Attorney General Nicola Roxon has hit back at comments by the head of one of Australia's largest agriculture companies comparing the Prime Minister to a ''non-productive old cow'', saying they are silly and stupid.

Australian Agricultural Company chief executive, David Farley, triggered the controversy after he made the remarks against Julia Gillard during a 30-minute lecture at an agriculture conference in Adelaide on Thursday.

David Farley. Credit:Glen Hunt

Mr Farley yesterday told The Saturday Age the comments were ''tongue in cheek'' and taken out of context.

The company has plans to build an abattoir near Darwin.