Agrium has raised the value of its offer for US rival CF Industries above $4bn after failing in another attempt to persuade its target to enter takeover talks.

The Canadian fertilizer group added $5.00 a share, equivalent to $240m, to its cash-and-share offer for CF, the second time it has raised the bid.

The latest offer values CF shares at $85.20, 15% above the $73.81 at which the stock closed on Friday. The bid is 53% higher than CF's stock price before Agrium first revealed its intentions in February.

CF said it would "evaluate the revised proposal carefully in the context of CF Industries` strategic plans to create shareholder value".

CF is attempting its own hostile offer, for US Terra Industries, another US fertilizer group.

'No choice'

Mike Wilson, the Agrium chief executive, said the raised offer followed another failed attempt to get CF to the negotiating table.

"CF has repeatedly rebuffed our efforts to meet, leaving us no choice but to take our increased offer directly to CF stockholders," Mr Wilson said, urging CF investors to raise the pressure for talks.

Agrium's proposal offered "far superior value" for shareholders than CF's alternatives, including the Terra bid, he added.

Agrium also extended to June 15, from May 19, the deadline for CF investors to accept its offer. Holders of only 1.27% of CF shares had accepted the offer so far.

'Not totally surprising'

Brian MacArthur, an analyst at UBS, said the bid increase was "not totally surprising".

"Agrium has sufficient cash and facilities in place to cover the increase in cash consideration,"

Nonetheless, Agrium's move failed to impress investors. CF shares were 2.3% higher at $75.52 in lunchtime deals in New York, well below the $84.85 offer price implied by Agrium's shares at the time.

Agrium stock was 0.8% lower at $44.85.