27 August 2013

6 minute read

Methane produced in the digestive system of ruminants accounts for almost half of greenhouse gas (GHG) emissions from Irish agriculture.It also represents a loss of energy for cattle and sheep, representing 2-12% of their gross energy intake.

Improved animal productivity and dietary manipulation are two strategies for reducing emissions indices.  

Experiments were undertaken to quantify firstly methane emissions by finishing beef cattle consuming a range of alternative diets, and secondly the emissions of these systems when account was taken of land use and land use change. In addition, in vitro rumen batch digestion tests were undertaken as a relatively cost-effective method of rapidly screening methanogens with a wide range of dietary ingredients and additives under standard conditions.

Finishing Beef Cattle

Although grass silage is the main winter-feed for cattle in Ireland, there are circumstances where forage maize or whole-crop small-grain cereal silages can be attractive.

From a GHG perspective this holds the additional attraction that increasing starch content in the diet has the potential to reduce enteric methane emissions intensity (g CO2-eq/kg beef carcass). Therefore, studies were undertaken where finishing beef cattle were offered maize or whole-crop wheat silages that differed in starch content, and their methane output was compared with cattle consuming diets based on grass silage (no starch) or cereal grains(very high starch) offered to appetite.

Methane output was measured using sulphur hexafluoride as a marker. Increasing the starch content of maize or whole-crop cereal silages significantly reduced methane emission intensity, with about half of this effect coming from an increase in the daily rate of carcass gain and the remainder coming from a reduction in the absolute quantity of enteric methane emitted daily.

This effect of dietary starch content was maximised where cereal grains were fed to appetite, with the methane emission intensity being reduced by 24-65% compared to the forage-based diets. This effect is not simply one of starch content per se, but also reflects the net energy content of these diets. Hence, for example, the grass silage was associated with an energy content and corresponding methane emission intensity that was in the middle of the values obtained with the whole-crop wheat silages assessed.

Beef Systems Assessment

The reduction in the intensity of enteric methane emission summarised above needs to be considered within the overall context of the total beef production system. Therefore, a lifecycle assessment (LCA) was undertaken to account for GHG emissions from enteric fermentation, manure, soils, indirect nitrous oxide emissions and the production of inputs. The methodology used estimated both financial performance and emission efficiency for seven beef finishing strategies.

Methane, nitrous oxide and carbon dioxide contributed 42-59, 23-29 and 17-29% of total GHG emissions across the beef systems modelled. The calculated GHG emissions were lowest with the diets based on high-starch maize silage and cereal grains fed to appetite (18.3 and 19.2 g CO2 -eq/kg beef carcass, respectively), intermediate for the grass silage diet (21.7 g/kg) and highest for the low-starch whole-crop wheat silage diet (24.0 g/kg). However, when account was taken of potential or estimated carbon sequestration by permanent pastures and carbon loss due to land use change from permanent pasture to arable crops, the rankings changed.

In this case, the corresponding whole-farm system GHG emissions for the four diets previously mentioned was 16.8, 18.4, 15.6 and 19.3 kg CO2 -eq/ kg beef carcass. These findings showed the attractiveness of the system based on permanent grassland, and highlighted the necessity to undertake a fully holistic assessment of how GHG mitigation strategies impact on whole-farm system emissions. An incomplete assessment can lead to erroneous conclusions being made.

In vitro studies

In vitro rumen techniques attempt to simulate conditions in the rumen, and the batch digestion version does this under conditions where simulated intake, passage rate, absorption and rumination are considered similar for each treatment.

It therefore helps produce a methane output value for a feed ingredient, derived under standardised conditions. In the series of studies undertaken it was shown that neither the grass species (n = 5) nor perennial ryegrass variety (n = 7) evaluated influenced methanogensis when each was managed under a similar simulated intensive grazing regime throughout the season.

However, doubling the water-soluble carbohydrate content in grass reduced methane output (mmol CH4 /g of apparent DM digested) by 14%, and this effect was more marked with fructan than sucrose. Managing grassland so that grazing cows were offered a sward allowance of 15 or 20 kg dry matter (DM)/day throughout the grazing season did not alter the methanogenic characteristic of the herbage. In contrast, managing the grass so that herbage mass at the commencement of each grazing was 2,400 rather than 1,600 kg DM/ ha resulted in a 6% increase in methane output.

In each of the above cases where assessments were made throughout the grazing season there were seasonal effects on the methanogenic character of grass, although this could not reliably be attributed to any single measured chemical composition trait. White clover has been associated with less enteric methane production than grasses grown with inorganic nitrogen (N) fertiliser.

This was confirmed in the present series of experiments where three different white clovers averaged 25% lower methane output than four grass species (perennial ryegrass, cocksfoot, meadow fescue and timothy). A surprising outcome when a range of mixtures of each white clover with each grass were assessed was that the methane output was higher than predicted based on the methane outputs measured with the monocultures. In the case of the 50:50 mixture of clover and grass this averaged at a 5% over yield of methane.

This finding needs to be assessed under in vivo conditions. Both reseeded and old permanent grassland swards in Ireland contain some content of non-legume forbs. Six commonly found species were assessed on two occasions during the grazing season, but none were found to provide consistently low methane outputs. Compounds such as nitrate in herbage can act as an alternative hydrogen sink to methanogenesis, thereby reducing methane output for herbage. This was evident when assessing the effects of applying inorganic N fertiliser to grassland (0 and 100 kg N/ha resulted in 60.0and 57.2 ml CH4 /g DM digested, respectively) and when comparing autumn grass with primary growth grass (52.9 and 63.2-64.4 ml CH4 /g DM digested, respectively).

Grass can be utilised at a range of growth stages, and advancing growth stage increased methanogenesis. Thus, grass sampled from the primary growth on 12 May and 7 July had digestibilities of 789 and 551 g/kg, respectively, with corresponding methane outputs of 49 and 55 ml CH4 /g DM digested. Approximately 28% of grassland in Ireland has at least one harvest of silage taken each year, and in the in vitro studies ensilage reduced the methane output associated with herbage by 9%.

Finally, a series of chemical and biological agents were co-incubated with different dietary ingredients. Lauric and linolenic acids, and two halogenated methane analogues, generated the more marked reductions in in vitro rumen methane production.

Acknowledgements

Funding for these studies was provided under the National Development Plan through the Research Stimulus Fund administered by the Department of Agriculture, Food & the Marine (DAFM; 05-224, 06-361 and 07-517). The research was undertaken in collaboration with the School of Agriculture, University College Dublin; AGRIC, Teagasc, Moorepark, Co. Cork; DAFM, Backweston Farm, Co Kildare; Instituto de Ganadería de Montana (CSIC-ULE), Universidad de León, Spain.

To Read More From Teagasc Research - Click Here