Planting time and stand density effect on radiation interception and use efficiency of maize and sunn hemp intercropping in semi-arid South Africa

•Radiation use efficiency (RUE) of maize/sunn hemp intercropping evaluated by the row-crop radiation transmission (RCRT) model;.•Optimal planting time and stand density in sole versus intercrop systems;.•RUE at 1.38 and 1.37 g MJ−1 in intercropping at the V15 maize growth stage and at 32 plants m–2, respectively;.•Sunn hemp at an extinction coefficient (k) of 0.37.