The protracted courtship between fertilizer makers continued Monday, growing ever more hostile.

CF Industries

(CF) - Get Report

once again rejected a buyout bid from rival

Agrium

(AGU)

Friday night, which had sweetened its offer to $40 a share in cash from $35, plus one Agrium share. As of Monday afternoon, the deal was worth about $91 a share.

Agrium's president and chief executive, Mike Wilson, issued a statement Monday morning in response to the spurning. "The fact that CF will not engage with us reinforces our belief that, left on its own, CF will not act in the best interests of its stockholders," Wilson said. "It's now up to CF stockholders to protect their own interests by demanding that CF come to the table to negotiate a mutually beneficial merger agreement with Agrium."

He noted that the most-recent offer represents a 59% premium to CF's share price the day before Agrium announced its first bid, on Feb. 24. Agrium has already tried to persuade CF shareholders to withhold votes for board candidates at CF's annual meeting last month.

Eager to remain independent, CF has made its own play to acquire a rival, nitrogen-fertilizer maker

Terra Industries

(TRA)

, for $30.50 a share. CF has accused Agrium of trying to sabotage its effort to combine with Terra and create a more formidable competitor. But Terra, for its part, has also been eager to remain independent, and has consistently rejected CF's overtures.

Said Agrium's Wilson in the Monday statement, "The inescapable fact is that Agrium's offer ... delivers more value to CF stockholders than any of the strategic alternatives articulated by CF, including remaining independent or acquiring Terra Industries. CF's unsupported assertions to the contrary do not change this reality."

CF has called Agrium's various bids "grossly inadequate." In rejecting the latest offer, CF said in a statement Friday that its board of directors, after considering the proposal, "concluded that the offer continues to substantially undervalue the company and is not in the best interests of CF Industries and its stockholders."

CF went on to argue that the sweetened bid represents a premium of "only" 58 cents a share over CF's cash position in the just-ended first quarter.

Shares of CF were trading Monday at $81.87, up $2.12, or 2.7%, while Agrium shares had jumped $2.83, or 6%, to $51.01, on heavier-than-average volume. Terra shares, meanwhile, were at $29.02, up 75 cents.

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