Allelochemicals regulate the productivity of crop ecosystems. A screen house experiment was conducted (2016) at the National Crops Resources Research Institute, Namulonge, Uganda to determine the effects of NERICA 1 rice (an interspecific hybrid between Oryza sativa and O. glaberrima species), Cymbopogon nardus (C), Desmodium uncinatum (D), Mucuna pruriens (Mc) and LONGE 6H, Zea mays (Mz) on crop relative growth rates (RGR), nitrogen (N), phosphorus (P) and potassium (K) nutrient levels. One field study was conducted on a farm (2017) to establish the allelopathic interactive effects of RCDMcMz on Striga hermonthica (a parasitic weed), crop competition and productivity. Data was collected on striga, RMz growth, nutrient levels and yield. Potted rice reduced (30%-47%) in root length but Mz leaf length increased (31% & 15%) with Mc & D. RMc reduced (73%) striga and increased rice RGR (14-42 days). RD similarly reduced (67%) striga. RC increased (96%, 44% & 73%) rice NPK uptake, RGR (14-42 days), reduced (57%) striga and increased (1.56) the combined land equivalent ratio (CLER) and rice grain yields. RMz reduced (16%, 38% & 38%) rice NPK reserves, RGR (14-42 days), CLER (1.0), grain yields and increased (36%) striga. RD recorded higher CLER (1.56). MzMc reduced (15% & 27%) maize P uptake and NP uptake increased (42% & 9.3%) under MzC & MzD (73% & 29%). RMc increased rice RGR (14-42 days). Maize RGR (14-28 days) increased under MzD, MzMc & MzC and reduced (28-42 days) under MzD, RC & MzMc.. The ecosystems’ productivity was attributed to allelopathy.