The preservation of biomasses through ensiling has a long history, and its sustainability has many aspects. One that is rarely considered is that the seeds of a wide variety of plants can enter the ensiling process with the plant biomass. This concise review provides an overview of the probability of seed survival in various types of silage since the 1930s. All data extracted from the reviewed studies are made available in a repository. The key finding from the 90 years of research is that ensiling can reduce the viability of plant seeds, but the seeds of some plant species can survive ensiling. Thus, silage production is both (1) a potential tool to ensure the sustainable, i.e., weed-free, use of plant biomass in agricultural production, including animal production chains, and (2) a potential gateway for weed spread, especially with regard to the closed material loops in circular bioeconomy approaches. The search for seed-borne factors and ensiling conditions that promote seed survival or killing is still ongoing and should be the subject of future research.