(Reuters) - (This September 22, 2015 story was refiled to correct the ninth paragraph to reflect that a Dec. 4 hearing, to be held in a room used by the House Agriculture Committee, is being organized by the National Coalition for Food and Agriculture Research. The briefing is aimed at congressional staff and members of Congress, but is not being held by the committee itself.)

Monsanto's Roundup weedkiller atomizers for sale at a garden shop near Paris, June 16, 2015. REUTERS/Charles Platiau

Concerns about the world’s most popular herbicide continue to mount, as U.S. agricultural experts note spreading weed resistance to glyphosate.As the key ingredient in Monsanto Co’s Roundup herbicide products as well as about 700 other products, glyphosate is widely used on farms as well as residential lawns.

But the chemical has come under increasing scrutiny in recent years in part because scientists and environmentalists have warned that weed resistance to glyphosate has become a significant problem that impacts crop production.

In the latest account of glyphosate-resistant weeds, U.S. weed scientist Dallas Peterson said this week that resistance is increasing rapidly in the key farming state of Kansas. The trend is a worrisome sign as weed resistance spreads from the southern U.S. into the Midwest and Plains farming states, he said.

Peterson, who is both a weed scientist at Kansas State University (KSU) and president of the Weed Science Society of America, said Kansas soybean farmers in particular are experiencing weed problems, particularly with a type known as Palmer amaranth. Wet weather along with the weed resistance contributed to the problem, he said.

“It’s really kind of exploded,” he said.

Farmers in other Midwestern states, including Missouri, Nebraska, and Illinois have reported mounting problems with weed resistance as well.

Weeds can choke off nutrients to crops hurting production, and raise costs for farmers who often use added chemicals or other means to combat the troublesome weeds.

Weed resistance across U.S. farmland is becoming such a significant problem that a briefing on the matter is being planned for Dec. 4 in a meeting room of the U.S. House of Representatives agriculture committee.

The U.S. Department of Agriculture said that reliance on glyphosate by many farmers is the primary factor for the problem. Fourteen glyphosate-resistance weed species have so far been documented in U.S. crop production areas, according to USDA.

The use of glyphosate by farmers surged after Monsanto introduced glyphosate-tolerant “Roundup Ready” soybeans and other crops in the mid-1990s.

Monsanto and DowAgroSciences, a unit of Dow Chemical, are bringing new herbicides to market, combining glyphosate with dicamba from Monsanto, and glyphosate with 2,4-D from Dow.

Peterson warned, however, that tests at KSU showed that these combinations still had trouble controlling Palmer amaranth weeds.

Both companies said research shows their new herbicide combinations are highly effective, but they also advise farmers to use multiple strategies to fight the troublesome weeds.