Studies on the ammoniation of sugar-cane bagasse by urea. Effects of moisture, urea levels, urease source and treatment periods on composition, in vitro dry matter digestibility and evolution of ureolytic bacteria

Abstract

This work was conducted in two experiments. In the first experiment about 500 g (for each treatment condition) of dried bagasse (90% of dry matter (DM)) were sprayed with urea solution plus 30 ml of cattle rumen fluid kg −1 bagasse dry matter, or without the addition of rumen fluid. Four levels of urea were applied: 70, 88, 106 or 124 g kg −1 DM. Urea solutions (with or without rumen fluid) were added to increase the moisture content to 500, 600 or 700 g kg −1. The samples were stored outside in sealed plastic bags at ambient temperature (about 24°C) for 2, 4, 6 or 8 weeks. All parameters were investigated in a 3 × 4 × 2 × 4 factorial design. When the plastic bags were opened, unhydrolysed urea (Urea op.) was measured and the moisture content controlled. After drying (4 days at 50°C) samples were measured for chemical composition, residual urea (Urea dr.) and in vitro dry matter digestibility (IVDMD). Untreated bagasse was used as control. Rumen fluid had not significantly ( P > 0.05) affected measured variables. The level of rumen fluid was probably too low and perhaps had too little ureasic activity. Urea op. was not affected by the treatment period ( P > 0.05). But the degree of urea hydrolysis was significantly ( P < 0.01) increased by moisture, whereas urea level had a significantly adverse effect ( P < 0.01 on hydrolysis. Urea dr. was significantly affected ( P < 0.01) by treatment period, moisture and to a lesser extent ( P < 0.05) by urea level. Urea hydrolysis during the drying phase continued with more intensity as the treatment period was longer and the moisture lower. Total nitrogen content corrected for Urea dr. content, was significantly ( P<0.01) increased when treatment period and urea level increased, and significantly decreased ( P<0.01) when moisture increased. Neutral detergent fiber (NDF) and acid detergent fiber (ADF) contents were significantly affected ( P<0.01) by treatment period, moisture and urea level. The treatment has decreased the NDF content whereas it increased the ADF content (from 90 to 85% and from 54 to 57% DM, respectively). The IVDMD significantly increased ( P<0.01) when treatment period, moisture and urea level increased and reached about 39–40%, whereas untreated bagasse was 31%. In the second experiment, about 100 g of dried bagasse (95% DM) for each treated sample were sprayed with urea solution. Two levels of urea were applied, 66 or 104 g kg −1 DM. Urea solutions were added to increase the moisture content to 500 or 700 g kg −1. The samples were stored in sealed plastic bags at 26°C for 2, 4, 6 or 8 weeks. When the plastic bags were opened, unhydrolysed urea, ureolytic bacteria and DM content were measured. After drying (4 days at 50°C), samples were measured for residual urea and ureolytic bacteria. The DM content was very different after the treatment. It decreased with more intensity as the treatment period was longer and moisture higher. After 8 weeks of treatment, DM content on average was 22.5 and 47.5% for respective initial values of 30 and 50%. The degree of urea hydrolysis, was increased by treatment period and moisture. When moisture was high (70%), more than 90% of urea was hydrolysed after 2 weeks. Residual urea measured after drying was not very different compared with the unhydrolysed urea value. The ureolytic bacteria population was affected by the urea treatment. The number of bacterial colonies increased with treatment time and was a little higher when moisture was 70%. Urea levels did not affect bacterial growth. On the contrary, the ureolytic population decreased after the drying phase. The results of these two experiments showed that urea treatment can improve the nutritive value of sugar-cane bagasse, increasing both total nitrogen content and IVDMD. They also show that ureolysis reaction is dependent upon ureolytic bacterial activity, and in our treatment conditions no urease is needed.