SUMMARYThe effects of weed competition, harvest date and cultivar on sweet potato (Ipomoea batatas(L.) Lam.) yield and components of yield were investigated in three factorial experiments planted during the wet seasons of 1984 (Expt 1) and 1986 (Expts 2 and 3) in the tropical lowlands In Expts 1 and 2, the commencement of regular hand-weeding (every 14 days) was delayed for varying lengths of time following planting. In Expt 3, the duration of hand-weeding from planting onwards was varied. The main competing weed species following planting were Cyperus rotundus, C. brevifolius, Euphorbia geniculata and Portulaca oleracea. After canopy closure, E. hirta, Imperata cylindrica, Eleusine indica, Sida rhombifolia, Mollugo pentaphylla and Mimosa invisa were the predominant weeds to emerge and persist. Cultivars differed in their sensitivity to weed competition. In L431 both mean marketable root tuber weight and number of tubers/plant were severely reduced. However, in L44, tuber number was more sensitive than tuber weight: marketable tuber weight was significantly reduced only by prolonged weed competition (no weeding until 56 days after planting (DAP)). In Expts 1 and 2, delaying the commencement of weeding beyond 14 DAP significantly decreased vine weight, total yield and mean number of tubers/plant. In Expt 3, prolonging regular weeding until 14 DAP and longer significantly increased total and marketable tuber yield and mean number of tubers/plant.Effects of weed competition in the early part of the season during the first 28–42 DAP substantially reduced crop production in the later part of the growth cycle (after 110 DAP). This was probably due to a partial suppression of vine growth early on, which reduced tuberous root initiation and thus limited the number of root tubers that could develop in the later stages of crop growth. Hand-weeding during the period 28–42 DAP also appeared to have a detrimental effect on tuber initiation as a result of mechanical root disturbance at this sensitive period of root differentiation. There was also a trend towards lower yields when the crop was regularly hand-weeded after 56 DAP, probably due to disturbance of the vine canopy.The critical period for weed competition commenced at or before 14 DAP, possibly as early as 7 DAP, and did not continue beyond 56 DAP. In order to avoid weed competition and the detrimental mechanical effects of hand-weeding, an optimum hand-weeding programme for sweet potato in the tropical lowlands of will probably involve (i) keeping the crop weedfree for the first 14–21 DAP; (ii) no crop disturbance during the period of maximum tuberous root initiation (from c. 21–28 DAP until 42–56 DAP); (iii) clean weeding at c. 56 DAP; and (iv) minimal or no weeding after 56 DAP. Weeding during the period 7–14 DAP is particularly important.
Effects of Varieties, Growth Duration, and Topping for Occurrence of Skin Ridge on the Tuberous Root in Sweet Potato (Ipomoea batatas Lam.)
