Spraying the shoots at the base of the tree with Roundup could damage the entire tree. Roundup is a systemic herbicide. Even if you spray only the undesirable shoots, the herbicide will be absorbed by the needles and spread throughout the tree. In addition to dieback of the shoots, you may observe yellowing, browning, and disfiguration of other parts of the tree. A much better option is to prune off the unwanted shoots at ground level.