By Carey Gillam

(Reuters) - Farmers in important crop-growing states should consider the environmentally unfriendly practice of deeply tilling fields to fight a growing problem with invasive "superweeds" that resist herbicides and choke crop yields, agricultural experts said this week.

Resistance to glyphosate, the main ingredient in widely used Roundup herbicide, has reached the point that row crop farmers in the Midwest are struggling to contain an array of weeds, agronomists say.

Extreme controls are needed to fight herbicide-resistant weeds in some areas, University of Missouri weed scientist Kevin Bradley said in a report to farmers. One particularly aggressive weed that can grow 1-2 inches a day is Palmer amaranth.

“Palmer amaranth is our No. 1 weed to watch in Missouri and the Midwest right now,” Bradley said.

He said farmers facing extreme out-of-control weeds should try deep tillage, a practice that removes weeds but can also lead to soil erosion and other environmental concerns.

Farmers moved away from heavy tillage of the land decades ago, and the more sustainable 'no-till' farming has become the norm. But it relies on heavy use of herbicides like glyphosate, and the U.S. Department of Agriculture says 70 million acres of U.S. farmland had glyphosate resistant weeds in 2013.

Palmer amaranth is also "exploding" across Kansas this year, according to Dallas Peterson, a weed specialist with Kansas State University. “We have had numerous calls about poor control of Palmer amaranth with glyphosate this year,” he said.

Weed resistance has grown as farmers have increased their use of glyphosate in conjunction with the Monsanto Co's introduction of an array of crops genetically altered to tolerate the herbicide.



(Reporting by Carey Gillam; Editing by Dan Grebler)