Supplementation of Compost for Agaricus subrufescens Cultivation

Introduction

Mushroom production has been gradually increasing worldwide due to the development of new technologies based on local needs for cultivation. ¹ In particular, the mushroom Agaricus subrufescens has aroused considerable interest within the scientific community due to its medicinal properties. ² A limiting factor for large-scale production and cultivation of A. subrufescens is the lack of research on production techniques to identify phases and cultivation steps that maximize productivity with a short harvest period. ³ Only a handful of countries—including Brazil, China, United States, Thailand, and South Korea—commercially cultivate A. subrufescens, which may contribute to the lack of technological development for its cultivation.

A. subrufescens production technology is poorly defined and is based on the same approach adopted for Agaricus bisporus production: two composting phases (I and II) followed by inoculation and mycelium growth, and casing and harvesting. The entire process takes approximately 120 days. ⁴ Supplementation of the compost, a technique performed before adding the casing layer, increases mushroom productivity and quality. ⁵ This study analyzed the effects of compost supplementation for A. subrufescens mushroom production using two doses and two strains.

Materials and Methods

Compost was obtained at the end of Phase I from a commercial farm in Canada. Phase II composting was performed at the Mushroom Research Facilities of Vineland Station, at the University of Guelph. Compost formulation followed the standard composition: wheat straw (74%), chicken manure (23%), and gypsum (3%). Phase II was performed in a pasteurization tunnel, where the compost was incubated at 60°C for 8 h and at 45°C for 5 days. At the end of Phase II, the compost exhibited 68% humidity, following the quality indicators proposed by Zied et al. ⁶

For inoculation, 300 g of spawn per 26 kg of compost (wet weight) were placed in trays. Strains CS1 (isolated in the city of Vitoria in Espírito Santo) and CS2 (isolated in the city of Belo Horizonte in Belo Horizonte) were chosen for compost inoculation because they differed genetically according to random amplified polymorphic DNA (RAPD) analysis. ⁷ In addition to the spawn, 2% or 4% of the commercial supplement Sylvan (Sylvan, Inc.; Kittanning, PA) was added to the compost (based on the compost's dry weight), resulting in 167- or 333-g per tray additions, respectively. Each tray was covered by a plastic film to protect the compost and was incubated in an acclimatized chamber at 25°C, 95% humidity, and 3,000 ppm of CO₂.

After spawn run, a casing layer composed mainly of peat moss (65.8 kg peat moss, 65.8 kg limestone, and 227 L water) was added to the compost, following the same procedures described for A. bisporus. For colonization of the casing layer, the same conditions described previously for compost colonization were used. After a casing run, scratching was performed, and the same environmental conditions were maintained until the casing layer was completely colonized. For fruiting induction, temperature was adjusted to 19°C for 4 days; CO₂ concentration was adjusted to 1,000 ppm; and 95% humidity was maintained. After the 4-day period, the temperature was readjusted to 25°C (air temperature); the same conditions for CO₂ concentration and humidity were maintained until the end of the experiment.

The temperature of the compost was monitored to ensure that it did not exceed 28°C. The cultivation cycle lasted for 60 days, after which the first mushroom was harvested. Mushrooms were harvested when they reached a maximum size (above 5 cm), before collapse of the veil and while the sides of the pileus were still parallel. Any excess casing layer was removed from the stipe base, and the mushrooms were counted and weighed to determine productivity and biological efficacy [(kg fresh mushrooms/kg dry composting)×100].

The experiment had a completely randomized 3×2 factorial design (three supplement concentrations and two mushroom strains, totalling six treatments) with six replicates in each treatment. The results were analyzed using Tukey's test with a 5% probability level using the Statgraphics Plus v. 4.1 (Statistical Graphics Corp.; Princeton, NJ) software package.

Results

The results for productivity and biological efficiency (Table 1) demonstrated a positive effect of compost supplementation. However, the effect was strain-dependent. The CS1 strain showed higher productivity and biological efficiency in response to supplementation compared with the CS2 strain, though not significantly different from the control.

The CS1 strain showed an increasing response in relation to the amount of supplement incorporated into the compost. Without supplement, the compost exhibited 6.08% productivity, and when supplemented with Sylvan at 2% or 4%, the compost exhibited 9.43% or 11.64% productivity, respectively. Thus, productivity was nearly doubled in the treatment supplemented with 4% Sylvan compared with the control treatment. However, the strain CS2 did not show the same response; supplementation did not have a significant effect on the productivity and biological efficiency values.

It is worth mentioning that at day 7 post-inoculation, all treatments were colonized, although the compost with no supplementation and inoculated with CS2 was colonized at a lower level than the other experimental treatments. These findings indicate that CS2 exhibited a different physiological response to the supplement compared to CS1. The colonization rate of CS2 was similar to that of CS1 upon compost supplementation; however, this rate was not sufficient to guarantee similar productivity. Nevertheless, it is important to note that although it was not significant, a positive trend in productivity could be observed with supplementation with 2% or 4% of the commercial product. Supplementation with 4% of Sylvan showed an increase (not significantly different from the control) above 50% in productivity. It is likely that higher concentrations of the supplement would have led to a significant increase in the productivity of CS2.

Conclusions

These findings suggest that compost supplementation is an interesting strategy for A. subrufescens mushroom cultivation. To obtain the best cost-benefit ratio, the strains that exhibit a better response to the treatment should be selected for supplementation, as supplementation increases production costs due to the cost of the product used as the supplement and the extra labor needed. In Brazil, several strains are commercially available, and they most likely respond differently to supplementation. Thus, the companies responsible for compost production should select a good productive strain that responds well to supplementation.

Additionally, it is important to consider the composition and physicochemical quality of the compost, which may vary among countries. In Brazil, bunkers are generally not used during Phase I of composting, and in Phase II, there is no control inside the tunnel, as is usually the case in more developed countries. One must also consider the commercial availability of the supplement to be used. In this study, a commercially available compost indicated for A. bisporus cultivation was used. In addition, it should also be considered that in Brazil the use of composts with higher carbon-to-nitrogen (C/N) ratios has been indicated for A. subrufescens cultivation. ⁵

Considering that in Brazil there are no commercial products for compost supplementation, the use of crushed legumes such as soy and the common bean has been proposed; however, there is great concern regarding the risk of contamination from such supplements. In addition to favoring slow release of nutrients, selection of a supplement should favor a low susceptibility to contaminants. As a result, supplementation at casing instead of at spawning has been suggested; as in the former cultivation stage, the compost would be completely colonized and thus would be less susceptible to contamination. Conversely, supplementation at casing presents the great disadvantage of increasing labor costs because it requires the complete breakdown of the compost to incorporate the supplement.

In conclusion, compost supplementation should be considered an important technology strategy to improve A. subrufescens mushroom production. However, further studies are necessary to ensure that the technology is appropriate to the local reality for mushroom cultivation, especially in less-developed countries. Moreover, strains exhibiting higher productivity and better responses to supplementation should be selected.