Monsanto Co., the world’s biggest seed maker, said Wednesday its third-quarter profit fell 14 percent and disclosed plans to cut 900 jobs, or about 4 percent of its work force.

The layoffs and declining profits were a stark turnaround for Monsanto, which has reported improved earnings over the last few years by selling patented, genetically engineered crops worldwide.

Executives said they were caught off guard by the deteriorating performance of the herbicide Roundup, which was once a mainstay revenue source.

Monsanto expects profit from Roundup to drop by half this year.

Monsanto shares fell $3.14, or 4 percent, to $76.16 in trading yesterday.

Chief Executive Hugh Grant said Monsanto will consolidate Roundup operations into a new division, dedicating less money and fewer workers to an increasingly volatile herbicide market.

Monsanto expects to reap the vast majority of its future profits from genetically modified seeds, releasing one or two new varieties every year.

Company spokesman Lee Quarles said each new strain of engineered seeds generated $300 million annually.

Genetically altered seeds also offer another advantage. Unlike Roundup, the company still has patents on the genes and seeds, giving it a monopoly over their sale. The company plans to replace lines of seed with aging patents with new versions, staying ahead of the curve that allows generic brands to enter the market.

“In most of the world, we’ll be replacing old technology acres with new,” Grant said.

He said the company will decide which jobs will be trimmed before the end of the year. The restructuring is expected to cost between $350 million and $400 million, but will save as much as $180 million in future costs.

Revenue fell 11 percent to $3.16 billion from $3.54 billion a year ago. Declining revenue from herbicides like Roundup were only partially offset by increased revenue from seed and trait products sold through its U.S. soybean, cotton, corn and vegetable businesses.