre-emergent herbicides*

Plant counts (25 Aug)

(m-2)

Head counts (1 Oct)

(m-2)

TOS1

TOS2

TOS3

TOS1

TOS2

TOS3

Nil

441 a

239 ab

439 a

585 a

285 b

287 b

Sakura (118 g ha-1)

179 ab

176 ab

241 ab

140 bcd

58 cde

53 de

Sakura (118 g ha-1) + Avadex Xtra (2 L ha-1)

112 b

109 b

139 b

47 de

28 e

19 e

Boxer Gold (2.5 L ha-1)

128 b

241 ab

176 ab

171 bc

66 cde

55 de

*Rates listed in this table are for trial purposes. If using these products commercially ensure you follow the rates listed on the registered labels.

These trials have demonstrated that early sowing of wheat with an effective pre-emergent herbicide package can provide at least as good a control of annual ryegrass compared to later sowing with an additional knockdown herbicide. The reason is that early sown wheat is more competitive than later sown wheat and tends to reduce the number of seed heads produced per plant. The higher yields that typically occur with earlier sowing of wheat mean that more crop is produced for the same number of weed seeds.

There are risks associated with this strategy, particularly in very weedy paddocks. The most important is the increased risk of patchy moisture conditions after sowing leading to poorer performance of pre-emergent herbicides. Boxer Gold can also struggle with high moisture conditions in the middle of the season leading to additional germination events after the herbicide is dissipated. A robust pre-emergent package appears essential for early sowing of wheat.

Hybrid canola and pre-emergent herbicides for grass weed control in wheat

Clethodim resistance in annual ryegrass has become a major concern for canola production. During 2013 and 2014 we conducted trials to examine potential new herbicides for the control of clethodim-resistant annual ryegrass in TT and Clearfield® canola at Roseworthy (SA). These trials were sown on 17 May 2013 and 23 May 2014. The varieties used were ATR Stingray (TT) and Pioneer® 45Y84 hybrid (Clearfield) and were sown to achieve plant stands of 50 plants m-2 for the TT canola and 35 plants m-2 for the Clearfield canola. Several pre-emergent herbicide options used alone were compared with current usual practice of a pre-emergent herbicide followed by post-emergent herbicides. The population was tested as resistant to clethodim, but was also clearly resistant to Group B and Group D herbicides.

The results of these trials were that pre-emergent herbicides alone would be ineffective at managing annual ryegrass in canola. However, it became clear that the surviving annual ryegrass plants set a lot more seed in TT canola than it did in hybrid Clearfield canola (Table 3). Typically there was more than twice as much ryegrass seed produced in the open-pollinated TT canola than in the hybrid Clearfield canola. This was a result of the slower-growing open-pollinated TT canola achieving canopy closure much later than the hybrid Clearfield canola. The result was that each surviving ryegrass plant had more opportunity to set seed.

Table 3. Annual ryegrass plants in crop, annual ryegrass seed production and canola yield at Roseworthy in 2013 and 2014. Different letters in each column for each year indicate significant differences in means (there was no significant difference for yield of Clearfield canola in 2014).

Pre-emergent herbicide**

Ryegrass plants
(m-2)

Ryegrass seeds
(x1000 m-2)

Yield
(t ha-1)

ATR Stingray

45Y82 (CL)

ATR Stingray

45Y82 (CL)

ATR Stingray

45Y82 (CL)

2013

Usual practice*

171 a

47 a

1.82 a

1.40 a

2.15 a

1.73 a

Rustler (1 L ha-1)

96 a

63 a

15.83 b

4.58 b

1.68 b

1.62 ab

Experimental A

269 b

186 c

21.70 b

5.80 bc

1.62 b

1.48 b

Experimental B

381 c

198 c

34.82 c

8.86 d

1.30 c

1.60 ab

Experimental C

133 a

101 b

22.31 b

7.37 c

1.65 b

1.60 ab

2014

Usual practice*

522 ab

632 a

6.79 a

5.40 a

1.69 a

1.71

Rustler (1 L ha-1)

354 a

553 a

32.78 b

17.27 ab

1.49 ab

1.65

Experimental A

864 b

1697 b

51.47 c

1.15 b

1.41

Experimental B

869 b

1643 b

51.19 c

27.11 b

1.26 b

1.36

Experimental C

767 b

1088 b

54.53 c

26.45 b

1.31 b

1.62

* Usual practice was Atrazine (1.5 kg ha-1) pre followed by 240 gai/L Clethodim (500 mL ha-1) post for TT canola (ATR Stingray) and Trifluralin (2 L ha-1) + Avadex Xtra (2 L ha-1) pre followed by Intervix (750 mL ha-1) + 240 gai/L Clethodim (500 mL ha-1) post for Clearfield canola (45Y82).
**Rates listed in this table are for trial purposes. If using these products commercially ensure you follow the rates listed on the registered labels.

In 2015 a further trial was conducted at Roseworthy (SA) that included three TT canola cultivars: ATR Stingray (open-pollinated), Hyola 559TT (a hybrid) and Hyola 750TT (a high biomass hybrid). The trial was sown on 15 May 2015 with a target population of 35 plants m-2. There were three herbicide management strategies employed: Herbicide Treatment 1 - no herbicides; Herbicide Treatment 2 - Atrazine (1.5 kg ha-1) pre followed by Clethodim (500 mL ha-1) post; and Herbicide Treatment 3 - Rustler (1 L ha-1) pre followed by Clethodim (500 mL ha-1) + Factor (80 g ha-1) + Atrazine (1.1 kg ha-1) post.

In this trial there was a significant effect of both cultivar (p <0.0001) and herbicide treatment (p <0.0001) on the number of annual ryegrass spikes present at maturity. The annual ryegrass population at the site was resistant to clethodim, so post-emergent treatments were not very effective. The high biomass canola (Hyola 750TT) significantly reduced the number of annual ryegrass spikes at harvest compared to the other two cultivars in the absence of herbicides (Herbicide Treatment 1), demonstrating the impact of extra competition provided by this cultivar (Figure 1). Where Herbicide Treatments 2 and 3 were employed (Figure 1) there was about twice the number of annual ryegrass spikes at maturity in the ATR Stingray plots compared with the two hybrid cultivars. Simply changing from an open-pollinated cultivar to a hybrid canola has the potential to reduce annual ryegrass seed set by half.

South Australia experienced a hot and dry spring during 2015 and so canola yields in this trial were low. The early finish to the season did not suit the longer season cultivars and in addition yield of Hyola 750TT was affected by frost. There were significant effects of cultivar (p = 0.042) and herbicide treatment (p <0.001) on canola yield; however, the highest yield was only 1.17 T ha-1 for Hyola 559TT with Herbicide Treatment 3.

Figure 1. Effect of canola cultivar and herbicide treatment on annual ryegrass spike numbers at maturity at Roseworthy in 2015. Herbicide Treatment 1: no herbicides; Herbicide Treatment 2: Atrazine (1.5 kg ha-1) pre followed by 240 gai/L Clethodim (500 mL ha-1) post; and Herbicide Treatment 3: Rustler (1 L ha-1) pre followed by 240 gai/L Clethodim (500 mL ha-1) + Factor (80 g ha-1) + Atrazine (1.1 kg ha-1) post.

Figure 1. Effect of canola cultivar and herbicide treatment on annual ryegrass spike numbers at maturity at Roseworthy in 2015. Herbicide Treatment 1: no herbicides; Herbicide Treatment 2: Atrazine (1.5 kg ha-1) pre followed by 240 gai/L Clethodim (500 mL ha-1) post; and Herbicide Treatment 3: Rustler (1 L ha-1) pre followed by 240 gai/L Clethodim (500 mL ha-1) + Factor (80 g ha-1) + Atrazine (1.1 kg ha-1) post.

Acknowledgements

The research undertaken as part of this project is made possible by the significant contributions of growers through both trial cooperation and the support of the GRDC, the authors would like to thank them for their continued support.  The authors would also like to thank collaborators at the Hart Fieldsite Group for hosting and managing a field trial and Pacific Seeds for providing canola seed and funding additional weed suppression trials.

Contact details

Dr. Christopher Preston
University of Adelaide
School of Agriculture, Food & Wine, University of Adelaide , PMB 1 Glen Osmond SA 5064
Ph: (08) 8313 7237
Fx: (08) 8313 7109
Email: christopher.preston@adelaide.edu.au

® Registered trademark

Varieties displaying this symbol beside them are protected under the Plant Breeders Rights Act 1994.

GRDC Project code: UCS00020