Sweet potato (Ipomoea batatas) forage

The vines are separated from the roots after harvest and provide a nutritive and relished green feed for ruminants (Nguyen Thi Tinh et al., 2006). It is a suitable protein supplement for animals receiving low quality forage.

Digestibility and degradability

Dry matter digestibility was found to be 70% (Ffoulkes et al., 1978a) and dry matter degradability was in the 40-50% range (Etela et al., 2008a). Rumen protein degradability of fresh sweet potato forage was quite high (about 70%), but the undegradable dietary protein was lower than that of other protein supplements of similar degradability (cottonseed meal and Gliricidia sepium). This resulted in less digestible true protein being available to the animal for metabolism after digestion and absorption (Kabi et al., 2005).

Dairy cows

Sweet potato forage can be fed to dairy cows as a supplement to forages such as Guinea grass (Megathyrsus maximus) or sorghum silage (Etela et al., 2009; Etela et al., 2008b; Ashiono et al., 2006). Such supplementation results in lower DM intake, but higher ME utilization for milk production (Etela et al., 2009; Etela et al., 2008b). While it has no negative effect on milk quality, it can only sustain low levels of milk production and does not compare favourably with dried brewers' grains or cassava forage as a supplement for milk production, though it could help in saving production costs for smallholders (Etela et al., 2009; Etela et al., 2008b; Lopez et al., 1998).

Sweet potato vines can efficiently supplement sorghum silage in dairy cow diets, and they have great potential for improving milk yield at up to 70% inclusion (Ashiono et al., 2006; Göhl, 1982).

Growing and fattening cattle

Heifers

Heifers fed Napier grass (Pennisetum purpureum) and supplemented with sweet potato forage had a lower DM intake than heifers fed on grass alone. Growth performance (500 g daily weight gain) was similar on grass and grass + sweet potato and lower than on grass + alfalfa (Kariuki et al., 1998).

Bulls

In Zebu bulls, supplementation with sweet potato forage of a diet based on sugarcane stalks increased voluntary intake and live weight gain, which was explained by the protein in the sweet potato forage acting as a by-pass nutrient. The physical nature of this forage also improved rumen function (Ffoulkes et al., 1978a). Addition of N urea in association with sweet potato forage supplementation resulted in further live weight gains. This was possibly due to the improvement in the rumen ecosystem which may encourage microbial growth and create a demand for fermentable N (Meyreles et al., 1979).

Calves

Sweet potato vines were found to be a high quality feed for calves due to their high yield, palatability and crude protein content. When offered ad libitum to calves fed Napier grass, they did not significantly affect the average daily gains of calves, and up to half of the milk was saved when sweet potato vines were fed as a milk replacer (Orodho et al., 1996). Calves fed Napier grass and supplemented only with sweet potato forage did better than those fed Napier grass alone, but less well that those fed Napier grass and supplemented with mixtures of sweet potato vines and other legume forages such as alfalfa, desmodium, Leucaena leucocephala and Sesbania sesban (Lanyasunya et al., 2006).

Sheep

The carrying capacity of sweet potato plots pruned for sheep feeding was comprised between 100 and 165 head/ha with a grazing interval of 6 weeks, which was found to optimize yield and quality of the forage (Olorunnisomo, 2007a). Mixing sweet potato forage and roots resulted in better nutrient utilization and the cost per kg of live weight gain was reduced. A 50:50 sun-dried mix maximized economic returns from cultivating sweet potato for sheep feeding (Olorunnisomo, 2007b). Sheep appeared to prefer cassava leaves to sweet potato leaves (Haryanto et al., 1982).

Lambs grazing sweet potato leaves recorded daily weight gains of around 50-60 g (Rondon et al., 1989).

Goats

Though fairly relished by goats, sweet potato vines provide a low CP intake, poor N retention and poor CP and DM digestibility (Katongole et al., 2009a). They compared unfavourably with cassava leaf meal or Sesbania grandiflora as they resulted in lower DM intake and lower live weight gain (Vo Lam et al., 2004). However, better results in DM intake and animal performance have resulted when goats have become accustomed to sweet potato vines (Kebede et al., 2011). Poor animal performances (less than 15 g daily weight gain) were also recorded when sweet potato vines were used to supplement a diet of Napier grass (Pennisetum purpureum), maize bran and Leucaena leucocephala leaves (Katongole et al., 2009a). Higher animal performances (44 to 82 g daily weight gain) were obtained when sweet potato vines supplemented a cottonseed cake and maize bran mixture, at a ratio of 1:4 (Katongole et al., 2009b), or a low quality grass (Ischaemum aristatum var. indicum) (Aregheore et al., 2004). Hence, sweet potato vines provide sufficient crude protein and metabolizable energy to sustain goat meat and milk production in tropical conditions, even during periods of feed scarcity, when conventional feeds are lacking (Katongole et al., 2009b; Katongole et al., 2008; Nambi et al., 2001). Feeding goats with sweet potato vines provides cheap nitrogen and increases feed efficiency (Aregheore et al., 2004). Sweet potato vines profitably replaced 50% concentrate feed (78.4% wheat bran, 20.6% noug seed cake) in male goat diets (Kebede et al., 2011).

Goats appeared to prefer cassava leaves to sweet potato leaves (Haryanto et al., 1982).