What can we learn from case studies investigating patch management of glyphosate resistant weeds

Author: | Date: 21 Jul 2016

Take home messages

  • There are examples from the northern grain region of successful patch management of glyphosate resistant weeds
  • Patch management of weeds involves a strong commitment from grain growers to drive weed seed banks down by focusing on stopping weed seed production repeatedly for at least 5 years
  • Effective patch management often involves a good combination of chemical and non-chemical tactics. Although additional effort is focused on the patch of weed, this is often superimposed over standard paddock-wide weed control treatments that provide additional control.
  • Farm weed hygiene is critical. Despite good efforts to control weeds in paddocks, extra effort is required for non-crop areas that can be a source of weeds back to paddocks.
  • To control glyphosate resistant weed patches, additional costs are required. However, continually applying glyphosate on fallows will lead to larger glyphosate resistant patches and lower subsequent crop yields. Therefore, controlling these patches is likely to be more economically beneficial than continued use of glyphosate.

Introduction

Literature reporting broad-acre glyphosate-resistant weed control infrequently provides research or advice on the management of small patches. More often, they assume weeds have spread beyond the manageable patch stage and have a paddock-wide viewpoint. Furthermore, it would be more prudent to have information that could prevent new glyphosate-resistant weed patches spreading beyond the early phases of establishment.

Many of the glyphosate-resistant issues are emerging from the northern grains region. This is predominantly due to the overuse of glyphosate in the fallow period and the diversity of weed species in this region. There are many examples where glyphosate-resistant weeds are affecting a small proportion of a paddock. Developing patch management advice to growers in the north is therefore a high priority.

There is a good variety of weed management tactics that can be used for patch management. Combinations of chemical and non-chemical methods are being used by grain growers with varying degrees of success. Although there are few examples of eradication, success can also be seen as containing or reducing the size of patches, preventing them from affecting large proportions of the farm.

In this paper, four case studies were examined. All of these are based within the northern grains region. Each focused on a different glyphosate-resistant weed. The species investigated were annual ryegrass (Lolium rigidum), awnless barnyard grass (Echinochloa colona), feather-top Rhodes grass (Chloris virgata), and liverseed grass (Urochloa panicoides). An economic evaluation of two patch weed scenarios was scrutinised, comparing a proactive approach and one that continues to rely on glyphosate as the only fallow option, allowing the patch to get larger.

Case study: glyphosate resistant annual ryegrass in Central West NSW

This case study highlights the attempts to manage glyphosate resistant annual ryegrass in a mixed farming system. The 18000 hectare property, located near Coolah, has 5000 ha devoted to cropping and 3000 ha for dual purpose cropping and pastures. There is opportunity to rotate from winter to summer crops (sorghum) for management flexibility and herbicide choice.

Glyphosate resistance was suspected in 2009 and tested soon afterwards. It was thought to have originated on the property due to over reliance on glyphosate based products. The rotation in the past had three seasons of sorghum followed by wheat then canola. Patches can be seen in the middle of paddocks and along fence lines. Approximately 30 to 40% of cropping paddocks are affected by GR annual ryegrass with 10% being termed “a real problem”. Some of the ARG movement can be attributed to movement down a slope following heavy rainfall. Sheep and the occasional cultivation have been used as non-chemical methods for control, although the manager admits the cultivation was used mainly for other purposes.

What was the approach to manage the GR ARG?

The cropping rotation has changed to a canola/sorghum/chickpeas/barley/dual purpose wheat cycle. This allows annual ryegrass control with a wider range of modes-of action chemistry. The ARG populations on this property were still susceptible to Group A herbicides and most pre-emergence options. Paddocks that are not infested by GR ARG, are treated as though they have GR ARG, with similar cropping system changes to ensure herbicide choice flexibility. The farm manager has also considered dryland cotton to add more summer options.

Some difficulties encountered were the availability and timeliness for spraying operations. Machinery is a limiting resource and the advent of double knocking has further restricted the availability of spray rigs.

Fence line weed management was a reasonable concern. The manager did not know of alternative options for this situation. However, whilst the case study interview was being conducted, NSW DPI advised him of alternative options. Furthermore, there were some fallow pre-emergence options that were discussed that would find a nice fit into the cropping rotation.

The most pleasing aspect of this case study was the understanding that the manager knew that stopping weed seed set was the key to success. In summary, the manager has minimised the spread of this weed but wants to take it one step further and reduce numbers down significantly.

Case study: glyphosate resistant liverseed grass in North West NSW

A 600ha farm, located at Pallamallawa was acquired by the current owner in 1998. Approximately 10 paddocks are used in a barley / chickpea / wheat / long fallow / summer crop rotation. Resistance was suspected and tested on the summer of 2007/08 and confirmed resistant at a 7-fold level.  The resistance is believed to have been the consequence of many years of consecutive applications with poor cultivation thereafter (not full disturbance).

The patch of liverseed grass is relatively small, about one hectare. The farmer now has managed the property for 18 years and has recently said in the case study interview, “I’m sure I have won this battle.”

What was the approach to manage the GR liverseed grass?

For this case, the patch was contained and very small, thus allowing the farmer to “throw the kitchen sink at it”. The objective of the farmer was to eradicate the patch; most of the tactics mentioned below were specifically directed at the patch. Thereafter, in 2009 the patch was treated with herbicides using alternatives modes-of-action to glyphosate. Cultivation was used frequently and changed to full disturbance (wide sweeps) so that nothing could survive. Very importantly the patch was monitored closely to check that the job resulted in 100% control. Double knocking was used to good effect along with pre-emergence herbicides.

One year in the rotation a winter cereal was brown manured and the biomass was used as mulch in the period that liverseed grass would normally emerge.  

The farmer stated one memorable comment in relation to managing his resistance, “Get onto it”, confirming his intention to avoid procrastination and to take control. Result of this case study is what could be seen as eradication. However, since being affected by resistance this farmer is always on the lookout for other potential small patches of resistance or hard to control tolerant weeds.

Case study: glyphosate resistant awnless barnyard grass in Western Downs QLD

A mixed farming property located near Tara, Queensland, consisting of 1750 ha cultivation/farming paddocks and 500 ha pasture which has been dealing with GR awnless barnyard grass and feather-top Rhodes grass. Glyphosate resistance in awnless barnyard grass was confirmed in 2012 and again in 2015, reportedly surviving glyphosate CT at 4.5L/ha. Feather-top Rhodes grass was first seen in 2012 and has progressively got worse. Glyphosate was the only herbicide used and often it was applied 3 to 4 times per fallow. The owner and agronomist blame the lack of chemical group rotation for their problems.

The family owned property has been owned since the 1950’s and was once predominantly based around sheep production; however it moved into cropping in the mid 1990’s. The cropping rotation is wheat / wheat / wheat / long fallow / sorghum / sorghum / long fallow.

What was the approach to manage the GR awnless barnyard grass?

Major changes to chemical and non-chemical options have led to improved results. Inclusion of an occasional long fallow, has allowed the option to use cultivation if weed ‘blow-outs’ occur. However, pre-emergence herbicides are now frequently used, namely Flame® (with deference to re-cropping restrictions on the product label) and Dual Gold® (in preparation for sorghum). WeedSeeker® technology is used for isolated patches but requires high rates of glyphosate to manage their problem.

The results are mixed. It appears the feather-top Rhodes grass can be managed with a combination of residuals, cultivation and camera sprayer technology. The awnless barnyard grass still remains the issue. The owners treat the affected paddocks differently to the rest of the farm. Currently the practice of extremely high rates of glyphosate via a camera sprayer are working well, but they struggle to justify the cost of the more expensive Group A options and understand most of them are not registered for use. Adjustments to some activities have been implemented since resistance. Now sheep are used as a second knock after the initial spray whereas previously they were used as the first knock, making plants less susceptible to herbicides when used as the second knock.

Attempting to patch spray many isolated patches on a large property was a difficult task and many outlier plants were missed and allowed to set seed. Therefore, over time some patches have slowly expanded. The rule used within this property is to make overall changes to paddock-wide treatments so that the time consuming effort of spot treating can be avoided. Fence line weed management is very active on this farm, either the choice is to allow native grasses to colonise these areas to provide some competition to the weedy grasses, or they use an array of herbicides with no consistent pattern around the farm.

In summary, the owners of this property are still reasonably happy with their outcome as they know that if they continue down the glyphosate based fallow strategy, the property would nearly be fully infested with awnless barnyard grass and feather-top Rhodes grass. They surmised the main treatment for control in this situation would be to resort to cultivation on every paddock several times per year.

Case study: glyphosate resistant feather-top Rhodes grass in Eastern Downs QLD

This case study highlights the excellent management of small patches of feather-top Rhodes grass (FTR). A 1350 ha farmer owned property near Dalby was used for this report. The split between winter and summer cropping is about 75:25. Prior to the discovery of FTR the cropping rotation was sorghum / sorghum / sorghum / chickpea / long fallow. The first sighting of the weed commenced soon after the floods of 2011. Rapid change was made in response to this new weed incursion as FTR was widely known as a troublesome weed around the Darling Downs. The largest patch on the property was 5ha.

What was the approach to manage the GR feather-top Rhodes grass?

A change in cropping rotation was the first action. One sorghum crop was substituted for wheat, followed by barley, then mungbeans. As such the new rotation is now sorghum / sorghum / chickpeas / wheat / barley / mungbeans. By incorporating additional winter crops, more summer fallows can allow better surveillance of the summer growing weed and permit the use of some pre-emergence herbicides. Some paddock-wide treatments are employed in the summer fallow to gain the first control. The pre-emergence herbicide Flame® is sometimes used depending on plant-back restrictions. (Flame is registered for the control of several other summer grass weeds however does not have a specific registered label claim for the control of feathertop Rhodes grass). Thicker patches are also treated with a Group A herbicide followed by paraquat as a double knock (Haloxyfop as per APVMA Permit 12941 which applies in Qld. only, or Shogun (propaquizafop)). Importantly, some of the critical management tactics used are hand chipping the occasional isolated plant or using fire to burn isolated mature plants that have a good seed load. If patch size warrants, the farmer patch cultivates.

Delayed sowing of summer crops is a tactic this farmer uses so that cultivation can be used. Other summer crop options used for general grass weed control include the use of Stomp® and Spinnaker® in mungbeans and Dual Gold® and atrazine in the sorghum phase. None of these products have a specific label claim for the control of feathertop Rhodes grass and Spinnaker is known to be ‘usually poor’ on FTR.

Fence line management is either the use of a Group A herbicide (Shogun® is registered for control of FTR in fallow situations when double knocked by a bipyridal herbicide) or promoting non-weedy grasses as a form of competition against FTR. The property seemed nearly 100% clean of FTR along the fence lines so internal farm hygiene must be a high priority.   

The five hectare infestation that was the largest is now confined to some isolated plants, largely due to sacrificing part of a chickpea crop by using glyphosate then burning the dry remains.

In conclusion, this farmer is very strict about his FTR patch management. He used combinations of non-chemical and chemical tactics and sets rules for various scenarios. For example, if the FTR is between 5 and 15% of the paddock he would use blanket treatments, if between 1 and 5% the camera sprayer would be used and on infestations less than 1%, the grower would consider hand removal.

This farmer had shown that monitoring, determination and setting plans are crucial factors to success.

Economic justification to control patches

There are many combinations and permutations of possible patchy weed infestations that could be investigated for an economic evaluation. An exhaustive list of factors to consider could include:

  • Weed species
  • Ability of the weed to spread
  • Initial size of patch
  • Effectiveness, frequency, type and cost of control measures
  • Timeframe for control
  • Longevity of seed bank
  • Gross margin of crops grown

For ease of reference for most growers and agronomists, this paper will investigate two weed species, awnless barnyard grass and feather-top Rhodes grass. The contrasting hypothetical management scenarios for each species are to;

  • either continue to spray fallows with glyphosate as the only treatment, or
  • use alternative paddock-wide treatments with additional ‘focused’ treatments to control the weed patch.

The assumptions made in calculating weed patch density and size over time and economic return are listed in Table 1.

Table 1. Assumptions used to calculate economic management of patches.

Weed species: Feather-top Rhodes grass patches

Weed species: Glyphosate resistant awnless barnyard grass patches

Seedbank decline in patches = 99% per annum (with no additional seed input)

Seedbank decline in patches = 85% per annum (with no additional seed input)

Patch expands at 50% pa in glyphosate only scenario. In the patch management scenario, size is constant but density reduces

Patch expands at 30% pa in glyphosate only scenario. In the patch management scenario, size is constant but density reduces

Winter cereal continuous cropping

Winter cereal continuous cropping

500 ha paddock with a solid (10000seeds/m2) 10ha patch of weeds

500 ha paddock with a solid (10000seeds/m2) 10ha patch of weeds

Glyphosate only fallow – 3 applications at 2L/ha (450g/L formulation)

Glyphosate only fallow – 3 applications at 2L/ha (450g/L formulation)

Patch management scenario – Balance® 100g/ha + Paraquat 2.4L/ha (applied once per year to the entire paddock). Glyphosate applied twice per year to areas without FTR.  )

Patch management scenario – Flame® 200mL + Paraquat 2.4L/ha (applied once per year to the entire paddock) per year to areas without glyphosate resistant ABYG.

FTR patches - Cultivate patch 3 times per fallow in for first two years then hand chip 3 times per fallow until year 5 (cost per ha =$10, for both techniques)

Glyphosate resistant patches - using a camera sprayer with a high rate of Group A  fb paraquat (3 times per season)

Note: assumptions common to both species are; weed patch is 10ha out of a 500ha paddock, weed seed bank density is 10000 seeds/m2, continuous winter wheat, soil available moisture just prior to winter cereal is 100mm (clean fallow) and after weedy fallow is 20mm, wheat potential average yield 2.9t/ha and priced at $275/t, water use efficiency is 10kg/ha/mm and variable costs to grow winter cereal is $400/ha (excludes fallow weed control costs). Optical sprayer savings trigger when BYG density is at 1 plant/m2, using only 30% of total potential spray, then each year costs reduce by 5% each year due to thinning density of BYG.

Table 2. Predicted outcomes (physical and economic) over a 10 year period to control a glyphosate resistant feather-top Rhodes grass patch using glyphosate only in the fallow.

Year 2

Year 4

Year 6

Year 8

Year 10

Weed patch info

Size of patch (ha)

15

33.8

75.9

170.9

384.4

% of paddock

3

6.8

15.2

34.2

76.9

Density of seedbank in patch (seeds/m2)

10000

10000

10000

10000

10000

% unaffected by weeds

97

93.3

84.8

65.8

23.1

Size of unaffected area (ha)

485

466

424

329

116

Economics

Yield in weedy area(t/ha)

2.1

2.1

2.1

2.1

2.1

Income per ha($)

577.5

577.5

577.5

577.5

577.5

Less Variable costs($)

400

400

400

400

400

 Gly 2L/ha - 3 times ($)

35.61

35.61

35.61

35.61

35.61

Gross margin per ha ($)

141.89

141.89

141.89

141.89

141.89

Yield in unaffected area(t/ha)

2.9

2.9

2.9

2.9

2.9

Income per ha ($)

797.5

797.5

797.5

797.5

797.5

Less Variable costs ($)

400

400

400

400

Gly 2L/ha - 3 times ($)

35.61

35.61

35.61

35.61

35.61

Gross margin per ha($)

361.89

361.89

361.89

361.89

361.89

Total g. margin weedy patch($)

2128

4788

10774

24243

54547

Total g. m. unaffected area ($)

175516

168731

153464

119112

41822

Total gross margin($)

177645

173520

164238

143355

96369

Total per ha ($)

355.29

347.04

328.48

286.71

192.74

Cumulative gross margin per ha($)

712.78

1411.81

2079.90

2678.39

3120.25

Table 3. Predicted outcomes (physical and economic) over a 10 year period to control a glyphosate resistant feather-top Rhodes grass patch using a strategic patch management approach.

Year 2

Year 4

Year 6

Year 8

Year 10

Weed patch info

Size of patch (ha)

10

10

10

10

10

% of paddock

2

2

2

2

2

Density of seedbank in patch (seeds/m2)

100

0.01

0

0

0

% unaffected by weeds

98

98

98

98

98

Size of unaffected area (ha)

490

490

490

490

490

Economics

Yield in weedy area (t/ha)

2.89

2.9

2.9

2.9

2.9

Income per ha ($)

795.3

797.5

797.5

797.5

Less Variable costs ($)

400

400

400

400

400

3 cultivations @ $10/ha - first 5 years ($)

30

30

0

0

0

Balance® 100g + paraquat 2.4L($)

51.48

51.48

51.48

51.48

51.48

Gross margin per ha($)

313.82

316.02

346.02

346.02

346.02

Yield in unaffected area(t/ha)

2.9

2.9

2.9

2.9

2.9

Income per ha($)

797.5

797.5

797.5

797.5

797.5

Less Variable costs($)

400

400

400

400

400

Balance® 200mL + paraquat 2.4L($)

51.48

51.48

51.48

51.48

51.48

Gly 2L/ha - 2 times ($)

23.74

23.74

23.74

23.74

23.74

Gross margin per ha($)

347.89

347.89

347.89

347.89

347.89

Total g. margin weedy patch($)

3138

3160

3460

3460

3460

Total g. m. unaffected area ($)

170466

170466

170466

170466

170466

Total gross margin($)

173604

173626

173926

173926

173926

Total per ha ($)

347

347

348

348

348

Cumulative g. m. per ha($)

690

1385

2080

2775

3471

Net gross margin per ha ($) compared to glyphosate based fallow

-23

-27

0

97

351

In years 3 to 5, it was assumed that patch hand chipping was at the same cost of cultivation as it was estimated that there would be very fey weed survivors.  Ie cost per hectare remains at $10/ha.

Table 4. Predicted outcomes (physical and economic) over a 10 year period to control a glyphosate resistant awnless barnyard grass patch using glyphosate only in the fallow.

Year 2

Year 4

Year 6

Year 8

Year 10

Weed patch info

Size of patch (ha)

13

22.0

37.1

62.7

106.0

% of paddock

2.6

4.4

7.4

12.5

21.2

Density of seedbank in patch (seeds/m2)

10000

10000

10000

10000

10000

% unaffected by weeds

97.4

95.6

92.6

87.5

78.8

Size of unaffected area (ha)

487

478.03

462.87

437.25

393.97

Economics

Yield in weedy area (t/ha)

2.1

2.1

2.1

2.1

2.1

Income per ha ($)

577.5

577.5

577.5

577.5

577.5

Variable costs ($)

400

400

400

400

400

3 applications gly 2L/ha ($)

35.61

35.61

35.61

35.61

35.61

Gross margin per ha ($)

141.89

141.89

141.89

141.89

141.89

Yield in unaffected area (t/ha)

2.9

2.9

2.9

2.9

2.9

Income per ha ($)

797.5

797.5

797.5

797.5

797.5

Variable costs ($)

400

400

400

400

400

Gly 2L/ha - 3 times ($)

35.61

35.61

35.61

35.61

35.61

Gross margin per ha ($)

361.89

361.89

361.89

361.89

361.89

Total gr. margin weedy patch($)

1844.57

3117.32

5268.28

8903.39

15046.72

Total g. m. unaffected area ($)

176240

172994

167508

158236

142568

Total gross margin($)

178085

176111

172776

167140

157615

Total per ha ($)

356.17

352.22

345.55

334.28

315.23

Cumulative gross margin per ha($)

713.66

1420.34

2115.21

2790.15

3431.37

Table 5. Predicted outcomes (physical and economic) over a 10 year period to control a glyphosate resistant awnless barnyard grass patch using a strategic patch management approach.

Year 2

Year 4

Year 6

Year 8

Year 10

Weed patch info

Size of patch (ha)

10

10

10

10

10

% of paddock

2

2

2

2

2

Density of seedbank in patch (seeds/m2)

1000

10

0.1

0

0

% unaffected by weeds

98

98

98

98

98

Size of unaffected area (ha)

490

490

490

490

490

Economics

Yield in weedy area (t/ha)

2.82

2.89

2.89

2.9

2.9

Income per ha ($)

775.5

797.28

797.49

797.5

797.5

Less Variable costs ($)

400

400

400

400

400

3x Optical sprayer:Verdict®+ para ($)

50.5

50.5

15.13

10.08

5.03

Flame® 200mL + paraquat 2.4L ($)

25.87

25.87

25.87

25.87

25.87

Gross margin per ha($)

325

346.78

382.37

387.43

392.48

Yield in unaffected area (t/ha)

2.9

2.9

2.9

2.9

2.9

Income per ha($)

797.5

797.5

797.5

797.5

797.5

Less Variable costs($)

400

400

400

400

400

Flame® 200mL + paraquat 2.4L ($)

25.87

25.87

25.87

25.87

25.87

Additional gly 2L/ha – twice ($)

23.74

23.74

23.74

23.74

23.74

Gross margin per ha($)

347.89

347.89

347.89

347.89

347.89

Total g. margin weedy patch($)

3250

3467.8

3823.728

3874.25

3924.75

Total g. m. unaffected area ($)

170466

170466

170466

170466

170466

Total gross margin($)

173716

173933

174289

174340

174390

Total per ha ($)

347.43

347.86

348.57

348.68

348.78

Cumulative g. margin per ha($)

690.90

1386.60

2083.71

2781.02

3478.53

Net gross margin per ha ($) compared to glyphosate based fallow

-22.76

-33.74

-31.50

-9.12

47.16

Comments: glyphosate based fallow for feather-top Rhodes grass control (Table 2)

The initial patch is estimated to take nearly 10 years to spread over most of the paddock. Although the yield predictions remained understandably constant for the weedy area and unaffected area (averaging a 0.8t/ha penalty), the spread of the weed over the paddock has meant the average return per hectare in the paddock has reduced more than $150/ha.

Comments: patch management approach to feather-top Rhodes grass control (Table 3)

The initial patch remained a constant size to due constant efforts to stop seed production. More importantly, due to the very short lived seed of this species, the density of the seed bank was minuscule at year 4. Subsequently yields of wheat from weed affected areas soon recovered to reach yield potentials in a rapid time frame. Although more money was spent to cultivate regularly for the first two years and more expensive treatments were used throughout the entire paddock, the relative performance of this scenario surpassed the glyphosate based fallow near year 6. This was due to lower yields from the poorly maintained fallow, once weeds dominated the area.

Comments: glyphosate based fallow for awnless barnyard grass control (Table 4)

Awnless barnyard grass took much more time to spread over the paddock. After a 10 year period it was predicted that about 20% of the paddock would be infested. The average return over the paddock had reduced by approximately $40/ha due to the spread of resistant plants over the paddock.

Comments: patch management approach to feather-top Rhodes grass control (Table 5)

The time to recover costs associated with patch management of awnless barnyard grass was much longer than feather-top Rhodes grass. This is greatly affected by the more residual seed bank and the more expensive alternative options to control glyphosate resistance in this species. A decision not to cultivate the patch was made because awnless barnyard grass does persist more when buried compared to feather-top Rhodes grass.

Discussion

There is no set recipe for successful patch manage of glyphosate resistant weeds. However common themes appear within many of these case studies and can be highlighted in the hypothetical economic analysis of two scenarios.

It appears some keys to winning the battle include:

  • Maintain the pressure on the weeds by using effective treatments regularly
  • Be flexible with treatment choice, most successful stories come from those that undertake chemical and non-chemical options
  • Get the weed patches early, if possible
  • Consider more pre-emergence and cultivation herbicides in fallows
  • Optical Spot Spray technology is useful
  • Fence line management is critical
  • Consider the battle may go on for at least 5 to 10 years
  • Other farm hygiene practices should be used
  • Monitor every paddock after every treatment
  • Patch management treatments can be complementary to paddock-wide treatments
  • Stop weed seed set

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

Tony Cook
NSW DPI
4 Marsden Park Road, Calala, NSW, 2340
Ph: 02 67 631 250 or 0447 651 607
Fx: 02 67 631 222
Email: tony.cook@dpi.nsw.gov.au

GRDC Project code: UQ00069