SAN FRANCISCO — The refusal of a convicted steroids dealer to testify at Barry Bonds’ perjury trial has already left the prosecution without key evidence, and the defense wants even more of it barred.

Just how much damage Greg Anderson’s decision to go to prison on contempt charges rather than testify against the baseball slugger will be determined at two pivotal court hearings over the next month.

In the first hearing set for today, court filings show Bonds’ attorney will seek to exclude from the March trial a massive trove of evidence prosecutors say helps prove the former San Francisco Giant knowingly took steroids as he pursued the single-season and career home run records.

U.S. District Court Judge Susan Illston has ruled that without Anderson testifying that he took the sample directly from Bonds’ to the Bay Area Laboratory Co-Operative lab, it’s impossible to prove that the test results belong to the baseball player. The 9th U.S. Circuit Court of Appeals upheld that decision in a 2-1 ruling dismissing prosecutors’ appeal.