Agriculture biotechnology giant Monsanto Co. is slashing hundreds of jobs for a third time in 14 months as it shrinks its Roundup weed killer business in response to declining sales.

The company, based in Creve Coeur, said in a regulatory filing Tuesday that it is eliminating as many as 700 jobs, including about 150 in the St. Louis area and taking other steps to reduce costs. The steps are expected to save Monsanto as much as $90 million a year beginning in fiscal 2012.

The job cuts come on top of 1,800 positions eliminated last summer, or 8 percent of its work force — part of a sweeping plan to reorganize its herbicide business. The downsizing is in response to a flood of Chinese-made herbicide that has flooded the U.S. during the past 18 months, offering farmers a cheaper alternative to Monsanto's higher-priced product.

"It is a highly competitive marketplace," spokeswoman Kelli Power said in an interview. "We continue to look at our business to determine how to best align our resources to meet our business goals and serve our customers."

Tuesday's announcement continues a swift and dramatic decline for Monsanto's Roundup business, one that forced CEO Hugh Grant to retreat from a pledge to investors to double the company's 2007 gross profit by 2012.

Investors have taken notice. The Roundup struggles and a slower-than-expected launching of new seed products have contributed to a 36-percent decline in Monsanto shares this year. That includes a drop of $3.25, or 5.8 percent, on Tuesday — the steepest single-day decline this year.

Executives have tried hard to stabilize the Roundup business while shifting attention to the company's more profitable seed lineup.

Beside eliminating jobs, Monsanto slashed prices for Roundup to $10 to $12 a gallon from more than $20 and set aside at least $100 million in incentives to win back farmers who may have switched to less expensive brands.

"Monsanto is changing (its) whole approach to this business," said Edward Jones analyst Jeff Windau. "Roundup has always been a good premium product. But given that you have a lot of oversupply and you can't demand the premium price, they're going more toward that generic-type solution."

It wasn't long ago that Roundup was a cash-generating machine that helped fuel the company's rapid growth.

Monsanto developed the popular weed killer in the 1970s when it was solely a chemicals maker. Roundup helped drive the scale-up of industrial agriculture, allowing farmers to save time by spraying entire fields without damaging crops.

The company was the exclusive maker of glyphosate-based herbicide for more than 20 years and later developed seeds that are genetically engineered to resist its effects.

Today, the Roundup-resistance trait is at the heart of Monsanto's seed business. But patents on the weed killer expired a decade ago, allowing rivals to manufacture cheaper generic products.

That wasn't too much of a concern until recently. In fact, Monsanto launched the $200 million expansion of a glyphosate manufacturing plant in Luling, La., in 2008 when the chemical was in short supply. The project was completed in March.

Then, millions of gallons of generic glyphosate-based herbicide entered the U.S. market from China.

Monsanto lowered its forecast range for gross profit for the Roundup business to $250 million to $300 million a year. Only a year ago, the company recorded $1.8 billion in gross profits from the sale of Roundup and other glyphosate-based herbicides.

The company said Tuesday's cost cuts will include closing some small offices and manufacturing operations and writing down the value of assets.

Specific details weren't provided.

The restructuring will result in $180 million of pretax costs for severance and related benefits, facility closures and asset impairments. Of that, $150 million will be realized in the fourth quarter.

Monsanto said earnings for the fiscal year that ended Tuesday will be in the range of $2.40 to $2.45 per share instead of $2.40 to $2.60 as previously forecast. The adjusted forecast includes a 22 cent-per-share impact from restructuring charges.

An additional $30 million in restructuring costs will be recorded in 2011.