Methane emissions, feed intake and nitrogen retention in Yellow cattle given a diet of cassava root chips and fresh cassava foliage and either urea or potassium nitrate as source of NPN

Cheat Sophal, Duong Nguyen Khang*, T R Preston** and  R A Leng***

Faculty of Animal Science and Veterinary Medicine,
Royal University of Agriculture (RUA), Phnom Penh, Cambodia
sophalcheat@yahoo.com
* Nong lam University, Vietnam
**Finca Ecológica, AA#48, Socorro, Santander, Colombia
***University of New England, Armidale NSW, Australia

Abstract

This study aimed to evaluate the effect of potassium nitrate as a source of fermentable nitrogen in replacing urea on enteric methane emissions and nitrogen balance in local Yellow cattle fed a basal diet of cassava root chips and fresh cassava foliage. Four male cattle with average weight of 140±19.9 kg  were allocated in a triple changeover design to either potassium nitrate fed  at 5% of diet DM or urea fed at 1.4% of diet DM. The period on each treatment was 15 days.

DM intake tended to be less on the nitrate diet (P=0.12). Apparent coefficients of digestibility of DM and organic matter were reduced by 3.2 and 5.1%, respectively,  for the nitrate compared with the urea  diet. Emissions of enteric methane were apparently reduced by 43% and N retention per unit organic matter digested was increased by 80% when nitrate replaced urea as the source of NPN.

Key words: carbon dioxide, climate change,  greenhouse gas, N-balance, rumen ammonia, VFA

Introduction

The research describe here  is part of a series of studies aimed to extend knowledge of the role of nitrate as a competitive sink for  hydrogen produced  in ruminal fermentation of carbohydrate, as hypothesised  by Leng (2008).  Previous concerns over the risk of developing methaemoglobinaemia caused by nitrite, which is produced as an intermediary compound in the reduction of nitrate in the rumen, have been allayed by recent research showing that if the adaptation to nitrate is done gradually over a period of 2 weeks and the requirements for sulphur are increased, toxicity is not a problem (Hao Trinh Phuc et al 2009).  

Subsequent research has confirmed that feeding nitrate salts to dairy cows (Van Zijderveld  et al 2011), growing cattle (Hulshof et al 2010),  sheep (Nolan et al 2010; Van Zijderveld et al 2010) and goats (Nguyen Ngoc Anh et al 2010) results in reduced emissions of enteric methane. However, there appears to be only one report in which the lowered methane production in growing cattle was accompanied by better growth rates and feed conversion (Sangkhom et al 2012). 

Preston and Leng (1987 advocate  that feeding systems for ruminants based on the available resources are  most efficient when nutrients are provided  to optimize rumen function (and hence maximize microbial protein entering the  intestines for digestion) complemented with a source of protein that would bypass (escape) the rumen for enzymic digestion in the intestines thus augmenting the protein  to energy ratio in the nutrients absorbed  The diet used by Sangkhom et al (2012) fulfilled these conditions in that the carbohydrate source was sodium hydroxide-treated rice straw, with bypass protein from fresh cassava foliage and potassium nitrate as the source of NPN. The finding by Ffoulkes and Preston (1978), in which cattle grew at 850 g/day on a diet of molasses and urea supplemented only with fresh cassava foliage, was considered by these authors to be proof of the rumen bypasss properties of the protein in fresh cassava foliage. Many subsequent reports (Ho Quang Do et al 2002; Sath et al 2008 Tham et al 2008; ) have confirmed the stimulatory effect of fresh cassava foliage on growth rates of cattle fed rice straw.  

In the research reported here, it was hypothesized that in growing cattle fed a diet of cassava root chips as source of highly fermentable carbohydrate,  fresh cassava foliage as the source of bypass protein and potassium nitrate as the source of fermentable N, there would be both reduced emissions of enteric methane and a concomitant improvement in nutrient utilization reflected by an increased retention of nitrogen per unit of organic matter digested.. ;

Materials and methods

Location

The experiment was carried out in the Animal Research Station of the Faculty of Animal Science and Veterinary Medicine, Royal University of Agriculture (RUA), Phnom Penh city, Kingdom of Cambodia, from 1 April to 31 May, 2012.

Experimental design

Four cattle were allocated to a triple changeover design with 2 treatments (Table 1) and periods of  15 days on each treatment;10 days for adaptation and 5 days for collection of feces and urine. The basal diet was cassava root chips and fresh cassava foliage. The treatments were two sources of non-protein-nitrogen: 

U: Urea at 1.4 % of diet DM
KN: Potassium nitrate at 5% of diet DM (3.05% nitrate)

Animals and management

The cattle (uncastrated males) were of the local “Yellow” breed with a mean live weight of 140±19.9 kg at the beginning of the experiment. They were housed in individual pen designed for collection of feed refusals, feces and urine.  They had  free access to water. They were adapted to the feeds and pens over a period of 14 days, during which time the allowances of nitrate and urea were increased gradually to the projected levels after 14 days.

Experimental feeds and feeding

Cassava root chips were purchased from a farmer household in Kampong Cham province. The sweet variety of fresh cassava foliage (Photo 1) was harvested daily from plots in the RUA campus. Urea, salt, CaCO₃ and dicalcium phosphate were bought from a market near the experimental location. Potassium nitrate was bought in Vietnam. 

All animals received the basal diet of cassava root chips and mineral supplement offered ad libitum. Fresh cassava foliage was offered at a level of 1% of live weight (on DM basis). K-nitrate and urea were dissolved in water and sprayed onto the cassava root chips prior to feeding. All feeds were mixed together and fed to the animal in four meals daily. 

Data collection

Offered feeds were weighed daily before giving them to the cattle; feed refusals were collected and weighed the next morning. Representative samples of  feed and feed refusals were collected for chemical analysis. The live weights of the cattle were taken at the beginning and at the end of each collection period. Samples of rumen fluid were taken by stomach tube 2 hours after morning feeding on the last day of each period. The pH value was measured immediately with a portable digital pH meter.  The samples were then acidified with 1 ml of 50% H₂SO₄ added to 50 ml of rumen fluid and stored in a refrigerator at -20^oC prior to analysis for VFA and ammonia. 

Rumen gases were measured in the evening by placing the cattle in a plastic-covered cage (Photo 2) and after a period of 5 minutes for equilibration with the surrounding air, the concentrations of methane and carbon dioxide were recorded over a 10 minute period,  using a GASMET 4030 meter (Gasmet Technologies Oy, Pulttitie 8A, FI-00880 Helsinki, Finland), The CH₄ and CO₂ concentrations in background air in the building were also recorded. The ratios of methane to carbon dioxide were calculated according to the formula proposed  by Madsen et al (2010):  

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.

Chemical analysis

Samples of cassava root chips, fresh cassava foliage and feed refusals, were analyzed for dry matter (DM), nitrogen (N), ash, NH₃-N, VFA and HCN following the methods of AOAC (1990).

Data analysis

Data of feed intake, N intake, N retention, live weight, pH, NH₃-N, VFA were analyzed with the General Linear Model option of the ANOVA program in the MINITAB software (Version 13.31) (Minitab 2000). Sources of variation were cattle, periods, treatments and error. When there was a trend in animal responses to independent variables, linear regressions were calculated using the same MINITAB software. The data for ratios of CO_2: CH₄ were analyzed using the same GLM program in Minitab (2000), but with correction for repeated measurements which were placed in the "random" option.

Results and discussion

Chemical composition of the feeds

The crude protein (CP) in the fresh cassava foliage was lower than reported by Sangkhom  et al (2012) and Phuong et al (2012). This may have been due to variety or a higher proportion of  petioles and stem in the foliage.  

Feed intake

All the offered cassava foliage was consumed (recorded intakes were close to 1% of live weight as DM); however, intake of cassava chips was greater on the treatment with urea (1.56 % of LW as DM) compared with nitrate (1.21% of LW as DM) (Table 4; Figure 1). Overall there was a tendency (P=0.12) for DM intake to be reduced by nitrate.  The percentage of crude protein in diet DM was lower on the urea diet, but nevertheless apparently adequate for efficient rumen microbial protein synthesis (rumen ammonia levels were 123 and 127 mg NH₃-N/liter for the nitrate and urea diets, respectively).   

Apparent digestibility and N retention

 Coefficients of apparent digestibility for DM and OM were lower on the nitrate diet than on the urea diet (Table 5; Figure 2).  N intake was higher but urine N lower on the nitrate compared with the urea diet. Daily N retention was 50% greater when nitrate was fed compared with urea (Figure 3).  Expressed as N retention per unit OM digested, the efficiency of N retention was 80% greater on the nitrate diet (Figure 4). The N  retention as percentage of N intake and as percentage of N digested were both greater on the nitrate diet. 

Methane: carbon dioxide  ratios

The ratio of methane to carbon dioxide in the samples of gas obtained when the animals were enclosed in the plastic lined box was reduced by 43% by feeding potassium nitrate compared with urea (Table 6; Figure 5).

The apparent  reduction of methane emissions caused by feeding a nitrate salt rather than urea is in line with most reports in the literature (see review by Leng and Preston 2010 ). However,  the 50% greater N retention when potassium nitrate replaced urea as the source of NPN is reported here for the first time. An improvement in the nutritive value of the diet is to be expected when methane production is reduced, as the enteric methane production results in 8-12% loss of the gross feed energy from the ruminant digestion process (Blaxter and Clapperton 1965). In addition the reduction of nitrate to ammonia by nitrate-reducing bacteria generates ATP used in microbial growth and together this increases both energy and protein substrates for digestion by the animal.  However, in  other reported studies with cattle, where growth rate (Hulshof et al 2010) or milk production (Van Zijderveld et al 2011) were measured, the reduced excretion of methane was not accompanied by more efficient utilization of the diet.  The exception is the report by Sangkhom  et al (2012) in which cattle fed lime-treated rice straw and cassava foliage grew faster with better feed conversion when potassium nitrate replaced urea as the source of rumen fermentable N.

It can be hypothesized that in the present study and that of Sangkhom et al 2012, the feeding system was predicated on maximizing the P:E (protein:energy) ratio in absorbed nutrients, by ensuring efficient rumen function (supplying fermentable carbohydrate and fermentable N) supplemented with a known source of bypass protein (fresh cassava foliage).  In contrast, in most other studies efficient rumen function may have been compromised by feeding concentrates containing varying quantities of maize grain.  

Conclusions

• Providing supplementary NPN as potassium nitrate instead of urea to growing “Yellow” cattle fed cassava chips and fresh cassava foliage reduced emissions of enteric methane by 43% and increased the rate of N retention per unit organic matter digested by 80%.

Acknowledgements

The authors are grateful to the MEKARN project, financed by Sida, Sweden for the support for this research. The Royal University of Agriculture, Phnom Penh, Kingdom of Cambodia is acknowledged for provision of research facilities.

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