Sweet potato tubers can be used in poultry diets as a substitute for cereals, usually as dried meal. The lower protein and energy content of sweet potatoes compared to cereals have consequences on feed formulation, particularly on the need of appropriate protein supplementation (Woolfe, 1992). This can explain some of the variability observed in experimental results. At higher inclusion levels, the powdery texture of ground sweet potato can cause a decrease in feed intake (Ravindran et al., 1996).
High values for starch digestibility (97.4%) have been recorded in older animals (Szylit et al., 1978) and it was found that the low digestibility of raw sweet potato starch in young birds (74%) increased with the age of the animal (Yoshida et al., 1962). The lower energy value of sweet potato compared to maize in young birds has been noted by several authors (Panigrahi et al., 1996; Yoshida et al., 1962).
Sweet potato meal can be successfully used as a substitute for maize in broiler diets, but in most cases the highest substitution levels decrease performance. The recommended inclusion level is usually 20%. For example 25% sweet potato meal plus 10% molasses could profitably replace maize in growing chick rations (Latif et al., 1975 cited by Devendra, 1988). However, up to 30-40% sweet potato meal in the diet did not alter performance in some experiments (Gerpacio et al., 1978; Agwunobi, 1999; Ravindran et al., 1996), though the general relationship between sweet potato level and performance is generally negative. In some cases, inclusion levels higher than 10% reduced performance (Ayuk et al., 2009; Rosenberg et al., 1952).
The effect of thermal treatments was found to be variable. In an experiment where raw starch was already fully digestible (97%), steam pelleting did not augment starch digestibility, feed intake and weight gain (Szylit et al., 1978). In young animals, increasing the drying temperature of the tubers from 40 to 80°C did not result in significant improvements in animal performance, which remained lower than those obtained with maize (Panigrahi et al., 1996). Other authors found thermal treatments to be beneficial. Starch digestibility increased at temperatures higher than 68°C (gelatinization point) (Morimoto et al., 1954). Pelleting had positive effects, especially for young birds (Kwack et al., 1975 cited by Woolfe, 1992). It has been suggested (Woolfe, 1992) that in some cases the improvement resulting from the thermal treatment of raw sweet potato tubers could be due to the reduction in trypsin inhibition activity, which is high in some cultivars (Ravindran et al., 1995).
Sweet potato has been used in layer diets, but a general trend of decreasing performance has been reported. Safe inclusion rates should be limited to between 10 and 15% sweet potato meal in the diet, with proper protein and vitamin A supplementation. Replacing 50% of maize with a mixture of sweet potato meal (21% of the diet), wheat bran and sweet potato leaf meal was found to be acceptable though it significantly reduced egg production by 5.6%. Total replacement of maize (42% of the diet) decreased performance (Ladokun et al., 2007). A similar significant decrease in egg production (-11.6%) was observed when substituting maize with sweet potato meal at 20% of the diet, whereas the decrease (-4.2%) was not significant at 10% (Lee et al., 1979 cited by Woolfe, 1992). 15% inclusion of peeled sun-dried sweet potato meal maintained egg production, while higher rates tended to reduce performance, though only significantly at rates higher than 45% (Agwunobi, 1993).