In this paper, we propose a practical code design for a new relaying protocol, coined Dynamic Selective Decode-and-Forward (D-SDF), in half-duplex slow fading Multiple Access Multiple Relay Channel (MAMRC), defined as follows: (1) Multiple statistically independent sources communicate with a single destination with the help of multiple half-duplex relays; (2) The links between the different nodes are independent, and subject to slow fading and additive white Gaussian noise; (3) Sources and cooperating relays interfere (non-orthogonal transmission); (4) No channel state information at transmitter is available and no feedback is allowed. In D-SDF, each relay individually decides the end of its listening phase and the beginning of its cooperation phase, based on different source selection rules. Practical Joint Network Channel Coding (JNCC) schemes at the relays are proposed, as well as the corresponding Joint Network Channel Decoding (JNCD) algorithms at the relays and destination. The relays' and destination's receivers are designed to benefit from the signals of the previously activated relays to better decode the sources.