Time-resolved photoinduced reflectivity is measured for the itinerant antiferromagnet UNiGa5 (TN≈85K) from room temperature to 10K . The relaxation time tau shows a sharp increase at TN consistent with the opening of a spin-density-wave (SDW) gap. The temperature dependence of tau below TN is also consistent with the opening of a SDW gap leading to a quasiparticle recombination bottleneck as revealed by the Rothwarf-Taylor model. This contrasts with canonical heavy fermions such as CeCoIn5 where the recombination bottleneck arises from the hybridization gap.