You may have noticed that sweet potato vines will tolerate considerable shade. You may have wondered, “If I intercrop sweet potato with taller plants, how much will that reduce the productivity of the sweet potato?”
This question was addressed by scientists Ladokun, Ogunkeyede and Tewe at the University of Ibadan in Nigeria. (Tropical Root and Tuber Crops Bulletin, March 1997, Vol 9, #2, pp. 6-8). They intercropped sweet potato with corn (maize), with cassava, and with both corn and cassava.
Fresh weight of tuber for a local variety of sweet potato grown by itself was 4.24 tonnes/ha. This dropped 46% when intercropped with corn, 26% with cassava, and 77% with both corn and cassava. The comparable figures for an improved cultivar called TIS 80/733 yielding 1.43 tonnes/ha alone were decreases of 58%, 21% and 38%.
They also measured the effect of intercropping on weight of vines (which make good animal feed). The local variety gave 9.3 tonnes/ha alone and increased by 10% with corn, by 55% with cassava and decreased by 16% with both corn and cassava. Comparable figures for the improved cultivar, which yielded 22.4 tonnes/ha alone, were a decrease of 87% with corn, 59% with cassava and 80% with both corn and cassava.
What effect did the intercropping of sweet potato have on corn or cassava yields? The authors did not measure either, though they reference three studies which have shown that “maize reduces the yield of base crops grown with it while its own yield is hardly affected”.
The authors also measured a few indicators of nutritional value. Crude protein in the leaves was the same for both varieties under all conditions; likewise for the vines. “Interestingly, the crude fiber of vines is similar to that of leaves, which makes it a unique forage source…. The vines also contain very high amounts of [the essential amino acid] lysine, suggesting usefulness as a supplement to grain products.”
More details for those with special interest. In the study sweet potato cuttings were planted at a spacing of 100x50 cm, maize at 100x100 cm with 4 seed per hole thinned to 3 plants. The experiment included 10 treatment combinations replicated three times.