Critical period of weed competition in potatoes (Solanum tuberosum L.) summer plantation.
Two field trials were conducted on clay loam at Giza in summer 1991-92 to determine the critical period of weed competition in potatoes cv. Alpha. In the 1st trial, potatoes were kept weed-free for 2, 4, 6 and 8 weeks after crop emergence (WAE), after which they were allowed to become weedy or were kept weed-free throughout the season; in the 2nd trial, weeds were allowed to grow with potatoes for similar periods after crop emergence. During both seasons the existing weed species were identified and their densities and DWs were determined at 50 d after emergence (DAE) and at harvest, and various potato traits were recorded at 70 DAE and at harvest. The dominant weed species were Urtica urens, Amaranthus viridis, Malva parviflora, Convolvulus arvensis and Portulaca oleracea. The total weed biomass recorded in 1991 and 1992 was 117.14 and 126.17 g DW/m2, resp. Weed presence during the early stages of potato growth exhibited a detrimental effect on FW/plant and tuber FW/plant at 70 DAE. Total DW and yield values reflected the effect of weed competition periods on potato growth; in 1991 the highest total yields were produced in plots kept weed-free for the whole season (12.47 t/feddan) and for 8 WAE (11.55 t), while in 1992 the highest total yields were obtained in plots kept weed-free for the whole season, for 8 WAE and for 6 WAE (9.03, 8.19 and 8.11 t, resp.). A full season of competition caused 39.7 and 40.4% reduction in tuber yield during 1991 and 1992, resp. It was concluded that, to avoid significant yield losses, the crop should be kept weed free for 6-8 WAE. [1 feddan = 0.42 ha]