Weed control and its impact on yield

Author: Christopher Preston, Samuel Kleemann, Gurjeet Gill (School of Agriculture Food & Wine, University of Adelaide), Sarah Noack (Hart Fieldsite Group), Peter Hooper (Hooper Consulting) and Paul Breust (Southern Farming Systems). | Date: 27 Jul 2017

Take home messages

  • Crop competition is an excellent partner with pre-emergent herbicides for control of grass weeds.
  • Sowing on time to maximise wheat competition with annual ryegrass helps reduce weed seed set.
  • Hybrid canola can reduce annual ryegrass seed set by 40% compared with open-pollinated cultivars.

Early sowing of wheat for greater crop competition

As grass weeds become increasingly resistant to post-emergent herbicides, more reliance is being placed on pre-emergent herbicides for weed control. One issue that arises with the use of pre-emergent herbicides is the emergence of weeds after the effectiveness of the herbicides have dissipated. Obtaining early ground cover before the pre-emergent herbicide has dissipated will reduce the competitiveness and seed production of later emerging weeds. This can be achieved by increasing the competitiveness of crops, particularly cereals.

There are several ways to increase the competitive nature of cereal crops, such as increasing the seeding rate or reducing the row spacings. However, we have been investigating an alternative approach to increasing the competitiveness of wheat crops via early sowing. Later sowing of wheat generally results in slower growth as the soil temperature decreases going into winter. This means it takes longer for canopy closure to occur and this gives weeds a greater opportunity to use the resources for their own growth. Earlier sowing, when soil temperatures are generally warmer, results in more rapid growth and faster canopy closure. This provides an opportunity to increase the amount of competition against weeds without having to significantly change other aspects of the farming system.

With investment from GRDC, over the past three years we have run a series of trials at Hart, Roseworthy and Lake Bolac in collaboration with the Hart Fieldsite Group and Southern Farming Systems to look at the role of competition from early sowing to aid pre-emergent weed control. In the trials, wheat was sown about one month apart with several different pre-emergent herbicide options used. Data on weed numbers, crop establishment, weed seed production and crop yield were collected.

In all the trials, early sowing and late sowing resulted in the same number of annual ryegrass establishing in crop. This is despite an additional knockdown herbicide application between the two times of sowing. For example, the 2016 trial at Roseworthy was sown into a paddock with annual ryegrass resistant to trifluralin and all the post-emergent herbicides. The first time of sowing (TOS) was 6 May 2016 and the second TOS 1 June 2016 (Table 1). At Roseworthy, 2016 was a particularly wet winter and had a cool wet spring that was conducive both to grain production and annual ryegrass seed production.

Table 1. Annual ryegrass plant counts in August 2016, head counts in October 2016 and grain yield in Mace wheat sown on 6 May 2016 (TOS1) or 1 June 2016 (TOS2) at Roseworthy, SA.

Pre-emergent herbicide

Plant counts (August)

(m-2)

Head counts (October)

(m-2)

Mace wheat yield

(T ha-1)

 

TOS1

TOS2

TOS1

TOS2

TOS1

TOS2

Nil

341

374

347

685

5.7

4.8

Sakura® (118g/ha)

77

40

60

71

7.3

8.8

Sakura® (118g/ha) +

Avadex Xtra® (2L/ha)ɸ

18

13

4

21

7.5

8.8

Sakura® (118g/ ha) fb

Boxer Gold® (2.5L/ha)

49

12

32

23

7.1

8.7

Boxer Gold® (2.5L/ha)

116

60

116

112

7.0

8.5

Boxer Gold® (2.5L/ha) +

Avadex Xtra® (2L/ha)

94

89

114

114

7.5

7.8

TOS*

ns

P = 0.05

P <0.001

Herbicide

P <0.001

P <0.001

P <0.001

* TOS = time of sowing; fb = followed by.

ɸLabel rate for ryegrass is 3L/ha and when mixed with trifluralin is 1.6 to 2.4L/ha.

Annual ryegrass plant counts in August were not significantly different between the two TOS (Table 1). However, in October annual ryegrass head counts were significantly different, with on average twice the number of annual ryegrass seed heads in the second TOS compared with the first TOS. Sakura®+ Avadex Xtra® coupled with the early TOS was particularly effective at reducing annual ryegrass seed set. However, Boxer Gold® tended to struggle with the high rainfall and long season.

In 2014 and 2015, average wheat yields were significantly higher with the early TOS; by 42% in 2014 and by 45% in 2015. Both seasons were characterised by lower than average spring rainfall and higher than average spring temperatures. In 2016, there was higher than average rainfall in spring and cooler temperatures. In 2016, average wheat yield from the second TOS was significantly higher at Roseworthy (12%) and Hart (36%), but not significantly different at Lake Bolac. Choosing an appropriate wheat cultivar for the season length is essential in maximizing yield from early sown wheat.

Canola hybrids for crop competition against grass weeds

Clethodim resistance is increasing in annual ryegrass making control in canola difficult. In the absence of alternative post-emergent herbicides for grass control in canola, a series of pre-emergent herbicides were examined. In these experiments it was identified that hybrid canola crops tended to reduce annual ryegrass seed production compared to open pollinated varieties. A series of trials were conducted to specifically examine this issue.

For example in 2016 at Roseworthy, SA the trial was sown on 14 May with ATR Stingray and Hyola® 559TT as the open pollinated and hybrid cultivars, respectively, using a variety of herbicide strategies based around pre-emergent herbicides and post-emergent atrazine (Table 2). In this trial, there were high numbers of annual ryegrass even after the pre-emergent herbicides were used. There was no significant difference in the number of annual ryegrass plants present in the hybrid canola compared to the open pollinated canola. However, the number of seed heads produced was on average 40% lower in the hybrid Hyola® 559TT compared to the open pollinated ATR Stingray.

None of the pre-emergent herbicide strategies used were particularly successful at controlling annual ryegrass in canola in this trial. This meant that the high weed populations present had a large impact on canola yield. However, the yield of Hyola® 559TT was on average twice that of ATR Stingray in this trial. In our other trials, hybrid canola has always reduced annual ryegrass seed production by between 40 and 50% compared with open pollinated canola, but has only produced a yield advantage over open pollinated canola in 50% of the trials.

Table 2. Annual ryegrass plant counts in August 2016, head counts in October 2016 and grain yield in open pollinated canola (ATR Stingray) compared with hybrid canola (Hyola® 559TT) at Roseworthy, SA in 2016.

Herbicides applied

Plant counts (August)

(per m2)

Head counts (October)

(per m2)

Canola yield

(T/ha)

 

OP*

Hybrid

OP

Hybrid

OP

Hybrid

Nil

773

671

1186

748

0.17

0.96

Rustler® (1L/ha) pre

437

417

1062

733

0.24

1.07

Rustler® (1L/ha) +

Avadex Xtra® (2L/ha) pre

325

299

1135

694

0.45

0.94

Simazine (1.1kg/ha) pre +

Atrazine (1.1kg/ha) post

179

140

498

212

0.97

1.70

Rustler® (1L/ha) +

Simazine (1.1kg/ha) pre

386

321

753

510

0.54

1.12

Rustler® (1L/ha) +

Simazine (1.1kg/ha) pre +

Atrazine (1.1kg/ha) post

127

182

610

367

0.99

1.41

Cultivar

ns

P <0.001

P <0.001

Herbicide

P <0.001

P <0.001

P <0.001

*OP = open pollinated

Just relying on pre-emergent herbicides, even with the use of post emergent atrazine, in canola is ineffective at managing annual ryegrass. Our long term management trials have shown that it is essential to include clethodim post emergent (even though the annual ryegrass is resistant to it), or where available use Roundup Ready® canola, and follow that up with seed set control to reduce annual ryegrass numbers for future crops.

Useful resources

Hart Trial Results 2016 Early or delayed sowing for improved ryegrass control-summary of three seasons

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 author would like to thank them for their continued support.

Contact details

Christopher Preston
University of Adelaide
0488 404 120
christopher.preston@adelaide.edu.au

Table 1