Effects of treatment of rice straw with lime and/or urea on responses of growing cattle

Nguyen Xuan Trach, Magne Mo*, Cu Xuan Dan

 Faculty of Animal and Veterinary Sciences, Hanoi Agricultural University, 
Gialam, Hanoi, Vietnam
* Department of Animal Science, Agricultural University of Norway

Abstract

Two 3-month feeding trials were undertaken to evaluate effects of treatments using quick lime and/or urea on the feeding value of rice straw. In Trial 1, 20 young bulls were allocated into 4 groups to be fed on diets based on untreated straw (U0L0), 3% lime-treated straw (U0L3), 4% urea-treated straw (U4), or 3% lime plus 4% urea (U4L3). The straws were given ad libitum together with minimum supplementation of maize, fish meal, minerals and vitamins. In Trial 2, 15 crossbred dairy heifers were divided into 3 groups to be fed ad libitum on untreated straw (U0L0), 3% lime-plus-2% urea straw (U2L3) or 4% urea straw (U4), together with 0.5 kg of a compound concentrate and 0.4 kg dry matter (DM) of green grass per head per day. In both trials, in the middle of each month of the main experiment period, feed intake and total faeces excretion were measured and sampled for 7 days to determine apparent digestibility.

It was found in Trial 1 that straws treated with 3% lime, 4% urea or 3% lime plus 4% urea all brought about increased straw organic matter intake (OMI), organic matter digestibility (OMD) and digestible organic matter intake (DOMI). The average daily gain (ADG) was 74, 211, 280, and 303 g head/day for growing cattle fed on U0L0, U0L3, U4L0 and U4L3, respectively. The three treatments dramatically improved feed conversion ratio. In trial 2, U2L3 was more effective than U4 in terms of OMD, DOMI, ADG, and FCR. The ADG was 284, 458, and 502 g head/day for the crossbred dairy heifers fed on U0L0, U4L0 and U2L3, respectively.

 It is suggested that a combination of 3% quick lime and 2% urea can be used as an alternative to 4% urea for treatment of rice straw.

Keywords: Rice straw, cattle, urea, lime, treatment, intake, digestibility, growth rate

Introduction

Increased utilisation of rice straw as ruminant feed has been of growing interest with efforts devoted towards obtaining most of the potential feeding value of this abundant by-product. Low and slow rumen degradation of rice straw, which is largely attributed to high levels of its cell wall lignification, has attracted a great deal of research into treatment techniques as reviewed by Jackson (1977), Sundstøl and Owen (1984), Doyle et al (1986) and Chaudhry (1998). Most effective treatments have involved the use of sodium hydroxide or ammonia. However, so far these treatments have not been widely applied in practice by small-scale farmers in tropical countries due to various technical, economic and environmental reasons. In an attempt to search for possible alternatives, straw treatments using lime (3% or 6%) and/or urea (2% or 4%) have been tested at the laboratory, in-vitro, in-sacco and in-vivo levels in our previous studies (Nguyen Xuan Trach et al 2001a, b). Straw treatment using lime together with a low level of urea was of particular interest. In such a combination, it is expected that lime, an alkali cheaper than urea, may act as the main alkalinity enhancer and urea, in addition to its treatment and mould inhibiting effects, can supplement enough nitrogen for rumen micro-organisms. The present study was designed to verify this possibility by means of feeding trials with growing cattle.
 

Materials and methods

Animals and diets

Feeding Trial 1:

A total of 20 young bulls of a small tropical breed (Bos indicus) at 12-15 months of age with an average weight of 118 kg were allocated into 4 groups according to a 2 x 2 factorial arrangement. The factors were rice straw treated using 0% and 3% of quick lime (87% CaO) in combination with 0% and 4% (w/w) of urea (46% N). Individual treatments were:

• Untreated straw (U0L0)

• Straw treated with 3% quicklime (U0L3)

• Straw treated with 4% utea (U4L0)

• Straw treated with 4% urea aand 3% quicklime (U4L3)

The trial lasted for 90 days following a preliminary period of 14 days. The animals were individually fed rice straw ad libitum (20% above the average intake of the previous 5 days) twice a day at 8 am. and 4 pm. In addition, each animal was given 200g of ground maize, 50g fish meal, 25 g bone meal, and 25 g of a mineral-vitamin mixture. The supplements were provided individually before each straw feeding to make sure they were totally consumed. U0L0 and U0L3 were sprayed with required amounts of urea dissolved in water prior to feeding to equalize the N and moisture levels with those of U4L0 and U4L3 (around 80% of the added N remained in the straw as actually determined at feeding). Drinking water was supplied in free access at all times.

Feeding Trial 2:

This trial was mainly to examine 3% lime plus 2% urea as an alternative to 4% urea alone for rice straw treatment. Three groups each with 5 dairy crossbred (Friesian x Zebu) heifers from 10-14 month of age with an average weight of 119 kg were fed ad libitum on U0L0, U2L3 and U4L0. Each heifer was additionally given daily 2 kg elephant grass (20% DM) and 0.5 kg of a compound concentrate. The concentrate consisted of 38.5% maize, 20% rice bran, 20% cassava root meal, 5% soya bean meal,  5% fish meal, 10% groundnut cake and 1.5% mineral mix. The experiment duration and feeding regime were similar to those in Trial 1.

Straw treatment procedure

Sun-dried rice straw in the long form was sprayed with the respective solution/suspension of lime and/or urea, which was dissolved in required amounts of water to keep the straw moisture level at around 50%. The TS was then mixed thoroughly on a spread plastic sheet.  In trial I, the mixed straws were placed in double layered plastic sacks of around 10 kg each (on a dry straw basis) and sealed after being carefully pressed to remove the air. The sealed sacks were stored in a shed at an average ambient temperature of 18.7^oC for 3 weeks before the straws were fed to the animals. In trial II, straw was mixed with the respective treatment solutions and put layer by layer into 3-wall pits of 2.5 m³ each, carefully pressed, then covered with plastic films and stored at an average ambient temperature of 18.9^oC for 3 weeks before starting to feed.

Measurement of straw intake

In both trials, the straw was weighed at every feeding. Residues were collected and weighed before the morning feed every day for the whole feeding trial period. Organic matter intake (OMI) of straw was determined based on the daily amounts of straw fed, residues and their dry matter and ash contents determined in conjunction with digestibility determination. Based on OMI and organic matter digestibility (OMD) of the whole diet, total daily digestible organic matter intake (DOMI) per head was calculated.

Apparent digestibility determination

There were three faeces collection periods of 7 days each in the middle of each month of the trials. During the faeces collection periods 100g samples of straw and residues were taken every day from the trough at feeding to make composite samples for the whole period. Faeces was collected immediately after excretion and bulked daily for total weight determination and then a 5% representative sample was taken to make running composite samples for individual animals. All the straw and faecal samples were preserved in sealed polyethylene bags stored in deep freezers (-20^oC) until analysed for dry matter (DM) and total ash. Organic matter digestibility (OMD) was thus computed first for the whole diet and then for straw alone based on the assumption for simplicity that OMDs of the supplements were all equal to 79% (Pradhan et al 1996).

Measurement of eating and ruminating behaviour

Each of the experimental animals was observed continuously for two consecutive days to record eating and ruminating times. Two persons took turns to record the eating and ruminating times of two animals at the same time, using clocks and spreadsheets. Feed intake on those particular days was also determined to calculate the eating rate (kg straw DMI/h) and ruminating index (minutes/kg straw DMI).

Animal weighing

At the beginning and the end of the trials the animals were weighed for two consecutive days at 7 am before the morning feed, using an electronic weighing system (RUDDWEIGHT, Model 1200, Aust.). In addition, the animals were weighed every fortnight during the main experiment period to monitor the progress.

Chemical analysis

Fresh samples of the different types of straw were analysed for nitrogen (N) content (to determine amounts of urea added before feeding if needed). Representative samples of straws, residues and faeces from the three collection periods as well as the supplements were analysed for dry matter (DM) and total ash. All the analyses were made according to the respective AOAC Official Methods.

Statistical analysis

Data were analysed using the General Linear Model (GLM) procedure of the SAS (1996). A fixed 2x2 factorial model for analysis of variance (ANOVA) was applied for Trial 1 to detect effects of 3% lime and/or 4%urea. A one-way model was used for Trial 2 to compare 3% lime-plus-2% urea-TS with US (negative control) and 4% urea-TS (positive control). Group means were separated by the Ryan-Einot-Gabriel-Welsch (REGWQ) multiple range test. 

Results

Feed intake and digestibility

Organic matter intake (OMI) of straw, organic matter digestibility (OMD) and digestible organic matter intake (DOMI) obtained from the two trials are given in Table 1. In Trial 1, all treatments using 3% lime, 4% urea or their combination significantly increased straw OMI and OMD, resulting in an increase in DOMI. Combination of 3% lime and 4% urea did not significantly change OMI compared with either 3% lime or 4% urea alone (P>0.05). However, owing to highly increased diet and straw OMDs, the group fed on 3% lime plus 4% urea straw (u$L3) had the highest value of DOMI compared with the other groups. The additive effects of 3% lime and 4% urea on these parameters were reduced when the two chemicals were combined together for straw treatment. In Trial 2, straw OMI and OMD as well as total diet DOMI were also significantly increased due to treatment with 4% urea or 3% lime plus 2% urea. However, the increases in diet OMD were not statistically significant (P>0.05). The mean values of OMI and OMD for the 3% lime-plus-2% urea treatment were higher than those of 4% urea alone  but the differences were not statistically significant (P>0.05).  

Eating and ruminating behaviour

All the treatments of rice straw in both trials significantly reduced the eating and ruminating times. Also owing to the treatments the rate of straw eating was significantly increased and the ruminating index reduced (Table 2). Those fed on U0L0, although they consumed the least straw DM (see Table 1), spent the longest times eating and ruminating with slowest eating rates and highest ruminating indexes. Straw treatment with 3% lime gave significantly lower eating and ruminating times than the other two treatments in Trial 1. In Trial 2, straw treated with 3% lime plus 2% urea had significantly lower eating time, ruminating time and ruminating index and higher eating rate than straw treated wwith 4% urea. Straw treated with 3% lime, but not that treated with 3% lime plus 4% urea, resulted in the least eating and ruminating times, although U4 also significantly reduced these times compared with U0L0. 

Growth rate and feed utilization

In Trial 1, the effects of lime and urea as well as their interactions were highly significant (P<0.001) for ADG and FCR of the growing beef bulls. The combination of 3% lime and 2% urea supported the same growth rate as the 4% urea treatment, and both were better than 3% lime which was better than the control (Table 3). Feed conversion ratio showed the same pattern as growth rate. 

The combination of 3% lime and 2% urea appeared to be more effective than 4% urea alone based on ADG and FCR, although the differences between the two treatments were not significant.

Discussion

All the treatments in the two trials brought about positive effects on intake, digestibility, growth rate and feed utilization efficiency. The results are in accordance with the increased straw degradability and apparent digestibility together with an improved rumen environment for cellulolysis in the rumen (Nguyen Xuan Trach et al 2001 a, b). Chermiti et al (1994) have also demonstrated that treatment not only increased intake of straw and its cell wall digestibility to improve the energy status, but also decreased N losses in the rumen accompanied by an increased N flow to the duodenum mostly in the form of bacterial protein. Sahoo et al (2000) have observed that treatment increased the rate of degradation of straw OM and reduced methane production and methane energy loss. The growth rates obtained from ad libitum feeding of rice straw treated with 4% urea or combination of lime (3%) and urea (2 or 4%) with minimum supplementation as in the present study are quite acceptable for growing beef cattle of such a small tropical breed as well as for the crossbred dairy heifers, for which only limited growth rates are desired during a great part of the rearing period.

The present treatments, especially those using lime, highly reduced the eating and ruminating times per day, increased the rate of straw consumption and reduced the time to ruminate each unit of straw DMI. Teller et al (1993) have indicated that ruminants are unlikely to spend more than 16 h per day eating and ruminating. In the present study, time limitation appeared to be an important factor since those offered US had to spend more than this limit (16.5 h per day) eating and ruminating, i.e. no more could have been consumed because of the time limitation. The reductions in chewing activity during eating and rumination, as reflected by increased quantity of straw consumed per unit of time and reduced ruminating time per unit of straw DM ingested, could have resulted in increased efficiency of straw utilization and thus performance by reducing associated heat production (loss) as discussed by Ørskov and McLeod (1990).

The finding that lime-treated straw gave higher intake, digestibility and growth rate in cattle than untreated straw is in agreement with previous studies (Dumlao and Perez 1976; Pacho et al 1977; Djajanegara 1985). However, 3% lime was not as effective as 4% urea in improving animal growth rate, although our previous studies (Nguyen Xuan Trach et al 2001a) showed that 3% lime was almost comparable to 4% urea in improving in-sacco degradability and in-vitro gas production of rice straw. That N supplied by urea-treated straw is more efficiently incorporated into bacterial protein than N in the form of urea sprayed just prior to feeding (Chermiti et al 1994) may explain the better growth response to 4% urea-treated straw as compared with 3% lime complemented with urea at feeding.

The responses to feeding rice straw treated with lime plus urea in the present feeding trials were not as high as one may have expected based on the encouraging results of previous in-vitro, in-sacco and in-vivo studies (Nguyen Xuan Trach et al 2001a, b). However, a combination of 3% lime with 4% urea was more effective than 3% lime alone in increasing intake, digestibility, growth rate and feed utilization. That is, urea had some additive and/or compensatory effects in this mixture in spite of the presence of negative interactions. Similarly, Singh et al (1982) have reported much higher growth rate in response to wheat straw treated with 4% lime plus 4% urea compared to 4% lime alone, although the degradability of the lime-treated straw  as measured in-sacco was as high as that of straw sprayed with sodium hydroxide. The good results from lime plus urea treatments have further demonstrated that lime and urea can be used together for straw treatment with better biological responses in comparison with either input used alone. Other researchers (Sirohi and Rai 1995; Zaman and Owen 1990, 1995; Pradhan et al 1997) have also found that ensiling straw with lime and urea highly increased its intake, in-vitro and in-vivo digestibility.

In Trial 1, a combination of 3% lime with 4% urea was not much more effective than 4% urea alone in improving biological responses, except for OMD. The ADG of cattle fed on 3% lime plus 4% urea was only 23g (8.2%) higher than that of those fed on 4% urea treated straw (P>0.05). That indicates there is not much benefit from adding lime to a standard treatment with urea. In Trial 2, the level of urea was reduced to 2% as a minor treatment input in addition to 3% lime with no more urea added prior to feeding. This treatment brought about apparently better effects compared to 4% urea. It is possible that when 3% lime is already used, a high level of urea as an alkali becomes unnecessary for effective treatment of straw since high alkalinity can be induced by lime itself. Moreover, in such a combination urease activity may be more or less inhibited by lime (Zaman and Owen 1995; Pradhan et al 1997). Urea can supply supplemental non-protein nitrogen (NPN) to straw, but 4% urea would be too much for rumen microbes and the animal (Nguyen Xuan Trach et al 2001a). Pritchard and Males (1985) have shown no improvement in straw digestibility or microbial protein production in the rumen of beef cattle when the CP content of a straw diet was increased to above 8-10% DM. Therefore, as far as N content is concerned, 2% urea may be already more than enough to supplement NPN to rice straw. On the other hand, it has been shown that a level of at least 1.5% urea is needed to prevent mould in straw ensiled with lime (Zaman et al 1994). Therefore, the combination of 3% lime and 2% urea can be biologically justified, at least as an alternative to 4% urea alone, for straw treatment. In this combination lime acts as the main alkalinity enhancer and urea is a source of NH₃ for NPN supplementation, mould inhibition and additional treatment effects, if any. However, further research is warranted to verify the latter roles of urea, concerning possible inhibition of urease activity by lime. 

Acknowledgments

The present study was carried out with the financial support from the Norwegian Council of Universities' Committee for Development Research and Education (NUFU) through NUFU project 25/96. The authors would like to thank Dr Frik Sundstøl, Dr Le Viet Ly and Dr Nils Petter Kjos as the project coordinators for their help and advice. 

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 Received 9 September 2001

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