T-Jaye (TEE'-jay) Heller, who is 33, made the plea Monday at the opening of his trial in Medicine Hat.
The child suffered multiple blood force trauma to the head, external and internal bruising and other internal injuries in June 2011.
Heller was originally charged with first-degree murder, sexual assault, interference with a dead body and other charges.
The other charges were dropped.
The second-degree murder charge carries an automatic life sentence, but the judge must determine how long Heller must serve before he can apply for parole.
"Given the circumstances of this case and the circumstances of this offender, my position is that 20 to 25 years before he can apply to parole is appropriate," said Crown prosecutor Brandy Shaw.
Defence lawyer Lyndon Heidinger has indicated he'll argue for 15 to 17 years.
Heller is to return to court May 30.
(CJCY, The Canadian Press)
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