CF Industries Holdings Inc. said Friday it has begun its unsolicited $4.8 billion offer for fertilizer company Terra Industries Inc., which has already accepted a buyout offer from a Norwegian company.

Chemical maker CF, based in Deerfield, Ill., on Tuesday offered $37.15 in cash and nearly one-tenth of a share of common stock for each Terra share.

The exchange offer for all of Terra's outstanding shares will expire at 5 p.m. Eastern time on April 2.

CF's offer has a total value of about $47.46 per share based on Thursday's closing stock price, or about $4.8 billion. That's up from a total value of about $4.7 billion when the offer was first announced on Tuesday. CF's last offer, made in December, was valued at $4.6 billion.

CF made its latest bid for Terra after Terra agreed to be acquired by Norway's Yara for $4.1 billion, or about $41.10 per share.

Terra shares rose 85 cents to $45.65 in afternoon trading Friday.

Terra, based in Sioux City, Iowa, had repeatedly rejected CF's previous offers. CF had vowed to end its hostile bids, but went back on the offensive after Terra agreed to Yara's deal.

"Our offer is far superior to Yara's in value, timing and certainty of closing," CF President and CEO Stephen R. Wilson reiterated in a statement Friday.

Terra said Friday that it is advising its shareholders not to exchange their shares until the offer has been reviewed by its board. It said in a statement it won't comment further until the review is complete.