Effect of harvesting interval and defoliation on yield and chemical composition of leaves, stems and tubers of sweet potato (Ipomoea batatas L. (Lam.)) plant parts

Fifteen sweet potato varieties were grown for 120 days, in two planting seasons in Hue, Vietnam. Leaf, stem and tuber dry matter (DM) yields were measured and chemically analysed. In Experiments 1 and 2, sweet potato vines were harvested at intervals of 15, 20 or 30 days. In Experiment 3, 25, 50, 75 or 100% of the total number of above ground branches of sweet potato vines were harvested. At harvest, the petioles were separated from the leaves and were recorded together with the stems. The leaf and stem fractions were weighed separately. Sweet potato tubers were harvested at 120 days. Chemical composition of the leaves and stems was determined from the harvest taken 60 days after planting. The leaf, stem and tuber DM yields and leaf to stem proportion of sweet potato vines varied between varieties and between planting seasons. The local variety, T2(20), AB95002, 69-1, KL7 and KB1 gave a high leaf DM yield in the cool-wet season, while the varieties AB95002, T2(20), 440.176 and K51 gave the highest leaf DM yield in the hot-dry season. In the cool-wet season, the tuber DM yield was higher in the local variety, TQ1, K51 and KB1 than in the other varieties. In the hot-dry season all varieties produced a very low tuber DM yield. The 20 day harvesting interval resulted in the highest leaf and stem DM yields. Harvesting at 120 days resulted in a significantly lower leaf yield, but higher tuber yield. Leaf and stem DM yields were highest when harvesting 50% of the total number of branches, and tuber DM production was the highest when harvesting 25 and 50% of the branches. Sweet potato leaves had a crude protein (CP) content of 25.5-29.8% in DM, which was markedly higher than in the stems. The sum of essential amino acids of leaves accounted for an average of 13.7-14.4% of DM, but the content of lysine was limited, with average values of 4.3 g 16 g-1N. The sum of amino acids constituted from 85.9 to 91.9% of the CP. ADF and NDF contents of the leaves on a DM basis varied from 13.6 to 19.9% and from 23.5 to 29.8%, respectively.