Pleurotus ostreatus(oyster) is a member of Pleurotus species of mushrooms cultivated globally for food. According to Besufekud et al (2019), oyster mushrooms are the second largest commercially produced mushrooms in the world. They possess a rare capability to convert lignocellulosic waste residues into edible fruiting bodies, rich in nutrients. Oyster mushrooms are reported by Ramsbottom (1954) to contain high amounts of vitamins, particularly vitamin D and high protein contents because they contain enzymatic proteases that can breakdown complex macromolecules in agricultural wastes into individual amino acids. Of much importance is the absence of fatty acids in oyster mushrooms, which makes them ideal for persons willing to maintain a low fat diet. Furthermore, oyster mushrooms contain the appropriate ratios of potassium and sodium, which are essential in reducing heart disease.
In Zimbabwe, mushroom cultivation is still at the grassroots level with most small-scale farmers producing enough for subsistence. Much knowledge is still lacking pertaining to the utilisation of substrate individually and in combination to optimise production and achieve commercial production leading to exportation.Govera (2020)
Mushroom substrate is the material or substance, which provides food for the growing mycelium. Oyster mushrooms can grow on a wide range of substrates compared to other mushrooms. They can grow on all wood types including sawdust and paper pulp sludge. They also grow on all types of cereal straw such as maize straw, wheatstraw and rice straw. They also grow well on other types of agricultural substrates such as corncobs, banana fronds, maize stalks and leaves, savanna veld grass, sage grass, cottonseed hulls and coffee grounds. The mechanism by which substrate is broken down, is by the extracellular release of enzymes.According to Onyeka et al (2017) the extracellular enzymes secreted by fungi contain amorphous homo and heteropolysaccharides often associated with fungal protein. These enzymes bind to cellulose, lignin and hemicellulose, and break them down into simple sugars and peptides.
Sayner et al (2019) alludes that the yield and quality of oyster mushroom is largely dependent on the chemical and nutritional content of the substrate. In instances where, a nutrient lacking in one substrate it is then provided by another substrate. There is need to establish a consensus by, pulling high yielding substrates together and then come up with the best formulation.
Mushroom yield is closely associated with the chemical and biological composition of the substrate (Bhattacharjya et al 2015) Most mushroom farmers in Zimbabwe today are using cotton hulls to grow oyster mushroom. As more people are becoming knowledgeable about cotton hulls as substrate, it creates a demand for this commodity. Tapiwa (2021). As a result of the increase in demand for cotton hulls there is increase in price of cotton hulls. It becomes economically non profitable to venture into mushroom farming due to expensive substrate. The price of wheat straw and cotton hulls is increasing every year, as many more people become mushroom farmers. This in turn affects the pricing of the mushrooms themselves.
Dead plant matter has different composition of nutrition. There are many factors, which determine selection of a particular substrate. These are nutrition, cost, availability, accessibility and storage. A single particular substrate may be readily available but costly to the farmer. The length of the mushroom crop or the number of flushes depends on the substrate used for the cultivation of oyster mushrooms. For example, wheatstraw has good nutrition but it has a limited cropping life. Banana fronds also have good nutrition but they has low water retention. Sawdust has high nutrition but it takes longer to fruit. Cotton hulls are the best for fruiting, spawn run, have good nutrition but they are very expensive and seasonal (Pant et al 2006). There are limitations in nutritional composition when only one substrate is used to grow mushrooms. A formulation of substrates might contribute to better nutrition in oyster mushrooms. The best composition of substrates should have a short spawn run, a high biological efficiency and a high benefit cost ratio. C/N ratio directly affects the mushroom spawn run, yield and biological efficiency.
Spawn run
Spawn run is the time it takes for the substrate to be fully covered by the mushroom mycelia. It is affected by qualities of the substrate and qualities of the spawn. For a shorter spawn run a substrate should have a high surface area and good nutrition. Banana leaves require 19 days for spawn run.(Asmamaw et al 2014).Sawdust takes 30 days for spawn run. (Hoa et al 2015).Cotton hulls takes 18 days for spawn run.(Muswati et al 2021). Wheatstraw takes 16 days for spawn run.(Girmay et al 2016). Corn cobs takes 40 days for spawn run. (Itelima 2011)
Biological efficiency
Biological efficiency is defined as the percentage measurement of harvested fresh mushrooms relative to the dry weight of substrate.(Chukwurah et al 2012). In theory a 100% biological efficiency is achieved when fresh mushrooms having a moisture content of 90% are havested from a substrate with a moisture content of 75% of which 25% of dry substrate gives us the mushrooms.
BE = DP/M
BE-Biological Efficiency
DP-mass of fresh mushrooms produced in kgs
M-initial dry mass of substrate in kgs
In practice a 100% biological efficiency is achieved when a mushroom farmer collects 10 kgs of mushrooms from three harvests after cultivating on 10kgs of dry substrate. Outside factors such as temperature and humidity, affect biological efficiency. (Girmay 2016)
A biological efficiency above 75% will be economical for the mushroom farmer. This is only achievable by using a high spawn rate. Usually the first, second and third flushes are the best flushes, with each flush decreasing in yield form the first. For subsistence, small scale farmers can allow the mushroom bags to fruit until the substrate is spent. For commercial farming only three flushes are economical. (Zakil et al 2019)
B-C Ratio
B-C (Benefit-Cost) ratio is the ratio of benefit from selling output monetary terms relative to the total cost during production. However, as the value of B:C increases so does the feasibility of mushroom production in economical aspect. To find the ratio the yield of mushrooms by is divided by the the sum total expenditure of raw materials and substrate. Rice straw gives a B-C ratio of 3.498, wheatstraw 1.108, sugarcane bargasse 0.217 and banana leaves 1.408. Therefore, as the B-C ratio becomes greater then 1, the more profitable the project and as the B-C ratio falls below 1, the less profitable the project. (Dubey et al 2019).
Carbon to Nitrogen ratio
According to Kashangura et al (2005)for mycelial growth and fruiting body formation the carbon to nitrogen ratio should be 50: 1, 100: 1 or 500: 1. Most agricultural wastes such as corncob, grain straws, wood, sawdust and banana fronds have sufficient C; N ratios to support saprophytic mushroom growth. C: N ratio is the proportion of carbon to nitrogen in a substrate. (Hoa et al 2015)
Oyster mushroom mycelial growth is optimum in the presence of more carbon compared to nitrogen. Therefore, substrates comprised primarily of cellulose, hemicellulose and lignin are good candidates for oyster mushroom cultivation. In this study, we will investigate cotton hulls, sawdust, wheatstraw, banana fronds and maize cobs individually and in combination to determine potential in optimisation of Pleurotus ostreatus. We discuss the characteristic features and C: N ratios of the substrates under investigation below:
Banana fronds are the dried leaves from banana plants. During the dry season banana leaves lose a lot of moisture and as a result, the plant withholds water in the stem, leading to the lower leaves turning brown and then drooping down the plant. Banana farmers have no choice but to cut away the leaves and burn them because the leaves invite pests such as mosquitoes, rats and fungi, which feed on the dead leaves. The carbon to nitrogen ratio in banana leaves is 70:1. (ScienceMate 2022)
Wheatstraw is the grass or straw that farmers leave behind when wheat farmers harvest their wheat. Wheat is a winter crop with a very short season and farmers have to use combine harvesters to harvest the wheat before the pod breaks and disperses the seed. Most farmers use a bailer to collect the leftover straw and then store it for feeding their livestock. Wheatstraw has problems with storage since it easily burns and termites tend to feed on the wheat straw. Less knowledgeable farmers tend to burn the remaining wheatstraw left in the fields in preparation for the maize farming season in Zimbabwe. This however leads to veld fires and deforestation. The carbon to nitrogen ratio in wheat is 80:1. (Lentz and Lindsey 2016)
Maize cobs are the cob that remains after removal of maize seeds from the cob. Large and huge masses of this waste accumulates soon after maize has been harvested because fresh maize cobs are sold and cooked as breakfast and discarded along streets and at dumpsites creating a huge environmental problem. Most of the weight of the maize cob is in the cob itself and this proves that it has a lot of carbon. Grinding down the cob after drying it can drastically increase its surface area thereby allowing mushroom mycelia to spread and absorb nutrients from the cob. Postharvest, farmers discard the cobs after shelling and most small-scale farmers use them as firewood during the winter. The carbon to nitogen ratio of maize cobs is 100:1.Adjapong (2015)
The wood industry is reponsible for producing furniture and roofing material. However, during the cutting down, shaping and designing a lot of this wood is broken down into wood shavings and sawdust. However if the wood waste derived from this industry was to be saved stored and commercialised for mushroom farming we could archive food security. The carbon to nitrogen ratio of wood is in the range of 500:1. (Rynk 1992)
Cottonseed hulls are a waste product resulting from the pressing and crushing down of cottonseed into cottonseed cooking oil. Ginneries opt to burn or throw away the waste as the main method of disposal. However, cottonseed hulls have proved to contain high amounts of nitrogen and carbon essential for mushroom growth. The carbon to nitrogen ratio of cottonseed hulls is 59:1. (Haziran 2019)
This study will provide sufficient data to make decisions on the utilisation of different types of agricultural and forest waste into nutritional food. To analyse, evaluate and ultimately identify the most appropriate composition of substrate for cultivating Pleurotus ostreatus.