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Integrated weed management for the control of herbicide resistant annual ryegrass

John M. Matthews, R.S. Llewellyn, S. Powles and T. Reeves

C.R.C. for Weed Management Systems, University of Adelaide. SA 5064

Summary. A series of experiments to evaluate suppression of herbicide resistant annual ryegrass infestations with sustainable management activities was conducted. Ryegrass seed bank densities and crop yields for various treatments are reported. Reducing the density of ryegrass prior to sowing by a 3 week delay in sowing date was an effective treatment in reducing the seed bank and improving crop yields. Pre-emergent herbicide application reduced the seed bank and improved crop yields in pea crops as did crop topping. Seed catching reduced the ryegrass seed bank but not always below the crop damage threshold and crop yield responses were not consistent . Crop rotations containing legumes had higher weed seed densities than cereal only rotations in the absence of herbicide treatments.

INTRODUCTION

Many populations of annual ryegrass across southern Australia have developed herbicide resistance due the persistent use of selective herbicides in both cereal and legume crops (1). Recent surveys indicate that 40% of cropped paddocks contain populations of ryegrass with serious levels of resistance (2,3). The continued emphasis on herbicide application for weed control will increase this proportion. Herbicide resistant ryegrass has developed multiple resistance to many selective herbicides such that many populations have limited herbicide options for suppressing infestations and reducing weed competition. With widespread and difficult weeds, an appreciation of integrating cultural control methods and herbicides to provide sustainable weed management is required. The following series of experiments evaluated the effect of integrated management on a ryegrass seed bank over 2 years.

MATERIALS AND METHODS

Field experiments were established to evaluate the effect of delayed sowing, burning stubble residues and catching ryegrass seed at harvest (4) on the continuing ryegrass seed bank. These cultural methods were evaluated in a wheat, barley and pea rotation (Experiment 1, Roseworthy SA, mean annual rainfall 441 mm) and in a wheat, barley and faba bean rotation (Experiment 2, Auburn SA, mean annual rainfall 596 mm). The use of trifluralin pre-emergent herbicide (0.7 L/ha) with crop topping at late ryegrass anthesis (paraquat, 100 g a.i./ha) at weed anthesis with seed catching at harvest were examined in a wheat and pea rotation at Roseworthy (Experiment 3). The efficacy of reducing the soil seed bank in a pasture prior to cropping were examined at Roseworthy with one year of spraytopping, chemical fallowing and ploughing and two years of spraytopping (Experiment 4). Experiments 1, 2 and 3 were split plots (3 levels) and experiment 4 was a split plot (2 levels). Preliminary results of the ryegrass seed bank are presented, three 110mm diam. cores per plot were taken in autumn prior to rainfall. Crop sowing rates were standard for each area. Data was analysed by Anova on Genstat 5.0.

RESULTS AND DISCUSSION

There were many treatments and interactions from four experiments, the results are presented in table form showing treatments and interactions effects on ryegrass seed bank density and crop yields. The important observations are summarized below.

1. Seed catching reduced the ryegrass seed bank by approximately 55% across all experiments, see Tables 1, 2 and 3. Crop yields did not immediately reflect this reduction as seed densities were not reduced to below a damage threshold, (Tables 5,6 and 7).

2. Delayed sowing time; Tables 1, 2 and 3 show that a delay of 3 weeks was effective in reducing seed numbers in the soil. Crop yields were improved by delayed sowing, (Tables 5, 6 and 7) except when crops were competitive, barley, Table 5 and wheat, Table 7.

3. Legume crops in the cropping sequence increased weed seedbank numbers, (Tables 1, 2 and 3) in the absence of herbicides. Where effective pre-emergent and crop topping herbicides were available, ryegrass numbers were reduced (Table 3). Crop yields reflected the improved weed control, expt.3. Table 7.

4. Burning stubble did not significantly reduce seed numbers (data not shown) in the drier, more variable Roseworthy environment. At Auburn burning resulted in a significant reduction as sufficient stubble was present to fuel a destructive fire on ryegrass seed. The burning of stubble across all treatments resulted in 58% less ryegrass seed in the 1995 seedbank (P<0.001)(data not shown). Crop yields were inconsistent under burning treatments and showed no sig difference from unburnt treatments (data not shown).

5. There were significant interactions between crop species and seed catching as weed seed output was higher in some crops. Tables 1 and 2 show this and Table 3 shows the usefulness of seedcatching in conjunction with other treatments.

6. Pasture treatments; spraytopping, chemical fallowing and cultivated fallow as single treatments and spraytopping twice as followed by wheat and pea crops are described in Table 4. Each pasture treatment was differentially effective. The wheat crops and the pea crop on the ploughed treatment effectively suppressed the ryegrass seed bank as much as the second year of spraytopping, and to a greater extent with seed catching than without.

Table 1. Means of the ryegrass seed bank after 2 years of a wheat, barley and pea rotation at Roseworthy, (ryegrass seed/sq m). The initial ryegrass seedbank was 5300 seeds/sqm.

 

Time of sowing

 

Early

 

Late

Crop sequence

Catching seed

Seed not caught

 

Catching seed

Seed not caught

Barley - peas

2247

5972

 

1372

4459

Peas - wheat

842

2008

 

412

573

Wheat - barley

495

929

 

197

224

Crop sequence and time of sowing were significant (P<0.05), with a significant interaction between crop spp. and seed catching (P<0.05).

Table 2. Means of the ryegrass seed bank after 2 years of a wheat, barley and bean rotation at Auburn, (ryegrass seed/sq m). The initial ryegrass seedbank was 3127 seeds/sq m.

 

Time of sowing

 

Early

 

Late

Crop sequence

Catching seed

Seed not caught

 

Catching seed

Seed not caught

Barley - beans

2153

3867

 

1488

2560

Beans - wheat

4358

8817

 

2867

5752

Wheat - barley

1175

2210

 

767

2784

All treatments and crop sequences.were significant (P<0.05), there were no significant interactions.

Table 3. Treament means of pre-emergence herbicide application, crop topping and seed catching in a wheat-peas rotation at Roseworthy (ryegrass seed/ sq m). The initial seed bank was 76 ryegrass seeds/sq m.

 

Herbicide treatment

 

Pre-emergent

 

None

 

Crop topping

No crop topping

 

Crop topping

No crop topping

Crop sequence

Seed caught

Un-caught

Seed caught

Un-caught

 

Seed caught

Un-caught

Seed caught

Un-caught

Wheat-peas

0

26

254

395

 

219

623

754

2131

Peas-wheat

26

26

202

237

 

140

254

886

1736

Crop topping was significant (P<0.001), with a pre-emergent herbicide*seed catching interaction (P<0.01).

Table 4. Treatment means of ryegrass seed density following pasture treatments in the year prior to a wheat-peas rotation at Roseworthy (ryegrass seed/sq m). The initial ryegrass seedbank was 1644 seeds/sq m.

Year

Pasture treatment in year prior to planting

1993

Spraytopped

 

Chemical fallow

 

Ploughed fallow

1994 Spraytopped

 

Wheat

Peas

 

Wheat

Peas

 

Wheat

Peas

   

Catch

U/C

Catch

U/C

 

Catch

U/C

Catch

U/C

 

Catch

U/C

Catch

U/C

1995

362

 

167

377

1561

2657

 

202

228

570

1894

 

88

263

368

1350

All pasture treatments were significantly different (P<0.05), 1994 crop spp. was significant (P<0.001), seed catching was significant(P<0.01).

Table 5. Crop yields (t/ha) from Experiment 1 at Roseworthy.

 

Time of sowing

 

Early

 

Late

Crop

Catching seed

Seed not caught

 

Catching seed

Seed not caught

1993-4 Barley

 

2.64

   

2.36

1993-4 Peas

 

1.17

   

0.93

1993-4 Wheat

 

1.43

   

1.60

1994-5 Barley

1.20

1.11

 

1.02

1.10

1994-5 Peas

0.12

0.16

 

0.36

0.34

1994-5 Wheat

0.85

0.88

 

1.23

1.13

Time of sowing was significant for wheat and peas only (P<0.001).

Table 6. Means of crop yields (t/ha) from Experiment 2, Auburn.

 

Early

 

Late

Crop

Catching seed

Seed not caught

 

Catching seed

Seed not caught

1993-4 Barley

 

1.54

   

3.25

1993-4 Beans

 

-

   

2.13

1993-4 Wheat

 

1.9

   

2.41

1994-5 Barley

1.53

1.77

 

1.83

1.74

1994-5 Beans

0.37

0.46

 

0.66

0.83

1994-5 Wheat

2.40

2.59

 

2.32

2.03

Sig. crop*time of sowing interaction (P<0.001).

Table 7. Means of crop yields (t/ha) from Experiment 3, Roseworthy.

 

Herbicide treatment

 

Pre-emergent

 

None

 

Crop topping

No crop topping

 

Crop topping

No crop topping

Crop sequence

Seed caught

Un-caught

Seed caught

Un-caught

 

Seed caught

Un-caught

Seed caught

Un-caught

1993-4 Wheat

-

2.59

-

3.59

 

-

2.58

-

3.30

1993-4 Peas

-

0.73

-

0.99

 

-

0.70

-

0.83

1994-5 Wheat

1.04

1.13

1.05

1.14

 

0.80

0.77

0.62

0.52

1994-5 Peas

0.55

0.53

0.28

0.35

 

0.31

0.32

0.09

0.05

Crop topping treatments and pre-emergent herbicide sig. (P<0.001) and sig. crop * pre-emergent herbicide interaction (P<0.001)

Annual ryegrass is a fecund grass weed with a high proportion emerging from the seedbank in any season, therefore, the greatest influence on the seed bank and on crop yields (in the absence of effective herbicides) is the size of the seed set from the previous year. Integrating the low herbicide resistance risk management options of pre-emergence trifluralin application and crop topping with competitive crop species, delayed sowing and seed catching shows good prospects for long term management of annual ryegrass populations.

ACKNOWLEDGEMENTS

These projects were supported by the GRDC.

REFERENCES

1. Hall L., Holtum J. A. M. and Powles S. B. 1994. "Herbicide Resistance in Plants", Chapter 9. (Lewis Publishers, Boca Raton).

2. Nietschke B., Llewellyn R., Matthews J.M., Reeves T. and S. B. Powles. 1995. A Survey to determine the incidence of herbicide resistant wild oats and annual ryegrass. These Proceedings.

3. Llewellyn R. and Matthews J.M. 1995 A survey of herbicide resistance on the Eyre Peninsula region of SA. (unpublished).

4. 4 Matthews J.M., Llewellyn R., Reeves T. and S. B. Powles. 1995. Catching weed seeds at harvest: a method to reduce annual weed seed populations. Proc. 8th Agronomy Conf., Toowoomba, Qld.

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