How can I manage weeds effectively in a stubble-retained system

Ryegrass
Ryegrass. Source: Nicole Baxter.

The aim of weed management is to reduce the weed seed bank over time. Weed numbers are dynamic across a paddock. Any intervention to decrease weed seed set will take multiple seasons.

Weed management should be approached as part of the whole farm system and no single practice relied on for control.

Particular challenges of the adoption of stubble-retained systems are that surface germinating weeds such as fleabane and brome grass have become more common and that the presence of stubble creates a physical barrier that reduces herbicide efficacy.

Management of weeds in stubble-retained systems requires a detailed understanding of the biology of specific weeds that are favoured by stubble retention, techniques to maximise herbicide efficacy in stubble and the implementation of multiple other weed management tactics.

What weeds are favoured in stubble retention?

As part of the GRDC Stubble Initiative farming systems groups have focused on understanding the biology and management of problem weeds including ryegrass, flaxleaf fleabane, brome grass, barley grass, Maireana, onion weed and statice.

Ryegrass Integrated Management or RIM is a decision support tool, developed by the University of Western Australia, that allows farmers and advisors to evaluate the long-term cropping profitability of strategic and tactical ryegrass control methods. As part of the Stubble Initiative RIM software was developed for Brome and Barley Grass.

How can I improve herbicide efficacy in stubble?

Stubble, existing weed cover and crop cover (for post-sowing applications) in a zero or minimal till system will intercept some of the herbicide before it reaches the soil. The amount of herbicide intercepted will be proportional to the percentage of ground cover (Figure 1). Interception can have two negative effects: firstly, herbicide can be tied up on the stubble or in the canopy and will not be available for weed control; and secondly it can lead to uneven coverage on the soil surface, lowering herbicide effectiveness and increasing potential weed escapes.

Herbicide interception increases as stubble groundcover increases

Figure 1. As groundcover increases more herbicide is intercepted before it reaches the soil. Source: Shaner 2013.

The availability of a herbicide is an interaction between its solubility, how tightly it is bound to soil particles and organic matter, soil structure, soil chemical properties such as cation exchange capacity, water content and pH, its volatility (loss to the atmosphere) and the rate at which it is applied. The EPARF Herbicide efficacy in cereal stubbles (PDF 404kb) has a table of solubility and soil water movement potential of key herbicides.

Reduced efficacy can be largely overcome by adopting appropriate spraying techniques and herbicides for the situation.

Management options to increase herbicide activity in paddocks with high stubble loads include:

  • increasing chemical and water rates
  • using nozzles that provide a larger droplet size to increase spray coverage
  • reducing the height of the spray boom or stubble height so herbicides reach the soil surface easier and cover the soil more evenly.

Standing stubble allows greater penetration of herbicide than stubble that is spread across the surface. Consider stubble management practices at each stage during the production cycle to optimise herbicide efficacy:

  • At harvest minimise the amount of ground cover and spread trash evenly across the header swathe. If header trails become too thick to achieve good herbicide efficacy, consider using stripper fronts or windrow burning to maximise contact between herbicide and the soil or weeds. Lodged stubble takes up a greater surface area than standing stubble.
  • In autumn limit grazing so that the stubble can remain upright in high stubble load situations
  • At seeding minimise soil disturbance through zero-till practices to reduce the number of weed seeds germinating

What are the options for reducing weed seed populations at harvest?

Herbicides are only one tool for weed control; growers should try adopting an integrated weed control package that includes non-chemical control methods such as harvest weed seed collection or hay cutting.

Harvest weed seed control (HWSC) techniques rely on the weed seeds being collected by the harvester. Harvest low to maximise weed seed collection.

Crop topping

Crop topping can be used to control remaining weeds before harvest. It is essential to get the timing right to achieve adequate weed seed-set control without damaging the crop.

Increased herbicide resistance is a major risk of crop topping especially for herbicides such as glyphosate that are used at other cropping stages. It is vital, wherever possible, to rotate with different herbicide groups (such as paraquat or diquat) or combine topping with other practices (such as windrow burning) to limit the number of escapes.

Hay cutting

A herbicide application, in conjunction with cutting for hay, will reduce ryegrass heads that would have set seed. Cutting hay before weed seeds mature will have greater weed seed bank reduction than baling straw after harvest.

Chaff carts

Chaff carts allow growers to retain stubble cover leading to the positive effects of stored soil moisture, reduced wind and water erosion, fewer weeds and increased soil microbial activity.

Narrow windrow burning

Studies in windrow burning have shown that the success of this treatment is reliant on the temperature of the burn.

The temperature needs to exceed 400°C for 10 seconds to kill ryegrass and 500°C to kill wild radish (Table 1). Research by SARDI, EPARF, UNFS and SAGIT has shown that cereal windrow burning (PDF 473kb) will achieve these target temperatures in most situations.

Table 1: The effect of temperature and duration of exposure, on the percentage germination of annual ryegrass and wild radish. Source: Walsh and Newman 2006.

Annual ryegrass

Temperature (°C)

 

200

225

250

275

300

400

Duration (seconds)

% Survival

10

-

-

-

-

77

0

20

92

70

55

57

5

0

40

90

26

15

6

0

0

60

89

1

0

0

0

0

80

74

0

0

0

0

0

Wild radish

Temperature (°C)

 

300

350

400

450

500

 

Duration (seconds)

% Survival

10

89

88

85

22

0

 

20

89

67

1

0

0

 

60

1

1

0

0

0

 

Whole paddock burning

Burning a whole paddock of cereal stubble is an option if the grass weed burden is very high, snails are an issue, or high stubble-borne disease inoculum levels were present the previous season and the paddock is to be cropped to another cereal. Nutrients will be lost through the burning process – see How do I manage nitrogen in a stubble-retained system?

Weed seed destructor technology

To manage herbicide-resistant weeds, seed destructor technology (seed mills) provide an alternative solution to other HWSC techniques. They grind the chaff and weed seeds coming through the header. This enables one pass weed seed processing to eliminate all weed seeds that are picked up with the crop.

More Information

Farming Systems Groups (weed management resources)

Farming Systems Groups (herbicide resources)

Other resources