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Evaluering af mulige tiltag til reduktion af landbrugets metanemissioner

1 Methane emissions from dairy cows

• 1.1 Introduction
• 1.2 Factors affecting the methane production in dairy cows
  • 1.2.1 Sugar and starch
  • 1.2.2 Fat
  • 1.2.3 Feeding level
  • 1.2.4 Genetic potential
  • 1.2.5 Conclusions from the literature survey
• 1.3 Simulation of methane production
  • 1.3.1 Effects of stage of lactation, feeding level and feed composition
• 1.4 Changes from 1991 to 2002
• 1.5 Discussion and conclusion
• 1.6 References

Allan Danfær, Danmarks JordbrugsForskning, Afd. for Husdyrsundhed, Velfærd og Ernæring

1.1 Introduction

The feed composition affects methane production in the digestive tract of dairy cows as described by Weisbjerg et al. (2005). Here, the chemical composition of dairy cow rations in Denmark 1991-2002 is estimated from winter-feeding plans reported to The Danish Cattle Organization. According to these calculations, the composition of the feed for dairy cows has changed considerably during this period so that the dietary content of sugar has decreased (from 20.0 to 8.5% of feed dry matter (DM) and the starch content has increased (from 7.6 to 15.1% of DM). These changes are caused by an extensive replacement of fodder beets (about 70% sugar in DM) by maize silage (about 2% sugar and 30% starch in DM). A substitution of starch for sugar in the feed ration is expected to decrease the methane production in the rumen due to a changed fermentation pattern, which results in a higher propionate and a lower butyrate production (Møller, 1969; Sutton et al., 1993). Weisbjerg et al. (2005) used three different prediction equations to estimate the methane production based on the reported winter-feeding plans. Sugar and starch are explicit input parameters in only one of these equations (published by Hindrichsen et al., 2004), and results with this equation show that the methane production is reduced by 19% in the winter period (200 days) from 1991 to 2002. This corresponds to 10-11% lower methane emission on a yearly basis, but the calculations must be regarded with caution because of the assumptions that had to be made in order to apply the equation (Weisbjerg et al., 2005). ThR_refore, there is need for a further examination of whether it is probable that changes in feeding has substantially reduced the enteric emission of methane from dairy cows in Denmark during the period 1991 to 2002.

The purpose of the present analysis is to investigate this question partly by reviewing the literature and partly by using models to predict the methane production from dairy cows. The literature review is focused on experiments in which cows were fed different levels of sugar and starch and in which the methane production was measured or can be estimated from other parameters. However, reference is also given to studies that examine other aspects of nutritional and genetic changes that have taken place in Denmark during 1991-2002. The criterion for selection of models is that sugar and/or starch are included as independent variables.

1.2 Factors affecting the methane production in dairy cows

The change in the composition of the winter-feed during the last 10 years is characterized by decreased sugar content, increased starch content, a minor increase of the fat content and also an increased feeding level (Weisbjerg et al., 2005). At the same time, the breeding program for dairy herds has resulted in an increase in the genetic capacity for milk yield (Dansk Landbrugsrådgivning, 2004). The impact of these factors on the methane production is examined in the following by a survey of relevant published data.

1.2.1 Sugar and starch

The fermentation pattern in the rumen affects the extent of methane production. The formation of acetate (C₂) and butyrate (C₄) increases, while the formation of propionate (C₃) decreases the methane production (Hungate, 1966). The proportion of the ruminal concentrations of the individual short-chain fatty acids (SCFA) reflects to a large extent their relative rates of formation (Kristensen et al., 1996; Kristensen et al., 2003). ThR_refore, the conditions for methane formation in the rumen can be assessed from the concentrations of acetate, propionate and butyrate in the rumen fluid. An increased sugar content in the feed ration by use of fodder beets, molasses or sucrose increases the ruminal concentration of butyrate in dairy cows (Owen et al., 1967; Møller, 1969; Piatkowski et al., 1977; Piatkowski and Voigt, 1978; Krohn and Konggaard, 1987; Beever, 1993), in calves (Keusenhoff et al., 1988; Khalili and Huhtanen, 1991a) as well as in goats and sheep (Chamberlain et al., 1985). By contrast, Huhtanen (1988) found a lower butyrate concentration in the rumen of male calves by feeding beet pulp + molasses instead of barley. Feeds with high sugar content often decrease the acetate concentration (Møller, 1969; Piatkowski and Voigt, 1978; Chamberlain et al., 1985; Krohn and Konggaard, 1987; Keusenhoff et al., 1988; Khalili and Huhtanen, 1991a), have variable effects on the propionate concentration (Møller, 1969; Chamberlain et al., 1985; Huhtanen, 1988) and decrease the NH₄⁺ concentration in the rumen (Møller et al., 1973; Chamberlain et al., 1985; Keusenhoff et al., 1988; Khalili and Huhtanen, 1991a). Apparently, the lower NH₄⁺ concentration is related to a higher microbial protein synthesis in the rumen obtained with fodder beets (Huhtanen, 1988; Keusenhoff et al., 1988; Khalili and Huhtanen, 1991a). Microbial protein synthesis from NH₄⁺ involves use of reduction equivalents (H+ and e-) and thereby a lower methane production (Demeyer and Van Nevel, 1975; Mills et al., 2001). However, feeding with sugar results in higher methane production than does starch feeding (Müller et al., 1994; Kirchgessner et al., 1994a; Torrent et al., c.f. Johnson and Johnson, 1995).

Normally, a high dietary starch content lowers acetate and increases propionate concentration in the rumen so that the ratio (C₂+C₄)/C₃ decreases (Owen et al., 1967; Sutton et al., 1988; Bergman, 1990; Beever, 1993; Sutton et al., 1993). Fermentation of starch yields less methane than fermentation of NDF (Moe and Tyrrell, 1979; Gordon et al., 1995b). However, Sutton et al. (1998) found the same methane production in dairy cows fed different proportions of grass silage and whole crop wheat silage.

Stoichiometric fermentation equations published by Baldwin et al. (1970), Murphy et al. (1982) and Bannink et al. (2000) show concordantly that fermentation of sugar results in higher production of butyrate and lower production of propionate than fermentation of starch. On this basis, replacement of sugar by starch in the feed ration is expected to decrease meth-ane production as actually shown by simulation with dynamic, mechanistic models (Mills et al., 2001; Kebreab et al., 2004) in which the fermentation equations of Bannink et al. (2000) were applied.

1.2.2 Fat

An increased dietary content of fat (especially unsaturated fat) reduces methane production (Holter and Young, 1992; Giger-Reverdin et al., 2003), but Johnson et al. (2002) found no consistent effect on the methane production in dairy cows by increasing the fat content (from 2.3 to 5.6% in DM) with supplements of whole cotton seeds and grinded rape seeds. Weisbjerg et al. (2005) have reported the major reasons for the reduction of methane formation by dietary fat:

• Unsaturated, long-chain fatty acids increase propionate formation, inhibit fermentation of cell wall carbohydrates and inhibit activity of methanogenic bacteria in the rumen
• Fatty acids reduce the number of protozoa, which produce high proportions of butyrate
• Reduction equivalents are used in hydrogenation of unsaturated fatty acids in the rumen
• Fatty acids are not fermented in significant amounts and thR_refore do not contribute to formation of methane.

1.2.3 Feeding level

An increased feeding level apparently changes the fermentation pattern in the rumen of dairy cows so that the ratio (C₂+C₄)/C₃ decreases (Sutton et al., 1988). Furthermore, high feeding levels increase the ruminal passage rate of microbial matter and undigested material leading to lower digestibility of organic matter (OM) in the rumen (Gabel et al., 2003). Both the changed fermentation and the increased passage rate contribute to a reduced formation of methane per kg DM when the feeding level is increased (Schiemann et al., 1970; Schiemann et al., 1971; Gordon et al., 1995b; Cammell et al., 2000; Yan et al., 2000; Giger-Reverdin et al., 2003). This effect of feeding level is also demonstrated by use of a mathematical simulation model (Mills et al., 2001).

1.2.4 Genetic potential

Possible influences of the genetic capacity for milk yield on the fermentation pattern and in turn on the methane production in dairy cows have not been examined to any large extent. Two studies have shown no difference in the methane energy loss (as percentage of gross energy (GE)) in cows with different breeding indexes for milk yield (Grainger et al., 1985; Gordon et al., 1995a). On the other hand, Ferris et al. (1999) found a lower methane production (6.3% of GE) in cows with a high than in cows with a medium breeding index (7.0% of GE), but this difference in methane loss may be explained fully or partly by a little higher feed intake of the high potential cows.

1.2.5 Conclusions from the literature survey

Replacement of sugar by starch in feed rations for dairy cows can be expected to cause a decreased ratio (C₂+C₄)/C₃ in the rumen and in turn a lower methane production. Increased dietary fat content and the feeding level reduce the production of methane per kg DM. The genetic capacity for milk yield in cows has no consistent effect on the methane formation.

1.3 Simulation of methane production

1.3.1 Effects of stage of lactation, feeding level and feed composition

The simulation model Karoline, which is a dynamic, mechanistic whole animal model of lactating cows, has been used previously by Weisbjerg et al. (2005) to predict the methane production from different feed rations. In all cases, the model simulated higher values of methane production than two selected empirical regression equations (IPCC, 1997; Kirchgessner et al., 1994b). The model has been adjusted since then and is at present in the process of being published (Danfær et al., 2005). The methane production in Karoline is calculated on the basis of stoichiometric fermentation equations for individual nutrients that are fermented in the rumen and the hindgut. The predicted methane formation is corrected for the use of reduction equivalents for microbial cell synthesis, synthesis of microbial fatty acids and hydrogenation of unsaturated dietary fatty acids. The latest version of Karoline (MixKarO, October 2004) has been changed further from the published version (MixKarH) as described below:

• Increased use of reduction equivalents for microbial syntheses and hydrogenation of unsaturated fatty acids
• Changes in the stoichiometric fermentation equations for starch and sugar (lower acetate and higher propionate and butyrate formation from starch; lower propionate and higher butyrate formation from sugar).

In the following, some simulation results obtained with Karoline (version MixKarO) are presented. The purpose is to evaluate the model's ability to give realistic predictions of the methane production in dairy cows depending on stage of lactation, feeding level and feed composition.

Table 1.1 shows input parameters for the simulations: cow weight, stage of lactation, feeding level and composition of the feed ration. The basal ration is composed (on DM basis) of 37.5% barley, 10.0% rape seed cakes, 2.5% soybean meal and 50.0% clover grass silage. The cow weight is 600 kg and the stage of lactation is 18 weeks after calving except for simulations 4 and 5. Simulations 1-3 are with the basal ration at different feeding levels (15, 20 and 25 kg DM/d, respectively), simulations 4 and 5 are with the basal ration (20 kg DM/d) and different stages of lactation (4 and 32 weeks after calving, respectively), simulations 6 and 7 are with low and high proportion of concentrates in the ration, simulations 8 and 9 are with grass silage of low and high digestibility, simulations 10 and 11 are with low and high dietary fat content, simulation 12 is with high dietary sugar content and simulation 13 is with high dietary starch content. All rations are balanced with respect to protein level (AAT and PBV) as well as physical structure (fill) according to Danish feeding standards (Strudsholm et al., 1999).

Results from the 13 simulations are collected in Table 1.2. The first three simulations show that the methane production per kg DM or as per cent of GE decreases with increasing feed intake, which is in agreement with experimental data as quoted above. Simulations 4 and 5 show that stage of lactation has no effect on the total methane formation, but the production per kg milk or as a percentage of net energy (NE) increases during lactation because of the decreasing milk yield. Studies of Cammell et al. (2000) and Sutter and Beever (2000) showed no consistent effect of lactation stage on the methane production. Cammell et al. (2000) recorded an increasing methane loss as percentage of digestible energy (DE) from week 6 to week 24 after calving, but this can be explained fully or partly by a concurrently decreasing feed intake. A high proportion of concentrates in the ration reduces the methane production per kg DM and as a percentage of GE (shown in simulations 6 and 7). This is confirmed by experimental results (Holter and Young, 1992; Ferris et al., 1999; Hindrichsen et al., 2004) and by other model simulations (Mills et al., 2001). The methane loss (g per kg DM or per cent of GE) is expected to increase with increasing digestibility of the fibre fraction (Holter and Young, 1992). Simulations 8 and 9 demonstrate this even though the feed intake is higher for the silage with high digestibility. The lowest methane production (15.9 g/kg DM) is obtained with a high dietary fat content (simulation 11) in agreement with data from the literature (Holter and Young, 1992; Giger-Reverdin et al., 2003). The last two simulations show that replacement of fodder beets + clover grass silage by barley + maize silage decreases the methane production both in absolute amount and per kg DM as also found by Müller et al. (1994) and Kirchgessner et al. (1994a).

It is concluded in the light of these 13 simulations that the model Karoline gives realistic predictions of the methane production in dairy cows at different feeding levels and with different feed compositions.

Klik her for at se Table 1.1

Klik her for at se Table 1.2

1.4 Changes from 1991 to 2002

In order to examine whether the methane production from dairy cows in Denmark has been reduced from 1991 to 2002, the estimated average combination of feedstuffs in the winter-feed for dairy cows in 1991 and 2002 is shown in Table 1.3. These estimations are based on information from the Danish Cattle Organization on the nutrient composition of the winter-feed for dairy cows during this period (see Weisbjerg et al., 2005) and show that the ration content of fodder beets + beet pulp is reduced by 3.70 kg DM, while maize silage is increased by 4.24 kg DM per cow daily. These changes result in a decrease of the sugar content of the ration from 198 to 84 g/kg DM and an increase of the starch content from 74 to 152 g/kg DM as shown in Table 1.4. The dietary nutrient composition calculated from Table 1.3 can be compared with the corresponding figures reported by the Danish Cattle Organization for 1991 and 2002, respectively.

The information in Tables 1.3 and 1.4 is the basis of calculations of the methane production in dairy cows during the winter periods 1991/92 and 2002/03 by means of four different models. The models 1-3 are empirical regression equations, and model 4 is the simulation model Karoline (version MixKarO). The regression equations are defined as follows:

Model 1 (Kirchgessner et al., 1994a):
Y (g CH₄/d) = 172 + 9,1 * sugar (kg/d) + 92,5 * digestible crude fibre (kg/d)

Model 2 (Johnson and Ward, 1996):
Y (Mcal CH₄/d) = 0,54 + 0,39 * digestible sugar (kg/d) + 0,08 * digestible starch (kg/d) + 0,68 * digestible cell wall carbohydrates (kg/d)

Model 3 (Hindrichsen et al., 2004):
Y (g CH₄/d) = 91 + 50 * digestible cellulose (kg/d) + 40 * digestible hemicellulose (kg/d) + 24 * starch (kg/d) + 67 * sugar (kg/d).

Table 1.3. Estimated composition of the winter-feed for dairy cows in 1991 and 2002.

Table 1.4. Chemical composition (g/kg DM) of the winter-feed for dairy cows in 1991 and 2002.

1) Calculated from the feed composition in Table 3, the Danish feedstuff table (Møller et al., 2000) and digestibilities of crude fibre (Andersen and Just, 1983)
2) Calculated by Weisbjerg et al. (2005) based on reports from The Danish Cattle Organization.

The independent variables can be derived from Table 1.4. Digestible sugar is calculated as sugar * 0.98, digestible starch as starch * 0.92, and digestible cellulose and hemicellulose are calculated as digestible cell wall carbohydrates times 0.2 and 0.3, respectively, as assumed by Weisbjerg et al. (2005). Inputs to the Karoline model are the feed rations given in Table 1.3, a cow weight of 600 kg and a lactation stage of 18 weeks after calving.

Results from the four models are shown in Table 1.5. The daily methane production per cow decreases from 1991 to 2002 with model 2-4, but not with model 1, whereas all four models show a decreasing methane production per kg DM and as a percentage of GE. The predicted decline in methane energy (as percentage of GE) varies from 6.0 to 22.1%, lowest with model 1 and highest with model 3. Models 2 and 4 show that the methane production (per cent of GE) in the winter feeding period is decreased by approximately 10% from 1991 to 2002.

Table 1.5. Predicted methane production in dairy cows, winter periods 1991 and 2002.

Model 1: Kirchgessner et al. (1994a), Model 2: Johnson and Ward (1996), Model 3: Hindrichsen et al. (2004), Model 4: Karoline, version MixKarO (2004).

1.5 Discussion and conclusion

The choice of models for calculation of methane production in dairy cows was based on whether dietary sugar and/or dietary starch were independent variables, but also on whether the models were available for use. There are other mechanistic simulation models than Karoline that are able to predict methane production in dairy cows (Mills et al., 2001; Kebreab et al., 2004), but these models were not immediately available.

Out of the four models used here, Karoline is the only one, which takes into account all the nutritional factors that have changed from 1991 to 2002: feeding level as well as dietary contents of sugar, starch and fat. Furthermore, it is concluded from the literature review and the simulation results in Table 1.2 that Karoline can give realistic predictions of various nutritional effects on the methane production in dairy cows. Model 1 (Kirchgessner et al., 1994a) contains sugar, but not starch as an independent variable, and the basis for model 3 (Hindrichsen et al., 2004) is feed rations that are very different from those normally used for dairy cows in Denmark. Moreover, the assumptions used in the calculation of digestible cellulose and hemicellulose are problematic as pointed out by Weisbjerg et al. (2005). The mean result from the four models is a 12% decline from 1991 to 2002 in the methane production (expressed as per cent of GE), which is close to the simulation result obtained with Karoline (Table 1.5).

In conclusion, the enteric methane loss (per cent of GE) from dairy cows in the winter feeding period in Denmark is likely to have decreased by approximately 10% from 1991 to 2002. This is equivalent to a 5-6% decrease on a yearly basis if a winter feeding period of 200 days is assumed.

1.6 References

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