OTTAWA - The Senate has voted in favour of changing a Conservative crime bill that would have imposed a mandatory six-month minimum sentence on people convicted of growing as few as five pot plants.

By a vote of 49-44, the Liberal-dominated upper chamber agreed to amend Bill C-15 to give judges greater discretion in sentencing convictions for growing between five and 200 plants.

The amendments also provide aboriginal convicts with an exemption from the minimum sentences, require judges to explain why they are not imposing the suggested minimums, and add in a cost-benefit review of how the legislation is working after five years.

However, mandatory minimum sentences of nine months still remain in place if there are aggravating factors to the marijuana grow-op, such as endangering the health of a child or creating a public safety hazard.

The Conservative government has sharply criticized the Senate for tinkering with legislation that has already been passed by the elected House of Commons.

Justice Minister Rob Nicholson called the changes "an early Christmas present from the Liberals to the people in the grow-op business -- it sends out the complete wrong message in my opinion."

However Nicholson did not rule out accepting the Senate amendments.

He would only say that the changes mean the legislative debate will drag into the new year before the bill eventually gets passed into law.