Journal of Research, SKUAST–J
Year : 2005, Volume : 4, Issue : 1
First page : ( 1) Last page : ( 12)
Print ISSN : 0972-7469.

Nutritional strategies for reducing methane production by ruminants – A review

Sharma R.K.

Division of Animal Nutrition SKUAST–J, R.S. Pura, Jammu-181 102

Abstract

Methane a normal endproduct of nutrient fermentation in the rumen of ruminants not only affects on the environment but also on the economy of animal production. After carbon dioxide, methane is the second major green house gas contributing to global warming but methane is 20–40 times more potent as green house gas. It is impossible to completely stop methanogenesis in rumen as it is the integral component of rumen fermentation and essential but wasteful process. Various strategies suggested for reducing methanogenesis include decreasing number of ruminants, increasing non ruminant animals, genetic manipulation of ruminal methanogenic bacteria, development of less methane producing breeds and dietary/nutritional manipulations. Dietary manipulations seem to have significant potential in future because of its simplicity and practical feasibility. The nutritional manipulations for suppressing methanogenesis include provision of feed additives (chemicals, organic acids, probiotics), diets rich in unsaturated fatty acids (UFA), modification of feeding practices and supplementation of straw based diets with deficient nutrients. Antibiotics including ionophores reduce methanogenesis but their effects are short lived. Fats and oils can be used in medium amounts as these in large amounts can depress fibre digestion.

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Key words

Methane, ruminants, nutritional manipulations, feed additives.

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Introduction

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Dietary/nutritional manipulations

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Feed additives

Chemicals as feed additives

Several compounds such as chlorinated methane analogues, pyromellitic diimide, trichlory ethyl adipate and bromoethane sulphonic acid [17] have been reported to be toxic to methanogens. However, ruminal microbial populations in vivo have been found to adapt to or degrade many of these compounds and positive effects on animal performance have rarely been observed. The ionophores such as monensin and lasalocid are considered to be most potential modifiers of ruminal fermentation, because they decrease methanogenesis and increase propionate production. Propionate is important for ruminant nutrition as a glycogenic substance, because most of soluble carbohydrates ingested by ruminants are fermented to organic acids in the rumen. The ionophores are to be used with caution as these antibiotics were found to depress fibre digestion and inhibit protozoan growth [18]. In addition the effect of ionophores is short lived being lost on continued use [19] or repeated use [20]. Ruminal bacteria appear to develop resistance to ionophores [21,22]. Moreover, polyether antibiotics are absorbed, which implies that the ionophore or their metabolites may remain in meat or be transferred to milk [22]. Accumulation of ionophores in the animal body may affect metabolism of affected animals as well as humans consuming milk/meat from these animals [23]. A peptide ionophore, aibellin was found to reduce methanogenesis without depressing digestion of various nutrients [24,25], but its production cost is too high. Recently mevastatin and lovastatin have been used in low doses to inhibit methanogenesis in vitro [26]. These are likely to have potential in future as these did not inhibit fiber degrading microorganisms [14]. Attempts have been made to defaunate the rumen to reduce methanogenesis. The chemicals used for defaunation include copper sulphate, sodium lauryl sulphate, manoxal and dioctyl sodium sulphosuccinate [27,28]. However, as with many of the inhibitors of methanogenesis, many of the defaunation agents are toxic to the animal and as a result defaunation methods that can be practically used to reduce methane emissions are yet to be developed [13].

Organic acids as feed additives

Another potential approach for suppression of methanogenesis is to enhance the utilisation of hydrogen and formate by microbes other than methanogens [29]. In vitro addition of fumarate increased propionate production and decreased methanogenesis in a stoichiometric manner [30,31,32,33,34]. Malate was as effective as fumarate, because malate was readily converted to fumarate [34]. In vivo study in dairy cattle revealed 20 per cent reduction in methane production on 2 per cent dietary addition of fumaric acid without significant depression in feed intake, fibre digestion or protozoan numbers [31,35]. Simultaneous dietary provision of formate and nitrate reduced methanogenesis [36]. There is need to screen the available feeds for their aspartate, fumarate and malate content.

Probiotics as feed additives

Probiotic is defined as a live microbial feed supplement that beneficially affects the host animal by improving its intestinal microbial balance. Some aerobic viable yeasts or fungi (Saccharomyces or Aspergillus spp.) added in very small amounts have been reported to reduce methanogenesis in the rumen [37,38]. It is desirable to seek acetogens that have high affinity for hydrogen to serve as probiotic [14]. It is possible to increase carbon flow from sugars to propionate via the succinate pathway by provision of Selenomonas species as probiotic [39]. There is potential for use of Veillonela parvula and Wolinella succinogenes as probiotic, as these microbes have higher affinity for formate than methanogens [31].

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Provision of fats

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Modification of feeding practices

Supplementation of straw based diets

In India, straws of cereals are the common and major feed for ruminants. Supplementation of straw based diets with deficient nutrients modify the rumen fermentation which results in reduced methane production and increase the effciency for production. Research in India and Bangladesh demonstrated doubling of milk production in cows on supplementation of straw based diets with urea molasses blocks [45] which may be indicative of reduced methane production. [6] demonstrated that strategic supplementation with urea molasses blocks could promote efficient fermentation digestion and thereby reduce methane generation per unit feed by 30–50 per cent of digestible energy. Moreover, fermentation of soluble carbohydrates such as molasses in rumen lead to narrower acetate-propionate ratio [43] which implies lower methane production on feeding of urea molasses blocks with fibrous diet.

Concentrate straw ratio

The type of feed consumed can have significant effect on the proportion of energy emitted as methane. The ratio of concentrate and roughage in the ration can have major effect on the proportion of energy emitted as methane. Since the acetate: propionate ratio in rumen fermentation products decreases with forage: concentrate ratio, it is anticipated that methane production decreases when grains are fed. Methane emission do falls drastically from a level of 6–7 per cent of energy intake when forages are at maintenance to as low as 2–3 per cent when high grain concentrates are fed at near ad libitum intake levels [46]. [46] indicated that a high grain grain diet and/or the addition of soluble carbohydrates with the resulting shift in fermentation pattern in the rumen are associated with a more hostile environment for the methanogenic bacteria in which passage rates are increased, ruminal pH is lowered and certain populations of protozoa and methanogenic bacteria may be eliminated [46]. Methane production tends to decrease when mature dried forages are fed. Methane emissions are lower with ensiled than with dried forages. Methane losses are higher for coarsely chopped rather than finely ground or pelleted diets [47]. [48] have concluded that increased amounts of methane are produced when low quality forages are upgraded by chemical treatments such as sodium hydroxide or ammonia but the amount of methane is decreased relative to digestible organic matter. Therefore increasing concentrate proportion of the diet can help in reducing the methane production in ruminants, but this will increase the cost of feeding. This method will increase the cost of feeding but any method to reduce the methane production will involve some extra money, but at the same time extra concentrate feeding will increase production too.

Feeding frequency

This is an important feeding strategy which can affect ruminal methane production. Low frequencies of feeding tend to increase propionate production and lower methane production [49]. The effect is probably associated with lowering the poulation of rumen protozoa and possibly rumen fungi since multiple feedings prevent the drastic fluctuations in rumen pH which can be inhibitory to these microorganisms. This has been confirmed [50]. Provision of ration once or twice a day can help in reducing the methane production in ruminants but this is normal routine followed in villages of J&K state and country as whole.

Feeding level

It has been reported that negative regression existed between methane production and feeding level. [46] obseved that the proportion of gross energy intake emitted as methane declined 1.6 per cent units for each multiple of intake. Generally high intake results in increased passage rates of feed particles out of the rumen [49]. Consequently, the extent to which microbes access substrate is decreased which in turn, reduces the rate and/or extent of ruminal dietary fermentation and thus methane production. However, this is not an economic and viable modification for reducing methanogenesis in rumen.

Increased amounts of green fodder

Indian livestock are mostly low milk producers and their requirements can be met through combination of green fodders and straws. Green fodder and grasses are available in plenty during rainy season througout the country. The green fodder and grasses on feeding to ruminants modify the fermentation pattern towards more propionate and hence increasing green fodder in diet can reduce methane production in ruminants. Supplementation of straw based ration with green fodder reduced methane production by 11–27 per cent [51].

Augmentation of dietary nitrate, nitrite and sulphur reduction

In the rumen, nitrate is reduced to nitrite. This sequential reaction is efficient for the consumption of reducing equivalents, because 4 moles of reducing equivalents are used when 1 mole of nitrate is reduced to ammonia. Reduction of 1 mole of nitrate to ammonia should decrease hydrogen production by 4 moles, which is equivalent 1 mole (22.4 litres) of methane. Conversion of nitrate to ammonia not only provides available nitrogen for rumen bateria but also energy to nitrate reducing bacteria through electron transport phosphorylation [50]. Plant feeds commonly consumed by ruminants contain nitrate exceeding 0.5 per cent DM or 37 per cent of total nitrogen. As much as 1.2 per cent of DM (expressed as nitrate-N) has been reported to be present in corn stalks and turnip tops, which makes it easy under some conditions for ruminants to consume lethal amounts [51]. However, the rate of nitrate reduction to nitrite in the rumen is much higher than the rate of nitrite reduction to ammonia, resulting in the accumulation of nitrite, a toxic and mutagenic substance [52]. Consequently ingestion of feed containing a high level of nitrate causes acute intoxication in the host animal, which is commonly referred to as nitrate poisoning. Nitrite has also been reported to inhibit the fermentation and growth of ruminal microbes when present in concentrations above 3 mM [53]. Animals adapted to gradual elevated dietary nitrate levels used efficiently both nitrates and nitrites. To use nitrate efficiently it is important to enhance the nitrite reduction rate. On stimulation of nitrite reduction, methanogenesis can be efficiently reduced without any toxic effect by feeding nitrate rich roughages or even by adding nitrate in the diet. In vitro experiments with mixed ruminal microbes showed that the rate of nitrate reduction was 2.5 fold higher than the rate of nitrite reduction [53]. If nitrate is given twice daily to a lactating cow with a 200-litre rumen at an initial level of 5 mM, methane production is likely to decrease by 45 litres or 13 per cent as calculated from a value of 350 litres of methane/head/day. Stimulation of nitrate and nitrite reduction was observed on addition of fumarate. Simultaneous addition of 10 mM nitrate and 30 mM fumarate resulted in a 40 per cent decrease in methanogenesis with little depression in fermentation or fibre digestion [53]. Activities of nitrate reductase and nitrite reductase per bacterial nitrogen were found to be elevated by feeding a high nitrate diet or by incubating mixed microbes in the presence of nitrate or nitrite. Protozoa also stimulated nitrate and nitrite reduction in cultures of mixed ruminal microbes mainly because of increased hydrogen production. Ruminal bacteria known to reduce nitrate are Veillonella parvula, Wolinella succinogenes and Selenomonas ruminantium [52]. It is important to stimulate the growth of theses bacteria, especially W. succinogenes, with its high ability to reduce nitrite in the rumen. It may be beneficial to introduce these bacteria in the rumen as probiotics. Ruminants ingest considerable amounts of sulphate from plant material, which is utilised by ruminal microbes as a sulphur source. Excess sulphate is reduced to hydrogen sulphide. Bacteria resembling Desulfovibrio sp. have been isolated from the rumen [53,54]. However, sulphate reduction appears to be insignificant in the rumen, because the sulphate levels are insufficient for sulphate reducing bacteria to outcompete the methanogens [55,56]. Since hydrogen sulphide is toxic to animals and it is theoretically difficult to convert hydrogen sulphide to a non toxic substance in the rumen, it is undesirable to increase sulphate reduction.

There is an urgent need for suppression or reduction of methanogenesis in the ruminants as methane is responsible for global warming and reduced ruminant productivity. Nutritional manipulations seem to have potential because of practical application and simplicity. The nutritional manipulations for suppressing methanogenesis involve provision of feed additives (chemicals, organic acids, probiotics), diets rich in unsaturated fatty acids (UFA), modification of feeding practices and supplementation of straw based diets with deficient nutrients. Antibiotics including ionophores reduce methanogenesis but their effects are short lived. Ruminal bacteria appear to develop resistance to ionophores. Moreover, these antibiotics or their metabolites may remain in meat or be transferred to milk of animals for human consumption. Accumulation of ionophores in the animal body may affect metabolism of affected animals as well as humans consuming milk/meat from these animals. Fats and oils can be used in medium amounts as these in large amounts can depress fibre digestion. However, fats are high cost ingredients and their addition does not appear to result in increased energy availability to the animal, therefore dietary lipids are not viable for suppressing methanogenesis in ruminants. Dietary provision of organic acids can reduce methanogenesis by reducing substrates for methanogenic bacteria but this is quantitatively of less importance. Under Indian conditions the supplementation of straw based rations with critical nutrients through concentrates, urea-molasses-mineral blocks and green fodders are of potential practical significance. Control of bacterial fermentation and metabolism as well as the use of specific bacteria as probiotics is likely to have future potential.

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