Agrium has launched another salvo aimed at securing a takeover of CF Industries, raising its offer by more than $150m and soliciting investors to put pressure on the board of its Illinois-based rival.

The Canadian fertilizer group added $3.30 in cash a share to its offer for its US-based rival. The addition takes the value of its cash-and-shares option to nearly $74 a CF share at closing prices on March 26. CF shares closed at $74.00 a share, giving the group a market capitalisation of $3.58bn.

The group has raised its cash-only option to $$75.30 a CF share, and increased the exchange ratio for CF shareholders wanting to be paid in Agrium shares to 1.8685 from 1.7866, a separate letter to investors showed.

And the group said it might sweeten the offer further if the CF board, which has declined invitations to enter talks, made a strong enough case for a higher bid.

"We do not understand why CF would summarily reject our offer without any attempt to negotiate a mutually beneficial transaction," Mike Wilson, the Agrium chief executive, said.

Call for inaction

Mr Wilson added in a letter to CF shareholders that their board's refusal to enter talks had left Agrium with "no choice but to take our offer directly to you".

"Withholding your vote for all of the nominees will send a clear message to the [CF] board that it should give proper consideration to our proposal," Mr Wilson said.

On average, companies included in the Standard & Poors 500 index could expect slightly less than 5% of votes at annual meeting votes to be withheld, he added. Agrium already holds a stake in CF Industries.