I would advise against pruning these new stems as they will flower next year. This is a shrub rose that does not grow in quite the same way as a hybrid rose. I leave mine until the branches are less vigorous and then prune out 2 or 3 each year. This encourages new shoots to form from the base and keeps the flowering wood younger. This method of pruning mean that you never have those old knarled unproductive branches.