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Seven experiments were conducted to investigate the interaction of components of maize and sweetpotato intercropping under semi-arid conditions. Early (Katumani) and mid-late (A511) maize varieties were interplanted with sweetpotato simultaneously, 15 days, 30 days, and 45 days after emergence (DAE) of maize, using an additive type of intercropping. Grain yield of maize was not affected by simultaneously-interplanted sweetpotato, but later interplanting decreased grain yield particularly in very dry years. Intercropping reduced tuber yield by more than 50% regardless of planting time. Land equivalent ratio (LER) was consistently higher than 1.5 in sweetpotato-Katumani mixtures, because sweetpotato was less affected by Katumani compared to A511. About 80% of the joint LER was contributed by maize. There was no difference in water use between intercrops and sole crops, and water use was much lower than the available soil water, when compared with maize plants grown with suppressed inter-row evaporation. This study indicated that higher LER values, relatively stable yield over seasons, and a high amount of feed from sweetpotato vines would be obtained if early-maturing maize varieties were interplanted with shade-tolerant sweetpotato simultaneously in long-growing seasons.