CF Industries has sweetened the terms of its offer for rival Terra Industries for a second time, and reaffirmed its opposition to its own suitor, Canada's Agrium.

The Illinois-based group raised to $30.50, subject to some constraints, the value of Terra shares under the all-share bid compared with the $27.50 proposed two weeks ago in the first revision to its offer.

The latest terms value Terra at more than $3.0bn, compared with the $2.1bn under CF Industries' initial approach in January.

"We continue to believe that a business combination between CF Industries and Terra is a compelling combination with a number of strategic benefits," Stephen Wilson, the CF Industries chairman and chief executive said in a letter to the Terra board.

"We are confident that Terra's stockholders will agree with us."

In a separate letter, Mr Wilson fleshed out CF Industries' opposition to Agrium's $3.5bn approach, which he dismissed as "grossly inadequate", "disadvantageous" to investors and "opportunistic".

"Agrium's chief executive has acknowledged publicly that Agrium is attempting to buy CF Industries at a low valuation multiple, at a low point in the fertilizer cycle and at a low point in the stock markets," Mr Wilson said in a letter to CF Industries shareholders.

CF Industries' strategy of focusing on higher-margin business had been "very successful", he said, contrasting it with Agrium's focus on bulk business.

"CF Industries' record demonstrates the success of its business strategy," Mr Wilson said, adding that CF Industries shares had nearly doubled between the company's flotation in 2005 and January 15, when it approached Terra. Agrium shares had risen by 34% in the same period.

CF Industries bid for Terra is conditional on a maximum exchange rate of 0.4539 CF Industries shares per Terra share and a minimum rate of 0.4129.