Sweet potato vines are one of the most promising protein sources coming from tropical forages for pigs (Bui Huy Nhu Phuc, 2000). Because of its high crude protein content, high CP digestibility (above 65%) and amino acid profile, sweet potato foliage in fresh, dried or ensiled form can be used in low fibre pig diets in tropical areas as a valuable source of protein and amino acids (Le Van An et al., 2004; Rodriguez et al., 2003; Barrios et al., 2002).
In small-scale pig farms, sweet potato foliage can 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, its use resulting in lower production costs and higher net income (Ngo Huu Toan et al., 2007). In Cuba, the sweet potato crop, including roots as energy source and vines as a protein source, can compete with maize in pig feed (Dominguez, 1992).
Fresh sweet potato vines
Growing and fattening pigs
Fresh sweet potato vines can be profitably included in growing and finishing pig 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 very high and compares favourably with other tropical foliages such as white mulberry leaves (Morus alba) (Régnier et al., 2010 (unpublished); Chiv Phiny et al., 2010).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foliage intake is about 3 kg/d (500 g DM/d) for a 50 kg pig, and 750 g/d for fresh chopped vines (Régnier et al., 2010 (unpublished); Nedunzhiyan et al., 2000).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 either as the sole protein supplement, or 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 appeared to have a lower nutritive value than cassava leaves, stylo (Stylosanthes guianensis) and duckweed (Du Thanh Hang et al., 2009).
Fresh sweet potato foliage offered to weaned piglets can replace 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 or 50% of soybean meal in a sweet potatoes and soybean meal diet resulted in a decreased dry matter intake that was probably due to the bulkiness of the diet, but the feed conversion ratio was unaltered at the 25% replacement level (Göhl, 1982).
Gilts and sows
Sweet potato leaves were included at up to 50% in the diets of gilts or pregnant sows, and up to 20% for lactating sows. They also replaced 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, either fresh, dried or ensiled, is a valuable ingredient for pig diets. Though dried and ensiled sweet potato foliage has 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 made with sweet potato foliage: sweet potato silage can be prepared with leaves alone or with lysine added, sweet potato leaves and cassava leaves, or 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 vines can replace more than 70% of the protein from fish meal (or 35% of total diet CP for growing pigs), thereby reducing feed costs without deleterious effects on growth or carcass quality (Nguyen Thi Hoa Ly et al., 2010). A 30% inclusion rate of 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) reduced 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 a 30-40% inclusion is advisable in growing-finishing diets (Mora et al., 1992 cited by Le Van An et al., 2005).