St. Louis, Oct. 11 (Bloomberg) -- Monsanto Co., the biggest developer of genetically engineered crops, reduced earnings forecasts as U.S. demand slowed for its Roundup weedkiller and sales of seeds dropped in Latin America.

Full-year profit, excluding some costs, will be about $1.15 to $1.23 a share, compared with a previous estimate of $1.50, Monsanto said in a statement. Losses from unpaid bills in Argentina will take 38 cents a share off net income and reorganization costs will slice off another 32 cents, it said.

Dry weather in the U.S. Midwest in the summer months reduced weed growth, hurting sales of Roundup, the company said. The St. Louis-based company halted seed and herbicide sales on credit in Argentina earlier this year after farmers failed to pay bills as the country's economy slumped.

Shares of Monsanto, which was spun off from Pharmacia Corp. in August, fell 35 cents to $15.20 as of 10:10 a.m. in New York Stock Exchange composite trading.

Monsanto had a third-quarter loss, excluding certain costs, of 54 cents to 59 cents a share, below the year-earlier period's loss of 15 cents a share, the company said.

Roundup sales in the U.S. this year will probably be between 36 million and 37 million gallons, down from the company's previous forecast of 41 million, spokeswoman Lori Fisher said.

Monsanto wrote down the value of acquisitions related to corn and wheat units by $1.8 billion in the first quarter. As a result, the company expects to have a net loss for the year of about $6.10 a share. That forecast will be updated when the company reports final third-quarter results Oct. 30, Fisher said.