War On Weeds Wages On

Worldwide distribution of resistant weeds
  • 261 resistant weeds
  • 157 different weed species
    * 95 broadleaf weeds
    * 62 grassy weeds
  • 210,000 fields

The war on weeds for many of us goes back to when we become old enough to wrap our little fingers around a hoe handle. Long hours in the summer sun whacking on tough tall weeds explains my continuing obsession with these vermin of the plant world. Today I prefer to attack these pests with more sophisticated tools aided by machines and technology. More sophisticated tools, such as herbicides, come with increased responsibility to use and manage them wisely.

Choosing an herbicide program is often a combination of 'what weeds do I have,' and 'what is the least amount of money I can spend?' Those were my parameters as I managed the county right-of-way vegetation management program until the day I found herbicide resistant kochia in the spray zone. Three years of minimal rates of Oust, a SU-type herbicide (Group 2), caught up with me.

Similar use patterns in agricultural herbicides have similar results. Around the world, it is estimated there are 261 resistant weed biotypes made up of 157 weed species. Resistant weeds include 95 broadleaf weeds and 62 grassy weeds.

In Oregon, there are currently 13 different resistant weeds. It is estimated there are 1,830 sites and more than 127,800 acres infested with herbicide resistant weeds in Oregon. They infest cropland, grass seed, kentucky bluegrass, mint, and wheat.

The most widespread resistant weed of Oregon is Russian Thistle which infests an estimated 10001-100000 acres and is found primarily in wheat. Smallseed falseflax with resistance to Finesse (Group 2) is the most recently discovered new resistant weed in Oregon.

Glyphosate, a widely used product in dryland production areas, continues to see new areas of resistance developing around the world. During 2002, researchers in Australia studying a glyphosate resistant ryegrass found it to also be 25% resistant to Group 1 herbicides such Hoelon and Assure II. The big surprise was it was also 100% resistant to Group 2 herbicides such as Glean.

Table 1. Glyphosate resistant weeds, world status 1996-2001

Weed Country Sites, crop Acres Year
Italian Ryegrass Chile 2-5 sites, orchards 101-500 2001
Horseweed (marestail) Deleware, USA 2-5 sites, soybeans 51-100 2000
Goosegrass Malaysia 205 sites, orchards 101-500 1997
Rigid Ryegrass South Africa 11-50 sites, vineyards 501-1000 2001
Rigid Ryegrass California, USA 2-5 sites, almonds 11-50 1998
Rigid Ryegrass New South Wales, AU 11-50 sites, apples, wheat 1001-10000 1997
Rigid Ryegrass Vicoria, AU 2-5 sites, grain sorghum, wheat 11-50 1996

Source: http://www.weedscience.org/in.asp

The possibility of resistant weeds was largely ignored by myself until they were in my personal spray zone. It is difficult to believe the problems we see others facing might come to our area. Yet, it may happen in our dryland fields unless we become more proactive active in our weed campaigns.

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