module 17B

Integrated weed management in pastures

Objectives

At the end of this module, you will:

Module Outline

Introduction

The first thing to be said about weed management in perennial pastures (these the primary focus of this sub-Module), is that weed control per se is not the desired outcome of pasture management. Instead the maintenance of a productive pasture that adequately feeds the livestock which graze it. As well, an integrated approach will of necessity need to be developed on a case by case basis, depending on:

Nevertheless, where different techniques can be easily combined then these will be emphasised.

General definitions of weeds have been given in Modules 1, but in considering Integrated Weed Management (IWM) in pastures, it is useful to define what is a weed in a pasture. Whereas in an annual crop a weed will probably be any plant other than the cultivated crop plant, and in a natural ecosystem be any plant not native to that particular region and vegetation type, these plants may be highly desirable species within a pasture. A good example of this is annual ryegrass (Lolium rigidum) which:

Likewise, in that state,

  • capeweed (Arctotheca calendula)
  • barley grass (Hordeum spp.),

    and even
  • Paterson's curse (Echium plantagineum)

are considered to be desirable weeds of pasture in most situations. While weeds do contribute enormously to livestock production and wool growth in Australian farming systems, there is ample evidence to suggest that pastures based on legumes provide better overall nutrition for livestock.

Weed invasion of a pasture often indicates that there is some problem affecting the growth of the desirable pasture plants. This may be:

Identification and correction of the problem will lead to reduced levels of weeds on a long term basis; however, such pasture renovation may need to be supplemented with management methods aimed directly at weeds, such as herbicides.

Practical classification of pasture weeds

Michael (1970) conveniently classified weeds of Australian pastures and grazing lands into two groups according to the type of impact they have and the situations in which they have their impact viz:

Weeds of established pastures

These are essentially two kinds of weeds of established pastures:

The first group includes a wide range of poisonous plants both native and introduced. The toxic principles of these plants include:

In many cases the toxic chemicals are unknown. Some examples of these toxins in plants are as follows:

Some further examples of poisonous plants and their effects in livestock:

Poison plant   Effect
Soursob
(Oxalis pescaprae)		 Toxic to sheep, particularly in South Australia
Variegated thistle
(Silybum marianum)		 May induce nitrite poisoning in grazing stock
St. John's wort
(Hypericum perforatum)		 Photosensitisation. Occurs in dry temperate forest regions in NSW, Victoria and South Australia
Bracken
(Pteridium esculentum) 		Haemorrhaging in cattle
Caltrop
(Tribulus terrestris),
Khaki weed
(Alternanthera pungens)

Mechanical injuries to livestock (both external and internal) in north-western NSW
Native Stipa and Aristida and
introduced barley grass
(Hordeum leporinum) 		Spikelets can penetrate the skins of livestock and get in their eyes, causing blindness
Skeleton weed
(Chondrilla juncea) 		  May cause choking in cattle. Abundant in south-western NSW

Noogoora burr
(Xanthium occidentale)

and

Bathurst burr
(Xanthium spinosum)



Burrs adhere to wool. Prevalent along water courses in NSW, Queensland and South Australia

The second group of weeds which occupy ground which would be utilized by more useful species include:


annual thistles
biennial thistles
Paterson's curse
barley grass
silver grasses (Vulpia spp.)

as well as perennial introduced shrubs such as:


blackberry
(Rubus fruticosus sp. agg.)
African boxthorn
(Lycium ferocissimum)
sweet briar
(Rosa rubiginosa)
lantana
(Lantana camara)

All are widespread in southern Australia but lantana is found in found in coastal areas of northern NSW and southern Queensland.

Weeds which interfere with the establishment of pastures

There are essentially two kinds of weed that interfere with the establishment of pastures:

Within the context of the first kind, 'unimproved land' in the Australian context is land which has not been fertilised with superphosphate and/or sown to legumes. Many of the plants in this group may be described as weeds of low fertility. The most important of these are the native woody shrubs which present difficulties in clearing and subsequent operations, but may also include useless plants like serrated tussock (Nassella trichotoma) which is at its worst in the central and southern Tablelands of NSW.

Within the context of the second kind, many of these plants may be described as weeds of high fertility. Perhaps the most important weeds in this group are the:

Strategies for control

The following is a brief introduction to the main methods available for weed management in pastures. The bulk of information on each strategy should be extracted from the associated readings. Strategies for control are:

Farm hygiene

Most emphasis in this module is placed on management of existing weeds, but the prevention of spread of weeds through hygienic farm management practices is of vital importance in the fight against weeds, particularly those species which may not yet be widespread. The guidelines for the prevention of spread of giant Parramatta grass (Sporobolus indicus) on the north coast of NSW are used as an example.

icon
Reading

"Guidelines to restrict the spread of giant Parramatta grass" by T Schmitzer, P Popovic, and T Launders, 1994, Agnote Reg 1/008, NSW Agriculture, 4 pp
Pasture competition

The essential principle of any weed management program must be the provision of a dense, vigorous and competitive pasture, particularly in those periods that coincide with the bulk of weed emergence and seedling establishment (commonly autumn in southern Australia). In other words, pasture weeds are often best controlled by the pasture plants themselves, which reduce light, moisture, and nutrient availability to the germinating weed seedlings.

Grazing should be managed to provide lighter stocking rates at these times in paddocks known to have high weed populations. The reduced stocking rates can promote pasture growth and significantly increase the level of weed seedling mortality. For example, deferred autumn grazing (until winter or spring) (see Module 11) has been used successfully to control:

  • Cape weed (left)
  • Paterson's curse (centre), and
  • some thistles (right)
   

In general, weeds are weakest or most susceptible to control when at this early seedling stage. Appropriate fertiliser applications can also promote pasture growth and lead to reduced weed numbers, although timing of applications would appear to be critical. The following reading demonstrates the efficacy of fertiliser application in enhancing pasture competivity.

icon
Reading

‘Is fertiliser the best herbicide? - The effect of fertilisation on weed competition in pastures’ by W King and S Priest 1999, Twelfth Australian Weeds Conference Papers and Proceedings, pp. 170-173.

Pasture species for sowing in a particular area should be selected on the basis of their:

Where the rainfall is sufficiently distributed throughout the year to support perennial grasses such as phalaris (Phalaris aquatica) or tall fescue (Festuca arundinacea), then these should dominate the pasture mix. Likewise, lucerne (Medicago sativa) is well known for its ability to suppress the growth of many weeds.

The timing of re-seeding of pastures (if that is required) needs to be carefully planned to avoid germination of and early competition from annual weeds. For example, spring sowing can often avoid the problem of annual grasses and thistles in some areas. Once improved pastures are established, they must be maintained and carefully managed to prevent their degradation and re-infestation. Isolated weeds are likely to continue to appear and should be removed systematically either by grubbing or spot spraying. As an exercise, have a look at some nearby pastures and see if the level of weed infestation relates to the amount of standing pasture dry matter. The following two readings examine issues related to the success and management of thistle species in pastures in Australia. The second reading in particular demonstrates use of herbicides to control thistles and the problems associated with this management technique. Clearly it is not appropriate to try to manage thistles in pastures just with herbicide.

icon

Go to Activity 17B-1

As an exercise, have a look at some nearby pastures and see if the level of weed infestation relates to the amount of standing pasture dry matter.

icon
Media

View the clip on parthenium weed creeps towards New South Wales, on the DVD 'Weeds in the Way' available from the UQ Gatton Librry.

Grazing

Apart from using grazing management to control weeds through increased pasture competition, livestock can also be used to graze weeds directly. While in general, few pasture weeds are grazed in preference to the sown species, extra grazing pressure for short periods (e.g. mob stocking) may be able to be used to force stock to eat plants that may otherwise be left. Mob stocking (see Module 11) has been used successfully to control:

Running different types of stock can also be exploited to apply grazing pressure to some weeds. In particular, goats graze a variety of weeds, often reducing plant numbers and preventing seed production. Although alternative more palatable pasture may be plentiful, less palatable species such as thistles, will still be eaten.

icon
Reading

‘Goats in fight against difficult weeds’ by C Allan and P Holst 1995, Pasture plus; the complete guide to pastures, Kondinin Group, Belmont, Western Australia, pp. 326-328.
Chemical control

Even with the establishment of improved pastures, herbicides may be required to control weeds that establish with the sown species, or which infest the pastures in particularly bad weed years. However, repeated annual applications of herbicides aimed at exhausting the soil seed reserves in heavily infested paddocks may weaken the pasture, making it more liable to future re-invasion.

Overuse of herbicides might also encourage the development of herbicide resistant weed populations, as found with Paterson’s curse, Echium plantagineum, in Australia and nodding thistle, Carduus nutans, in New Zealand. It is best therefore, if spraying is strategic and integrated with other control tactics.

The aim of chemical control, except where pastures are being re-sown, is to selectively control the undesirable weeds but to leave the desirable pasture plants relatively unharmed and sufficiently competitive to permanently replace the weeds. Selective broadleaf weed control can be achieved using herbicides such as 2,4-DB followed by spelling and fertilising.

Paraquat can be used to selectively remove annual grass weeds but there may be some temporary injury to the perennial grass and annual legume pasture component. Removal of perennial grass weeds from perennial grass-based pastures is more difficult, but flupropanate has been used to control African lovegrass (Eragrostis curvula) and serrated tussock (Nassella trichotoma).

Pasture weeds are best sprayed usually either in autumn or spring when growth is rapid and annual weeds are small and more susceptible to chemical control. Once plants begin to elongate, in late spring or summer, they become much more difficult to control by chemical means. The choice of herbicide and rate depends largely on the:

Selectivity can be achieved using non-selective herbicides if the desirable perennial grasses are in a dormant state and the weeds are still actively growing, or when there is a height difference that can be exploited using some type of blanket or wick wiper. Woody shrub weeds can be selectively controlled by spot spraying or stem application.

Spray-topping (or pasture-topping), the application of low rates of herbicide to weeds prior to or in early flowering can substantially reduce the number of viable seeds set by plants and hence the soil seed bank for future years. If spraying is correctly timed (e.g. green bud to early flowering stage in saffron thistle) viable seed production can be reduced by 90-100%. As the majority of seeds of most annual grass weeds are only viable for one or two years, spray-topping with glyphosate or paraquat in spring during flowering can be very effective in reducing seed carry-over to the following year.

However, the sensitivity of spray-topping to time of application reduces its versatility, especially when more than one species is present. Hence, spray-topping may need to be repeated several times where weeds are maturing at different times. Even after spray-topping dense thistle stands, grazing capacity can be reduced for some time because of the remaining standing plants.

Spray-grazing on the other hand, is the application of sub-lethal doses of herbicides such as 2,4-D, MCPA, or glyphosate, to weeds to increase their sugar content and palatability, thereby encouraging livestock to graze them in preference to other pasture plants. Spray-grazing needs careful application for good results. The herbicide should be applied to actively growing weeds that are not yet elongating. Stock need to be introduced after 7 to 10 days and the area heavily grazed until weed populations are reduced, but before pasture begins to suffer.

Correctly applied, spray-grazing can produce outstanding results with some weed species. The following reading discusses the place of strategic herbicide use to maintain and improve pasture composition.

icon
Reading

‘Productivity requires long-term focus’ by I Logan1995, Pasture plus; the complete guide to pastures, Kondinin Group, Belmont, Western Australia, pp. 301-302.
Physical control

There are three commonly-used methods of physical control and they include:

Method Details
Slashing Effectiveness on pasture weeds is variable and will depend on the:
  • weed species concerned
  • timing
  • number of passes over the pasture
  • ability of the weeds to re-shoot
It can be used to reduce:
  • flowering and seeding
  • the underground storage reserves of some weeds
Slashing:
  • may be made difficult because of the sequential flowering of different species.
  • is usually only effective in preventing seeding of thistles if done either immediately prior to, or when plants are just starting to flower. Otherwise flowering may only be delayed. If left too late in the flowering process, viable seeds may still develop in capitula following cutting.
  • is of benefit in dense weed stands as grazing stock can gain better access to pastures.
  • can induce annual plants to survive through to the next year and these become even more resistant to chemical control. Slashing of weeds like bracken (Pteridium esculentum) which has large underground reserves is ineffective.
Ploughing Only used in conjunction with the sowing of a new pasture and in that case, disc ploughing has been found to better control perennial weeds than tined implements by cutting below the plant crown.
Hand-weeding and chipping This method is still practised for some weeds of pastures, particularly where weed numbers are relatively small and on small-sized holdings, for example, fireweed (Senecio madagascariensis) in coastal pastures of NSW.
Burning

Burning is potentially very useful for controlling woody weeds (shrubs and trees) and some herbaceous species, particularly in the semi-arid regions of Australia where techniques other than grazing are unlikely to be economical. The degree of reduction of woody weeds can be related to the proportion of resprouting species present and the level of recruitment of hard-seeded species. Because of the associated risks, widespread burning has not been readily adopted in Australia.

Biological control

Classical biological control is a viable additional method to those already mentioned for pasture weeds. Graziers should contact their state Departments of Agriculture to find out if biological control agents have been distributed in their region. Most classical biological control agents (insects or diseases introduced from overseas (host area) and self-sustaining) have been aimed at broadleaf weeds of pastures.

Grasses have up until now not received much attention because most biological control agents which attack weedy grasses would not be sufficiently selective too avoid damaging the bulk of desirable perennial grasses. Biological control has not been used commonly in annual crops because there are no host plants available for much of the year and insect lifecycles are likely to be disrupted by cultivation and other cropping operations. Such constraints are not so important in perennial-based pastures.

icon
Reading

‘Overview and use of biological control in pasture situations’ by DA McLaren 1993, Plant Protection Quarterly, vol. 8, pp. 159-162.

Integrated management of pasture weeds

As already stated, the integration of weed management strategies will need to be made on a case by case basis and so no prescriptions can be given here. Nevertheless, it is evident from the foregoing discussion and readings that certain techniques do combine well to improve the efficiency of weed management, such as competitive pastures, herbicides, and grazing. Of foremost importance is:

Other techniques will fit well with this strategy but will only be effective in the long-term where such a pasture is present. The aim therefore is to create pastures that are less sensitive to invasion by weeds and in which weeds do not persist. The following reading suggests ways to go about this. It will be clear from the reading that a focus on weed management as a separate issue in pastures leads to the ready invasion of pastures by weeds.

icon
Reading

‘Weed-proofing pastures: how can we go about it?’ by DR Kemp, WMcG King, DL Michalk and Y Alemseged, Twelfth Australian Weeds Conference Papers and Proceedings, pp. 138-143.

Most other weed management techniques, perhaps with the exception of burning and ploughing, can be added to these two with little conflict of interest. However, some problems do arise when attempting to combine biological control with practices such as:

where these practices interfere with the survival and lifecycles of the biological control agents. Research is currently underway to try and marry these techniques in a better way (see Smyth and Sheppard 1996).

Summary

Weeds of cropping can contribute considerably to livestock production in pastures if they are palatable. Nevertheless, other pasture species may provide better overall nutrition. Weeds can be conveniently classified according to the type of impact that they have on pastures and the livestock grazing them. For any weed management program to be effective, it must be based on the provision of a dense, vigorous, and competitive pasture.

This can be achieved through integration of a range of tactics.

Biological control shows much promise for weed management; however, most agents to date have been aimed at broadleaf weeds. Integration of weed management methods needs to be considered on a case by case basis.

References and further reading

Dowling, PM, Michalk, DL & Sindel, BM 2000, ‘Weed management in pasture systems’, in BM Sindel (ed.) Australian Weed Management Systems, RG and FJ Richardson, Melbourne, pp. 307-328.

Kemp, DR & Michalk, DL (eds) 1993, Pasture management. Technology for the 21st Century, CSIRO Information Services, Melbourne, Australia.

Michael, PW 1970, ‘Weeds of grasslands’, in RM Moore (ed), Australian grasslands, Australian University Press, Canberra, pp. 349-360.

Popay, I & Field, R 1992, ‘Grazing animals as biological control agents’, Proceedings of the 1st International Weed Control Congress, pp. 273-277.

Sheppard, AW, Smyth, MJ & Swirepik, A 2001, ‘The impact of a root-crown weevil and pasture competition on the winter annual Echium plantagineum’, Journal of Applied Ecology, vol. 38, pp. 291-300.

Smyth, MJ & Sheppard, AW 1996, ‘The effect of simulated spray-grazing on the Paterson’s curse crown weevil, Mogulones larvatus Schultze’, Proceedings of the 11th Australian Weeds Conference, pp. 291-293.

Wheeler, JL, Pearson, CJ & Robards, GE, (eds) 1987, Temperate pastures: their production, use and management, Australian Wool Corporation/CSIRO, Melbourne, Australia.

icon

Self assessment

Do you know:

  • the IWM practices in a pasture situation
  • the impact of different weed management methods on pastures
  • the principles of IWM in pastures