How to handle weeds depends on many factors

How to handle weeds depends on many factors


Cropping
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A given weed can be a bad weed for some properties, but on another farm it is more practical to learn to live with it.

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Like most properties, we have many different weeds and if we tried to eliminate all of them we would have an impossible task and go broke in the process.

St John's wort is one of many common farm weeds, generally requiring a strategy for long-term control in many areas.

St John's wort is one of many common farm weeds, generally requiring a strategy for long-term control in many areas.

Some weeds we can comfortably live with, some we try very hard to eliminate, and some we endeavour to keep their impact on productivity at a low level.

How to handle a weed problem depends on many factors such as; is it possible to eliminate at a reasonable cost and effort? What is the real impact of that weed on productivity? And can one run a successful business at a given weed level?

A given weed can be a bad weed for some properties, but on another farm it is more practical to learn to live with it, at least for the time being when other pressing issues are at hand.

On our property (Central West NSW) our policy is to eliminate two weeds - blue heliotrope and St Barnaby's thistle. Both have in recent years been relatively new invasions and with early detection it is feasible to eliminate them. The same goes for any new weed, in our case galvanised burr, St John's wort and boxthorn.

Blue heliotrope, a deep-rooted perennial broadleaf weed that can smother other pasture plants and at times be poisonous to livestock, especially in dry times when little other herbage is available, is a potential big threat.

Spot treatment with granule herbicide Tebuthiuron, provided not within 50 metres of trees or gullies, can be very effective for small outbreaks with other herbicide options near trees or gullies.

Where heliotrope outbreaks are extensive, three years of cropping combined with fallow herbicide elimination and sowing down to summer active perennial grass pastures is a better long-term strategy. A strong summer growing perennial pasture can largely out-compete any lingering heliotrope seedlings emerging from residual seed.

St Barnaby's thistle is so entrenched on some farms it is very difficult to eliminate and a 'live with' strategy is chosen by many landholders. In a new invasion case, options to eliminate include regular property surveillance from the beginning of flowering (early spring onwards for many areas) and physical removal or herbicide spot treatment before seeds form.

Provided surveillance is over several years (weed has some "hard" seed) it can be successful. An in-pasture herbicide option, recommended to me by Mudgee agronomist Ed Blackburn, is "EcoPar" + MCPA amine for use in sub clover (little damage to clover). A bit costly ($30/ha) but effective.

A multitude of common nuisance summer weeds like Bathurst burr, other burrs, caltrops, dwarf marigold, spiny burr-grass, melons, various amaranths, khaki weed, datura, fleabane, and mintweed are best addressed by competitive summer growing pastures.

Paterson's curse requires an integrated control program involving introduced predators, herbicides and competitive pastures for best long term control.

Paterson's curse requires an integrated control program involving introduced predators, herbicides and competitive pastures for best long term control.

On arable country our approach is to crop for three years combined with clean summer fallows via herbicides after every rain event (often five treatments per summer). Then sow to tropical grass. Well established and managed these have prevented these weeds re-invading.

On non-arable land, encouraging strong perennial native grasses provides strongest competition against weeds. Sometimes this involves more subdivision to allow rest and recovery of valuable grasses and not grazing them too hard, which reduces their root reserves and ability to recover competitively. Weeds favour lack of competition.

Winter weeds like capeweed, thistles, Paterson's curse and barley grass also can be reduced by strong competition, in their case from winter/spring growing pastures. Again, in our case we use winter cropping with effective in-crop herbicides, plus effective fallowing to reduce the weed seed bank before sowing to perennial grass pastures with winter legumes. Starting with an effective weed kill in late winter/spring before the cropping phase is a good starting point.

There are many more troublesome weeds than detailed here (some native like rock fern) but control strategies mostly follow similar strategies as described above.

Early weed identification is a must, an understanding of its growth pattern (e.g. summer or winter grower, perennial or annual) and an assessment of likely weed ramifications (e.g. competitiveness, poisonous) are all part of good control strategies.

Next week: Wind and water erosion. Long-term strategies for upgraded control.

A given weed can be a bad weed for some properties, but on another farm it is more practical to learn to live with it, at least for the time being when other pressing issues may be at hand. - Bob Freebairn

  • Bob Freebairn is an agricultural consultant based at Coonabarabran. Email robert.freebairn@bigpond.com or contact (0428) 752 149.
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