Animal Nutrition and Feed Technology
Year : 2001, Volume : 1, Issue : 1
First page : ( 61) Last page : ( 68)
Print ISSN : 0972-2963.

Fate of ligno-cellulosic components and urea-nitrogen in urea-ammoniated wheat straw

Elangovan A. V.*, Kishan Jai, Sahoo A.¹

Department of Animal Nutrition & Feed Technology College of Veterinary Science & Animal Husbandry Mathura - 281 001, India.

*Reprint request: Dr. A. V. Elangovan, Division of Nutrition & Feed Technology, Central Avian Research Institute, Izatnagar - 243 122, India. Tel: +91-581-447261; Fax: +91-581-447321; E-mail: elangocari@rediffmail.com

¹Present address: Division of Animal Nutrition, Indian Veterinary Research Institute, Izatnagar-243 122, India.

Received:  13  ,  2000.

Abstract

Effect of both incubation and exposure time on the loss of ammonia-N was tested during the process of urea-ammoniation of wheat straw. The straw chaffed to a size of 5 mm length was ammoniated with 5% urea at 40% moisture level and incubated for 1, 2, 3 and 4 weeks in a air tight glass desiccator having a capacity of 4 kg. Periodically, the incubated lots were exposed, and aired over 3 × 3 sq. ft. area for the determination of N. Representative samples of ammoniated straw were collected after 1,2,3,4,5,6,7,10,14 and 21 days of exposure to determine the DM, CP, CF, ADF, NDF and lignin content in the straw. Due to air drying there was a similar trend in moisture disappearance in all incubated samples with a DM content of 60% on day 1 which air dried to reach a figure of 93% on day 21. The samples from 1 week incubation showed incomplete ammoniation with a CP content of 16.39% on day 1 and it reduced to 10.19% on day 21, a reduction of only 38% due to higher percentage of residual urea-N (51%) compared to 3 and 4 weeks incubated samples. Up on aeration, the CP content on 21st day post exposure was reduced by 38% in 1 and 2 weeks incubated samples compared to 60% in the later. There was a declining trend in CP content which remained almost constant at 6 to 7% in both 3 and 4 weeks incubated lot. No significant difference in CF, ADF and lignin content of treated and untreated straw was seen. However, a reduction of 2to 3 units in NDF and lignocellulose fraction was seen in 3 and 4 weeks incubated samples. From the study of N input to the straw a meagre of 15% N was retained in urea-ammoniated straw resulting in a net gain of only about 55% N by the straw which is available for animal feeding.

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Key words

Urea, arnmoniation, straw, ligno-cellulose, nitrogen.

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INTRODUCTlON

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Materials and Methods

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Results and Discussion

Chemical composition

The moisture content in the urea-treated straw before incubation was 41.06% which reduced to 40.2, 39.6, 38.3 and 37.3% in 1, 2, 3 and 4 weeks incubated samples respectively, suggesting that the chamber was more or less air-tight with very marginal loss of moisture that increased with the increase in incubation time. Due to post-exposure air drying all the samples dried to an extent of 92.4–93.3% (Table 1). The CP content in post-exposure treated straw was 6.2%. The higher value in 1 and 2 weeks incubated samples was attributed to higher percentage of unhydrolysed urea-N (50.7 and 30.9%, respectively). Similar CP content in urea-ammoniated straw was also observed in earlier studies (Mandal et al, 1997; Bhar et al, 1998).

The fibre components showed variable results with no significant change in ADF, lignin and cellulose content between control and treated straw, but a reduction in NDF and hemicellulose content by 2–3 units in treated straw (lc3 and lc4) exposed to 3 and 4 weeks of incubation. A possible alkali hydrolysis of hemicellulose may have resulted in its solubilization in neutral detergent solution which resulted in decrease in NDF and hemicellulose. Cloete and Kritzinger (1984) and Caneque et al (1998) also observed decreased hemicellulose and NDF content in urea-ammoniated straw. The non-significant change in fibre components in 1 and 2 weeks incubated samples (Icl and lc2) was also indicative of incomplete ammoniation process which complete after 3 and 4 weeks of incubation. An increase in moisture level favours ureolysis (Ibrahim, 1986; Hassoun et al 1990) and may thus increases hemicellulose degradation (Van Soest et al 1984). To the contrary, effect of urea treatment on ADF and ADL fractions are variable with no change (Nelson et al 1985; Caneque et al 1998), a slight increase (Mandell et al 1988) or a slight decrease (Reid et al 1988).

Fate of urea-N

From the CP content in 2, 3 and 4 weeks incubated samples (13.9, 13.8 and 13.6, respectively), it was quite evident that urea hydrolysis reached a maximum of 70% at 2 weeks of incubation and a lot of free ammonia-N got escaped into the atmosphere untrapped even after 4 weeks of incubation (Table 1). An increased CP value (16.4%) in 1st week incubated sample was contributed by unhydrolysed urea which remained as incompletely hydrolyzed urea molecule on the external surface of whreat straw. This was further supported by the findings that the analysis of residual urea in the treated straw gave more concentration in the 1st week incubated sample (Table 3). A lower CP value in the straw minus the residual urea-N concluded for incomplete trapping of ammonia as a result of ureolysis, which was observed to be complete by 3 and/or 4 weeks of incubation. Most of nitrogen retained in the straw is in the form of free ammonia (Solaiman et al 1979) which lost during airing, storing or processing. In the present experiment 37.80, 38.84, 55.07 and 54.41 per cent of free ammonia of the treated straw was lost during airing, which accounted to 47.16, 41.34, 58.18 and 56.65% loss of the added urea-N. A lower value in 1 and 2 weeks incubated samples was indicative of incomplete ammoniation. It was further noticed that the loss of free ammonia from the exposed lot became almost complete after 7 days of exposure (Table 1). The total ammonia loss to the atmosphere starting from day 1 of incubation to day 21 of post exposure of the ammoniated wheat straw was summed to be 48.23,61.24, 78.85 and 78.85% of the added urea-N. The loss was considered to be of major concern for the environmentalists as well as the nutritionists and efforts are therefore, being made to reduce the total loss through various methods of trapping the free ammonia (Borhami et al 1982; Cloete and Kritzinger, 1984; Yadav and Virk, 1994, Mandal et al 1997; Bhar et al., 1998; Dass et al, 2000).

On the other hand, retention of nitrogen, which always misquoted to be 75–80% was a meagre 15% in the post-exposure treated straw. Above all, a net gain of only 55% was observed in the treated straw that had undergone 4 weeks of incubation. Caneque et al (1998) observed a maximum N-retention of 39.69% as a percentage of added urea-N at maximum ureolysis, which they obtained at a moisture level of 40% and temperature 35°C. Chermiti et al, (1989) also observed the N-retention between 25 and 31%. In the present study, the moisture level was maintained at about 40% and a similar expression of N-retention was observed to be 14.35% in 4 weeks incubated sample. Contrary to earlier findings the lower value was attributed to post exposure loss of ammonia N. Bhar et al (1998) advocated differentisealing methods to check the loss of ammonia-N during incubation, and they could be able to reduce the loss to 32% from as high as 48% and the moisture level was also better preserved to give a net N loss of 43% after 72h of air exposure. The residual urea-N which adds to the increased N content of treated straw even after airing for 21 days, contributed significantly to total N availability from treated straw.

Urea-ammoniation process was thus found to be complete after 3 weeks of incubation at a moisture level of around 40% and temperature 30–38°C.

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Tables