Effects of treatment of rice straw with lime and/or urea on its intake, digestibility and rumen liquor characteristics in cattle

Nguyen Xuan Trach, Magne Mo*, Cu Xuan Dan 

Faculty of Animal Science and Veterinary Medicine, Hanoi Agricultural University
* Department of Animal Science, Agricultural University of Norway

Abstract

Two trials were carried out to compare voluntary intake and digestibility of rice straw which was treated according to a 3 x 3 factorial design using unslaked lime (0, 3, and 6%, w/w) and urea (0, 2, and 4%, w/w). In Trial 1, 27 growing beef bulls were divided into 9 groups to be fed on the 9 types of straw. Voluntary intake was first measured, followed by digestibility determination when a restricted level of straw (47 g OM/kg W^0.75/day) was fed. In Trial 2, three rumen-fistulated adult oxen were fed ad libitum (20% in excess) on the same 9 types of straw to determine intake and digestibility concurrently. 

It was found that both lime and urea significantly increased straw intake, digestibility, and thus its energy availability. The intake and digestibility of 2% urea-treated straws and especially 4% urea-treated straws were higher than those of untreated straw. A level of 3% lime significantly increased straw intake and digestibility. Treatment with 6% lime continued to increase apparent digestibility, but depressed straw intake compared with 3% lime in Trial 1. In addition, rumen liquor analyses showed that rumen ammonia (NH₃) and total volatile fatty acids (VFA) were decreased by the treatments. 

The findings suggest that both lime and urea are effective in increasing straw intake and apparent digestibility.

Keywords: Rice straw, urea, lime, cattle, intake, digestibility

Introduction

Treatments of rice straw with lime and/or urea were effective as judged by chemical composition, in-vitro gas production and in-sacco degradability (Nguyen Xuan Trach et al 2001). However, a measure to improve the feeding value of straw, as a roughage, must then result in increased voluntary intake, digestibility and consequently available energy. This is because animal performance is the product of supply, nutrient concentration, intake, digestibility, and metabolism (Mertens 1994). Voluntary intake and digestibility are thus of great importance in evaluating roughage quality. Direct measurements or estimates of them have always been of major interest to nutritionists (Burns et al 1994; Cochran and Galyean 1994; Weiss 1994; Ørskov 1998). Measurements of these properties would provide convincing indicators for evaluation of different treatments to improve rice straw quality. 

The present paper reports two intake-digestion trials, one on growing and the other on adult cattle, to further evaluate straw treatments using lime and/or urea. In addition, rumen liquor parameters were measured to provide additional insights into the mechanisms of the treatment effects.

Materials and methods

Straw treatment

Sun-dried rice straw (90% DM) in the long form was treated with quick lime (87% CaO) at  0%, 3% or 6% (w/w) in combination with urea (46% N) at 0%, 2% or 4% (w/w) according to a 3 x 3 factorial design. The treatment chemicals were dissolved in required amounts of water to attain 50% moisture content for the treated straw. The respective solution/suspension was then sprayed onto the straw with a watering can and thoroughly mixed manually. The mixed straw was then placed in polyethylene sacks (90 cm x 120cm), which each had been put inside another woven plastic sack of the same size, and sealed after being carefully pressed to remove as much air as possible. The sacks were then stored in a shed for 3 weeks at an average ambient temperature of around 25^oC.

Animals and feeding regimes

In Trial 1, a total of 27 growing bulls of a tropical beef breed (Bos indicus) at 12-15 months of age with an average live-weight of 116kg were allocated into the 9 groups to be fed on the straws. All animals were dewormed prior to commencement of the experiment. The animals underwent a transition period of 3 weeks to stabilize straw intake and after that voluntary intake measurements were made for 10 days. For the following 10 days all the animals were fed straw at a restricted level of 47g OM/kg/W^0.75/day (equal to 80% of the determined voluntary intake of untreated straw) in preparation for a total collection period of 11 days, during which time the animals remained on restricted intakes. In Trial 2, three fistulated adult yellow oxen of the same breed (251, 269 and 236kg) were fed ad libitum (20% in excess) in turn on the same 9 types of straw in a random order. For each type of straw, voluntary intake measurement, faeces collection and rumen liquor sampling were concurrently made within 11 days following a 15-day adaptation period.

In both trials, animals were kept in a well-ventilated shed with a cement floor and fed in individual pens. Rice straw was the sole energy source in the diets, which were supplemented with 1% of bone meal and 1% of a vitamin-mineral mixture. Urea, 32 g and 16 g/kg straw respectively, was additionally given to the 0% and 2% urea-treated straws prior to feeding to equalize the total N level to that of 4% urea-treated straw (20 % urea-N already lost after treatment) in order to exclude the effect of nitrogen supplementation. The added urea was dissolved in required amounts of water before spraying to maintain the moisture of untreated straw similar to that of the treated straws (approximate 50%). Drinking water was freely supplied at all times.

Straw voluntary intake measurement

Voluntary intake of straw was measured according to Burns et al (1994). Straw was given twice a day at 7 am and 4 pm with 20% in excess of the average intake of the previous 5 days. All the residues left from the previous day were removed from the feeding trough in the morning and weighed. A sample of 100 g was then taken to create a pooled sample for the whole collection period. Newly fed straw was also sampled from the trough with 100 g taken every day to create a composite sample for each type of straw. Animal weights were recorded at the beginning and end of the intake and/or digestion periods. Straw dry matter (DM) and organic matter (OM) intakes were expressed both as a percentage of live-weight (% LW) and in g per kg metabolic weight (g kg^/W^0.75).

Digestibility determination and energy estimation

Digestibility determination was organized according to Cochran and Galyean (1994).  Representative samples of the straws were taken at feeding and saved as part of the "running" composite samples for 10 days. Faeces collection began 1 day following the start of straw sampling.  It was quantitatively collected immediately after excretion and 5% of the bulked faeces were then taken to make a composite sample for 10 days for each type of straw per animal. The running samples of straw and faeces were then stored at -20^oC for subsequent analyses. Before chemical analysis the representative samples of straws and faeces were thawed and mixed thoroughly. A portion of each composite sample was taken and pre-dried at 55^oC for 72h in a forced air draught oven, then left to cool for 4 h and ground to pass a 1mm screen.

The apparent digestibility was calculated according to Cochran and Galyean (1994) for organic matter (OMD) and neutral detergent fibre (NDFD). The formulae proposed by van Es (1978) for estimation of metabolisable energy of a low protein roughage was used to estimate rice straw energy availability, which is ME (MJ) = 15.1 * DOM, where ME is metabolisable energy in Mega-joule (MJ) and DOM is digestible organic matter. Metabolisable energy of straw in the present case was calculated particularly on an organic matter (OM) basis to exclude the differences in the ash content of straw DM due to lime treatment.

Rumen liquor sampling

Rumen liquor samples were collected on two consecutive days in the middle of each collection period in Trial 2 between three and four hours after the morning feeding. Samples were taken with a 50 ml syringe connected to a 50 cm long plastic tube introduced through the fistula. The pH value of rumen liquor was determined immediately after the sample was taken using a portable pH meter. Each sample of 25 ml was then placed in a 30ml bottle acidified with 5 drops of concentrated sulphuric acid and then stored at -20^oC until analysed for ammonia (NH₃) and total volatile fatty acids (VFA).

Chemical analysis

Straw and faeces samples were analysed for dry matter (DM) and crude ash according to AOAC (Cunniff 1997). NDF content was determined according to Van Soest and Robertson (1985). Ammonia was separated from rumen liquor by steam distillation, collected in boric acid solution and determined by titration with standard acid (Preston 1995). Total VFA was also determined by steam distillation according to AOAC (Cunniff 1997).

Statistical analysis

Data were analysed using the GLM (General Linear Model) procedure of the SAS package (1996). For trial 1, a fixed 3x3 factorial model of analysis of variance (ANOVA) was applied. Factor level means were separated by the Ryan-Einot-Gabriel-Welsch (REGWQ) multiple range test. In addition, the two levels of urea (2% and 4%) as well as the two levels of lime (3% and 6%) were linearly combined together to contrast with non-urea or non- lime treated straw, respectively. For Trial 2, similar procedures were applied, except that the animal factor was included in the model as a block.

Results

Voluntary intake of straw

Both lime and urea increased straw OMI in both trials (Table 1). The intake of 6% lime-treated straw tended to be lower than that of 3% treated straw in growing cattle and almost the same between the two lime levels in adult cattle. Growing cattle tended to respond better to straw treatment in terms of intake. For example, treatment with 2% urea plus 3% lime increased OMI  by 32% in growing cattle compared to 24% in adult cattle. No significant interaction between lime and urea was found in adult cattle, but it was significant in the growing cattle of Trial 1. When urea was combined with lime, the differences in OMI between the two levels of urea (2% and 4%) were no longer apparent.    

Apparent digestibility and energy availability

It was found in Trial 1 that both lime and urea increased OMD and NDFD (Table 2). When urea was used in combination with lime no significant difference was found between the two levels of urea. The differences between 3% and 6% lime were not statistically significant. In Trial 2 (ad libitum feeding) the OMD and NDFD appeared to be lower than in Trial 1 (restricted feeding) (Table 3). For instance, 3% lime plus 2% urea increased OMD by 13.0 percentage points in Trial 1, compared to only 10.5 percentage points in Trial 2. Treatment effects in Trial 2 were not as clear as in Trial 1. In both trials OMD was significantly (P<0.001) increased by both lime and urea without any significant interaction found between the two chemicals. NDFD was also increased  (P<0.001) by both lime and urea with a significant interaction (P<0.05) to reduce their additive effects in Trial 1. In Trial 2 the effect of lime on NDFD was still significant (P<0.01), whereas urea apparently increased NDFD in absolute terms but the effect was not statistically significant (P = 0.08).

Straw ME was greatly increased by lime and/or urea treatment (P<0.001). As a result of higher digestibility due to restricted feeding, the straw ME values were higher in Trial 1 than in Trial 2. Compared to untreated straw, the value of ME was increased by 32% and 26% due to the best treatment in Trial 1 and Trial 2, respectively. In the absolute terms, combination of 6% lime with 4% urea (in Trial 1) or 2% urea (Trial 2) resulted in the highest values of straw ME. However, when lime and urea were combined for treatment, the differences in effect on straw ME between 4% and 2% urea or 6% and 3% lime were not significant.

Rumen liquor parameters

Rumen pH and NH₃ tended to decline while VFA increased with increasing application rates of urea for treatment, although the effects of lime and urea on rumen pH were non-significant (Table 4). Rumen NH₃ concentration was lowest for diets based on 4% urea-treated straw and highest for urea-supplemented untreated straw diets. The contrast setting revealed a significant influence of lime treatment on rumen NH₃ concentration (P<0.05). Both lime and urea significantly increased rumen VFA. However, no significant differences between the effects of 3% and 6% lime on the three parameters of rumen liquor were detected.

Discussion

Effect of treatment on straw voluntary intake

Urea treatment may increase intake of straw in the range of 15 to 50% as reviewed by Chenost and Kayouli (1997). Straw intake was increased by urea treatment in the present study in the lower half of this range. This is probably because in the present study, untreated straw was supplemented with urea at feeding. In non-supplemented rice straw the crude protein content is too low to meet the requirement of rumen microbes and thus supplementation of NPN increases digestion and thus intake due to increased microbial protein production (Doyle et al 1986; Djajanegara and Doyle 1989). Therefore, it has usually been uncertain to what extent the increased nutritive value of urea-treated straw is a result of NPN supplementation and to what extent it is a result of changes in the structure of the straw due to treatment effects (Schiere and Nell 1993). However, the increases in straw intake found in the present study were probably due only to treatment effect because N was no longer a limiting factor in all the straws under comparison owing to urea supplementation prior to feeding.

The increased straw intake due to present treatments may thus be explained by virtue of its increased degradability in the rumen as previously reported (Nguyen Xuan Trach et al 2001). An increase in the outflow of straw cell walls into the abomasum as a result of alkali treatment has also been reported (Males 1987). These possible effects of alkali treatment can aid in explaining the increases in straw intake in the present study.

Effect of treatment on apparent digestibility

The present findings show that both lime and urea are effective in increasing OMD and NDFD. In general, the higher the level of lime and/or urea applied in the present study, the more digestible the treated straw became. This was in agreement with the increased delignification, in-vitro gas production and in-sacco degradability of straw due to the treatments (Nguyen Xuan Trach et al 2001). Increased straw digestibility due to urea treatment has been well documented previously (Sundstøl and Coxworth 1984; Doyle et al 1986; Schiere and Ibrahim 1989; Chenost and Kayouli 1997; Madrid et al 1997). That lime treatment in the present study increased apparent digestibility of rice straw is in agreement with Selvendran et al (1977), Saadullah et al (1981) and Chaudhry (1998). Improvements in straw apparent digestibility as a result of treatment with lime and urea in combination have also been reported by Zaman and Owen (1990) and Sahoo et al (2000). 

Since the rumen is the primary site for fibre digestion, the increases in apparent digestibility of the treated straws were presumably due to increased rumen degradability resulted from increased susceptibility of structural carbohydrates of straw cell walls to rumen fermentation as well as more energy being made available for better growth of rumen microbes which degrade straw (Silva and Ørskov 1988; Rai and Mudgal 1988). The rumen retention time is actually not sufficient for the maximal fermentation of the substrate, thus an increase in degradation rate, as a result of increased straw degradability and better microbial activity, would cause a substantial improvement in digestibility and also in voluntary intake (Ørskov 1994).

In the present study, NDFD was slightly higher than OMD. This may be because measurements of apparent NDF digestibility over-estimated the digestibility of original cell wall material for treated straws since the compounds, which are solubilised, are unlikely to be completely digested (Djajanegara and Doyle 1989). Moreover, the determination of NDF is not confounded with any endogenous or microbial sources in the faeces.

Although not compared statistically, the apparent digestibility of straw cell wall components was generally higher in Trial 1 than in Trial 2. This is in agreement with the finding by Misra et al (1995) that digestibility of NDF was higher under restricted feeding of wheat straw and rice straw compared with ad libitum feeding regimes. This may be related to longer retention time under restricted feeding since the extent of degradation can be increased if rumen retention time is prolonged (Ørskov 1994).

Effect of treatment on rumen liquor parameters

The general decreases in rumen liquor pH and increases in VFA (Table 4) due to the treatments were probably a reflection of the improvements in the ruminal fermentation rate as previously found (Nguyen Xuan Trach et al 2001), which resulted in increased OMD and NDFD (Tables 2 and 3). Those groups with higher digestibilities showed higher values of VFA and lower values of pH. This is presumably because VFA was the end product of rumen microbial degradation of straw, and the more the VFA produced, the lower the resulting rumen pH. Jayasuriya et al (1987) have concluded that the output of VFA increases with the increase in feed intake. This explains the increased VFA content in the present observation.

Although the N level in straw was deliberately equalized prior to feeding, urea-treated straws resulted in lower rumen NH₃ concentrations than urea-supplemented straws. Singh and Gupta (1988) have also found significant lower ammonia nitrogen concentration in strained rumen liquor in buffaloes fed on ammonia treated straw compared with untreated straw. The higher rumen NH₃ resulted from supplemented straws may have been due also to greater part of the urea from the supplemented diets converted to free ammonia than the N provided by the urea-treated straws  (Chermiti et al 1994). More favourable conditions for rumen microbes to grow and thus capture more free ammonia in the rumen liquor (Singh and Gupta 1988; Chaudhry 1998) may be another explanation for the reduced rumen ammonia concentration due to lime and/or urea treatment.

Level of lime application for straw treatment

Results of our previous study (Nguyen Xuan Trach et al 2001) indicated that the effect of lime on in-sacco degradability and in-vitro fermentability of rice straw was increased with increasing application level. However, the present in-vivo results showed that 6% lime was not significantly better than 3% lime in increasing straw intake, digestibility and VFA. A level of 6% lime even reduced straw intake in growing cattle, compared with 3% lime. This would suggest some negative responses in-vivo to straw treated with too high a level of lime. This situation is similar to that of NaOH treatment as reviewed by Ribeiro (1989) where an application level above 6% resulted in in-vivo digestibility and voluntary daily intake tending to level off or even decrease while the IVDMD continued to increase.

The discrepancy between the in-vivo digestibility and in-vitro or in-sacco results further indicate that 6% lime is too high a level to maintain favourable rumen conditions for actual rumen degradation, although it can highly improve degradability/ fermentability of straw as a substrate. As previously shown (Nguyen Xuan Trach et al 2001), the rate of in-sacco degradation of hay as a standard substrate was lower when the animals were fed on 6% compared to 3% lime or urea-treated straws. The similar apparent digestibility and VFA content that resulted from feeding 6% and 3% lime-treated straws have probably been a reflection of a compromise between the highly increased degradability of straw and the less favourable rumen conditions associated with 6% lime. Unpalatability of straw treated with too high a level of lime (6%) may be a possibility for reduction in voluntary intake of the straw (Hove personal communication,  2000). However, a reasonable level of lime may improve straw palatability due to an effect of lime in reducing oxalates (Mudgal et al 1996).

Information is limited to elucidate why 6% lime did not have more positive effects on straw intake and in-vivo digestibility compared to 3% lime.  In general, high concentrations of dietary calcium are tolerated well by cattle (NRC 1996), and most of the additional calcium is excreted in the faeces (Djajanegara et al 1984). However, Ammerman et al (1963) have reported that protein and energy digestibility were reduced when cattle were fed a diet containing 4.4% Ca. In addition, Alfaro et al (1988) have also found some negative effects of high dietary Ca (2.35%) on metabolism of phosphorus, magnesium and certain trace elements, although the changes were relatively small. Although ruminants can tolerate wide Ca: P ratios (Call et al 1978), a ratio of Ca: P higher than 7:1 has been reported to reduce growth and feed efficiency (Wise et al 1963; Ricketts et al 1970). Another possible cause for 6% lime being no better than 3% lime is that a larger effect of a high lime level on straw cell wall degradability may, at the same time, be offset by a stronger effect on the lignin molecule to release phenolic acids, which are toxic to rumen microbes (Akin et al 1988; Chaudhry 2000).  

Whatever the reasons may have been, since 6% lime is not superior to 3% lime in increasing straw intake and digestibility and if a maximum tolerable concentration of Ca in feed for beef cattle is 2% as indicated by NRC (1984), a level of 3% lime should be maximum for rice straw treatment. However, further research in this area is needed before a firm conclusion can be made.

Conclusions

Based on the present study, 3% lime should be used in combination with urea to ensure the overall effectiveness of straw treatment. The additive effects of lime and urea on apparent digestibility of straw create the possibility that lime and urea can be used in combination as alkalis for treatment together with NPN and Ca supplementation of rice straw.

Acknowledgments

The authors would like to thank the Norwegian Council of Universities' Committee for Development Research and Education (NUFU) for the financial support to the present study. Special thanks are extended to Dr. Frik Sundstøl, Dr. Le Viet Ly, and Dr. Nils Petter Kjos for their facilitation and advice during the experimentation and manuscript preparation.

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Received 20 July 2001

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