The success of modern farmers has become the misery of traders at the world's biggest agriculture merchants.

Profits are shrinking as years of bumper harvests sap volatility and trading opportunities, forcing companies to tighten their belts. Trading houses such as Cofco International Ltd. have reorganized their agriculture units this year, leading to the departure of several business heads. Some are leaving to test their luck elsewhere, while others have thrown in the towel and retired.

"The market has changed," said Miroslaw Marciniak, a consultant at InfoGrain in Warsaw and a former grains trader. "The results aren't what they used to be. That's prompting firms to look to cut costs and make savings. There's pressure and not everyone can stand it."

At least 40 senior managers and executives in agriculture left their positions at trading houses such as Archer-Daniels-Midland Co. and Louis Dreyfus Co. this year, based on a tally of news stories published by Bloomberg. While in many cases the positions have been refilled, it represents an unprecedented changing of the guard.

Agriculture accounts for almost half of all the job changes in commodities this year, compared with other businesses like oil, metals and gas and power, according to recruiter Commodity Appointments Ltd.