Sweet potato vines are one of the most promising protein sources coming from tropical forages for pigs (Bui Huy Nhu Phuc, 2000). Thanks to its high crude protein content, high CP digestibility (above 65%) sweet potato foliage in fresh, dried or ensiled form can be used as a valuable protein and amino acids source in low fibre pigs diets in tropical areas (Le Van An et al., 2004; Rodriguez et al., 2003; Barrios et al., 2002).
In small-scale pig farms, sweet potato foliage could replace up to half the protein normally provided through conventional feedstuffs such as soybean meal or fishmeal (Preston, 2006). Sweet potato foliage can be fed to pigs without any negative effects on health (Renaudeau, personal communication). In Vietnam, sweet potato foliage is much appreciated by the poorest pig farmers because it can be fed year-round and its use results in lower production costs and higher net income (Ngo Huu Toan et al., 2007). In Cuba, sweet potato crop, including roots as energy source and vines as a protein source, can compete with maize in pig feed in Cuban pig production (Dominguez, 1992).
Fresh sweet potato vines
Growing and fattening pigs
Fresh sweet potato vines can be profitably included in growing and finishing pigs diets as they enhance basal diet palatability, overall DM intake, nutrient digestibility and animal performances (Chiv Phiny et al., 2010; Chhay Ty et al., 2007; Nedunzhiyan et al., 2000). They are generally offered ad libitum (Chiv Phiny et al., 2010; Chhay Ty et al., 2007). Fresh sweet potato foliage palatability is rather high and compares favourably with other tropical foliages such as mulberry leaves (Régnier et al., 2010 (unpublished); Chiv Phiny et al., 2010). The average daily fresh sweet potato foliage intake is about 3 kg/d (500 g DM/d) for a 50 kg pig, and 750 g/d for fresh chopped vines (Régnier et al., 2010 (unpublished); Nedunzhiyan et al., 2000).
Fresh sweet potato vines can be used as the sole protein supplement, combined with other foliage such as mulberry leaves or cassava leaves or associated to a protein-rich (20-23%) supplement (Chiv Phiny et al., 2010; Chhay Ty et al., 2007; Gonzalez et al., 2003). However, they appear to have a lower nutritive value than cassava leaves, stylo (Stylosanthes guianensis) and duckweed (Du Thanh Hang et al., 2009)
Sweet potato fresh foliage offered to weaned piglets can substitute 10 % of the cereal concentrate with satisfactory animal performance (weight gain, feed conversion, mortality and herd culling) (Göhl, 1982). However, because of its bulkiness, fresh sweet potato foliage cannot be included in large amounts in the diets of young pigs (Renaudeau, personal communication).
Fresh foliage replacing 25 and 50 % of soybean meal in a sweet potatoes and soybean meal diet resulted in a decreased of dry matter intake that was probably due to its bulkiness, but the feed conversion ratio was conserved at the 25% replacement level (Göhl, 1982).
Gilts and sows
Sweet potato leaves inclusion can be as high as 50% in the diets of gilts or pregnant sows, and up to 20% for lactating sows. They can also replace up to 50% soybean meal during pregnancy and lactation (Hoang Nghia Duyet et al., 2010; Hoang Nghia Duyet, 2003).
Preserved sweet potato foliage
Sweet potato foliage in fresh, dried or silage is a valuable ingredient for pig diets. Though dried and ensiled sweet potato foliage have slightly lower lysine digestibility, the three products have similar palatability and overall nutritive values (Le Van An et al., 2004).
Many types of silages can be done with sweet potato foliage: sweet potato silage can be prepared with leaves alone or in mixtures: leaves and lysine, sweet potato leaves and cassava leaves, sweet potato leaves and sweet potato roots (Nguyen Thi Hoa Ly et al., 2010; Le Van An et al., 2005). Ensiled cassava leaves and sweet potato vine can replace more than 70% of the protein from fish meal (or 35% of total diet CP), reducing feed costs without deleterious effects on growing pig performances or carcass quality. (Nguyen Thi Hoa Ly et al., 2010). 30% inclusion rate for ensiled sweet potato foliage should not significantly affect growth performance in growing pigs (Le Van An et al., 2005). Higher inclusion rates (40 and 60% DM) would reduce growth by 16 and 30%, respectively (Hoang Huong Giang et al., 2004).
In post-weaning diets, sweet potato inclusion (meal or silage) should not exceed 10% and to 30-40 % inclusion is advisable in growing-finishing diets (Mora et al., 1992 cited by Le Van An et al., 2005).