CHICAGO, Jan 23 (Reuters) - An investigation into accounting practices in Archer-Daniels-Midland's Nutrition segment could not come at a worse time for the company as sinking crop prices look set to erode profit for its core grain trading and processing businesses this year.

Before news of the accounting issues broke and sent ADM shares tumbling 24% on Monday, the biggest fall since 1929 according to the Center for Research in Security Prices, the company had been forecasting the Nutrition unit it has been expanding for much of the past decade would return to profit growth in 2024.

The recovery in the business segment that generated about 11% of profit for ADM in 2022 would have helped cushion the blow from thinning margins in soybean crushing and ethanol, and from lower crop prices as global supplies of corn and soy rise, analysts said.

"It is now uncertain whether Nutrition operating profits will return to (year-over-year) growth in 2024," said Arun Sundaram, senior equity analyst at CFRA Research.

"We expect the investigation and uncertain outlook to cast a shadow over ADM's shares, as the Nutrition segment was once the fastest growing and most profitable segment," he said.

CFRA cut its 12-month price target for ADM to $61 a share from $76 previously, one of several analysts that downgraded ADM share targets Monday.

ADM and its crop processing and trading rivals cashed in on historically wide soy crushing margins over the past two years due to strong demand for vegetable oil to make biofuel, and reduced soy product supplies from drought-hit Argentina. Those margins are now thinning due to expanded U.S. processing capacity and a projected crop rebound in Argentina.

Meanwhile, margins for producing ethanol biofuel, a cornerstone of ADM's portfolio, have narrowed and a global grain glut has curbed crop exports from the United States, home to the bulk of ADM's operations.

Rivals including Bunge have more of an export base in Argentina and Brazil.

LOWERED OUTLOOK

ADM on Sunday put its CFO Vikram Luthar on administrative leave and postponed the upcoming release of fourth-quarter results and its annual 10-K filing with the U.S. Securities and Exchange Commission (SEC).

It also cut its adjusted earnings forecast to $6.90 per share for 2023 from an "excess of $7 a share" view earlier and withdrew all of its forward-looking outlooks for Nutrition.

ADM has invested billions of dollars over the past decade in Nutrition, the smallest yet fastest growing of its three main business units, starting with its $3 billion acquisition of WILD Flavors in 2014. In that time, annual adjusted earnings per share swelled from $2 to $3 a share to a record $7.85 in 2022.

ADM executives frequently tout the segment as the future of the company, aiming to capitalize on healthier eating trends and rising consumer demand for natural ingredients and flavorings.

The unit also provided more earnings stability as company results were tied less directly to the highly cyclical commodities market.

It was unclear if two recent Nutrition unit acquisitions due to close early this year would be impacted. ADM announced the purchase of Revela Foods, a Wisconsin-based developer and manufacturer of dairy flavor ingredients, and UK-based flavor and ingredient firm FDL late last year. Analysts also struggled to gauge future returns for the Nutrition segment.

"If we can't rely on the financial statements, it's hard to judge the return that they are getting for all these acquisitions if there is going to be a massive restate of profits that affects multiple years," said Seth Goldstein, strategist with Morningstar.

(Reporting by Karl Plume in Chicago; Additional reporting by David Gaffen; Editing by Chris Reese)