Smallholder irrigation schemes in semi-arid Zimbabwe are vital for food security but increasingly constrained by declining soil fertility, inefficient water use, and climate variability. Closing nutrient loops through manure recycling offers a potential pathway to improve system sustainability. This study evaluated the effects of manure recycling on maize grain yield and water productivity (WP) under supplemental irrigation at Silalatshani, Zimbabwe, using APSIM simulations across historical and mid-century climates. Simulations included manure rates (0-10 t ha(-1)), nitrogen levels (20-150 kg N ha(-1)), irrigation regimes, and climate scenarios, with improved irrigation guided by smart water management (SWM) tools. Grain yield was significantly influenced by nitrogen, manure, and irrigation (p < 0.05), with strong climate & times; nitrogen & times; manure interactions. Yields declined under increasing climate stress, predominantly under unimproved irrigation. Manure improved yield and water productivity, with greatest benefits at 2.5-5 t ha(-1), beyond which responses diminished. Water productivity gains were modest and constrained by water availability. Economic benefits were limited to moderate manure rates with adequate nitrogen. Combining moderate manure application with appropriate nitrogen fertilisation and improved water management, supported by SWM tools, provides a reliable and economically viable pathway for enhancing productivity, profitability, and resilience under climate variability.