Updated from 10:56 a.m EDT

CF Industries ( CF) rejected Agrium's ( AGU) $3.6 billion acquisition offer as "opportunistic" and said it will continue to pursue a tie-up with Terra Industries ( TRA) instead.

In a letter to Agrium's board of directors, CF Industries Chairman and CEO Stephen Wilson called the acquisition proposal "grossly inadequate" and not in the best interests of CF Industries stockholders. Last month, Agrium had offered $72 a share in cash and stock for CF Industries.

In a separate letter, Wilson told Terra's board that CF Industries intends to continue to pursue a business combination with the company. Earlier this month, Terra rejected an offer CF Industries made in January worth 0.4235 of a share for each share of Terra.

Wilson said CF Industries has tweaked its offer, saying it will now pay between 0.4129 and 0.4539 of a share using an exchange ratio that values each Terra share at $27.50.

"CF Industries also believes that Agrium's proposal is a transparent attempt to interfere with CF Industries' proposed business combination with Terra," Wilson wrote in his letter to Agrium.

Shares of CF Industries gained 1.6% to close at $61.56, a level that is roughly 20% above its trading price before Agrium made its original offer. Agrium shares rose 3.7% to $32.58, while Terra Industries gave back 3.6% to close at $25.17.

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