English

Abstract

Post-harvest management of sweet potatoes (SP) crop residues preserves nutrients, deactivates inhibitor compounds and improves rumen degradation. The aim of the study was to determine effects of drying crop residues and heating roots on forage value of a bio-fortified orange fleshed sweet potato (OFSP) variety in South Africa. The crop was harvested at maturity and roots separated from crop residues. Roots were washed, sliced and divided into three portions as SProots that were frozen at -4°C for 4 weeks, SP70- oven dried at 70oC for 8 days and SP80 -80oC for 7 days. Aboveground crop residue were separated into portions of vines and leaves (SPVLf) and leaves and petioles only (SPLf). A subsample of each portion was air dried for 7 days (SPVLd and SPLd, respectively). Chemical composition and in sacco organic matter disappearance were determined. Crude protein (CP) was higher (P<0.05) in SPLd (24.9% CP DM) compared to fresh material with 6.5%. Neutral detergent fibre (NDF) and insoluble CP (NDFICP) were higher after drying, non-fibre carbohydrates (NFC) declined and vitamin A declined. Effective degradability (ED) was higher than for Lucerne hay and differed between SPVLd and SPLd 77.6% and 81.3% at kp=0.05; respectively. The SProots were low in CP, ether extracts and fibre; had higher NFC (77% DM) and gross energy (4.1 Mcal/kg DM) compared to SP70 and SP80. The SP80 roots had the least NFC (P<0.01) and highest amount of fibre. Calcium, phosphorus and vitamin A were negligible post heating. Rate of degradation (c h-1) and ED was highest with SP80 (0.22 and 91.3%; kp=0.03) and lowest with SProots (0.135 and 81.7%). Drying OFSP crop residues and heating roots affected nutrient profiles however, forage degradability improved. Key words: Sweet potato, vines and leaves, vitamin A, rumen degradation, non-structural carbohydrates.