DETERMINATION OF SUITABLE HARVESTING AGE OF SELECTED FORAGE SWEET POTATO CULTIVARS TO ACHIEVE HIGH NUTRITIVE VALUE IN RUMINANTS

Abstract

Protein is a major limiting nutrient in livestock production in Kenya especially during the dry season. Sweet potato (Ipomoea batatas (L) Lam) is a protein rich forage and grows well under many farming conditions. The study objective was to determine the suitable harvesting age of forage sweet potato cultivars that will result in high nutritive value in ruminants. Nutritive value was measured through in vitro organic matter digestibility (IVOMD), metabolizable energy (ME), volatile fatty acids (VFA) and microbial crude protein (MCP) yield.  Cultivars 99/1, K049, K158, Marooko, Mugande and Wagabolige were planted in randomized complete block design (RCBD) with three replications. These cultivars were sampled at 90, 120, 150 and 180 days growth and subjected to in vitro gas production technique according to Makkar (2004). Gas volume produced during 24 hr sample incubation was used to calculate IVOMD, ME, VFA and MCP using recommended equations. The data was subjected to analysis of variance and means separated using least significant difference. Harvesting age affected the studied parameters. Most cultivars recorded their optimum IVOMD (711-851 g/kg DM), ME (7.43-8.13 MJ/kg DM), VFA (0.71-0.84 mg/L) and MCP (85.4-103 g/kg DM) at 120 days. Wagabolige maintained similar ME (8.18-8.24 MJ/kg DM), VFA (0.82-0.84 mg/L) and MCP (102-102 g/kg DM) across the harvesting ages which were, also, the highest among the cultivars. Lowest IVOMD (684-711 g/kg DM) and MCP (7.43-7.83 g/kg DM) was recorded in K158 at all four harvesting ages and lowest ME (7.25-7.43 MJ/kg DM) and VFA (0.68-0.71 mg/L) were in 99/1. It was concluded that as nutritive value significantly decreased with increased age, then 120 days was recommended as suitable harvesting age for these cultivars. Wagabolige recorded the highest nutritive value hence it is recommended as the most suitable cultivar in ruminant nutrition.