Effects of urea concentration, moisture content, and duration of treatment on chemical composition of alkali treated rice straw

Nguyen Xuan Trach*, Cu Xuan Dan *, Le Viet Ly^** and Frik Sundstøl^***

^* Hanoi Agricultural University, Gialam, Hanoi, Vietnam
^** National Institute of Animal Husbandry, Tuliem, Hanoi, Vietnam
^*** Agricultural University of Norway.

Abstract

An experiment was carried out to examine the effects of two levels of alkali (5% urea and 3% urea plus 0.5% calcium hydroxide), three ratios of water/straw (0.5:1, 0.75:1 and 1:1), and three treatment durations (10, 20 and 30 days) on chemical composition of rice straw after aeration and pre-drying at 65 ^oC. It was found that the levels of the different factors did not affect the neutral detergent fibre (NDF) and acid detergent fibre (ADF) content, although these two measurements were slightly lower in treated straw than in untreated straw. The more urea and the longer the treatment duration, the higher was the content of chemically fixed nitrogen (P<0.05). However, the nitrogen content was not significantly affected by the change of water:straw ratio from 0.5:1 to 1:1. Verification of these findings in practical feeding trials is needed as there are economic advantages from replacing some of the urea with calcium hydroxide.

Key words: urea treatment, rice straw, NDF, ADF, nitrogen content

Introduction

Although ammoniation with urea has been considered to be a method of choice for improving the feeding value of rice straw as feed for ruminants (Schiere and Ibrahim 1989; Sundstøl et al 1993; Chenost and Kayouli 1997), the optimal conditions for the treatment is not yet fully defined. This may be one of several reasons why urea treatment of rice straw has not been widely applied in spite of expectations of rapid implementation in many developing countries (Preston 1995). Therefore, further technical and economic considerations of urea treatment of rice straw should be undertaken before it may become a routine practice for ruminant holders in the tropics.

The present paper reports a laboratory experiment carried out to determine the effects of urea concentration, moisture content, and duration of treatment on chemical composition of straw of a rice variety grown under irrigation. This was a first step in a series of studies to investigate related aspects of treatment of rice straw as feed for ruminants under the particular circumstances of the humid tropics of Vietnam.

Materials and methods

Treatment of straw

Sun-dried straw of a rice variety called C70 (88% dry matter) was chopped to a length of 5-7 cm and treated in air-tight polyethylene bags at room temperature (15-21^oC) according to a 2 x 3 x 3 factorial design (18 different treatment combinations) involving:

• 2 levels of alkali: 5% urea and 3% urea plus 0.5% calcium hydroxide/dry straw,
• 3 ratios of water/ straw: 0.5:1, 0.75:1 and 1:1, and
• 3 treatment durations: 10, 20 and 30 days.

After treatment the straw was aerated for 24 hours and pre-dried at 65^oC for another 24 hours to allow evaporation of free water and free and loosely bound ammonia. The main purpose of the aeration and pre-drying was to help detect the amount of nitrogen which was chemically fixed to the straw structure after treatment. In addition, untreated straw samples were also taken and pre-dried for parallel chemical analyses.

Chemical analyses

Dry matter (DM), ash and Kjeldahl-nitrogen were determined according to the AOAC methods (AOAC 1990). Neutral detergent fibre (NDF) and acid detergent fibre (ADF) were determined according to Goering and van Soest (1970). All the analyses were run in triplicates.

Statistical analyses

Fixed effects of the different factor levels of moisture, alkali, duration of treatment, and their interactions on the contents of NDF, ADF and fixed nitrogen (% of DM) of the treated straws were analysed by means of a 3-way layout analysis of variance (ANOVA). Pairwise multiple comparisons of means were made using the least significant difference (LSD) method.

Results and discussion

Effects of alkali level

In the present experiment rice straw was treated with either 5% urea or 3% urea plus 0.5% Ca(OH)₂, regarded as two different alkali levels. The latter treatment aimed at reducing the cost of chemicals as calcium hydroxide is a cheap alkaline agent. In addition, since urea hydrolyses to ammonium carbonate, a weaker base than ammonia which yields ammonium hydroxide in water as a saponifying agent, ammonium carbonates are more effective when used with calcium hydroxide, which removes carbonate and generates hydroxide (Van Soest 1994).

The contents of NDF and ADF (Table 1) were not significantly different between the two alkali treatments, but the nitrogen content was influenced by the concentration of urea (P<0.01). This result indicates that the amount of nitrogen in straw after treatment increased with the concentration of urea applied, being in agreement with results reported earlier (Saadullah et al 1981; Sundstøl and Coxworth 1984). However, the nitrogen content in this study was increased by only 0.16 percentage units on a DM basis (equivalent to 1% crude protein) at the expense of 2% urea, i.e. only 17.4% of the additional urea nitrogen increment was fixed. The low increment of nitrogen was, on the one hand, due to the fact that the nitrogen detected was only the fraction of nitrogen which was chemically fixed to the cells of the straw and insoluble in water, thus being unrepresentative of the total nitrogen in the treated straw as usually determined. On the other hand, the nitrogen fixation ratio usually falls with the increase in the urea level because large amounts of free ammonia (not fixed yet) built up within the straw matter may stop or hinder hydrolysis of the urea (Chenost and Kayouli 1997). Therefore, treatment with 3% urea plus calcium hydroxide may be more economical if it has good effects on digestibility and intake of straw by the ruminant.

Table 1: Chemical composition (%DM) of rice straw as affected by levels of urea, moisture and treatment duration
	NDF	ADF	Nitrogen
Untreated	72.9	45.5	0.590
Effects of alkali level
5% urea	70.9a	44.9 a	1.11 a
3% urea plus 0.5% Ca(OH)2	71.3 a	45.1 a	0.95 b
Effects of water/straw ratio
0.5:1	70.8 a	45.1 a	1.05 a
0.75:1	71.2 a	44.8 a	1.03 a
1:1	71.4 a	45.2 a	1.02 a
Effect of treatment duration
10 days	71.6 a	45.0 a	0.98 a
20 days	71.3 a	45.0 a	1.00 a
30 days	71.3 a	45.1 a	1.12 b
ab Means in the same column within each treatment factor bearing the same superscript are not significantly different at the 5% level.

Effects of moisture level

There were no significant differences among the three ratios of water to straw on the average contents of NDF, ADF and nitrogen (Table 1). It appears that ratios in this range of 0.5:1 to 1:1 are acceptable. This range falls within the limits usually recommended for practical application (Schiere and Ibrahim 1989; Chenost and Kayouli 1997). According to these authors, a ratio greater than 1:1 may lead to a poor final product with dark, unpalatable and pasty straw. Too much water would make straw too soft and, in addition, the ammonia being hygroscopic may be trapped by the water before fixing itself to the cell walls. On the other hand, hydrolysis of the urea can only occur if sufficient water is present. An adequate amount of moisture also helps in the compaction of straw driving out the air, thus increasing the concentration of ammonia.

Effects of treatment duration

Cell wall constituents (NDF and ADF) did not differ significantly between the three treatment durations (10, 20, and 30 days). The fixed nitrogen content increased over the treatment time, especially after 20 days (P<0.05). This finding suggests that the added nitrogen existed mainly in the form of urea at the beginning with free ammonia being released after that. Later, more and more nitrogen from free ammonia would be fixed together with the straw structure, and only this fixed nitrogen could be detected by the Kjeldahl analysis after the sample was aerated and pre-dried, resulting in evaporation of the free and loosely bound ammonia. It is therefore likely that with short treatment times the response to urea ammoniation may be reduced. A rough calculation showed that, on average, about 29% of the urea-nitrogen was really fixed within the straw structure after 1 month of treatment. According to the results of this experiment, the treatment duration should not be shorter than 20 days if the ambient temperature is lower than 21^oC.

Conclusions

The NDF and ADF cell wall constituents of rice straw were slightly reduced after treatment, but did not apparently vary in response to the source of alkali (5% urea or 3% urea plus 0.5% Ca(OH)₂₎, the treatment duration (10, 20 and 30 days) and the water:straw ratio (range of 0.5:1 to 1:1).

The amount of the nitrogen which was fixed to the straw structure increased significantly over the time of treatment up to at least 30 days.

When the urea level was increased by 2% units from 3% to 5% of the straw, only 17.4% of the additional urea nitrogen was fixed. This means too much nitrogen is lost when the level of urea applied is high.

Partial replacement of urea with Ca(OH)₂ could be technically and economically justified. However, it warrants more detailed comparative studies.

Chemical analyses alone are not enough to evaluate the improved feeding value of rice straw after alkali treatment. Other methods of evaluation are needed to define a good set of conditions for alkali/urea-ammoniation treatment of rice straw.

Acknowledgements

The authors are grateful to the Norwegian Council of Universities' Committee for Development Research and Education (NUFU) for the financial support for the proposed series of studies on better use of crop residues as feed for ruminants in Vietnam.

References

Received 9 February 1998

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