Chemical composition and in vitro ruminal fermentation of red grape pomace spent mushroom substrates
Abstract
Red grape pomace (GP) is a by-product of grape wine and juice making industries. The pomace is produced in large quantities and thus tends to have detrimental effects on the environment when it is disposed into the landfills or when it is incinerated. The use of GP in ruminant diets can mitigate the negative effects it has on the environment whilst at the same time, improving food and nutrition security in South Africa. However, the alternative use of GP as a feed source or ingredient in ruminant rations may be limited by high levels of structural carbohydrates and condensed tannins, which reduce nutrient digestibility and utilization. It is, therefore, important to evaluate innovative strategies to reduce the negative nutrient effect of these anti-nutrients such that high levels of GP can be included in ruminant diets for economic and environmental sustainability. One such previously unexplored strategy is the use of mushrooms to reduce fibre content and modify condensed tannins in GP. The aim of the current study was, therefore, to examine the influence of oyster mushroom (Pleurotus ostreatus) spawning rates and duration on the chemical composition, in vitro ruminal gas production and in vitro ruminal dry matter degradability (DMD) of red grape pomace (Vitis vinifera L. var. Shiraz). The red GP substrates were inoculated with oyster mushroom spawns at 0, 200, 300, 400, and 500 g/kg. The first chapter of the thesis provides a background on the grapes and ruminant industries, the problem statement, the justification and objectives of the study. The second chapter is a literature review on ruminants, the nutritional composition of grape pomace and the utilization of oyster mushrooms to improve the nutritive value of fibrous feeds for ruminants. The third chapter investigated the chemical composition of red GP treated with incremental spawn levels. The results from chapter three showed that the oyster mushroom (OM) spawning rates and incubation duration influenced the chemical composition (fibre and crude protein) of spent GP mushroom substrate (SMS). Both linear and quadratic trends (P <0.05) were observed for dry matter (DM), organic matter (OM), neutral detergent fibre (NDF), acid detergent fibre (ADF), acid detergent lignin (ADL) and crude protein (CP) in weeks 3 and 4.
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Higher OM spawning levels (20 – 50%) and longer incubation periods (3 – 4 weeks) improved the NDF, ADF, ADL and CP content for the treated substrates relative to the untreated substrates. The fourth chapter investigated the in vitro ruminal gas production and the in vitro ruminal dry matter degradability parameters of the SMS, which was only evaluated using week 4 based on the maximum effectiveness of the oyster mushroom on the chemical composition of the red GP in week 4. The spawning rates linearly increased the rate of gas production at 12, 24 and 48 h of in vitro incubation. Quadratic trends were observed for the rate of gas production across all weeks. The treated substrates had a higher rate of gas production relative to the untreated substrates in week 2, 3 and 4. There were significant linear trends for the immediate fermentable fraction (a), the slowly fermentable fraction (b), the fermentation rate of fraction (c), the potential gas production (Pgas) and the effective gas production (Egas) in weeks 2 and 4. Quadratic responses were also observed in weeks 1, 2, 3 and 4 for the in vitro ruminal gas production kinetics. Inoculation with spawn (20 - 50%) had an effect (P <0.05) on the in vitro ruminal gas production kinetics substrates. Linear and quadratic trends were observed for the immediately degradable fraction (a), DMD48, the effective degradability (EDeg), the in vitro organic matter degradability (ivOMD) at 36 and 48 h, and the partitioning factor (PF) at 48 h, respectively. The oyster mushroom spawn treatment had an effect (P <0.05) on the fraction a, the slowly degradable fraction (b), potential degradability (PDeg), EDeg and PF (at 12, 24, 36 and 48 h). Substrate GP40 had a higher (P <0.05) fraction b (277.8 g/kg DM) and PDeg (147.5 g/kg DM) than the control post-incubation. It was, therefore, concluded that inoculating GP with graded levels of oyster mushroom spawn and incubating it for a sufficient amount of time (28 days) has the potential to reduce the fibre content of GP and improve the CP content. Furthermore, it has the potential to improve the in vitro ruminal dry matter degradability and fermentation efficiency of the red GP waste. However, it was not possible to deduce the optimum spawning rate to improve the chemical composition and in vitro fermentation of red grape pomace spent oyster mushroom substrate.