Evaluation of forage sweet potato cultivars as feed for ruminants

Abstract

There is potential in Kenya to utilize forage sweet potato (Ipomoea batatas (L) Lam) cultivars as livestock feed. There are many cultivars of forage sweet potato in Kenya whose feeding values have not been evaluated. The objectives of the current study were: 1) evaluate forage sweet potato cultivars for crude protein (CP) yield and digestibility at different harvesting ages, 2) evaluate their conservation potential as silage or hay and 3) determine sheep growth performance and goat milk yield when fed on these cultivars. Cultivars K158, Marooko and Wagabolige recorded high CP yield (1597-1679 kg/ha) and digestibility (711-851 g/kg DM) at the four harvesting ages and their optimum content of dry matter (DM) (166-168), organic matter (OM) (880-882) and CP (170-171 g/kg DM) was at 120 days. Using weighted scoring method K158, Marooko and Wagabolige (scoring 21-24) were ranked the three most superior forage cultivars and were recommended for further evaluation. Calcium (Ca) (4.40-8.41), magnesium (Mg) (3.20-5.15), phosphorus (P) (31.01-37.38), potassium (K) (11.80-25.01), sodium (Na) (4.31-8.75 g/kg DM); cobalt (Co) (247.2-252.6), copper (Cu) (22.7-56.6), iron (Fe) (278.0-326.3), manganese (Mn) (116.2-217.4) and zinc (Zn) (136.4-180.8 mg/kg DM) in all the cultivars decreased with age. For the conservation using silage, the 5% molasses rate provided adequate water soluble carbohydrates for optimal silage fermentation (pH 4.0). Marooko (NH3-H, 5.5) fermented the best quality silage followed by K158 (6.4) then Wagabolige (6.6 % of total N). Shredding increased DM (298.8-306.2) in all the cultivars and Wagabolige dried more rapidly (330.2) than K158 (310.0) and Marooko (263 g/kg DM) when dried whole or shredded. These cultivars differed in CP (156.8-170.4), neutral detergen fibre (NDF) (373.0-397.1), acid detergen fibre (ADF) (280.3- 283.4), acid detergen lignin (ADL) (85.0-117.1 g/kg DM) and there were minimal losses due to spoilage and crop respiration during hay making. These cultivars had similar DM (93.8-98.1), OM (82.0-85.6) and NDF (37.6-38.8 g/kg W0.75) intake, DM (740.3-744.1) and OM (747.0-751.7 g/kg DM) digestibility and rumen fermentation characteristics (pH 6.80-6.86). The cultivars differed in CP (10.7-16.9 g/kg W0.75) and metabolizable energy (ME) (0.94-0.99 MJ/kg W0.75) intake, CP digestibility (737.4-821.1 g/kg DM) and nitrogen balance in sheep (2.7-4.5 g/day). Cultivars K158, Marooko and Wagabolige provided high ME (9.7-10.2 MJ/kg DM), recorded superior intake of digestible OM (74.5-75.8), CP (13.8-16.1) and NDF (27.4-30.8 g/kg W0.75). Sheep fed xixon these cultivars attained average daily gain (ADG) and feed efficiency (FCE) beyond 143.0 gand 13.0 kg which were considered high. Cultivar K158 had the highest ME (10.2 MJ/kg DM) and ME (1.17 MJ/kg W0.75) intake in goats. These cultivars affected the milk yield and composition, digestibility of CP (750.0-826.1) and NDF (627.4-680.9 g/kg DM), their gross (17.6- 19.5) and digestible intake (13.8-16.1 g/kg W0.75) by goats but did not affect the rumen fermentation characteristics. The goat daily milk yield was 680, 585 and 492 g when fed on K158, Marooko and Wagabolige, respectively. The goats fed on K158 produced milk containing high butter fat (BF) (35.7), protein (44.7), lactose (53.9), solid non-fat (SNF) (108.9 g/kg) and freezing point depression (-0.63oC) and goats fed on the test cultivars differed in milk total solids (TS) (107.4-141.2 g/kg). According to milk yield, the cultivars ranking in descending order was K158, Marooko and Wagabolige but according to milk composition qualities the rank was K158, Wagabolige and Marooko respectively. Cultivars K158, Marooko and Wagabolige yielded high CP and recorded high digestibility and should be harvested at 120 days to maintain high nutritive quality. Silage making using 5% molasses was the suitable conservation method as hay making was unsuccessful. The ADG and feed efficiency were similar for the sheep on cultivars K158, Marooko and Wagabolige indicating that any one of them was recommended to feed growing sheep. Among the three forage sweet potato cultivar evaluated in the current study, K158 was the most suitable to feed dairy goats. These cultivars have a potential as feed resource for ruminants in Kenya.