Effect of different levels of supplementary potassium nitrate replacing urea on growth rates and methane production in goats fed rice straw, mimosa foliage and water spinach

Iv Sophea and T R Preston

Kampong Cham National School of Agriculture, Cambodia
iv_sopheakcnsa@yahoo.com

Abstract

Potassium nitrate at levels of 0, 2, 4 and 6% replaced iso-nitrogenous amounts of urea in a diet of fresh mimosa foliage, rice straw and water spinach fed to growing goats in individual cages (n = 12) over an experimental period of 84 days. Methane production was estimated at the end of the experiment from mixed samples of eructed gas and air using an infra-red detection meter (Gasmet).

Intakes of DM ranged from 29.6 to 31.2 g//kg live weight, with average proportions (% as DM) of 70, 20 and 10 for mimosa, rice straw and water spinach (excluding the NPN sources). There were no differences among treatments in live weight gain or DM feed conversion, and no signs of ill-health, even at the highest levels of potassium nitrate. Concentrations of methane and carbon dioxide in the mixed eructed gas and air were lower for all diets containing potassium nitrate compared with the urea-only control. The ratio of methane: carbon dioxide decreased with a curvilinear trend as urea-N was replaced by nitrate-N. The relative rate of methane reduction showed a positive curvilinear trend with up to 60% reduction when all the urea was replaced by nitrate.

Key words: Climate change, eructed gas, nitrite, toxicity

Introduction

The rumen is a complex organ which contains a dense population of bacteria, fungi and protozoa. There are as many as 20 species of protozoa and possible 200 species of bacteria which break down feed particles to derive energy they need for their metabolism (Hungate 1966).  Rumen microorganisms can degrade β-linked polysaccharide into simple sugars and can use non-protein nitrogen for microbial protein synthesis (Preston and Leng 2009). Consequently, ruminants can use nutrient resources which cannot be used directly by humans or non-ruminant animals.

Since the start of the industrial era in 1750, human activities have contributed to climate change by the liberation of gases, mainly carbon dioxide and methane, causing global warming through the greenhouse effect (IPCC 2007).. Livestock contribute about 9% of total carbon dioxide  emissions, but 37% of the methane, and 65% of the nitrous oxide (Steinfeld  et al 2006). There is therefore a great incentive to reduce methane emissions from livestock.

In a recent review Leng (2007) postulated that nitrate could replace carbon dioxide as an electron acceptor in the rumen with the generation of ammonia instead of methane. In this reaction,  nitrate is reduced to nitrite and then to ammonia, resulting in lower methane gas emission.

Therefore, it was hypothesized that a nitrate salt could potentially replace urea as a source of non–protein nitrogen because, as with urea, it would provide a fermentable nitrogen source for microbial protein synthesis; it possesses a higher-affinity than carbon dioxide to accept hydrogen, resulting in lower methane production.

Materials and methods

Location

The experiment was conducted in Kampong Cham National School of Agriculture, about 125 km from Phnom Penh capital city, Cambodia.

Animal housing

Twelve individual cages were built from bamboo and wooden materials with one meter width and one and a half meter long (Photo 1).  A feeding trough was placed in front of each individual cage and was covered by net so that feed refusals did not fall to the ground. There was a bucket in every individual cage to supply water throughout the whole day .

Photo 1: Experimental cage and feeding trough

Animals

Twelve young male goats were used in the experiment with average body weight of 12 kg. All animals were bought from the same herd of one owner.

Experimental design and treatments

The four treatments in a completely randomized design were iso-nitrogenous levels of either urea or potassium nitrate according to the following pattern (on DM basis):

• KN0    : 1.8% urea

• KN2    : 2% potassium nitrate+ 1.2% urea

• KN4    : 4% potassium nitrate+ 0.6% Urea

• KN6    : 6% potassium nitrate  

The basal diet was ad libitum rice straw, water spinach (0.3 % of LW as DM), mimosa (Mimosa pigra) foliage (2% of LW as DM) and a mineral supplement (Table 1). The rice straw was chopped and soaked with 10 % diluted sugar palm for a few seconds before feeding freely to the animals. To facilitate the consumption of the potassium nitrate and urea, these were dissolved in 200ml water and sprayed on the water spinach which was completely consumed by the goats. 

Feeds and feeding system

All animals were slowly adapted to the NPN supplements by step-wise increases in their concentration in the diet to reach the assigned level after a 14 day period before starting the experiment. Water spinach, sugar palm syrup, rice straw and urea were bought from local farmers. Potassium nitrate was purchased in the market in Cantho city, Vietnam. Mimosa foliage was collected every very morning from plants growing wild in the neighborhood.

The nitrate and/or urea solution was sprayed on the water spinach, previously chopped in small pieces,  and fed in equal amounts at 7.00 am, 8.00 am and 2.00 pm. Rice straw was chopped and soaked with 10% sugar palm syrup for a few seconds and fed at 10.00 am, 01.00 pm, and 04.00 pm. Mimosa foliage was fed in a small amount at 9.00 am with the remainder of  the daily allowance at 3.00pm. Among all diet ingredients, mimosa foliage was the most preferred by the goats. .

Data measurement

The goats were weighed every 7 days during the 84-day trial. Individual feeds and residues were recorded daily. Sample of feeds (rice straw, water spinach, and mimosa foliage) and feed residues were analyzed for DM and N at 7-day intervals. . Samples of rumen fluid were taken by stomach tube after 56 days of the experiment, 3 hours after the morning feed . pH was measured immediately, after which a drop of concentrated sulphuric acid was added to preserve the sample for subsequent ammonia analysis.

Rumen gas was collected in expired air using a plastic sleeve and bottle (Photo 2). Gas sampling was done on the last day of the experiment, from 4.30 pm after all diets were fed.  Each goat was permitted to breath  into the bottle for 3 minutes before it was tightly closed. The samples were then taken to the laboratory in Nong Lam University, Ho Chi Mih city where the ratios of methane to carbon dioxide were measured using a GASMET infra-red meter (Photos 3-6). The carbon dioxide and methane in background air were measured at the same time. The ratios of methane to carbon dioxide were calculated as:

CH₄: CO₂ = (a-b)/(c-d)

Where "a" is methane concentration in mixed eructed gas plus air, "c" is carbon dioxide concentration in mixed eructed gas plus air, "b" is methane in background air and "d" the carbon dioxide in background air.

As feed intake did not differ among treatments it was assumed that carbon dioxide production was also similar and could be used as internal marker as described by Madsen et al (2010).

Methane reduction was derived from the equation proposed by Leng and Preston (2010) in which it was assumed that:

If the methane production rate is "A", the carbon dioxide entry rate "B" is the same on all diets, and the ratio of methane to carbon dioxide is "R",  then the  following equations apply:

For the urea-fed animal .........................................A_U (urea)=B*R1

For the nitrate-fed animal .......................................A_N (nitrate) =B*R2

The percent methane reduction rate is then: .......................B(R1-R2)/(BR1*100

(R1-R2)/(R1*100

Photo 3: Passing the gases from the sample bottle to a closed plastic bag	Photo 4: Forcing the  gases from inside plastic bag into gas meter for analysis
Photo 5: Analyzing the gas sample	Photo 6: Ending the process when all the gas had been passed through the meter

Statistical analysis

The data were analyzed by Analysis of variance (ANOVA) using the General Linear Model (GLM) procedure of the Minitab software version 14, following  correction for initial live weight  using covariance analysis.  Sources of variation were treatments and error. Regression analysis was done with the same software.

Results

Chemical composition of experimental diets

The crude protein content of the water spinach (Table 2) was lower than typical values in the literature (eg: ranging from 18% in DM [Nguyen Tuyet Giang and Preston 2011] to  22% in DM [Kean Sophea and Preston 2001]).  The crude protein in mimosa was within the range (7 to 22% in DM) reported by Nguyen Thi Thu Hong et al (2008).

Feed intake and daily weight gain

There were no differences among treatments in the intake of individual feeds nor in total DM as a proportion of live weight (Table 3). Mimosa foliage represented 70% of the DM intake on all treatments (Figure 1).

Figure 1. Proportion of diet DM from individual feed ingredients other than the NPN and mineral sources

The growth curves were linear following the first 2 to 3 weeks when the goats were adapting to the housing and feeding system (Figure 2).  There were no differences among treatments in live weight gain or DM feed conversion (Table 4), and no signs of ill-health even with the highest level of potassium nitrate. These findings are in  agreement with recent reports in goats (Hao Trinh Phuc et al 2009;  Nguyen Ngoc Anh et al 2010) and in cattle  (Le Thi Ngoc Huyen et al 2010; Do Thi Thanh Van et al 2010). In all cases there was gradual adaptation to the nitrate-rich diets which appeared to be an adequate precaution for avoiding nitrite toxicity.

Concentrations of methane and carbon dioxide in the mixed eructed gas and air were lower for all diets containing potassium nitrate compared with the control with only urea as the  NPN source (Table 6). When corrected for the concentrations in background air (Madsen et al 2010), the ratios of methane: carbon dioxide showed linear decreases with increasing substitution of urea-N by nitrate-N (Figure 3).  Expressing the data in terms of relative rates of methane reduction (Leng and Preston 2010) indicated a positive curvilinear trend increasing to 60% methane reduction when all the urea was replaced by nitrate (Figure 4). 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).

Conclusions

• There were no differences in live weight gain or DM feed conversion, and no signs of ill-health, when potassium nitrate was supplied at up to 6% in the DM of a diet of mimosa foliage, rice straw and water spinach fed to growing goats.

• Concentrations of methane and carbon dioxide in the mixed eructed gas and air were lower for all diets containing potassium nitrate compared with the control with only urea as the  NPN source.

• The ratio of methane: carbon dioxide showed linear decreased with increasing substitution of urea-N by nitrate-N.

• The relative rate of methane reduction indicated a positive curvilinear trend increasing to 60% reduction when all the urea was replaced by nitrate

Acknowledgements

The senior author would like to express his grateful thank and gratitude to SIDA-SAREC and the MEKARN program for financial support. He would like to thank for students and staffs of department of animal science in Kampong Cham National School of agriculture for help and laboratory use.

References 

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