Field experiments were conducted in 2014 through 2015 to investigate yield response trends of Sweetpotato and Cassava in a rotational cropping sequence under one-time application of five rates of composite manure (M) (0, 2, 4, 6 and 8 t ha-1) and four rates of NPK 15:15:15 fertilizer (F) (0, 200, 400 and 600 kg ha-1) on a degraded Ultisol at Umudike, Southeastern Nigeria (SEN). The experiment was a 5x4 factorial in a randomized complete block design (RCBD) with three replications. The composite manure was an equal mixture, by weight of chicken droppings, swine waste and cow dung. The treatments were applied to the 1st crop of sweetpotato and no further fertilization of the 2nd crop of cassava, except incorporation of after-harvest residue of the previous crop as surface mulch. The results showed that neither manure nor fertilizer application alone, except their combinations, significantly affected sweetpotato yield. Maximum sweetpotato yield (15.9 t ha-1), which was significantly higher than the control by 103.1%, was obtained on M2F2 (4 t ha-1 composite manure and 400 kg ha-1fertilizer) â€“treated plots. In contrast, both composite manure and NPK fertilization alone and in combinations had a very highly significant (P<0.001) residual influence on cassava root yield. NPK fertilization, significantly increased cassava yield over the control with increasing rate by 12.2, 20.3 and 21.4%, respectively, indicating that the highest fertilizer rate (600 kg ha-1), maximized cassava yield (48.28 t ha-1) but had similar yields with those obtained at lower rates. However, for sole composite manure application, cassava root yield followed almost the same trend as in NPK fertilization, except a drop at 2 t ha-1, while only rates beyond 4 t ha-1 gave significantly higher yield than the control. This indicates that the highest manure rate (8 t ha-1) also maximized cassava yield (50.29 t ha-1). Similarly, the highest composite manure and NPK fertilizer combination (8 t ha-1 M and 600 kg ha-1 F) maximized cassava root yield (57.69 t ha-1) and significantly out-yield the control by 89.9%, but induced similar yield responses to several other rates, especially M3F1 (6 t ha-1 M and 200 kg ha-1 F) and M3F2 (6t ha-1 M 400 kg ha-1 F) which equally produced exceptionally high cassava root (>50 t ha-1) of 51.45 and 54.18 t ha-1, respectively. The almost always occurrence of yield maxima at the highest application rates of manure and fertilizer alone or in combination, is attributable to the progressive improvement in soil condition and nutrient status towards higher doses of the inputs. It was concluded, on the basis of this study, that composite manure induced superior yield responses to NPK fertilizer and that though highest rates of the treatments, either alone or in combination, almost always maximized crop yields, lower application rate (s) with lower but comparable yields could be recommended, on economic consideration and practicability of technology, for sustainable sweetpotato-cassava production on the degraded SEN Ultisol at Umudike, under a double rotational cropping system. Key words: Sweetpotato- cassava, rotation system, integrated composite manure and fertilizer management strategy, degraded Ultisol, yield response trends.
Keywords: Sweetpotato- cassava, rotation system, integrated composite manure and fertilizer management strategy, degraded Ultisol, yield response trends.