My understanding of mulch is vegetation that is placed on top of garden soil to prevent weeds and retain moisture. Are you talking pine needs that have been raked up, fresh cut pine shavings or freshly made shavings from deadfall. If standing, then the shavings won't absorb the urine so it should be piled and dried for a year with the occasional turning. If from deadfall they are may be fine, you'll just have to see how well they work. Bedding is mainly about how well it absorbs urine so it doesn't run all over the whole stall.