Standard & Poor's raised its ratings on AGCO Corp. and Viterra Inc. to investment grade ratings, doubling the number of companies that have been upgraded from speculative ratings this year, the agency said in a report.

The 4 companies have $2.34 billion in rated debt. AGCO, a U.S. agricultural equipment manufacturer, saw its ratings raised to BBB-minus from BB-plus as well.

Viterra, a Canadian grain handler, was raised last month to BBB-minus from BB-plus with a stable outlook.

"Despite recent positive developments, we currently expect more credit quality deterioration than improvement," analysts said.