Livestock Research for Rural Development 24 (2) 2012 Guide for preparation of papers LRRD Newsletter

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Effect of potassium nitrate and urea as fermentable nitrogen sources on growth performance and methane emissions in local “Yellow” cattle fed lime (Ca(OH)2) treated rice straw supplemented with fresh cassava foliage

Sangkhom Inthapanya, T R Preston*, Duong Nguyen Khang** and R A Leng***

Souphanouvong University, Lao PDR
* Finca Ecológica TOSOLY, AA 48 Socorro, Colombia
** Department of Animal Physiology and Biochemistry, Nong Lam University, Ho Chi Minh City, Vietnam
*** University of New England, Armidale NSW, Australia


Sixteen male local “Yellow” cattle with initial weight 63-100kg were fed lime-treated rice straw and fresh cassava foliage in a randomized complete block design (RCBD) with to two treatments: potassium nitrate or urea as NPN source.  The NPN sources were dissolved in 100 g molasses diluted with 500 ml of water.  The experiment lasted 120 days at the end of which concentrations of methane and carbon dioxide were determined in eructed gas mixed with aif in a closed chamber in which the animals were kept for 5 minutes prior to measurement of the gases so as to ensure equilibration of the eructed gases with the air in the chamber

Daily live weight gain and DM feed conversion were improved by supplementation with nitrate rather than urea. There was no difference between treatments in DM intake. Feed intake as g DM/kg live weight and growth rate were linearly and positively related to initial live weight.  The ratio of methane to carbon dioxide in the mixed eructed gas and air was decreased by feeding  nitrate with an overall 25% reduction in methane emission, for animals fed nitrate compared with those fed urea.

This is the first  research to report better growth rates and better feed conversion ratios when nitrate replaces urea in a low quality diet. This  result mabe related to the pattern of feeding of the straw /molasses  which were given every 6 hours.

Key words: Climate change, feed conversion, greenhouse gases, live weight gain, rumen ammonia


In SE Asia, most farmers raise livestock for their main source of income and draft power.  According to FAO (2005) over 75% of the population in Lao PDR  rely on agriculture as their primary source of income. Cattle and buffaloes are important on smallholder farms in developing countries to provide meat, milk, traction power and manure in integrated crop and livestock farming (Preston and Leng 2009). In Lao PDR the populations of cattle, buffalo and sheep-goats have increased significantly from year 2002 to 2007 at the rates of 1.9, 0.61 and 9.46 % per annum, respectively (Anon 2007). However, a number of impediments and constraints have been shown to affect  livestock productivity and efficiency. The main feed resources for the ruminants in Lao PDR are native grasses, legumes and tree leaves that are available in the natural grassland and forests (Phonepaseuth Phengsavanh and Ledin 2003). The availability of these feed resources is seasonally limited and both feed availability and quality are  low, especially in the cropping season.   

Preston and Leng (2009) and Leng (1997) have emphasized that the most appropriate ways to improve feed resources for ruminants are through efficient  utilization of crop residues and tree/shrub foliages.  However, to optimize performance correct feeding methods need to be applied ensuring that rumen function is efficient and secondly ensuring efficient assimilation of nutrients by providing a source of bypass nutrients (Preston and Leng 2009).

Rice straw is the most abundant crop residue in  Asia, particularly in Lao PDR. It is the main feed in the dry season when natural grasses are in short supply to animals. Rice straw is characterized by high fiber level (39-53 % ADF) and nutrient deficiencies, especially protein (2 to 4% crude protein), vitamins, minerals and soluble carbohydrates. The straw itself is 98% covered with silica which has to be disrupted (McAllister et al 1994). Thus rice straw has low digestibility for ruminants in the range 41- 59 % (Napasirth et al 2005; Susuki et al 2004; Bui Van Chinh et al 2001; Tran Quoc Viet et al 2001).

There are  two ways to improve the feeding value of rice straw: (i) by delignification treatments which also disrupt the silca covering, which may be physical, chemical or biological (Sundstol 1984; Doyle et al 1986); and (ii)  supplementation with limiting nutrients in the rumen and essential amino acids (bypass protein) in the animal (Preston and Leng 2009). Treatment with sodium hydroxide was used in early trials on delignification of straw but, ammoniating the straw with urea has been the most widely used method (Chenost and Kayouli 1997). The partial replacement of urea by lime was reported by Nguyen Xuan Trach et al (2001) and Le Thi Thuy et al (2005) to be equally effective and more economical then ammoniation methods.

Leng (2008) concluded that the inclusion of nitrate salts in feed supplements appeared to be  entirely feasible as a means of providing fermentable nitrogen and simultaneously reducing enteric methane emissions from ruminant livestock. Whilst there is a risk of nitrite toxicity, nitrate reduction to ammonia in the rumen should theoretically  improve microbial growth efficiency and retain the energy that is lost in methane in the nutrients absorbed.

The possibility of nitrate as an alternative hydrogen sink to carbon dioxide has been downplayed because of the possible toxic effects of nitrite, which is formed as an intermediate during the reduction of nitrate to ammonia (Lewis 1951).  However, several reports have recently examined the potential of nitrate as a methane-lowering feed additive, and it has been shown to lower methanogenesis consistently (Leng and Preston 2010).

Recent researches in Vietnam (Nguyen Ngoc Anh et al 2010) and Cambodia (Iv Sophea and Preston 2010) have confirmed that the long term feeding of nitrate salts supported the same growth in goats as when urea was the NPN source, but brought about  30% reduction in production of methane.  Much higher (50%) responses were reported in sheep in the Netherlands (Van Zijderveld et al 2010a) and in dairy cattle in Brazil (Van Zijderveld et al 2010b).  However, there appear to be no reports showing that the reduction in methane is accompanied by more efficient production of meat or milk, which should theoretically happen..

It has been postulated (Leng 2008) that nitrate salts will be most effective as an NPN source when the diet is low in other sources of rumen-fermentable nitrogen and that the additional protein needed by the animal, over and above that produced by rumen microbes should be provided in the form of bypass protein. Fresh cassava foliage has been shown to be an effective source of bypass protein in diets where the dietary N was mostly in the form of urea (Ffoulkes and Preston 1978). Cassava is widely grown in all tropical countries and has been shown to support increased growth rates in cattle fed on rice straw (Keo Sath et al 2008; Tham et al 2008).

In order to avoid the use of both urea and sodium hydroxide for straw treatment we showed in an earlier paper (Sangkhom et al 2011) that hydrated lime (Ca[OH]2) could effectively replace sodium hydroxide with similar improvements in the digestibility of the straw. The objective of the studies reported in this paper was to investigate the use of a combination of lime-treated straw and fresh cassava foliage as the basal diet for fattening the local "Yellow" breed of cattle, with associated mitigation of methane emissions by incorporation in the diet of potassium nitrate.

Materials and methods

Location and duration

The experiment was conducted in the farm of Souphanouvong University, 7 km from Luang Prabang city, Luang Prabang province, Lao PDR. This experiment was conducted for 4 months, from July to October 2011 excluding adaptation and organizing

Treatments and experimental design

Sixteen local male “Yellow” cattle (Photo 1) were assigned to 8 blocks according to live weight and within blocks,  in a randomized complete block design (RCBD),  to two  treatments, which were:

 The basal diet was lime-treated rice straw fed ad libitum, fresh cassava foliage at 300 g/kg diet (DM basis) and 100g of molasses/day diluted with 500ml of water as the carrier for the urea and nitrate salts

Animals and housing

The cattle had an initial weight in the range of 63 to 100 kg. They were confined in separate pens (Photo 2). Vaccination was done against epidemic diseases and drenching against internal parasites before the commencement of the experiment.

Photo 1. Local "Yellow" cattle

Photo 2. The cattle kept in individual pens

Feeding and management

The cattle were gradually introduced to the NPN sources over a period of two weeks.  The rice straw is offered at 120% of recorded intake the previous week. It was bought from farmers in the area. The straw was treated with lime (Ca(OH)2) at 4% of DM, dissolved/suspended in water (50% solution), and stored in sealed plastic bags for 14 days before feeding (Photo 3). The NPN sources was dissolved in the diluted molasses and sprayed over the straw prior to each new feed of straw, which was four times per day. Cassava foliage was harvested daily from farmer areas. It was offered two times a day, at 7.00 am and 4.30 pm and put in to individual wood feeders. Water was freely available. 

Photo 3. Storage of lime-treated rice straw

Photo 4. Cassava at the time of harvesting the foliage

Data collection and measurements

Feeds offered were weighed before giving them to the cattle. Feed refusals were collected each morning prior to offering fresh feed and weighed to measure the feed intake. The live weights of the cattle were taken at the beginning, every 2 weeks and at the end of the experiment, using an electronic balance. Samples of rumen fluid were taken by a stomach tube; two hours post feeding in the morning at the end of the experiment for determining rumen ammonia and pH At the end of the experiment, a sample  of mixed eructed and respired gas from each animal was analysed for methane: carbon dioxide ratio using the Gasmet equipment (GASMET 4030; Gasmet Technologies Oy, Pulttitie 8A, FI-00880 Helsinki, Finland), based on the approach suggested by Madsen et al (2008). The cattle were held for 5 minutes in wooden crates covered with polyethylene film before taking the measurements, so that the  gases emitted from the animal could equilibrate with the air in the box (Photo 5). Samples of air in the animal house were also analyzed for the methane: carbon dioxide ratio.

Photo 5. Wooden crates enclosed in plastic used to house the cattle during the 5 minute period of adaptation/measurement using the GASMET infra-red analyser.

Chemical analysis

Samples of feeds offered and residues were collected every 14 days to determine dry matter (DM), ash, crude protein (CP) following the procedure of Ly and Nguyen Van Lai (1997). Rumen pH was measured immediately after taking rumen fluid from the animal with a digital pH meter. Rumen ammonia was measured by steam distillation and titration with 0.1N H2SO4. Protein solubility was determined by shaking 3g sample with 100ml 1M NaCl  for 3 hours, filtering through Whatman No.4 filter paper and determining nitrogen in the filtrate.

Statistical analysis
The data were analyzed by the general linear model option of the ANOVA program in the Minitab (2000) software (version 13.31). In the model the sources of variation were blocks, source of NPN, straw treatment, the interaction NPN*straw and error. Live weight gains were calculated from the linear regression of live weight (Y) on days in the experiment (X).

Results and Discussion

Chemical composition of the feeds

The leaves of the fresh cassava had higher DM, CP, Ash, OM and nitrogen solubility than fine stems (Table 1). The DM and crude protein contents of the lime-treated rice straw were lower than was reported by Sangkhom et al (2011) but the solubility of the nitrogen was higher.  Two thirds (65.4%) of the fresh cassava foliage was in the form of leaves.

Table 1. Chemical composition of the feeds

    ----As percent in DM----  






N solubilit







Fresh cassava




















Feed intake

Intake of rice straw, of total DM and of crude protein was similar on both nitrate and urea treatments (Table 2). 

Table 2. Mean values of feed intake for local "Yellow" cattle fed lime-treated rice straw supplemented with fresh cassava foliage






DM intake, g/day


Rice straw



93 0.87

Cassava foliage



18 0.60



















DM intake, g/ kg LW





N*6.25 intake, g/day


Rice straw




Fresh cassava foliage




















CP (N*6.25) in DM, %





Growth rate

The growth rate was higher for cattle fed potassium nitrate compared with urea  (Table 3, Figures 1 and 2).  The animals used were of village origin and were  immature with a rough condition score of 2 out of a scale of 5. Overall growth rates were lower than reported for Yellow cattle  in Cambodia fed similar diets (250 g/day, Seng Mom et al 2001; 243 g/day, Sophal et al 2010).  However, initial weights were lower for the "Yellow” cattle in our experiment (83kg) compared with those used in Cambodia (range of 100 to 127 kg). The data in Figures 5 and 6 show clearly that both relative DM feed intake (as g/kg live weight) and daily live weight gain were increased linearly with increasing initial live weight. Thus cattle that had an initial live weight of 100 kg (as in the Cambodia data) had a growth rate of 205 g/day which is closer to the range of growth rates (243-250 g/day) reported for Yellow cattle in Cambodia (Seng Mom et al 2001; Sophal et al 2010).

DM feed conversion followed the same response pattern as for growth rate with better conversion for the potassium nitrate treatment compared with urea (Table 3; Figure 5Mc). The range of values were similar to those (16 to 26) reported for Bos indicus cattle fed similar diets (Ongol in the case of Keo Sath et al 2010; and Red Sindhi crosses in the case of Tham et al 2010). Seng Mom et al (2001) and Sophal et al (2010) reported a range of DM feed conversion of 13 to 17, better than in the present experiment and probably a reflection of the higher growth rates.

Table 3. Mean values for live weight and conversion ratio in local "Yellow" cattle fed lime-treated straw supplemented with fresh cassava foliage






Live weight, kg











Daily gain, g/day





DM intake, g/day





DM feed conversion





Figure 1. Growth curves of  local “Yellow” cattle fed lime-treated rice straw supplemented with cassava foliage and K-nitrate or urea as NPN source Figure 2. Live weight gain of local “Yellow” cattle fed lime-treated rice straw supplemented with cassava foliage and K-nitrate or urea as NPN source

Figure 3. Effect of initial live weight on feed DM intake (as g/kg live weight) by local “Yellow” cattle fed lime-treated rice straw supplemented with cassava foliage and K-nitrate or urea as NPN source Figure 4. Effect of initial live weight on daily live weight of local “Yellow” cattle fed lime-treated rice straw supplemented with cassava foliage and K-nitrate or urea as NPN source

Figure 5.  DM feed conversion of local “Yellow” cattle fed lime-treated rice straw supplemented with cassava foliage and K-nitrate or urea as NPN source
Rumen parameters and ratio of methane and carbon dioxide

There was no difference among treatment in the rumen pH value, but the concentration of rumen ammonia was  higher when urea rather than potassium nitrate was the NPN source (Table 5). The concentrations of methane to carbon dioxide in the mixed eructed gas and air were decreased by replacing urea with potassium nitrate (Table 4 and Figure 6). The reduction in methane mission due to nitrate was 27%. Similar beneficial effects of dietary nitrate to mitigate methane production in ruminants have been reported by Nolan et al (2010), Van Zijderveld et al (2010a;b), Nguyen Ngoc Anh et al (2010), Hulshof et al (2010) and Do Thi Thanh Van et al (2010).

Table 4. Mean values for rumen pH, ammonia, concentrations of methane and carbon dioxide  in mixed eructed gases/surrounding air from  local "Yellow" cattle fed lime-treated straw supplemented with fresh cassava foliage and either potassium nitrate or urea as the NPN source






Rumen pH





NH3, mg/litre





CO2 1626 1945 86  
CH4 109 107 5.1  
CO2 (air) 430 430    
CH4 (air) 2.13 2.13    
CO2 (corrected) 1410 1679 268  
CH4 (corrected) 107 104 16.9  
CH4/CO2 0.0679 0.0498 0.0048 0.02

Figure 6. Ratio of methane to carbon dioxide in expired breath of local “Yellow” cattle fed lime-treated rice straw supplemented with cassava foliage and K-nitrate or urea

Despite the expected increased efficiency in using feed energy from incorporation of nitrate salts in ruminant diets, there appear to be no reports in the literature of improved growth and feed conversion from feeding of nitrate salts. It is tempting to hypothesize that the response observed in the present experiment reflected the result of a feeding system planned to optimize rumen function with supplementation of bypass protein, whereas in other reported feeding trials concentrate supplements, rich in maize and soybean meal,  or based on maize silage, were  the basis of the diets. Such feeding systems improve animal performance but may compromise the efficiency of the ruminant microbial mode of digestion.  A response to increased microbial growth efficiency would only be discernible where it improved the N balance of the animal. In much of the high quality feeding systems protein is not limiting and often wastefully metabolized as amino acids in the animal with excretion of urea at an energy cost.



This research was done by the senior author with support from NUFU as part of the requirements for the MSc degree in Animal Production "Specialized in Response to Climate Change and Depletion of Non-renewable Resources". The authors acknowledge support for this research from the MEKARN project financed by Sida. Special thanks to Mr Thong vanh Bountum; Mr Keomany and Mr Bounlung Silivong who provided valuable help in the farm. We also thank the staff of Department of Animal Science, Faculty of Agriculture and Forest Resources, Souphanouvong University for providing the facilities to carry out this research.


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Received 1 January 2012; Accepted 5 January 2012; Published 7 February 2012

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