In vitro digestibility of rice straw after mushroom  cultivation and the utilization of nutrients of enriched rice straw
by Red Sindhi crossbred cattle

Abstract

Two experiments were carried out to investigate nutrient contents and in vitro digestibility values of  rice straw after it's use to cultivate mushrooms and to evaluate the effect of rice straw enriched by urea, molasses and minerals on in vivo digestibility and nutrient intake of cattle. In Experiment 1 a complete randomised design was used for the in sacco evaluation of rice straw at three stages: initial (R), at 20 days treated with water (RW20) and after harvesting mushrooms (MS). There were 8 replicates of each treatment. Faeces of a Murrah buffalo were  used as inoculant. In Experiment 2, four Red Sindhi crossbred cattle (138 ± 12 kg) were allocated in a 4 x 4 Latin Square design to four diets, which included rice straw (R), rice straw plus urea-molasses-mineral mixture (RS+UMM), urea-treated rice straw (URS) and urea-treated rice straw  plus molasses-mineral mixture (URS+MM).

The crude protein and ash in MS was higher and NDF lower than in R and RW20. In vitro digestibility values at 24 h were similar but were lower for MS at 96 h. The MS after drying was dark, dusty and had a bad smell and was not palatable to cattle. 

In Experiment 2,  adding urea, molasses and minerals to rice straw, or molasses and minerals to urea-treated rice straw,  increased intake, DM in vivo digestibility and N retention.

 Key words: Cattle, rice straw, mushrooms, digestibility, intake  
  

Introduction

The Mekong delta of Vietnam is the main rice bowl of the country, thus a large amount of rice straw is produced. Some is used for feeding ruminants, some is burned and the ash used as fertilizer, and a part of it is used for cultivation of mushrooms. The rice straw after harvesting the mushrooms is normally used as fertilizer. Neelakantan and Deodhar (1993) reported that white rot fungi (Actinomycetes) were able to biodegrade lignin and ligno-cellulose. From this arose the idea that growing mushrooms on rice straw might improve its nutrient value for feeding to ruminants.

Another approach to improving rice straw quality is to supplement it with molasses, urea and minerals (Thu and Udén, 2000).

This study, therefore, aimed to investigate: (i)  the nutrient content and in vitro digestibility values of the residue from straw used to grow mushroom; and (ii) and effect of supplementing rice straw with urea, molasses and minerals.
 

Materials  and methods

A complete randomized design was used to evaluate the nutritive value of rice straw used for growing mushrooms. Samples for analysis and in vitro digestibility were taken  at the initial stage (R), at 20 days after treatment with water (RW20) and after harvesting the mushroom (MS). There were eight replicates (batches of straw used for growing the mushrooms). Samples were analysed for DM, N and NDF by AOAC (1990) procedures and used to measure organic matter degradability by the in sacco method using faeces from Murrah buffalos fed natural grasses as inoculant (Thu and Udén 2003).

In the Exp.2 four Red Sindhi crossbred cattle (138 ± 12kg) were allocated in a 4 x 4 Latin Square design with four diets: rice straw (RS), rice straw  with added urea-molasses-mineral mixture (RS+UMM), urea-treated rice straw (URS) and urea-treated rice straw plus molasses-mineral mixture (URS+MM). Each experimental period was three weeks, including one week for adaptation. Feeds were offered twice a day at 7:15 and 14:15h. Feeds offered and refusals were collected daily and pooled weekly for analysis of DM to calculate feed intake. In vivo DM, OM and NDF digestibility values were estimated by total faecal collection for 7 d beginning 2 d after the start of feed intake recording (McDonald et al 1998). Rice straw was treated with 5% urea (100 kg straw, 5 kg urea, 100 kg water), and it was fed to experimental animals after 21 days of storage. Rice straw in the RS diet before feeding was added water to make it softer (36.8% water based on fresh matter basis). The RS+UMM diet included 1.2% urea, 10.6% molasses and minerals. For the URS+MM diet, 12.7% molasses and minerals were added 24hr before feeding.

In both experiments feed samples were dried at 105^oC overnight to determine dry matter (DM). The OM was determined by ashing samples in a furnace at 500^oC for 4 h. The crude protein was determined by the Kjeldahl method (AOAC 1980). Analysis of neutral detergent fiber (NDF) was by a method described by Van Soest et al (1991). Acid detergent fiber (ADF) was analyzed by the method suggested by Robertson and Van Soest (1981). Feeds offered and refusals were collected daily and pooled weekly for analysis of DM to calculate feed intake. 

In Experiment 1, the data were subjected to analysis of variance (ANOVA) using the General Linear Model (GLM) procedure of Minitab (1998). When the F test was significant (P<0.05), Tukey’s test for paired comparisons was used. The data from in vitro degradability were fitted to a non-linear model (Ørskov et al 1980):

DMD= a + b (1- e^{-ct )},

where DMD = DM disappeared after time (t), a = the intercept on the "y" axis, c= fractional degradation rate (h^-1) and a+b = the curve asymptote  representing the potential degradability. The Table Curve 2D V4 program (Table Curve 2D V4 1987) was used for curve fitting. In Experiment 2 data were subjected to analysis of variance (ANOVA) using the General Linear Model (GLM) procedure of Minitab (1998).
 

Results and discussion

Experiment 1

The cultivation  of mushrooms reduced the organic matter and NDF content of the straw but increased the crude protein content (Table 1).