Total global greenhouse gas emissions from livestock are estimated to account for 14.5 per cent of total anthropogenic emissions and 40 per cent of this is accounted for by enteric methane from ruminants.
Most (about 90 per cent) of the enteric methane has its origin in the rumen through the process of microbial methanogenesis.
Methane has a much shorter life-time than CO2 in the atmosphere, which makes it an attractive target for shorter-term gains in global warming reduction.
In addition to the environmental implications, ruminal methane production also represents a considerable loss of energy to the animal, ranging from 2 to 12 per cent of gross energy intake of ruminants, though values between 3 and 7 per cent are more realistic in intensive dairy production.
Ruminal methane is produced as a by-product of fermentation of feed components by rumen microbiota (bacteria, protozoa and fungi) to volatile fatty acids (VFA) in the rumen.
It is strongly influenced by dry matter intake, which determines volume of fermentable feed substrates and fermentability of feed.
Methane gas produced is eructated (burped out) by the animal, facilitating removal of excess hydrogen from the rumen. This is a natural process essential to the continuation of anaerobic rumen fermentation and microbial growth.
Fermentation of different energy sources in the rumen leads to alternative patterns of VFA production.
Fermentation of fibrous material in the rumen drives acetate and butyrate which lead to a net release of hydrogen, whereas increasing proportion of concentrates in the ration and development of starch-fermenting microbes leads to increased propionate production (an alternative hydrogen sink) and reduced hydrogen release for methane production.
Furthermore, increasing concentrate supplementation and starch intake can decrease ruminal pH leading to inhibition of methanogenic growth.
In contrast to other nutrient sources, dietary fat is unique in that it is not fermented to VFA in the rumen.
This characteristic offers a specific method of increasing energy supply without adding to fermentation-induced hydrogen production in the rumen.
However, the potential for methane mitigation using ‘rumen-active’ lipids is limited due to negative effects on animal production. In particular, the risks of reduced fibre digestibility, feed efficiency and formation of trans fatty acid isomers that cause milk fat depression with increasing rumen-active fat sources need to be considered.
Rumen-protected fat supplements
In contrast, rumen-protected fats were developed to overcome the negative effects on rumen fermentation and fibre digestibility associated with unsaturated ‘rumen-active’ oils.
Their development enabled supplementation of ruminant diets with lipid to increase energy density without the negative intra-ruminal and production effects associated with ‘free’ oils, with additional benefits including the ability to deliver a greater proportion of biologically-active unsaturated fatty acids (for example: C18:1, omega-3) through the rumen to the small intestine.
Rumen-protected fat supplements offer a unique approach to methane mitigation efforts.
As established ingredients for improving production and fertility aspects in dairy and other ruminant diets, a beneficial effect on methane reduction is achieved more as a ‘side-effect’ in addition to the performance gains for which these supplements are typically used for economic return.
With minimal effect on ruminal environment or microbiota, the primary mechanism for rumen-protected fat-mediated reduction of methane is via direct replacement of fermentable organic matter in the diet to reduce substrate availability for hydrogen production.
Established research work from the United States evaluated the effects of replacing 2.95 per cent ground maize with rumen-protected fat (Megalac; calcium salts of fatty acids) targeted to supply about 454g of fat supplement/lactating cow/day.
Fat supplementation increased milk yield by 2.3 kg/cow/day and reduced daily methane production by 7.5 per cent, translating to a 13.7 per cent reduction in methane intensity. Non-lactating cows had a tendency for lower methane per unit of gross energy intake (-5.1 per cent).
Rumen-protected fats with specific fatty acid profiles are also commonly targeted to improve fertility, with mechanisms including increasing energy supply, increasing progesterone production, and improving quality and survivability of fertilised eggs.
Improved fertility traits can translate to fewer replacement animals needed which has major implications on methane production at farm level.
In summary, rumen-protected fats are widely used in dairy and other ruminant diets to provide an economic return through enhanced productivity effects.
The indirect effect on methane reduction should be considered as an additional benefit without additional cost to the producer, and with expected persistent effects through the period of use.
Dr Richard Kirkland is a ruminant nutritionist and global technical manager for Volac Wilmar Feed Ingredients, based in the United Kingdom.