Effect of different processing methods (chemical and biological) on chemical composition, gas production parameters and digestibility of wheat straw

Crop residues such as straws are lignocellulosic by-products which results from crop cultivation. These compounds can be used as carbohydrate and energy sources in ruminant nutrition. The presence of strong physical and chemical bonds between lignin, cellulose and hemicellulose reduces the nutritional value of these materials. Straws produced as cereal by-products are an important feedstuff for ruminants. Howere, they are not preferred by animals due to poor digestibility, low nitrogen and mineral contents. Many of the treatments were conducted to improve the nutritional value of low feed quality including physical, chemical treatments and biological treatments. Several chemical treatment methods have been developed to improve the nutritive value of low quality forages by reducing the concentration of cell walls or increasing their digestion. Alkali treatment is the most widely used method for increasing the degradation of forage cell walls and the main alkalis used are ammonia and NaOH. The underlying principle of the method is partial solubilization of hemicellulose, lignin, and silica and hydrolysis of uronic and acetic acid esters by the alkali (Chesson et al., 1983). Various studies have demonstrated the effectiveness of dry and wet NaOH treatment processes at improving the nutritive value of low quality forages and crop residues (Wanapat et al., 1985; Moss et al., 1990). However, the use of ammonia and other alkalis for forage improvement has been limited by their corrosive nature and the hazards they pose to humans. Recently, several studies showed that the biological treatments of crop residues to improve the accessibility of cellulosic fractions, thus improving their digestibility and feeding value have been attracting the extensive interests. This study aimed to evaluate the effect of different processing methods (chemical and biological) on chemical composition, In vitro gas production parameters and digestibility of wheat straw.

wheat straw was prepared from minoodasht city. The following treatments were: 1) untreated wheat straw (control, CON), 2) CON processed with Urea, 3) CON processed with sodium hydroxide, 4) CON processed with hydrogen peroxide, 5) CON inoculated with Aspergillus niger for 21 days, 6) CON inoculated with Trichoderma harzianum for 21 days, 7) CON inoculated with Aspergillus niger for 40 days and 8) CON inoculated with Trichoderma harzianum for 40 days, in a completely randomized design. In order to processing with H2O2, first the samples pretreated with sodium hydroxide (NaOH, 80 g/kg DM) to attain and maintain a pH of 11.5, then 132 ml H2O2 (purity: 35%) were added. Treated samples were then placed into plastic bags, tied up and stored under anaerobic conditions. Then the bags were opened and samples dried by exposure to air. After processing the chemical composition of the samples was determined using the standard methods. In vitro cumulative gas production was determined using 120 ml serum bottles as described by Theodorou et al. (1994). A buffered mineral solution (Menke and Steingass 1988) was prepared and placed in a water bath at 39 ◦C under continuous flushing with CO2. Rumen fluid was collected before morning feeding from three ruminally fistulated steers fed on a forage diet at a concentration of 40:60. In vitro gas production was measured in triplicate on composite samples from the same treatment silos. For each replicate, a sample of 200 mg DM untreated and inoculated corn silage obtained from the d 45 and 90 mini silos were used. The bottles were then filled with 30 ml of incubation medium that consisted of 10 ml of rumen fluid plus 20 ml of buffer solution and placed in a water bath at 39 ◦C. Gas production was recorded at 2, 4, 8, 16 24, 48 and 72 h. Total gas values corrected for blank incubation and gas values expressed in ml g-1 of DM. A pressure transducer and LED digital read out voltmeter were used to measure the headspace gas pressure in the culture bottles. Volumes of gas at the top of the culture bottles were transferred into a syringe by withdrawal of the syringe plunger until the pressure became zero. A medium similar to one developed for gas production was used for batch rumen culture system to measure pH, and NH3-N and in vitro digestibility. The pH of the media was measured after 24 h incubation. After 24 h incubation, the contents of each glass bottle were empty, strained through four layers of cheesecloth and then 10 ml of strained rumen fluid was acidified by 10 ml of 0.2 N HCl for determination of NH3-N using the distillation method. Finally, all contents remaining in the bottles were filtered through nylon bags, oven dried at 60 °C for 48 h and analyzed for IVDMD and IVOMD.

Results showed that there were significant differences among treatments on chemical composition (p < 0.05). The highest and the lowest DM content were related to control treatment and soaked treatments. Sodium hydroxide and hydrogen peroxide treatment's had highest and urea treated wheat straw had lowest Ash content. Processing with two kind of fungi had no effect on chemical composition (p>0/05). Different processing methods had significant effect on potential and rate of gas production (p < 0/05). Soaked and trichodermal treated wheat straw had lowest and sodium hydroxid and hydrogen peroxide had highest gas production potential. Processing with Sodium hydroxid and hydrogen peroxide increased DM and OM digestibility significantly, Howere, Soaked wheat straw had lowest digestibility, PF and microbial crud protein.

Generally, processing with Sodium hydroxide and hydrogen peroxide had more effect on improving the nutritional value of wheat straw.