A NATIONALS senator has taken aim at Peter Slipper over the former Liberal MP's support for the deregulation of wheat exports.

"Peter Slipper probably wouldn't be able to tell the difference between wheat and barley," John Williams told reporters.

Mr Slipper, who resigned from the Liberal National Party last year to become Speaker in the lower house only to quit the position in controversial circumstances earlier in October, backed Labor's wheat bill yesterday.

In doing so he took aim at his former colleagues, saying they had no political credibility at all. He was especially scathing of Opposition Leader Tony Abbott.

Senator Williams said it was "just a big spray at Tony Abbott".

West Australian Nationals MP Tony Crook crossed the floor to join the government on the issue, alongside independents Rob Oakeshott, Adam Bandt and Mr Slipper.

The government has secured a deal with the Greens to guarantee the bill passes the Senate.

Under the agreement, the Wheat Export Authority will be abolished only after a mandatory code of conduct, overseen by the Australian Competition and Consumer Commission, is in place.