Coombs, Amy. May 20, 2012. Revenge of the Weeds. Plant pests are evolving to outsmart common herbicides, costing farmers crops and money.  the-scientist.com

Please read the above article, below are some excerpts:

Weeds are developing resistance to glyphosate—the most common herbicide on the market.  Glyphosate-resistant weeds now infest more than 17 million acres nationwide

• Alberta Canada: the kochia shrub has taken over at least 2,000 acres.
• Tennessee: 1 million acres of cropland have been taken over by Palmer amaranth.
• California: hairy fleabane has invated vineyards.

Many of these plants are also often able to resist other pesticides.

The chance that a single mutation will confer herbicide resistance is 1 in 100,000, making the likelihood of a double resistant mutant less than 1 in a trillion. Early industry-sponsored research suggested resistance to glyphosate was particularly unlikely because large mutations in the herbicide’s target, the EPSPS enzyme, would render it dysfunctional, killing the plant before it could reproduce.

“The claims made were naïve, and resistant weeds have indeed developed,” says David Mortensen, a weed scientist at Pennsylvania State University. “When a chemical is applied to such a wide area—to nearly all soybean and cotton, and a big percentage of corn—the selection pressure is too intense.”

Indeed, glyphosate use has increased dramatically, from the 4 million or so pounds that were applied to corn in 2000 to 65 million pounds last year, with use on cotton and soy fields also climbing.  To help farmers spray glyphosate directly over fields without harming crops, Monsanto released Roundup Ready soy and canola in 1996. Genetically engineered cotton and corn soon followed, and by 2001, the GE crops spanned millions of acres. This is when resistant weeds made their debut.

“Glyphosate has been around since the 1970s, but resistant weeds didn’t become a serious problem until the herbicide was packaged with genetically engineered crops,” says Mortensen.

Masters of survival

Weeds can’t grow wings or run away, so in order to survive harsh environments, they have developed diverse, generalized adaptation responses.

The rise of the resistant

If the situation wasn’t bad enough already, it appears to be snowballing. Weeds in nine different countries have independently developed resistance to multiple modes of action. Some stubborn survivors can now survive most of the chemicals used by farmers, and the infestations are spreading.