* Acknowledges spreading weed resistance problems

* Says will offer new weed-fighting product

* Critics say effort comes late and may not be enough

KANSAS CITY, Mo., May 27 (Reuters) - Monsanto Co MON.N, said on Thursday it will restructure its herbicide products in an effort to help combat the spreading environmental woes of herbicide-resistant weeds, also known as "super weeds," which many critics have blamed on the chemical giant.

“We need to get in front of this,” Monsanto chairman Hugh Grant said in a conference call with analysts.

Grant acknowledged that weed resistance was a problem for U.S. cotton farmers, and was emerging as a bigger problem for soybean farmers. But he said the issue was not yet a big problem for corn farmers.

Monsanto is the world’s leading seed company and purveyor of the popular Roundup herbicide.

The company’s development of glyphosate-tolerant biotech soybeans, corn and cotton and its push for farmers to use its glyphosate-based Roundup with those crops has accelerated herbicide usage and consequently weed resistance to an environmentally dangerous level, critics have charged.

Experts estimate glyphosate-resistant weeds have infested close to 11 million acres so far. More than 130 types of weeds have developed levels of herbicide resistance in more than 40 U.S. states, more resistant weeds than found in any other country, according to weed scientists.

“It is too bad it has taken this long for Monsanto to get serious about addressing this problem,” said Charles Benbrook, chief scientist at The Organic Center and former executive Director of the agriculture board of the National Academy of Sciences.

“If solid glyphosate-resistance management plans had been put in place three or four years ago, the worst of the problem could have been prevented,” he said.

Grant said Monsanto would create a new product offering that would combine Roundup with complementary chemicals for a new weed control regimen at a lower cost than farmers are paying today.

Monsanto officials said currently a southern U.S. cotton farmer is paying $40 to $60 an acre in weed control measures, up from $20 to $25 an acre a few years ago.

“There’s good news for farmers in all of this. We’re going to bring simplicity to weed management,” Grant said in a statement. “Weed resistance is real, but managing it doesn’t have to be complex.”

Weed resistance to glyphosate has been mounting across the United States in recent years as Monsanto’s genetically modified “Roundup Ready” corn, soybeans and other crops that withstand dousings of the herbicide have gained popularity with farmers.

The vast majority of U.S. acreage devoted to those crops is now planted with Roundup Ready varieties.

Monsanto introduced the genetically altered crops as efficiency enhancements for farmers.

But environmentalists and other critics claim with the Roundup Ready crops, herbicide usage has risen to such a level that weeds have developed resistance levels that make it difficult to kill them, which in turn leads farmers to boost use of other types of herbicides.

Last year, a report by the Organic Center (TOC), the Union for Concerned Scientists (UCS) and the Center for Food Safety (CFS) said that weed resistance was directly tied to the rapid increase of herbicide use, which grew by 383 million pounds from 1996 to 2008, with 46 percent of the total increase occurring in 2007 and 2008.

“The problem is already threatening the economic viability of the cotton industry in the southeast,” Benbrook said. “Farmers in the Midwest better really pay attention and deal with it if they want to escape further problems.”

Monsanto’s shares slid sharply Thursday as the company also said it was adjusting its profit outlook downward as it trims back its herbicide business overall.

Shares fell more than 8 percent to the lowest levels in more than three years after the company said it now expects fiscal-year 2010 earnings per share to be $2.40 to $2.60 on an ongoing basis, and $2.15 to $2.41 on an as-reported basis.

Monsanto shares are down more than 40 percent from a year ago. (Reporting by Carey Gillam, Edited by Rene Pastor)