CSIRO PUBLISHING
Animal Production Science, 2010, 50, 159–167
www.publish.csiro.au/journals/an
Experimental validation of the Intergovernmental Panel on Climate Change default values for ruminant-derived methane and its carbon-isotope signature F. Klevenhusen A, S. M. Bernasconi B, M. Kreuzer A and C. R. Soliva A,C A
ETH Zurich, Institute of Plant, Animal and Agroecosystem Sciences, Universitaetstrasse 2, 8092 Zurich, Switzerland. B ETH Zurich, Geological Institute, Sonneggstrasse 5, 8092 Zurich, Switzerland. C Corresponding author. Email:
[email protected]
Abstract. Two aspects regarding the ruminant’s contribution to global methane (CH4) emissions were investigated: (i) testing the accuracy of the Intergovernmental Panel on Climate Change default values for dairy cows fed different diet types and differing slurry storage temperatures; and (ii) providing carbon-isotope (C-isotope) signature data to contribute information on the characteristics of ruminant-derived CH4 as global source. The experimental diets, fed to 18 dairy cows, were separated into forage-only (hay, C3 plant) and forage-concentrate diets (barley, C3 plant; maize, C4 plant). Accumulated slurry was stored at either 14 or 27C. The hay diet had the highest CH4 conversion rate (Ym 7.9%). Negligible amounts of CH4 were emitted from slurries stored at low temperature. No diet effect was found at 27C (~33 L/kg volatile solids). The isotope ratios of enteric CH4 averaged 67.7‰ (C3 plants) and 57.4‰ (C4; maize). High temperature slurry storage resulted in different enrichment factors eCO2-CH4 for maize (33.2‰) and hay (35.9‰). Compared with the Intergovernmental Panel on Climate Change default values for Ym and slurry CH4 emission the results gained in the present experiment were higher and lower, respectively. Slurry-derived CH4 was less depleted in 13C than enteric CH4, which decreases the usefulness of this signature for global ruminant-derived CH4. Additional keywords: C3- and C4-plants, dairy cow, slurry, stable isotope.
Introduction Enteric methane (CH4) formation in ruminants as well as methanogenesis in manure, are considered to represent a major contribution to the global anthropogenic CH4 budget and considerably promote global warming (Duxbury et al. 1993; Steinfeld et al. 2006). In many countries measured values of CH4 emissions from agriculture including enteric and manure methanogenesis, as influenced by diet type, are missing or scarce. Therefore, the Intergovernmental Panel on Climate Change (IPCC 2006) offers so-called default values to calculate the annual CH4 budget as demanded when ratifying the Kyoto Protocol. The default values provided regarding enteric fermentation include a general CH4 emission factor per animal and year (EF; Tier 1 method), as well as the CH4 conversion rate (Ym; Tier 2 method), describing the percentage of gross feed energy lost as CH4. The IPCC (2006) encourages the use of the Tier 2 rather than the Tier 1 method to decrease the uncertainties in the budget. Both options distinguish between livestock categories and few aspects about feeding regime, but still large uncertainties of up to 50% for the Tier 1 and up to 20% for the Tier 2 method (IPCC 2006), respectively, are to be expected. Depending on animal species and diet, manure is supposed to have a maximum CH4 production capacity (Bo) of 240 L CH4/kg volatile solids (VS) (Bo 15%, VS 20%) for dairy cows located in North America and Europe (IPCC 2006). The actual CSIRO 2010
amount of manure-derived CH4, however, depends, among other things, on storage duration and seasonal temperature (Steinfeld et al. 2006). According to IPCC (2006) the variation in expected manure-derived CH4 emission ranges from 10 to 80% of Bo depending on temperature and the presence or absence of a natural surface crust layer on the slurry. Due to the large uncertainties in estimating CH4 emissions with the default values suggested by IPCC (2006), it seems valuable to enlarge the database of CH4 emission measurements from ruminant husbandry. Therefore, the IPCC default values might be replaced by diet-specific CH4 data in the future, resulting in improved budgets of CH4 emissions. Analysis of stable carbon-isotopes (C-isotopes) is an indicative tool for studying CH4 emissions from various sources. The variation in C-isotope composition of CH4 allows tracking the process of methanogenesis (Whiticar 1999) and can also be used to calculate global CH4 budgets, if the isotope composition of individual sources is known (e.g. Mikaloff Fletcher et al. 2004). It is, therefore, important to characterise the isotope fractionation associated with CH4-cycling processes to enable better differentiation among different sources contributing to atmospheric CH4. An important factor influencing the isotope signature (a measure of fractionation) of CH4 (d 13CH4) emitted by cows are the diets’ proportions of C3 and C4 plants, which largely differ in d13C (Levin et al. 1993;
10.1071/AN09112
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Animal Production Science
Bilek et al. 2001). The d 13C values of CH4 and carbon dioxide (CO2) produced in vitro during ruminal fermentation of C3 and C4 plant diets were shown to reflect the C-isotope composition of the feeds, and the fractionation factor between CO2 and CH4 was quite similar for diets with different proportions of C3 and C4 plants (Klevenhusen et al. 2009). Only few studies actually investigated the C-isotope signature of cattle manure and the fermentation gases produced during its storage (e.g. Levin et al. 1993). The objective of the present study was to generate values for the CH4 EF from dairy cows and their slurry when the animals are fed different diets representing typical Western European feed production systems which are based either on feeds from grassland or arable land. In doing so, forage-only and mixed forage-concentrate diets (1 : 1) were used. Further, in order to investigate the C-isotope fractionation during CH4 formation as another objective, diets were designed consisting exclusively of either C3 or C4 plants. These diets, though not exactly common in feeding practice, could be considered to satisfactorily reflect the production systems and diet types described above. In Klevenhusen et al. (2008) few selected parameters using a reduced dataset obtained during the present dairy cow experiment were already published. Materials and methods Experimental setup of the dairy cow experiment The experiment (conducted in accordance with the Swiss guidelines for Animal Health and Welfare) was carried out with 18 dairy cows of Holstein Friesian (11) or Brown Swiss (7) breeds weighing on average 649 53 kg (mean standard deviation). The stage of lactation was mid to end and the average milk yield amounted to 14.6 1.0 kg/cow.day, which adds up to ~6.3 t of milk/cow.year when applying the standard lactation curves given in Kirchgessner et al. (2008). This stage of lactation and milk yield was deliberately chosen to closely reflect the mean milk yield of cows across the entire reproduction cycle applied by the IPCC (2006) for Western Europe (6.0 t of milk/cow.year). Each of the 18 animals received one of the three experimental diets (n = 6). For practical reasons, three experimental runs lasting for 28 days each were carried out. In each run six cows were employed and all three diets were included. Eventually, for the maize diet, data of only five cows could be evaluated as one animal had to be excluded from the experiment. During the first 6 days, the pre-experimental diet was gradually changed to the experimental diets. In the following 14 days of the experimental run the animals were allowed to adapt to the new diet before the sample collection period of 8 days started. The three experimental diets (Table 1) were chosen to reveal differences in CH4 formation depending on a differing carbohydrate profile (ryegrass hay versus straw plus grain diets, i.e. easily degradable fibre versus hardly degradable fibre plus starch). In addition, the variability of d13CH4 and the dietary influence on C-isotope fractionation during CH4 formation was determined. The feed ingredients selected for the three diets were either based on C3 or C4 plants in order to use the natural variability in C-isotope contents (Bender 1971). Diets were balanced in contents of crude protein (CP) and, as far as possible, in net energy
F. Klevenhusen et al.
Table 1. Ingredient and analysed nutrient composition (g kg of the three experimental diets Dietary treatment Ryegrass hay Barley straw Maize straw
Hay
Barley
1
DM)
Maize
Forage 984 459 444 Concentrate
Barley grain Soybean meal Maize whole plant pellets Maize gluten Sugar beet molasses Sugarcane molasses Urea Vitamin–mineral mixtureA NaCl MgSO4 Dry matter (g/kg wet weight) Organic matter Crude protein Starch Total sugars Neutral detergent fibre Acid detergent fibre Total ash Gross energy (MJ/kg DM) NELB (MJ/kg DM)
266 238 368 151 21
14 2 Nutrients 841 846 211
