Fermenting rice straw with the fungus Pleurotus eryngii increased the content of crude protein and the digestibility of the straw

Nguyen Thi Huyen^1,2, Nguyen Thi Tuyet Le¹ and Bui Quang Tuan¹

¹ Department of Animal Nutrition and Feed Technology, Faculty of Animal Science, Vietnam National University of Agriculture, Trau Quy, Hanoi, Vietnam
² Animal Nutrition Group, Wageningen University, The Netherlands
nthuyencnts@gmail.com

Abstract

The objective of this experiment was to determine the digestibility of rice straw fermented for 28 days with the fungus Pleurotus eryngii. Four Phan Rang sheep with body weight of 20.5 kg (±0.42 kg) were allocated to a changeover design with three treatments (consecutive periods each of 20 days). In period 1 the diet was 100% of guinea grass (1:GG); in the second period it was 30% FTR + 70% GG (2:FTR); in the third period it was 30% RS and 70% GG (3:RS:GG). In each period, the first 14 days were for adaptation to the diet followed by 6 days of measurements of feed intake and digestibility.

The growth of the fungus reduced the content of NDF, ADF and ADL, enhanced the content of crude protein (from 4.2 to 7.1% in DM), and the digestibility of DM (from 43 to 53%) and of the crude protein (from 45.8 to 54.4%). Over the 28-day treatment period, 14.8% of the straw biomass was catabolized.

Keywords: biomass, catabolism, nutritive value, wood-rot fungi

Introduction

In 2017, 44 million tonnes of rice was produced in Vietnam. If it is assumed that the ratio of grain to straw is 1:0.8 with 50 % collectable rice straw, a total of about 17.6 million tonnes of rice straw could be used as a feed source for ruminants. If a daily intake of 10 kg rice straw per head is assumed, about 4.8 million heads of buffalo/cattle could be fed by rice straw. Therefore, improving the nutritional value of rice straw could be very beneficial in the development of ruminant production in Vietnam, especially in the dry winter season when the grazing resources are scarce.

Physical and chemical treatment of rice straw to increase the intake and digestibility by ruminants has been extensively studied inVietnam (eg: Nguyen Xuan Trach 2004). However, Tuyen et al (2012) stated that physical and chemical treatments can be expensive, harmful to users or unfriendly to the environment. Biological methods using white-rot fungi may be a more viable alternative to improve the nutritional value of rice straw. This method is environmentally friendly and potentially economical (Tuyen et al 2012).

Using white-rot fungi to treat rice straw has been studied by Jafari et al (2007), Akinfemi & Ogunwole (2012), Shrivastava et al (2012) and El-Bordeny et al (2015). However, the technology is relatively new in Vietnam.

As a starting point, several strains of Pleurotus were evaluated for their applicability and effectiveness for fermenting rice straw. The results (data unpublished) indicated that the nutritional value of rice straw was improved when incubated for 4 weeks with Pleurotus eryngii. The following experiment was conducted to confirm these interim results by determining the nutritional value of the fermented straw when fed to sheep.

Materials and methods

The experiment was conducted at the Goat and Rabbit Centre Research, Son Tay, Vietnam from February to April 2018. Rice straw (RS) was collected in AnLac village, Trau Quy town, Hanoi city. It was fermented with Pleurotus eryngii (P. eryngii; strain MES 03757) according to the procedure developed by Tuyen et al (2012). Rice straw was soaked in water for 24 h. The soaked rice straw was then removed and drained of water for 24 h, finally, was chopped into lengths of 2–3 cm. Two kilograms (fresh basis) soaked rice straw was packed in polyethylene bags (40 cm length and 30 cm diameter and 2.54 mm thickness), that was immediately tied up with a little cotton on the top of bag by nylon rope. The bags were autoclaved for 1 h at 121 °C. The autoclaved rice straw bags were cooled at 20 °C and then were inoculated with spawn at 2.5% of rice straw (fresh weight basis). All the bags were transferred to the fermentation room, which was maintained at 30 °C and the relative humidity of the room was maintained at 75 % for 4 weeks until all bags were colonized by the mycelia of P. eryngii. Then the bags were removed from the fermentation room and sun-dried for 3 days and stored at an ambient temperature of 20 – 25 °C until feeding to the sheep.

The fungus was sourced from the Wageningen UR Plant Breeding Center, the Netherlands. Guinea grass (Panicum maximum), cultivated at the Goat and Rabbit Centre, was collected daily at 40 – 45 days regrowth.

Experimental design, animals and management

Four male Phan Rang sheep with body weight of 20.5 (±0.42) kg housed in individual pens were allocated to a changeover design of three periods, each of 20 days. In period 1 the diet was 100% of fresh guinea grass (GG); in the second period the diet was 30% fungus -treated rice straw (FTR) and 70% GG (FTR:GG); in the third period it was 30% RS and 70% GG (RS:GG). In each period, the first 14 days were for adaptation to the diet followed by 6 days of measurements of feed intake and digestibility.

The sheep were housed in individual metabolism cages for both adaptation and measurement period. Each metabolism crate had a device for collecting feces and urine separately. The sheep were fed twice daily in equal portions at 06:00 h and 16:00 h. During the adaptation 14 days, all sheep were fed their assigned diets and allowed ad libitum intake. During the following 6 days, diets were offered at 95 % of ad libitum intake per sheep to minimize feed residues. The sheep had access to water and mineral blocks for 24 h per day. The sheep were weighed at the beginning and at the end of the experiment.

Measurements sampling and analytical procedures

During the measurement period, data on feed offered, feed residues and feces were recorded daily for each sheep.

Samples of feed offered and feces were stored at −20 °C before being oven dried at 60 °C for 72 h and ground in a cross-beater mill to pass through a 1mm sieve. Feed and feces samples were analysed for DM, ash and nitrogen according to AOAC (2005) methods. Neutral detergent fiber (NDF), acid detergent fiber (ADF) and acid detergent lignin (ADL) were determined according to Van Soest et al (1991).

The digestibility of separate diet components was measured by the “difference technique” (Khan et al 2003), calculated as follows:

Eg: For FTR ingredient

Digestibility (%) of FTR = [T – C × P/100] / D

Where: T = Digestibility of mixed FTR-GG diet; C = Digestibility of component GG (determined when fed alone); P = proportion of component GG in total diet (%), D = proportion of FTR in the diet (eg: 0.3).

Statistical analysis

All data were analysed by ANOVA using the General Linear Model procedure of SAS (SAS 2010). The model was:

Y = μ + T_i + ε_ij

where Y = the dependent variable, μ = the overall mean, Tj = the effect of treatment (i=1 to 3) and εij=the residual error term. The results are presented as the least squares means and standard error of the means. Differences among main effects were analysed using Tukey-Kramer’s multiple comparison procedure in the LSMEANS statement of SAS (SAS 2010) with effects considered significant at p≤0.05 and a trend at 0.05<pof

Results and discussion

Fungal treatment increased the CP content of the rice straw and the digestibility of the straw and the CP (Table 5). Similar results were reported by Akinfemi and Ogunwole (2012) who reported that CP content of rice straw was increased from 4.67 % to 7.69 % of DM when it was treated with Pleurotus ostreatus, Pleurotus pulmonarius and Pleurotus tuber-regium for 21 days. Similar improvements in CP content of fungal treated rice straw were also found by Jafari et al (2007) and Vorlaphim et al (2018). The increase of CP content could be due to the proliferation of fungi during substrate degradation and also the concentration effect due to the loss of substrate during the fermentation (Table 3).

The concentrations of ADF, NDF and ADL were reduced by fungal treatment. Similar results were reported by Akinfemi and Ogunwole (2012), Jafari et al (2007) and Vorlaphim et al (2018). The fungi require substrates such as cellulose, hemicellulose or other carbon sources for their growth, the end products being fungal protein and carbon dioxide, the latter accounting for the overall 15% loss of substrate DM during the fermentation (Table 3). Tuyen et al (2013) reported a DM loss ranging from 4.5 to 26.2% when rice straw was fermented with Pleurotus fungi.

Fungal treatment increased the apparent digestibility of DM, OM, CP, NDF and ADF (Table 5). These findings are supported by many in vitro studies (Zadrazil 1997; Ko et al 2005; Jafari et al 2007; Akinfemi and Ogunwole 2012). Lignin is one of the major barriers preventing microbial enzymes digesting the cell wall carbohydrates in rice straw. Delignification results in a change in cell wall structure that makes it more readily available to rumen microorganism.

Conclusions

• Fermenting rice straw for 28 days with the fungus Pleurotus eryngii increased the content of crude protein and reduced the content of ADF, NDF and ADL
  
  
• DM and CP apparent digestibilities were increased by fungal treatment
  
  
• Over the 28-day treatment period, the growth of the fungus resulted in the catabolism of 15% of the original straw biomass.

References

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Received 16 January 2019; Accepted 21 January 2019; Published 1 February 2019

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