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Protecting WA crops

Management of weed seeds at harvest

Figure 1. Catherine Borger, weed researcher from Department of Primary Industries and Regional Development (DPIRD), Northam.
Figure 1. Catherine Borger, weed researcher from Department of Primary Industries and Regional Development (DPIRD), Northam.

Harvesting provides an excellent opportunity to remove weed seeds from the system and prevent them from being spread across the paddock or farm. Collecting seed at harvest has the potential to be a useful component of an integrated weed management program.

Weed seed collection at harvest will not increase grain yield, as the weeds have already caused damage to the crop. This tactic can prevent increases to the seedbank though and it may give a subsequent yield advantage to the next season’s crop through reduced weed numbers during the season.

Harvest weed seed destruction/control

There are a number of methods that can be used to concentrate and control weed seeds during the harvest operation.

Narrow header trail

Narrow header trails (also known as trash windrows) are the cheapest, simplest form of collecting residue. It is done by disconnecting the straw spreaders on the header and allowing the straw, chaff and weed seeds to fall in a narrow trail behind the header. Adding a simple chute forces the residue into an even narrower, more discrete row.

The trail can then be burned during autumn. This has become common practice in many districts when harvesting canola. The high fuel load in the row results in a hot burn and a good weed seed kill. However, any unburned seeds will produce seedlings that will need to be controlled using an alternative tactic.

In a controlled traffic cropping system, straw can be spread while chaff and weed seed are deflected onto the wheel track where they are less likely to grow. This is known as chaff lining. During the subsequent growing season spray nozzles can be fitted to another implement to apply a non-selective herbicide to the wheel tracks, killing any germinated weeds. This system fits well where there is a preference for avoiding burning.

Chaff cart

Figure 2. Harvesting with a chaff cart at Tarin Rock (© DPIRD).
Figure 2. Harvesting with a chaff cart at Tarin Rock (© DPIRD).

In a chaff cart system the chaff and weed seeds are collected and thrown into a trailing cart. When the cart is full the gate is tripped and the cart self-empties to create a chaff dump. Chaff dumps are usually burned or fed to livestock, and both options require careful management.

Harrington seed destructor

Testing of the Harrington seed destructor (HSD) has shown that it has the ability to destroy 90-95% of the weed seeds in the chaff it processes. The HSD entered commercial production in 2012 and there is now a version of the mill available that is incorporated into a header. For more information refer to Integrated Harrington Seed Destructor, iHSD.

Factors affecting weed seed removal

The type of weed species has a major influence on the proportion of weed seed removed from the paddock when collected at harvest. For example, annual ryegrass is much easier to collect than wild oats, which tend to shed seed before harvest.

Successful seed collection and control depends on the weed:

  • Maturing at the same time, or later than, the crop being harvested.
  • Having seeds at a similar or greater height compared with the crop being harvested. This may be overcome by setting the header at a height so weed seeds are captured or using crop lifters.
  • Having seeds that do not shed or shatter before or during harvest.
  • Having seeds that can be threshed and are of a size that end up in the chaff component of the harvested crop.
  • Timing of harvest will affect the amount of seed removed from the paddock when residue is collected. As harvest is delayed, a greater proportion of the weeds presented will shatter or lodge, reducing the total proportion of seed able to be collected.

Harvest weed seed destruction – Does it work for great brome and barley grass?

Figure 3. Great brome grass in a wheat crop (Photo courtesy: Catherine Borger, DPIRD).
Figure 3. Great brome grass in a wheat crop (Photo courtesy: Catherine Borger, DPIRD).

Lessons learnt

  • Shedding of great brome and barley grass varies between seasons and ecotypes. Plant height also varies between ecotypes.
  • Harvest weed seed destruction will be effective for some populations, in some years, but growers need to be familiar with the ecological characteristics of their own weed populations to plan effective weed management strategies.

Brome grass and barley grass are ranked as the fourth and ninth most detrimental weeds nationally, with an annual revenue loss of $22.5 and $1.7 million respectively (Llewellyn et al, 2016).

In 2016-2018 research scientists Catherine Borger (DPIRD) and Abul Hashem (formerly DPIRD) investigated the shedding variation between populations of great brome and barley grass at Wongan Hills to determine if harvest weed seed destruction had potential as a control technique.

Great brome and barley grass were assessed for the cumulative percentage of seed shed from early October until harvest each year. By mid-November in each year of the trial, great brome had shed 64% of its seed in 2016, 70% in 2017 and 46% in 2018.  However, in 2017, when the crop was ready for harvest on the 9th November, great brome had only shed 28% of seed. All great brome seed heads formed above harvest height, with most heads at the same height or taller than the crop heads.

Table 1. The cumulative percent of seed loss by each weed species from early October to the date of harvest in 2016 at Wongan Hills trial. Note that the crop reached maturity (earliest possible date that harvest could have been completed) on 14 November 2016.
Species 7/10/16 7/10/16 7/11/16 14/11/16 25/11/16 1/12/16
Annual ryegrass 0 1 3 9 24 27
Great brome 0 0 1 64 100  
Barley grass 0 8 13 38 100  
Table 2. The cumulative percent of seed loss by each weed species from early October to the date of harvest in 2017 at Wongan Hills trial. Note that the crop reached maturity (earliest possible date that harvest could have been completed) on 9 November 2017.
Species 9/10/17 24/10/17 9/11/17 15/11/17 23/11/17
Annual ryegrass 0 1 3 13 13
Great brome 0 6 28 70 74
Barley grass 0 4 4 8 8
Table 3. The cumulative percent of seed loss by each weed species from early October to the date of harvest in 2018 at Wongan Hills trial. Note that the crop reached maturity (earliest possible date that harvest could have been completed) on 12 November 2018.
Species 5/10/18 19/10/18 1/11/18 12/11/18 28/11/18
Annual ryegrass 0 1 3 5 8
Great brome 0 0 14 46 53
Barley grass 0 0 0 1 5

Harvest weed seed destruction may be an effective control measure for great brome but as its seeds shed rapidly at maturity, the crop should be harvested as early as possible if great brome seed destruction is the goal. Timing of shedding varied between seasons for the single population at Wongan Hills, so the amount of seed captured will vary, and will depend on how early the crop reaches maturity.

Figure 4. Barley grass (© DPIRD)
Figure 4. Barley grass (© DPIRD)

Barley grass started shedding seed in the trial in October, but retained 62% of seed at the earliest possible date of harvest in 2016, 96% in 2017 and 99% in 2018. In this trial the barley grass was poorly competitive due to late emergence and the plants did not grow taller than 10-12cm, i.e. below the harvest height of 15cm. In 2016 and 2017, harvest caused sufficient agitation that the remaining barley grass heads shed from the plant, but fell onto the ground rather than being captured at harvest. In 2018 the barley grass was late to senesce, and most heads were retained during and after harvest.

The population of barley grass in the field trial shed later than great brome. The likely reason for this is that great brome in the field was at a similar height or taller than the crop at maturity, whereas barley grass in the field was about 10cm tall. These short plants were protected from wind and agitation against moving crop heads, which likely reduced shedding.

More research is required to determine the long term impact of harvest weed seed destruction for those species where seed capture at harvest is low or variable between years.

Research is also required to improve seed capture at harvest, which may involve; shorter harvest height and improved capture of short or lodged weed seed heads, swathing to retain weed seeds or use of early maturing crops for an earlier harvest date.

For more information on managing brome and barley grass refer to Grain Research and Development Corporation's (GRDC) Management of brome grass and Barley grass pages.

More information refer to GRDC's Weed seed management at harvest

Summer weeds

Figure 5. Fleabane (© DPIRD)
Figure 5. Fleabane (© DPIRD)

A survey was conducted to assess weed species incidence and density on the roadside of major roads within the WA grainbelt, over three consecutive summers from 2015 to 2017. Results are displayed in Table 4.

The survey was conducted each year during February to April, at least 6 to 8 weeks after a summer rainfall event of more than 20mm, to ensure summer weeds had germinated and reached maturity before assessment.

The survey identified a total of 144 separate species. There were 76, 72, and 129 species identified in 2015, 2016, and 2017, respectively.

Table 4. Summer weed species found at greater than 10% of sites surveyed in the Western Australia grainbelt from 2015 to 2017, the percent of sites each species was found in each year, and the percent of sites each species was found averaged over all years.

Common name

Scientific name

2015

2016

2017

Average

African lovegrass

Eragrostis curvula

48

70

59

57

Windmill grass

Chloris truncata

30

55

46

42

Fleabane

Conyza sp.

40

36

37

38

Wild radish

Raphanus raphanistrum

27

43

45

38

Stinkgrass

Eragrostis cilianensis

20

30

50

34

Sowthistle

Sonchus oleraceus

22

34

40

31

Wild oats

Avena sp.

15

22

37

25

Roly poly

Salsola australis

16

29

19

20

Capeweed

Arctotheca calendula

15

22

22

19

Couch

Cynodon dactylon

16

20

21

19

Afghan thistle

Solanum hoplopetalum

13

25

13

16

Stinkwort

Dittrichia graveolens

15

20

13

16

Panic grass

Panicum sp.

11

22

16

15

Afghan melon

Citrullus lanatus

12

11

21

15

Mulla mulla

Ptilotus polystachyus

5

18

24

15

Puncturevine

Tribulus terrestris

11

17

12

13

Goosefoot

Chenopodium sp.

4

12

22

13

Crowfoot grass

Dactyloctenium sp.

8

13

14

11

Paddymelon

Cucumis myriocarpus

11

7

13

11


The roadside survey showed some weed species were consistent with germinating, i.e. found in a site in every year, after rainfall. However, most summer weeds were not consistent. Some were initially absent in the first year and then introduced in the second year, or initially present and then absent in subsequent years. The weeds that are generally considered winter weeds (e.g. wild oats, wild radish etc.) occurred more consistently over each year.

Figure 6. Windmill grass (© DPIRD).
Figure 6. Windmill grass (© DPIRD).

The emergence of summer weeds varies from season to season. Much of the variation will depend on the timing and extent of late spring/early summer rainfall events plus the weeds interaction with suitable temperature ranges. However, there is also an effect of seed dormancy in some species of summer weeds.

Research scientist Catherine Borger (DPIRD) studied the dormancy and emergence patterns of five summer weed species during a GRDC co-funded project on emerging weed species (Figure 7). Seed was collected in 2015 and placed immediately into trays, lightly covered with wheat straw, to simulate stubble cover. The trays were watered at regular intervals and the germination/emergence recorded fortnightly.

Figure 7. Time and magnitude of emergence (%) of five weed species over a period of three summer seasons.
Figure 7. Time and magnitude of emergence (%) of five weed species over a period of three summer seasons.

The largest level of emergence, for all five species, was in the first growing season studied, 2016. Indeed, for the button grass and windmill grass, no further germinations were recorded after the first season.

Figure 8. Afghan melon (© DPIRD).
Figure 8. Afghan melon (© DPIRD).

Afghan melon, roly poly and caltrop did have 10 to 20 percent of seeds germinating in 2017 and 2018. These species are exhibiting seed dormancy and their seeds banks can last for at least three years.

The emergence of summer weeds can vary from season to season and the species that are present will also vary depending on the climatic conditions and seed dormancy characteristics. It is important to consider these factors when planning summer weed management.

Further information on summer weed management and individual species can be found on the following webpages:

Summer weeds 

Fleabane 

Roly poly   

Windmill grass 

For more information on the control of summer weeds contact Catherine Borger, DPIRD, Northam on +61 (0)8 9690 2220.