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 SP_roots 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 (SPVL_f) and leaves and petioles only (SPL_f). A subsample of each portion was air dried for 7 days (SPVL_d and SPL_d, respectively). Chemical composition and in sacco organic matter disappearance were determined. Crude protein (CP) was higher (P<0.05) in SPL_d (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  SPVL_d and SPL_d 77.6% and 81.3% at kp=0.05; respectively. The SP_roots 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 SP_roots (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.