Evaluation of nutritional value of spent oyster mushroom (Pleurotus spp.) substrate-based diets and nutrient digestibility by Tswana lambs

Cereal stover contributes immensely to ruminant nutrition but are harvested when they are low in nutritive value. This affects rumen fermentation and animal productivity. Bio-conversion of cereal stover through mushroom may increase its nutritive value index. This dissertation was a record of three studies. The first study was a survey that assessed the current uses and yields of crop residues by small-scale farmers in Kgatleng, Kweneng and Southern districts of Botswana. Survey data were collected through structured questionnaires that were administered face to face to the identified small-scale farmers in the mentioned districts. Survey questionnaire data were analysed using Statistical Package for Social Sciences software. Farming and utilisation of crop residues were dominated by male farmers 58.7% (37/63). Most of the farmers 66.7% (42/63) harvested and stored cereal crop residues for feeding animals after harvesting grains while 33.3% (21/63) left the crop residues to be grazed in situ by livestock. Cattle and goats were the most (50.8%) animals that benefited from feeding with crop residues. Maize crop residues were perceived to be of very poor quality, sorghum crop residues rated better in terms of nutritive value while 98.4% (62/63) of the farmers did not know the quality of millet crop residues. In trying to improve the value of crop residues, farmers added feed ingredients such as molasses, salt, and wheat bran while cowpeas stover and lablab were added to improve protein content of the crop residues. The second study was conducted to determine the effect of crop residue type on the nutritional composition of oyster mushroom spent substrate-based diets for growing Tswana lambs. The study was arranged in a 3 x 2 factorial design with residue type (millet stover, sorghum stover and maize stover) as main plot (Factor A) and crop residue treatment (mushroom spent substrate and untreated) as subplot (Factor B). Six treatment diets were formulated by mixing 55.0% of each of millet spent substrate based-diet (MLSS), sorghum spent substrate based-diet (SSS), maize spent substrate based-diet (MSSS), millet stover based-diet (ML), sorghum stover based-diet (S) and maize stover based-diet (MS) to 45% of other ingredients to achieve a total mixed ration. The nutritive values of the diets were determined using AOAC procedures while in vitro dry matter digestibility (IVDMD) of the diets was determined through incubation of test diets in DaisyII incubator for 48 hours and in sacco degradability of dry matter (DM) was determined by incubating test diets in the rumen of two cannulated steers for 3, 6, 12, 24, 48, 72 and 96 hours. Data were analysed using General Linear Model procedures in Statistical Analysis System (SAS). There were no significant differences (P>0.05) in crude protein (CP) contents (15.56 and 15.42%) between mushroom treated and untreated substrate based-diets respectively, while mushroom treated substrate based diets had significantly lower (P<0.05) acid detergent fibre (ADF) (15.88% compared to 18.11% of the untreated diet) and acid detergent lignin (ADL) contents (7.16% compared to 8.65% of the untreated diet). There was an increase in IVDMD (from 79.72 to 85.57%) and organic matter digestibility (from 87.06 to 89.06%) due to the mushroom treatment effect. Significant interaction effect (P<0.05) was observed in CP with SSS having the highest content (15.92%) and lowest in other diets (15.09-15.67%). Fat (13.96%), percentage digestible energy (92.79%) and gross energy (21.96 MJ/Kg) were high (P<0.05) in SSS, MSSS and ML respectively while OMD was high in MSSS (90.06%) and lowest in ML (84.68%). Treatment of crop residues increased (P<0.05) quickly digestible fraction (a), potential degradability (PD) and effective degradability (ED0.03 and ED0.05) of dry matter, which is likely to increase the supply of nutrients to the animal and improve productivity. The highest degradability response was for MSSS while ML had the lowest degradability parameters. The third study determined the effects of mushroom spent substrates based diets from local cereal crop residues on in vivo nutrient digestibility, nitrogen balance and growth of Tswana lambs. Twelve castrated Tswana lambs aged between 9-12-months were randomly assigned to three dietary treatments in a completely randomised design with four lambs per treatment: MS, MSSS and SSS for 42 days. Data were analysed using General Linear Model procedures in SAS. Treatment effect showed no significant difference (P>0.05) on final body weight (FBW), feed conversion ratio (FCR), total weight gain (TWG) and average daily gain (ADG). Lambs fed SSS and MSSS diets had significantly higher daily feed intake (DFI) (P<0.05) of 586.00 g/day and 581.50 g/day respectively than lambs fed MS diet (464.70 g/day). Significant differences (P<0.05) were observed in nutrient digestibility of Dry matter; MS, MSSS and SSS (55.84%, 51.72% and 48.12% respectively). acid detergent fibre (ADF) digestibility was higher in lambs fed MS (51.82 %) and lowest in lambs that consumed MSSS (44.50%) while acid detergent lignin (ADL) digestibility was highest (P<0.05) in lambs offered MSSS (29.46%) and SSS (26.36%) diet and lowest in those fed MS (19.33%). Fat (87.89%) digestibility and calcium (Ca) (84.00%) disappearance was significantly higher (P<0.0001) in lambs fed SSS diet compared to other diets. Phosphorus disappearance was 95.00%, 91.57% and 88.38% in lambs fed MS, MSSS and SSS respectively. Nitrogen intake of 14.38 and 14.39 g/day were observed in lambs fed MSSS and SSS respectively which were significantly higher (P<0.05) than of lambs fed MS diet. Nitrogen balance (12.23 and12.50 g/day) and retention (8.80 and 9.06 g/day) were also high in lambs fed MSSS and SSS respectively. The inclusion of mushroom spent substrates in complete diets did not support better growth performance of lambs but improved feed intake and digestibility of some nutrients such as ADF, ADL, fat and Ca disappearance by lambs. High inclusion rate (55%) could have limited high growth rate of Tswana lambs, therefore future research may test low inclusion rate of spent mushroom substrates together with small amounts of high quality protected protein such as fish meal or fatty acid coated protein that escapes rumen degradation in complete diets, to promote desired growth.