Livestock Research for Rural Development 9 (5) 1997

Citation of this paper

Assessment of various treatment conditions affecting the ammoniation of long straw by urea

M Hadjipanayiotou and S Economides

Agricultural Research Institute, 1516 Nicosia, P.O.Box 2016, Cyprus


The effect of moisture level (trials 1, 2 and 3) and of treatment period (trial 1) on the nutritive value of urea-treated straw (UTS) was studied in vivo and in sacco. In trial 2, untreated straw (US) sprayed with urea solution prior to feeding was compared with UTS and US. The effect of covering UTS with polyethylene sheet was studied in trial 4.

With the exception of crude protein content that was greater in UTS than US, there were no differences between US and UTS for crude fibre, ash, NDF or ADF. In vitro digestibility and in sacco degradability were greater in UTS than US. Maximum improvement was attained with the first level of moisture in trials 1 (200 litres/tonne) and 3 (300 litres/tonne), but in trial 2, UTS at 300 litres moisture/tonne had greater digestibility than UTS at 200 litres moisture/tonne. Although maximum digestibility and degradability of UTS were obtained in the second week of treatment, differences between treatment periods were not significant. There was superiority of UTS over US sprayed with urea prior to feeding (trial 2), as well as between UTS covered and non-covered (trial 4) following spraying.

Based on the findings of the present studies the following recommendations can be made for straw treatment under Mediterranean conditions. Treatment time of 1-2 weeks is required during summer months, and a moisture level of from 200 to 300 litres/tonne is sufficient for maximum response. Covered UTS is superior to non-covered, and UTS is also superior to urea-spraying prior to feeding.

Key words: urea treatment of straw, moisture level, reaction period, digestibility, degradability


The upgrading of cereal straws by means of ammoniation with gaseous or liquid ammonia has received considerable attention in many temperate countries (Sundstol et al 1978). An alternative method of ammoniation, using urea as the source of ammonia, has been reported by several research workers (Saadullah et al 1981; Kritzinger and Frank 1981; Hadjipanayiotou 1982a; Cloete and Kritzinger 1984; Dias-Da-Silva and Sundstol 1986). Many of the factors influencing the effectiveness of straw treatment with urea (Sundstol et al 1979; Hadjipanayiotou 1989; Cloete and Kritzinger 1984, 1985) like type and level of chemical, reaction period, ambient temperature, quantity of water (moisture level) and physical form (Kay 1972), are closely related to the economics of straw treatment (Hadjipanayiotou 1989). Despite the fact that both methods of ammoniation are widely applied in certain parts of the world (Tingshuang et al 1993; Tingshuang and Zhenhai 1996; Chenost and Kayouli 1997), the need for additional studies, particularly under local, and/or regional conditions, may be of importance for improving both the effectiveness and the economics of straw treatment. The objective of the present work was to assess various treatment conditions affecting the ammoniation of long straw by urea in the hot and dry climate of the Mediterranean region.

Materials and methods

The Norwegian stack method (Sundstol et al 1978) was used for treating rectangular bales (86 x 48 x 34 cm) of long straw of 10 - 12 kg each. The straw was sprayed mechanically with a solution of fertilizer-grade urea (40 kg urea/tonne of straw, as fed basis). The concentration of urea solution varied in different trials. Bales of straw were sprayed on the top surface of each layer. Polyethylene sheet was placed at the bottom of the stack on a layer of sand (1-3 cm) to prevent puncturing of the plastic and leakage of ammonia. A 20-30 cm strip of free plastic sheet was left to facilitate sealing of the stack. Special care was taken in folding the corners. The two plastic sheets (bottom and top) were then covered with soil. The stacks were opened 1-4 weeks following spraying and sealing, depending on the experiment. Representative samples were taken prior to treatment and at the opening of the stack. Treated samples were left in open plastic bags for 24 h before drying at 65 oC to constant weight. Total nitrogen (macro-Kjeldahl method) on urea-treated straw (UTS) was made before drying. The dried samples were ground to pass 1 mm sieve for the determination of ash, DM and nitrogen (MAFF 1973), in vitro digestibility (Tilley and Terry 1963), ADF, NDF and lignin (Harris 1970). Degradability of organic and dry matter was measured in the rumen of three Chios sheep using Dacron bags incubated in the rumen for 48 h (Mehrez and Orskov 1977). Apparent digestion coefficients were determined using castrated Chios rams placed in metabolic crates. Animal feeding, housing and management, urine, faeces and feed samples collection, processing and analysis were as outlined by Hadjipanayiotou (1982b).

Trial 1:

Twenty-four stacks of 32 bales each were made to study the effect of quantity of water (moisture level) and of treatment period on the nutritive value of urea-treated barley straw. Bales were sprayed with fertilizer-grade urea (40 kg/t of straw) diluted in three quantities of water (200, 300 and 400 litres/tonne of straw). Four stacks, representing four treatment (storage) periods (1, 2, 3 and 4 weeks) were sprayed for each quantity of water . Each treatment was replicated twice. For assessing treatment effects on in vitro digestibility, in sacco degradation and chemical composition a completely random design with factorial arrangement of treatments (4x5) was used. Measurements were taken in duplicate on representative samples collected from different parts of the stack. In the middle of the 24 stacks there was one stack with untreated straw (US, control), representing the treatments where no urea was sprayed/applied (urea/water: 0/0, 0/200, 0/300, 0/400; treatment period/water: 0/0, 1/0, 2/0, 3/0, 4/0). Sources of variation in the analysis of variance were factor A (four levels of water, 0, 200, 300, 400 l/t), factor B (treatment period, 0, 1, 2, 3 and 4 weeks), and their interaction (n=12). The crude protein and ash content of the straw used for treatment were 6.75% and 6.89%, respectively.

Trial 2:

Bales of barley straw were sprayed at stacking with a 20% (UTS200) or 13.3% (UTS300) urea solution at the rate of 200 or 300 litres/tonne, respectively. Both stacks were of 250 bales, and were opened for evaluation two weeks following spraying. The former two treatments were compared with US and US sprayed prior to feeding with an amount of urea equivalent to that retained on the treated straw (100 litres solution/tonne). The four treatments were evaluated in vivo using four mature castrated Chios rams per treatment. The straw used in this study had 7.22% crude protein and was highly contaminated with soil (ash content 15.8%).

Trial 3:

Forty kg of fertilizer-grade urea were used for each of three moisture levels (300, 400 and 500 litres/tonne of straw) for treating stacks of 1200 bales of straw each. The crude protein and ash content of US were 4.0% and 9.4%, respectively. Spraying, stacking and sealing of stacks with polyethylene sheet were as in trials 1 and 2. Stacks were opened for evaluation two weeks after treatment. Untreated, and urea-treated straws were evaluated in vivo using six castrated Chios rams per treatment. Rumen NH3-N and pH were measured on five rumen fistulated ewes offered the four straws ad libitum. To the animals on US and UTS, offered alone, 30 g of a vitamin-micro-macro element mixture were offered per animal daily. The vitamin-micro-macro element mixture was composed (g/kg) of 800 dicalcium phosphate, 87 NaCl, 87 Na2SO4, and 26 Vita 6 (Vita-Trace Nutrition, Cyprus). The potential of 2 kg of Vita 6 was 6 000 000 IU vitamin A, 1 000 000 IU vitamin D3, 8 500 IU vitamin E, 25 000 mg Mn, 1 750 mg I, 45 000 mg Zn, 30 000 mg Fe, and 2 000 mg Co.

Trial 4:

The effect of covering with polythene sheet on the feeding value of urea-treated straw was studied in a digestibility trial with four Chios castrated rams using three types of straws (US, urea sprayed straw covered [UTS-C] or not covered [UTS-U]). The crude protein and ash content of US were 7.03% and 5.88%, respectively. UTS was sprayed with a 10% urea solution applied at the rate of 400 litres/tonne of straw. Animals were offered the same allowance of the vitamin-macro-micro element mixture used in trial 3. Data collected were analyzed by one-way analysis of variance.



Treatment of straw took place in summer, when temperatures were high; the mean maximum was 35.3 oC (range 33.5-37.0 oC), and the mean minimum 19.9 oC (range 18.0-22.0 oC). The effect of treatment period and moisture level on the chemical composition, in vitro digestibility and in sacco degradation of UTS is shown in Tables 1 and 2. There were no differences in crude fibre, NDF or ADF content between US and UTS; but, as expected, the crude protein content was higher for UTS (P<0.001) than for US. Both in vitro OM digestibility and the in sacco DM and OM degradability after 48 h incubation in the rumen were greater (P<0.01) in UTS than US. Treatment period and moisture level had no effect on the chemical composition of the straw and the observed differences in digestibility were not significant.

The effect of two levels of water (40 kg of urea dissolved in 200 or 300 litres of water/tonne of straw), and of application of urea solution on US prior to feeding, on straw intake and in vivo digestibility is shown in Table 3. Although there was no difference in the liveweight of rams on the four diets, voluntary intake was greater by rams on the UTS sprayed with 300 litres/tonne straw (UTS300) of urea solution (13.33%) than on the other three diets. Similarly, straw sprayed with 13.33% urea solution at the rate of 300 litres/tonne had higher (P<0.01) DM and OM digestibility than the US, US sprayed with a urea solution prior to feeding (USS) and UTS sprayed with 20% urea solution at the rate of 200 litres/tonne (UTS200).

The data from trial 3 showed that the level of water (40 kg of urea dissolved in 300, 400 or 500 litres of water/tonne straw) did not affect DM, OM, CF, ADF and NDF digestibility (Table 4). All three UTS straws had greater digestion coefficients than US.

The effect of feeding the three UTS and the US on rumen ammonia nitrogen and pH is in Table 5. Rumen pH was greater (P<0.05) when feeding US than UTS500. The difference between US and UTS300 and UTS400 did not reach significance. Overall, rumen pH was greater (P<0.05) when feeding US than UTS (6.65 vs 6.41), whereas it was the opposite with rumen ammonia nitrogen (P<0.01). Rumen ammonia nitrogen was highest for the UTS400, followed by UTS300 and UTS500.

An increase in digestibility with urea treatment (P<0.001) was obtained without covering the stack with polyethylene sheet, but this increase was lower compared with the covered stack (Table 6).


There is variation in reported observations on urea-treated straw which has been ascribed by Mandell et al (1988) to differences in treatment conditions (level of ammonia, ambient temperature, straw moisture level) and straw characteristics (species, cultivar, quality). Waagepetersen and Thomsen (1977), Sundstol et al (1979) and Cloete and Kritzinger (1984) have shown that treatment time is affected by temperature. In previous studies (Hadjipanayiotou 1982a; Cloete and Kritzinger 1984) with temperatures lower than the ones prevailing in the present studies, an improvement in the in vitro DM digestibility was obtained after 6 to 7 weeks of treatment. Ammoniation was accelerated at higher temperatures, particularly at the higher moisture level (Cloete and Kritzinger 1984). The temperatures prevailing during straw treatment in the present studies were higher (maximum over 35 and minimum over 20 oC) than the ones prevailing in the former mentioned studies and could explain, therefore, the satisfactory response obtained after the first to second week of treatment. Completion of treatment in one week at a temperature greater than 30 oC has also been reported by Sundstol et al (1979). Since polyethylene sheet can withstand only a relatively short period of exposure to sunlight, the short treatment time required for straw treatment in the dry, hot Mediterranean climate would certainly facilitate treatment of many batches of straw using the same sheet, leading to lower treatment costs.

The effect of moisture level on the digestibility of UTS has also been variable. Williams et al (1984) reported a significant increase in straw degradability by lowering the straw DM from 750 to 450 g/kg, whereas Tinnimit (1987) found that DM digestibility with the least water (0.2litres:1 kg straw) was highest, and that digestibility tended to decrease with increasing amounts of moisture in UTS. Schneider and Flachowsky (1990), testing ammoniation at three levels of moisture (120, 300 and 450 g/kg straw), reported that the highest DM digestibility was obtained at 300 g moisture/kg. In the present studies, the degree of improvement was similar for the various levels of water used in trials 1 (200, 300 and 400 litres/tonne of straw) and 3 (300, 400 and 500 litres/tonne of straw). But in trial 2 there was an improvement in in vivo digestibility of UTS by increasing the amount of water from 200 to 300 litres/tonne. The only differences between trials 1 and 2 were the ash content of the straw and the size of stacks (see materials and methods). Knowing that excess water leads to difficulties in applying the urea solution, it is better to recommend a moisture level of 200 - 300 litres/tonne.

Urea that is converted to ammonia is only partly retained in the straw, and partly evaporated when opening the stack. Of the total urea-N applied on the straw around 65% was retained, and this is somewhat higher than previously reported values (Hadjipanayiotou et al 1997).

In line with previously reported data (Cloete and Kritzinger 1984; Hadjipanayiotou et al 1997) there was a marked superiority of urea treatment of straw as opposed to urea supplementation prior to feeding. This has been ascribed by rskov and Grubb (1978) to the fact that NPN in ammoniated straw is becoming available when the availability of the energy component in straw is increased.

Covering of UTS with polyethylene sheet improved its N-content (15.8 vs 13.4%) and in vivo dry matter digestibility (58.7 vs 50.1%) compared to UTS that was not covered with plastic; this might be due to higher temperatures in the covered stack, and greater losses of ammonia in the uncovered stack.

Overall, there was an increase of 8.8 percentage units (range 4.4-15.7) in in vivo DM digestibility as a result of urea treatment of straw; similar values were previously reported by others (Hadjipanayiotou et al 1997). The higher rumen pH in animals fed US than UTS (6.65 vs 6.41) can be ascribed to the fact that UTS was more digestible than US, whereas, the higher rumen ammonia-N in the animals on UTS than US must be associated with differences in N-content in the two straws.


Based on the findings of the present studies the following recommendations can be made for straw treatment under Mediterranean conditions.


The authors are grateful to A Photiou, M Theodoridou, M Karavia, G Kyprianou, I Antoniou and the staff of the Central Chemistry Laboratory for skilled technical assistance. The work had the generous support of the International Development and Research Centre of Canada (IDRC).


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Received 2 September 1997

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