Advances in methane emissions measurement from the enteric fermentation of ruminants in Latin America and the Caribbean

Currently, most efforts in the LAC region to reduce methane produced by ruminants are focused on quantification of emission volumes, determination of emission factors and calculation of national inventories, while few studies have focused on the development of mitigation strategies. This is because it was not until recently that governments and scientists in the area realized the importance of the role of cattle, sheep and goats in the production and emission of large quantities of methane into the atmosphere and its influences on climate change. In fact, the first regional conference on greenhouse gases from agriculture took place in Chile in 2014. The situation in LAC contrasts sharply with countries in Europe, North America and some in Oceania where a large number of studies have been carried out on the theme for several decades (^{Blaxter and Clapperton, 1965}).

The literature reviewed by the authors of this research shows that the first initiatives to know the magnitude of the problem, are directed towards the development of facilities and infrastructure, that will allow the researchers to measure the emissions of methane and thus generate the baseline in which the local governments will be able to negotiate the mitigation commitments within the existing international protocols. Reliable emissions inventories will reduce current uncertainty and allow monitoring of emissions from livestock production systems before and after the implementation of mitigation strategies, thus emissions reduction can be corroborated and the strategy effectiveness evaluated.

The generation of local factors of methane emission by ruminants is an emerging challenge for the countries of the LAC region, as it requires costly installations, scientific equipment and a large number of experiments with a large number of animals over relatively long periods of time that would allow an accurate characterization of emissions. A review of available literature reveals a small number of studies on the subject, although the agriculture and livestock sectors, are two of the main economic activities of the region.

One of the first studies on methane emission in Latin America took place in Argentina, a country with 51 million head of cattle, by ^{Bárbaro et al. (2008)}. These authors used the technique of sulfur hexafluoride (SF₆) (^{Johnson et al., 1994}) to measure the methane emission in a group of Aberdeen Angus steers 14 months old. Half of them were grazing native pasture and half cultivated grass, dominated by ryegrass (Lolium perenne) and white clover (Trifolium repens).

The study was carried out in a temperate climate region of Argentina. Methane production recorded was 227 and 248 L of CH₄ head^-1 day^-1, respectively, and an average daily weight gain for both groups of 1.3 kg day^-1. In a similar experiment carried out by ^{Bualo et al. (2014)} 20 beef cows with an average weight of 382 kg, half the cows had access for six hours at a grasses and legumes meadow and the other half had access to a sorghum (Sorghum spp.) meadow also for six hours. The SF₆ technique was also used in this experiment and the average methane emissions were very similar to the previous experiment, within the range of 291 and 234 L cow^-1 day^-1, respectively.

In Chile ^{Muñoz et al. (2015)} conducted one of the first studies to measure enteric methane production by dairy cattle grazing in that country. Similarly, they used the SF₆ technique in 24 cows grazing in a ryegrass meadow. These authors observed that increasing the level of supplementation with concentrated of 1 kg to 5 kg cow^-1 day^-1 has resulted in an increase in milk production and the total production of methane going from 452-500 L CH₄ cow^-1 day^-1, respectively, without affecting the methane production per unit of milk produced.

In a similar study, ^{Herrera et al. (2014)} evaluated the effect of concentrate supplementation on methane production by multiparous Holstein cows with an average live weight of 597 kg in their final stage of lactation (15 kg milk cow^-1 day^-1) grazing in a Lolium perenne meadow. These authors used the SF₆ technique to measure methane production and two levels of concentrate supplementation, 4 and 8 kg cow^-1 day^-1. As in the previous study, their results also showed that increasing levels of concentrate resulted in more methane produced, going from 406 L cow^-1 day^-1 in cows receiving 4 kg of concentrate to 449 L cow^-1 day^-1 in those with 8 kg of concentrate. However, in this paper less methane has been produced (p< 0.05) per kilogram of total mixed ration consumed per cow in the treatment with 8 kg of concentrate tan on the treatment with 4 kg concentrate (29.5 vs 33 L CH₄ kg^-1 diet).

Brazil is a special case because it has the largest commercial herd of cattle in the world, with more than 212 million head, reason why the enteric fermentation of this cattle species is responsible for more than 73% of all the anthropogenic methane produced in that country (^{Cerri et al., 2009}). This situation may explain why Brazil is one of the few countries in the region that has invested in the construction and operation of respiration chambers to measure in vivothe methane production and other colorimetric studies tending to evaluate various alternatives to mitigate methane emissions by livestock.

For example, a study conducted by ^{Gonçalves et al. (2014)} in order to determine the effect of the quality of silage made with Bermuda var. Tifton 85 forage in different growth stages (25, 45, 56, 74 and 90 days), on the production of methane by adult sheep. In this study, 25 sheep were used with an average live weight of 46.5 kg and the methane emission was measured with an indirect open circuit calorimetry system. Their results showed no significant differenc (p> 0.05) among treatments for the consumption of dry matter (DM) (1.081 g DM head^-1 day^-1) and methane production (23.8 L head^-1 day^-1).

In a similar study also with sheep, ^{Machado et al. (2011)} evaluated the effect of sorghum silage made with three different varieties of sorghum in three different growth stages in methane production. They found that methane emissions varied without statistical significance (p> 0.05) from 13.6 to 24.4 L head^-1 day^-1, while the consumption varied between 832 and 911 g DM head^-1 day^-1 on sheep of 47.5 kg live weight.

As for the dairy cattle in tropical regions of Brazil, ^{Primavesi et al. (2004)} reported that the methane emission of Holstein cows grazing Brachiaria spp., ranged from 564, 389 and 294 L head^-1 day^-1 in lactating cows, dry cows and heifers, respectively. In the same order, the dry matter intake was 16, 12 and 9.5 kg DM head^-1 day^-1, confirming that reported by Machado et al. (2009) regarding the relationship between DM consumption and methane production.

Furthermore, they found that methane production in Holstein*Zebu cows is not different (p> 0.05) from emissions observed in pure Holstein cows, for example, 463, 413 and 286 L head^-1 day^-1 in lactating cows, dry cows and heifers, respectively. In contrast, in a second experiment carried out by the same group of researchers in Brazil, compared again the amount of methane produced by purebred Holstein cows against Holstein*Zebu cows (^{Pedreira et al., 2009}).These researchers reported that Holstein cows produce more methane (p< 0.05) than the hybrid cows, 419 L day^-1 vs 376 L day^-1, respectively, despite the fact that both groups of cows received the same diet. The results of the second experiment suggest that the race might be a determining factor for the production of methane and should be considered in the calculation of the inventories of the region.

In the case of beef cattle in Brazil, ^{Demarchi et al. (2003)} observed relatively lower emission factors in Nellore cows compared to dairy cows, which ranged from 143 to 308 L cow^-1 day^-1.The same authors managed to reduce methane production to 93.5 L head^-1 day^-1 in Nellore cows by adding 7% fat in the diet, such as palm or soybean oil. However, these authors do not recommend the use of palm oil because it had negative effects on voluntary intake and animal yield (^{Demarchiet al., 2003}). Moreover, the higher emission factors were reported by ^{Canesin et al. (2014)} in adults Nellore steers of 399 kg live weight, grazing Brachiaria brizantha, supplemented with citrus pulp, cotton seed cake and urea. The amount of methane produced by adult Nellore steers ranged from 316 to 355 L day^-1 for an average intake of 7.7 kg DM.

In Mexico advances in estimating emission factors and inventories have been limited, it is certainly the most backward of the great cattle countries in the region, the few jobs that exist are based on in vitro studies (González and Ruiz, 1995) and modeling (Castelán et al., 2013), these authors estimate that the methane produced by the 32 million head of cattle in Mexico is about two tera grams. It was until 2014 that the first two breathing rooms were built at the Autonomous University of Yucatán in southern Mexico.

The new equipment allows to measure in vivo methane emissions in sheep and cattle in tropical regions of southern Mexico. The results of the first experiments carried out suggest that Cebu cattle methane production fed with tropical grasses ranges from 74 L head^-1 day^-1 in young animals with an average consumption of 4.4 kg DM day^-1 and 348 L head^-1 day^-1 in adult cows. The sheep emission factors range from 21 to 34 L head^-1 day^-1 (Ku-Vera 2014, Com. Pers.).

Finally, the emission factors in Uruguay where cattle outnumber people in a ratio of 3.6 to 1 are among the highest in the region. For example, ^{Dini et al. (2012)} reported that adult Holsteins with an average live weight of 536 kg fed with grass and legumes (76% Lolium multiflorum and 24% Lotus corniculatus) can produce up to 521 L head^-1 day^-1. These high volumes can be attributed to a diet rich in forage because most of the cattle production in the country is based on the grazing of native and improved grasslands.

Advances in the mitigation of methane emissions from enteric fermentation

Some studies have been carried out in the region to reduce enteric fermentation in ruminants with the use of oils and tannery plants with variable results. For example, in Colombia, ^{Rodríguez et al. (2014)} evaluated using the poli-tunnel technique (Murray et al., 2007), the in vivo effect of the addition of oregano oil (Lippia origanoides) on methane production by Holstein heifers. Oregano oil is known to inhibit methanogenesis in the rumen due to its direct effect on the growth of archaean methanogenic bacteria without affecting the degradation of forage fiber. However, these authors reported a significant effect (p> 0.05) by the addition of 25 mg kg^-1 of DM of essential oil oregano on methane production (175 L head^-1 day^-1) in heifers fed on a diet composed of 83% pennisetums (Pennisetum clandestinum) and 17% commercial concentrate, compared to those who did not received oregano oil (192 L head^-1 day^-1).

In Argentina, ^{Martínez et al. (2014)}, in an in vitro study evaluated the anti-methanogenic potential of essential oil extracted from four native plants: Aloysia gratissima, Lippia turbinata, Schinus molle and Tagetes minutas compared with monensin, an antibiotic that has proven effective in reducing the formation of methane in the rumen. It was observed that low dose Lippia (100 mg L^-1 of incubation medium) and mean doses of Aloysia show a similar effect to monensin without affecting digestibility of forage fiber. In Mexico ^{Ayala et al. (2014)} reported that the addition of 450 g DM head^-1 day^-1 of a meal prepared with the fruit of Parota tree (Enterolobium cyclocarpum) to the diet of Pelibuey*Katahdin sheep, reduces methane emissions up to 36% compared with the control diet.

Other strategies to mitigate enteric methane emissions used in LAC include the use of tree legumes and shrubs integrated in silvopastoral systems, which is believed to be a more sustainable form of production compared to traditional livestock systems. Shrubs improve the nutritional quality of grazing cattle diet, usually by increasing protein concentration, and help reduce methane production by the effect of secondary metabolites such as tannins and saponins present in these plants. For example, ^{Mayorga et al. (2014)} used the poly-tunnel technique to evaluate the effect of Guazuma ulmifolia, shrubby leguminous plant, on methane emission of Zebu steers of 10 months old and 191 kg liveweight. They observed that addition of 30% guazuma in a basal diet of Panicum maximum fodder reduces the emission of methane to 320 L head^-1 day^-1 compared to the control diet producing 368 L day^-1. The experimental diet also increased daily consumption of dry matter by 17.5%.

Finally, Leucaena leucocephala, a tropical legume native from Mexico, has also proven to be effective in reducing methane emissions by ruminants in tropical regions of LAC. ^{Moreira et al. (2013)} used the SF₆ technique to determine the effect of leucaena in methane production by St. Agnes lambs of eight months old and 28 kg liveweight. The lambs in the experimental group received a compound 5% soybean meal, 3% corn and 82% leucaena diet while the lambs in the control group received a diet with 71% soybean meal and 29% corn.

The results showed that leucaena reduces the production of methane up to 30%, but also to reduce the consumption of dry matter, 616 and 820 g DM head^-1 day^-1 respectively matter which is possibly explained by the high concentration of leucaena in the experimental diet. Moreover, high levels of leucaena on the diet might increase nitrogen concentration in the urine and feces of animals, which can be later converted to nitrous oxide (N₂O), a powerful greenhouse gas by bacteria in the soil. The potential of leucaena to reduce enteric methane production is promising; however, more research is needed before reaching conclusive results, in particular because of its effects on animal performance at high inclusion levels and its potential for N₂O production, also a potent pollutant.