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Rethink required for weed control – a serious threat

Posted by Bestprac on Oct 01 2012

Mick Alexander

Pre 1990, our farming systems included a program of ploughing, scarifying and cultivating multiple times before planting a crop. This methodology had its drawback including loss of carbon, often serious erosion and loss of topsoil during high rainfall events.

Then we discovered minimum tillage in the 1980’s, which meant we could use a chemical application to control weeds sometimes and maybe work our country sometimes. The season dictated how much chemical and how much tillage was undertaken. I remember in the 1980’s, the herbicide of choice was glyphosate, as it controlled all weeds in one treatment and it did a wonderful job. The cost of using chemicals in the 1980’s was high, but it was only used when necessary. Planting equipment was the biggest drawback, in the early days, as most of us had combines that could not handle large trash loads.

In the past 20 years, more producers have adopted zero-till farming, which generally means no tillage except for the planting process. The changes in planter design towards disc openers and narrow edge on tyne, parallelogram, press wheel configurations and air seeders has assisted in the uptake of the new paradigm. This move to zero-till farming is accepted as one of the best-practice farming methods and is promoted world-wide, as it has saved millions of tonnes of top-soil and protected our landscape. The drawback was that this system has relied on extremely large amounts of chemical to ensure weed free fallows; and the cost of glyphosate to control weeds has reduced significantly compared to the cost in the 1980’s.

And now, three decades later, this exciting farming system has brought with it a number of major problems which no-one (including the chemical companies) has the answers to. Specific herbicides such as glyphosate and 2,4-D have probably been sprayed too often, with reducing rates allowing some weeds to survive the first treatment.

“What doesn’t kill me, makes me stronger”

That is, during the past decades, specific weeds have developed a resistance to the herbicide group and are able to survive and breed even stronger plants in following seasons. Today, some weeds simply will not die from herbicide applications. A new series of studies released by Weed Science in the USA finds at least 21 weed species have become resistant to glyphosate and a growing number survive multiple herbicides, so-called "super-weeds." The same selection mechanism that created bacteria resistant to multiple antibiotics is leading to the rapid evolution of plants that survive modern herbicides.

In Queensland, our no-till/zero-till farming systems, that are reliant on chemical applications, are under threat from resistant weed populations, including ryegrass, feathertop Rhodes grass, windmill grass and barnyard grass. Until recently, there were 347 documented glyphosate-resistant populations of annual ryegrass, 58 of awnless barnyard grass, 49 of fleabane, 10 of windmill grass, 3 of liverseed grass and 1 of great brome. These resistant weeds are showing up in farming systems, horticulture, irrigated summer and winter crops, driveways, fencelines, gardens, around buildings, railways and roadsides.

Most of these resistant weed populations have occurred in situations where there has been intensive use of glyphosate, often over 10 years or more, few or no other effective herbicides used and few other weed control practices used. The following are the main risk factors leading to glyphosate resistance:

  • Intensive use of glyphosate - every year or multiple times a year for 10 years or more
  • Heavy reliance on glyphosate for weed control
  • No other weed controls

It has become such a part of the landscape, that best-practice farming systems in the cropping regions are heavily dependent on glyphosate for weed control in their chemical fallows. Farming practices under the vines in vineyards across Australia are also heavily dependent on glyphosate for weed control. Therefore, it is highly likely that if you are using glyphosate and no other chemical, you may have unconfirmed populations of resistant summer and winter weeds.

Many farmers are very concerned about their situations and ask about possible options to replace the use of glyphosate, as it is so easy to spray today and relatively low cost. Even the expert scientists, in the large chemical companies, are struggling with the problems that have been created. At this stage, there are no solutions to replace the low cost glyphosate in the production system. The more expensive and time consuming option is to use a number of herbicides to control specific weeds.

However, if you are an organic grower, an exciting new Australian based product, called Bio-Weed, has been trialled at several field days earlier in 2012. Bio-Weed has a different mode of action to glyphosate based products and is approved as an organic product to control many broadleaf and grass plants. It will reduce the photosynthetic ability of the plant and so kill the above ground portion, but not the roots. This may be an option for many growers.

In recent press releases, specialists have been recommending that anyone wanting to control weeds use a series of integrated weed management methodologies to reduce the incidence of super weeds. They should, where possible:

  • Rotate between chemical groups (modes of action)
  • Slash/mulch weeds
  • Graze weeds
  • Ploughing/cultivating
  • Rotate crops

Mick Alexander is an agricultural consultant and educator with Grazing BestPrac in Yeppoon. Contact Mick on 49383919

Last changed: Oct 02 2012



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