Characteristics of the experiment and treatments

Prickly pear glochids obtained from the 2018 and 2019 production cycle, from the locality of San Felipe Teotitlán, Municipality of Nopaltepec, State of Mexico, were used. According to ^{Ulloa-Leitón et al. (2021}), the glochids contain 41% (±0.2) cellulose, 41% (±0.2) hemicellulose and 5.27% (±0.2) lignin, with a density of 0.1574 g ml^-1.

The mycelium (F2) used was obtained by direct sowing (mycelium-mycelium) in sorghum grains and mixture of ground and whole glochids from the strain of the collection of the Instituto de Ecología, AC (INECOL) (^{Jesús-Rivera, 2020}; ^{De Jesús-Rivera et al., 2022}). The treatments consisted of mixing oat straw with proportions of ground (EM) or whole (EE) glochids as indicated in Table 1. The control consisted of using only oat straw, the traditional substrate for these production purposes. In total, seven treatments were used in a completely randomized experimental design with six replications.

Substrate preparation

Prior to the preparation of the treatments, the glochids were cleaned, removing the largest debris. In the oat straw, the stubble that contained soil and all material that was different from the required substrate were discarded, and it was divided into portions of 8 to 10 cm. The mixtures of each experimental unit were placed in cloth (organza) bags with a capacity of 6 kg, properly labeled. The dry weights were recorded and the bags were closed, then pasteurized for 20 min at 85 °C (Figure 1).

Figure 1 Preparation of the substrate: a) straw chopping; b) mixing and closing of sacks; and c) pasteurization. 

Once pasteurization was complete, the bag was suspended to remove excess water. The sowing and draining area were disinfected with soap and chlorine solution, the shelves were cleaned with antibacterial gel. The drained mixture was dispersed in a sifter in order to bring it to room temperature, then alkalized by sprinkling Ca(OH)₂ (Figure 2).

Figure 2 Preparation of the substrate, a) draining, b) cooling, and c) Aakalization with Ca (OH)₂. 

The mycelium (Figure 3a and 3b) was placed in a container to homogenize it, then sown by interspersing a layer of approximately 5 cm of substrate and a layer of mycelium, until reaching a weight between 2 and 3 kg, in transparent plastic bags of 50x70 cm. Eight holes of 1 cm in diameter were made in the bags to favor aeration, subsequently, the experimental units were closed, labeled and placed on incubation shelves (Figure 3c and 3d). Ambient temperature and humidity data were recorded (Table 2).

Figure 3 Sowing a) homogenized mycelium; b) sowing; c) labelling; and d) incubation area. 

Twenty-one days after the establishment of the experiment, the appearance of primordia began (Figure 4a) and based on this indicator, the irrigations were applied every 12 h to favor the increase in relative humidity. The collection of carpophores occurred nine days after the appearance of primordia, when their gill had thinned edges (Figure 4b). The collected oyster mushrooms were weighed and placed in 500 g bags for sale (Figure 4c).

Figure 4 a) appearance of primordia, b) carpophores with thinned edges; and c) carpophores for sale. 

Variables evaluated

The production parameters of Pleurotus sp. were evaluated based on earliness, biological efficiency, yield and production rate, taking as a reference what was proposed by ^{Gaitán-Hernández et al. (2009}). Earliness (E): defined as the time that elapses between the day of inoculation and the day on which the first primordia or carpophores appear. It is expressed in number of days.

Biological efficiency (%, BE): it expresses the degree of energy bioconversion from the biodegradation of the substrate, defined by the ratio between the fresh weight of the fruiting bodies and the dry weight of the substrate used for their production for the sum of the harvests (^{Barba-Chávez and López-Cruz, 2017}).

Yield (%, Y): it is the ratio in percentage between the fresh weight of the mushroom and the weight of the wet substrate.

Production rate (%, PR): this indicator provides important information on the replacement that exists between each lot that is grown and is related to the effectiveness of the process; through the time of sowing, incubation, fruiting and harvesting. The production rate is the ratio in percentage between biological efficiency and the time required for harvest, that is, it represents daily biological efficiency (^{Barba-López and López-Cruz, 2017}).

Statistical analysis

Because the response variables did not present continuous means, the values of the evaluated parameters were transformed into ranges for the analysis of variance. The means of the treatments and the corresponding standard deviation were estimated, the LSD test was also applied for the corresponding comparison with a confidence level of 95%. Data were analyzed with the statistical package Statistical Analysis Software (SAS) with license number 70074773.

Economic evaluation

The economic evaluation was carried out in order to estimate the profit obtained when alternative materials are used as improvers of the traditional production substrate that is normally purchased. According to the data reported by ^{Gaitán-Hernández (2007}) and the experience of the producer, the following parameters were considered: Installed capacity, production per production cycle, net production, costs of inputs for production, cost of substrate per production cycle, current price of the product to the consumer and net income per kilogram of fresh mushroom produced.

Production parameters

The analysis of the significance of the treatments on the variables evaluated, as well as the test of means LSD, indicated that at least two treatments were significantly different in three of the production parameters (Table 3).

In general, treatments with whole or ground glochid showed shorter sprouting time (25.2 to 27.8 days) with respect to the control (30.3 days), with an average of 26.6 days elapsed from sowing to the appearance of primordia, effect that was significant in four of the treatments. For this parameter, there were no differences between mixtures. In oat straw, ^{Gaitán-Hernández y Silva-Huerta (2016}) report 23 days of incubation, although in their mixture 80% oat straw-20% corn straw, the earliness was 27 days. The results obtained reveal that the addition of whole or ground glochid in the proportions tested contribute to accelerate the earliness of Pleurotus sp.

This production parameter is important because it is an indicator of the functioning of the substrate in terms of accelerating or delaying the production process and according to ^{García-Oduardo et al. (2011}), this colonization phase is the critical point of oyster mushroom production. Regarding the yield (Y%) parameter, there were no significant differences between treatments; however, the highest percentages were achieved with mixtures with EM, which, on average, reached 38.3% (±10.9) of production with respect to the control with 32.9% (±8.1). Although statistically there were no differences, it is clear that the use of ground glochids reduces the amount of traditional substrate that is frequently purchased and represents savings for the producer.

The results obtained also exceeded those reported by ^{Gaitán-Hernández and Silva-Huerta (2016}), who, in their best treatment (mixture 20% corn stover, 80% oat straw), achieved a yield of 31.56%. ^{García-Oduardo et al. (2011}) consider that substrate formulations in which yields greater than 10% are achieved are viable to be economically exploited. With this criterion, even treatments with whole glochid represent an economically feasible option.

Biological efficiency (BE) was another parameter that showed no significant differences between treatments; however, the highest average BE was obtained with mixtures of ground glochids. These promoted a BE of 124% (±30.7), while that of the control was 99.8% (±24.5). ^{Gaitán-Hernández and Silva-Huerta (2016}) obtained BEs similar to those obtained in this experiment when using oat straw substrate, with which they achieved a BE of 120.3% (±15.4) and with their best treatment mixture 20% corn stover-80% oat straw of 139.7% (±3.9).

Michel-^{Aceves et al. (2015}) confirm that BEs with values close to or greater than 100% are considered highly profitable and less than 65% unprofitable. In this context, ^{Garzón-Gómez and Cuervo-Andrade (2008}) report that the productive quality of a substrate is perceived as acceptable from biological efficiencies of 50%, which coincides with what was confirmed by ^{García-Oduardo et al. (2011}), concluding that formulations with BE greater than 50% and Y greater than 10% are economically feasible. The BE is an indicator that reflects whether the substrate is suitable for the production of the mushroom and the ability of the mushroom to use nutrients.

Considering these references, it is important to mention that mixtures with whole or ground glochids meet these production parameters so they can be considered viable options for the production of oyster mushrooms. Regarding the indicator of production rate (PR), the treatments with significantly higher PR were achieved with mixtures of EM in the proportion of 30 and 40%, with an average PR of 3%, while the control was 2.2% (±0.5). ^{Gaitán et al. (2016}) report a PR of 1.6% (±0.2) for oat straw substrate and the best PR in substrate mixture 20% corn stover 80% oat straw with 1.8% daily biological efficiency; for their part, ^{Romero-Arenas et al. (2018}) achieved a PR of 1.8 in wheat straw substrate. Although in this study the PRs obtained with EM stand out, it is important to mention that the seven treatments have PRs greater than 2, which exceeds the PRs reported by the aforementioned authors.

Economic analysis

Based on the two best treatments that resulted from the experiment, compared to the control, Table 4 shows the impact of the net profit obtained by reducing production costs for substrate. According to the data provided by the producer, they use bags with a capacity of 5 kg of dry substrate (RA) for an installation volume of 30 bags, which demands 450 kg of straw for an annual yield of 180 kg of fresh oyster mushrooms (in three production cycles) at a sale price per kilogram of $70.00. Using the mixture 60RA-40EM results in straw purchase savings of 270 kg with annual profits that increase from $6 137.00 to $9 490.75.

It is important to consider that the price of a kilogram of fresh oyster mushrooms varies according to the time of year (^{Gaitán-Hernández, 2007}) and ranges between $30.00 and $45.00; nevertheless, even with this price range, the use of the by-product of the processing of prickly pear as a complement to the traditional substrate would improve the net profit. The potential for edible mushroom production at the rural level is significant because it is a simple process with low implementation costs.

The reality shows that a significant proportion of rural producers have an unstable production due to the lack of economic support and technical advice, added to poor planning and commercialization of the product (^{Martínez-Carrera et al., 2000} and ²⁰⁰⁷), therefore, the decrease in costs for the purchase of substrate represents an area of opportunity for the producer. On the other hand, the results of this research are conclusive as to that the waste generated by the production of oyster mushrooms can be reincorporated as organic fertilizer (Figure 5) to the prickly pear production system, eliminating the environmental impact caused by its waste of origin.

Figure 5 Substrate remaining from the production of Pleurotus sp.