Evaluation of dietary strategies to reduce methane production in ruminants: A modelling approach

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1 Evaluation of dietary strategies to reduce methane production in ruminants: A modelling approach C. Benchaar 1, 2, C. Pomar 1, and J. Chiquette 1 1 Agriculture and Agri-Food Canada, Dairy and Swine Research and Development Centre, P.O. Box Route 108 East, Lennoxville, Quebec, Canada J1M 1Z3 ( cbenchaar@nsac.ns.ca); 2 Nova Scotia Agricultural College, Department of Plant and Animal Sciences, Truro, Nova Scotia, Canada B2N 5E3. Contribution no. 708 received 1 December 2000, accepted 1 August Benchaar, C., Pomar, C. and Chiquette, J Evaluation of dietary strategies to reduce methane production in ruminants: A modelling approach. Can. J. Anim. Sci. 81: The objective of this study was to use the modelling approach to assess the effectiveness of different existing nutritional strategies to reduce methane production from ruminants. For this purpose, a modified version of a mechanistic and dynamic model of rumen digestion was used. Simulated strategies included: dry matter intake (DMI), forage to concentrate ratio, nature of concentrate (fibrous vs. starchy concentrate), type of starch (slowly vs. rapidly degraded), forage species (legume vs. grass), forage maturity, forage preservation method (dried vs. ensiled), forage processing, and upgrading and supplementation of poor quality forages (straw). This study showed that mathematical modelling is a valuable tool to evaluate the impact of a given dietary manipulation not only on methanogenesis but also on the metabolism of the whole rumen system. Depending on the nature of the intervention, methane production can be reduced by 10 to 40%. Increasing DMI and the proportion of concentrate in the diet reduced methane production ( 7 and 40%). was also decreased with the replacement of fibrous concentrate with starchy concentrate ( 22% ) and with the utilization of less ruminally degradable starch ( 17%). The use of more digestible forage (less mature and processed forage) resulted in a reduction of methane production ( 15 and 21%). was lower with legume than with grass forage ( 28%), and with silage compared to hay ( 20%). Supplementation or ammoniation of straw did not reduce methane losses, but had a positive impact on the efficiency of rumen metabolism. The modelling approach demonstrated that reduction of methane production from ruminants is a complex challenge. Implementation of any strategy must take into account the possible consequences on the efficiency of the entire rumen system. Key words: Ruminants, methane reduction, modelling approach Benchaar, C., Pomar, C. et Chiquette, J Evaluation de stratégies alimentaires pour réduire la production de méthane chez les ruminants: approche modélisatrice. Can. J. Anim. Sci. 81: L objectif de cette étude est d utiliser la modélisation mathématique pour évaluer et expliquer l impact de différentes stratégies alimentaires appliquées pour réduire la production de méthane chez les ruminants. Pour cela, une version modifiée d un modèle dynamique et mécaniste de la digestion ruminale a été utilisée. Les différentes stratégies alimentaires évaluées sont: le niveau d ingestion, la proportion de concentré dans la ration, la nature du concentré (fibreux vs. amylacé), la dégradabilité ruminale de l amidon (lentement vs. rapidement dégradable), l espèce (légumineuse vs. graminée), la maturité et la méthode de conservation du fourrage, et le traitement chimique et la supplémentation de la paille. Les résultats des simulations obtenus montrent que la production de méthane peut être réduite de 10 à 40% selon la stratégie appliquée. L augmentation du niveau d ingestion et de la proportion de concentrés dans la ration réduisent les pertes d énergie sous forme de méthane ( 7 et 40%). La production de méthane est diminuée par le remplacement de concentré fibreux par du concentré amylacé ( 22%) et l utilisation d un amidon lentement dégradable ( 17%). La consommation d un fourrage plus digestible (moins mature ou broyé et cubé) s accompagne d une plus faible production de méthane ( 15 et 21%). Les pertes de méthane sont également moins élevées avec les légumineuses qu avec les graminées ( 28%) et avec l ensilage comparativement au foin ( 20%). La production de méthane n est pas réduite par la supplémentation ou le traitement de la paille à l ammoniac. Cependant, l efficacité des fermentations ruminales se trouve améliorée avec ces traitements. Cette étude a démontré que la réduction de méthane chez les ruminants demeure un défi complexe. L application de toute stratégie visant à réduire les émissions de méthane chez les ruminants devrait prendre en compte les possibles conséquences sur le métabolisme ruminal. Mots clés: Ruminants, réduction de méthane, approche modélisatrice 563 For many years, several experiments have been carried out in order to reduce methane production from ruminants. Initially, the objective of these studies was mainly nutritional, and aimed to improve feed efficiency by reducing the part of feed energy lost as methane by the animal. During the past few years, a renewed interest in reducing methane emitted from ruminants has been observed. Methane has been identified as a potent greenhouse gas contributing to climate change and global warming (Tyler 1991). To develop strategies to mitigate ruminant methane emissions, it is necessary to quantify methane production under a wide range of circumstances. Different techniques are currently Abbreviations: BW, body weight; DE, digestible energy; DM, dry matter; DMI, dry matter intake; GE, gross energy;, neutral detergent fiber;, organic matter; F, organic matter fermented in the rumen; VFA, volatile fatty acid

2 564 CANADIAN JOURNAL OF ANIMAL SCIENCE available to measure methane produced by digestive fermentation from ruminants (Johnson and Johnson 1995). However, their application is complex and requires costly installations. Therefore, mathematical relationships have been developed to predict methane production from feed characteristics (Moe and Tyrrell 1979b) or from digestibility of feed components (Blaxter and Clapperton 1965; Moe and Tyrrell 1979b). However, these equations cannot predict methane production under a variety of nutritional conditions. With the progress made during the past years in the area of modelling rumen function, mathematical models currently available allow simulation of ruminal fermentation processes under different nutritional conditions. Mechanistic and dynamic models of rumen digestion have been reported to be more accurate than existing regression equations to predict methane production (Benchaar et al. 1998). Hence, the modelling approach can be helpful in evaluating the impact of different interventions not only on methane production but also on the metabolism of the whole rumen system. The objective of this study was to evaluate the potential of using mechanistic and dynamic models of ruminant digestion to assess the effectiveness of different existing nutritional strategies to reduce methane production in ruminants. The systematic approach applied here was used to identify potential strategies for reducing methane production, and to explain the dietary effects on methane production as reported in the literature. MATERIALS AND METHODS The mathematical model of rumen digestion of Dijkstra et al. (1992) as modified by Benchaar et al. (1998) was used to simulate the effect of different nutritional strategies on methane production. In this modified version, methane production is predicted as proposed by Baldwin (1995). Briefly, ruminal methane production was predicted based on hydrogen balance. Excess hydrogen produced during the fermentation of carbohydrates and protein to lipogenic volatile fatty acid (VFA) (i.e., acetate and butyrate) is used for microbial growth, biohydrogenation of unsaturated fatty acids, and the production of glucogenic VFA (i.e., propionate and valerate). It was assumed that the remainder of hydrogen is completely used for the production of methane. Required input parameters of the modified model are the same as those required in the original model of Dijkstra et al. (1992). These parameters are daily DMI, chemical composition of the diet, solubility and degradability of protein and starch, degradability and degradation rates of protein, starch and neutral detergent fiber (). For this study, chemical composition of feeds was obtained from published tables of feed composition or from the literature. Kinetic degradation parameters of feed components were obtained from in sacco measurements reported in the literature. In addition to these nutritional input parameters, the model also required other input variables such as rumen volume and fractional passage rates of solids and liquids in the rumen. Therefore, the regression equation of Rémond (1988) was used to predict the volume of the rumen: V (liter) = 2.26 DMI (kg d 1 ) Ruminal passage rates of liquids (K l ), forage (K pf ) and concentrate (K pc ) were predicted based on the empirical relationships proposed by Sauvant and Archimède (1990; cited by Sauvant et al. 1995): K l (% h 1 ) = DMI F 2 K pf (% h 1 ) = DMI F 2 K pc (% h 1 ) = K pf, where DMI is in g kg 1 BW 0.75 and F is the percentage of forage in the diet. In the original model of Dijkstra et al. (1992), mean ruminal ph reached during the day had to be specified to run the model. Therefore, the regression equation (ph = VFA) reported by Tamminga and van Vuuren (1988) was used to predict mean ruminal ph value. For the purpose of this study, provisions were added to the original model of Dijkstra et al. (1992) to allow the calculation of gross energy (GE) intake and the digestibility of different nutrients and energy in the lower digestive tract. Intestinal digestion of nutrients that escaped rumen digestion and fermentation was calculated empirically by multiplying duodenal flows by a mean coefficient of intestinal digestion. Coefficients of digestion of nutrients entering the intestinal compartment are those reported by Baldwin (1995). GE content of feed and feces [kcal kg 1 of dry matter (DM)] was calculated from heats of combustion of the individual nutrients. GE intake and fecal energy were then obtained by multiplying the energy concentration of the ingested diet and the feces by the quantities of DM ingested and excreted, respectively. To simulate the effect of different nutritional strategies on rumen fermentation processes, different theoretical diets were formulated. The composition of these diets was desired to be as simple as possible because our objective was not to quantify methane production for a specific diet but rather to examine the effect of nutritional changes on methane production. The chemical composition of feedstuffs making up the diets used in different strategies is reported in Table 1. The effectiveness of a given dietary intervention was assessed for its potential to reduce methane production and also for its impact on rumen metabolism. Simulations were conducted for a 500-kg body weight cow. Unless indicated, all simulations were performed at intake of 15.0 kg of DM d 1. The extent of rumen digestion, ruminal ph, microbial efficiency, VFA and methane productions were predicted by running the model until steady state conditions were achieved. Simulations were performed using ACSL software (Mitchell and Gauthier 1995). Finally, it is important to note that no statistics can be applied to the results of the simulations. The mathematical used in the present study is a deterministic model and, hence, the predictions generated by the model apply specifically to the average animal in a population (Baldwin 1995). RESULTS Strategy 1: Increasing DMI To assess the effect of intake level on methane production, simulations were performed for diets based on 100% alfalfa

3 BENCHAAR REDUCTION OF METHANE EMISSIONS FR RUMINANTS 565 Table 1. General characteristics of feedstuffs making up the diets used to simulate the effect of different dietary strategies on methane production Composition (% DM) Degradation rates z (d 1 ) Strategy Feed CP Sugars + starch k k CP k Starch 1, 2, 3, 4, 6 Alfalfa hay y , 2, 3, 4 Corn , 2, 3, 9 Soybean meal , 4 Barley Beet pulp Alfalfa silage y , 7 Vegetative alfalfa hay x Midbloom alfalfa hay x Full inflorescence timothy hay x Long alfalfa hay w Pelleted alfalfa hay w Non-treated straw v Ammoniated straw v z Degradation rates (k) from: Tamminga et al. (1990), Nocek and Grant (1987), Sniffen et al. (1992), Shaver et al. (1986), Hoffman et al. (1993), Fondevila et al. (1994). Chemical composition from: y Harlan et al. (1991), x Hoffman et al. (1993), w Shaver et al. (1986), v Fondevila et al. (1994). hay or 30% alfalfa hay + 70% of concentrate mixture using different levels of intake: 9, 12, 15, and 17 kg of DM d 1. Concentrate mixture contained corn (45%) and soybean meal (55%). Results of simulations (Tables 2 and 3) show that manipulation of DMI altered rumen fermentation and digestion processes: for both diets, increasing daily DMI was accompanied by an acceleration of ruminal fractional passage rates of solids and liquids. The amount of organic matter () and its carbohydrate constituents (starch and ) degraded in the rumen was greater when DMI was increased. However, when expressed as a proportion of ingested dietary intakes, ruminal digestion was decreased as DMI increased. This effect was more accentuated with the concentrate-based diet than with the 100% alfalfa diet. Ruminal microbial efficiency was improved by increasing DMI and the effect was greater with the concentrate-based diet than with the 100% alfalfa diet. Ruminal ph decreased with increasing DMI. Finally, the formation of end-products of fermentation was also modified by the manipulation of feed intake. For both diets, production of individual VFA increased as daily DMI increased. Regardless of the type of diet, methane production in Mcal d 1 increased with increasing DMI. However, when methane production was expressed relative to GE intake or to digestible energy (DE), the opposite was observed, i.e., methane yields were depressed when DMI increased. The effect of increasing level of intake was greater with the diet containing concentrates than with the one consisting of alfalfa hay only. For example, increasing DMI from 9 to 17 kg d 1 decreased methane energy losses (% of GE intake) by 9% for the 100% alfalfa hay diet compared to a reduction of 23% when the alfalfa hay was supplemented with concentrates. Strategy 2: Increasing Proportion of Concentrates in the Diet The effect of increasing the amount of concentrate in the diet on methane production was investigated using different forage/concentrate ratios: 100:0, 80:20, 50:50 and 30:70 of a diet based on alfalfa hay supplemented with a concentrate mixture (45% corn + 55% soybean meal). The results of Table 2. Effect of DMI on rumen fermentation and methane production: diet consisting of 100% alfalfa hay DMI (kg d 1 ) (kg d 1 ) (kg d 1 ) GE (Mcal d 1 ) Passage rate (d 1 ) Liquids Solids kg d % intake kg d % intake Ruminal microbial efficiency (g N kg 1 F z ) Ruminal ph Total Acetate Propionate Butyrate Valerate Mcal d % GE intake % DE z F = organic matter fermented in the rumen. these simulations are presented in Table 4. Increasing the proportion of the concentrate in the diet reduced ruminal passage rates of liquids and solids. Ruminal degradation of starch (% of starch intake) was increased whereas that of

4 566 CANADIAN JOURNAL OF ANIMAL SCIENCE Table 3. Effect of DMI on rumen fermentation and methane production: diet consisting of 30% alfalfa hay + 70% concentrate z DMI (kg d 1 ) (kg d 1 ) Starch (kg d 1 ) (kg d 1 ) GE (Mcal d 1 ) Passage rate (d 1 ) Liquids Solids kg d % intake Starch kg d % starch intake kg d % intake Ruminal microbial efficiency (g N kg 1 F y ) Ruminal ph Total Acetate Propionate Butyrate Valerate Mcal d % GE intake % DE z Concentrate mixture contained 45% corn meal and 55% soybean meal y F = organic matter fermented in the rumen. was decreased with the increase of the proportion of the concentrate in the diet. Ruminal microbial efficiency was linearly depressed by increasing the quantity of the concentrate in the diet. As a consequence of these changes (nature of substrate available to rumen microbes, residence time of substrate in the rumen, and extent of ruminal digestion), the rumen fermentation pattern was also altered: increasing the proportion of concentrate in the diet linearly reduced ph of rumen fluid. Total VFA production increased as the proportion of concentrate in the diet increased. Prediction of production of individual VFA indicated that the amount of acetate was increased with decreasing forage/concentrate ratio up to 50:50, where it then declined for the 30:70 forage/concentrate ratio. Production of propionate and valerate increased linearly when the proportion of concentrate increased., expressed in Mcal d 1, increased when the proportion of concentrate in the diet was increased from 0 to 20%, and it then declined Table 4. Effect of forage to concentrate ratio on rumen fermentation and methane production z Forage/concentrate ratio 100:0 80:20 50:50 30:70 DM (kg d 1 ) (kg d 1 ) Starch (kg d 1 ) (kg d 1 ) GE (Mcal d 1 ) Passage rate (d 1 ) Liquids Solids kg d % intake Starch kg d % starch intake kg d % intake Ruminal microbial efficiency (g N kg 1 F y ) Ruminal ph Total Acetate Propionate Butyrate Valerate Mcal d % GE intake % DE z Concentrate mixture contained 45% corn meal and 55% soybean meal. y F = organic matter fermented in the rumen. for the highest proportions of concentrate in the diet. However, methane generated per unit of GE intake or DE decreased linearly as the proportion of concentrate in the diet increased. Strategy 3: Replacing Fibrous Concentrate with Starchy Concentrate in the Diet The influence of replacing fibrous concentrate by starchy concentrate in the diet on methane production (Table 5) was evaluated using a diet based on 30% alfalfa hay and 70% concentrate. The concentrate mixture contained 55% of soybean meal, and 45% of either barley (starchy concentrate) or sugar beet pulp (fibrous concentrate). The proportion of ingested starch degraded in the rumen was higher (+41%) with the barley diet than with the beet pulp diet, whereas the opposite was predicted for degradation ( 27%). Ruminal microbial efficiency was reduced ( 17%) when

5 BENCHAAR REDUCTION OF METHANE EMISSIONS FR RUMINANTS 567 Table 5. Effect of replacing fibrous concentrate (beet pulp) with starchy concentrate (barley) in the diet on rumen fermentation and methane production z Nature of concentrate Fibrous: beet pulp Starchy: barley DM (kg d 1 ) (kg d 1 ) Starch (kg d 1 ) (kg d 1 ) GE (Mcal d 1 ) kg d % intake Starch kg d % starch intake kg d % intake Ruminal microbial efficiency (g N kg 1 F y ) Ruminal ph Total Acetate Propionate Butyrate Valerate Mcal d % GE intake % DE z Diet consisted of 30% alfalfa hay, 38% soybean meal and 32% either barley grain or beet pulp. y F = organic matter fermented in the rumen. replacing beet pulp by barley. However, the difference in ruminal ph between the two sources of carbohydrate was small. The production of total VFA and acetate was not affected by the nature of concentrate. However, the production of propionate and butyrate was increased (+52% and +36 %, respectively), but that of valerate was reduced ( 11%) with the inclusion of barley grain in the diet. Finally, methane production in Mcal d 1 was reduced ( 14%) when beet pulp was replaced by barley. This depression was more accentuated ( 23%) when methane production was expressed relative to GE intake or DE. Strategy 4: Replacing Rapidly Degraded Starch with Slowly Degraded Starch in the Diet Utilization of slowly degraded starch rather than rapidly degraded starch as a potential strategy to reduce methane production in ruminants was simulated with a diet consisting of 30% alfalfa hay and 70% of either barley grain (rapidly degraded starch) or corn grain (slowly degraded starch). Table 6. Effect of replacing rapidly degraded starch (barley) with slowly degraded starch (corn) in the diet on rumen fermentation and methane production z Source of starch Barley Corn DM (kg d 1 ) (kg d 1 ) Starch (kg d 1 ) (kg d 1 ) GE (Mcal d 1 ) kg d % intake Starch kg d % starch intake kg d % intake Ruminal microbial efficiency (g N kg 1 F y ) Ruminal ph Total Acetate Propionate Butyrate Valerate Mcal d % GE intake % DE z Diet consisted of 30% alfalfa hay and 70% of either barley or corn grain. y F = organic matter fermented in the rumen. Results presented in Table 6 show that intakes of starch and were respectively higher (+21%) and lower ( 21%) with corn than with barley grain. Ruminal degradation of starch and (as a percentage of ingestion) was decreased ( 10 and 11 percentage units, respectively) when corn replaced barley. Ruminal microbial efficiency and total VFA production were slightly lower ( 10% and 6%, respectively) for corn. Amounts of different VFA produced were changed: acetate and valerate were decreased, whereas propionate and butyrate were not affected by the replacement of barley with corn. Substitution of barley with corn in the diet depressed total methane production ( 14%) as well as the part of energy lost as methane by the animal ( 16% and 17% of GE intake and DE, respectively). Strategy 5: Stage of Maturity of Forage The effect of forage maturity on methane production was evaluated using a diet based on 100% alfalfa hay harvested at two different stages of maturity: vegetative and midbloom. Simulation results presented in Table 7 indicate that

6 568 CANADIAN JOURNAL OF ANIMAL SCIENCE Table 7. Effect of forage maturity on rumen fermentation and methane production z Alfalfa hay maturity Midbloom Vegetative DM (kg d 1 ) (kg d 1 ) (kg d 1 ) GE (Mcal d 1 ) kg d % intake kg d % intake Ruminal microbial efficiency (g N kg 1 F y ) Ruminal ph Total Acetate Propionate Butyrate Valerate Mcal d % GE intake % DE z Diet consisted of 100% alfalfa hay harvested at vegetative or midbloom stage. y F = organic matter fermented in the rumen. utilization of alfalfa hay harvested at the vegetative stage instead of the midbloom stage provided less fiber to the rumen microbes, as illustrated by a lower ( 34%) intake. As a consequence, degradation in the rumen (% of intake) was higher for the vegetative hay (+46%). Ruminal microbial efficiency was slightly reduced ( 7%) when the diet was based on the vegetative hay. Ruminal ph and VFA production were not affected by the maturity of the hay. The replacement of the midbloom alfalfa hay with the vegetative hay had a small effect (+4%) on methane production when the latter was expressed in Mcal d 1. When expressed as percent of GE intake, methane losses were higher (+15%) for the vegetative hay. However, the opposite was observed if methane production was expressed per unit of DE, i.e., methane losses were lower ( 15%) with the vegetative hay than with the midbloom hay. Strategy 6: Forage Preservation Method (Hay versus Silage) Results presented in Table 8 indicate that intake of was lower ( 11%) with alfalfa silage than with alfalfa hay. of and (% of intake) were also reduced ( 21 and 9%, respectively) when alfalfa was preserved as silage rather than hay. Ruminal microbial efficiency was slightly enhanced (+9%) by the utilization of Table 8. Effect of forage method preservation on rumen fermentation and methane production z Method of preservation Hay Silage DM (kg d 1 ) (kg d 1 ) (kg d 1 ) GE (Mcal d 1 ) kg d % intake kg d % intake Ruminal microbial efficiency (g N kg 1 F y ) Ruminal ph Total Acetate Propionate Butyrate Valerate Mcal d % GE intake % DE z Diet consisted of 100% alfalfa harvested at vegetative stage and preserved as hay or silage. y F = organic matter fermented in the rumen. alfalfa silage. Ruminal ph was higher for alfalfa silage. The intensity of ruminal fermentation was quantitatively influenced by the method of preservation of alfalfa; total and individual VFA productions were lower with alfalfa silage compared to alfalfa hay. Total methane production (Mcal d 1 ) was depressed ( 33%) by the utilization of alfalfa silage instead of alfalfa hay. Fractions of GE intake and DE lost as methane were also lower ( 32 and 28%, respectively) with alfalfa silage than with alfalfa hay. Strategy 7: Forage Species (Legume versus Grass) Due to the difference in their chemical composition, and intakes were lower ( 24% and 51%, respectively) with the alfalfa hay diet than with the timothy hay diet (Table 9). Such difference in dietary intakes affected ruminal degradation of feed components. Indeed, proportion of ingested and degraded in the rumen were higher (+ 50% and +76%, respectively) for the alfalfa hay. Ruminal microbial efficiency was slightly lower for the alfalfa hay diet. Ruminal ph was relatively similar between the two forage species. Except for valerate production, which was greater with the alfalfa hay, production of the other of the individual VFA was not influenced by forage species. Methane losses expressed in Mcal d -1 or as % of GE intake

7 BENCHAAR REDUCTION OF METHANE EMISSIONS FR RUMINANTS 569 Table 9. Effect of forage species (legume versus grass) on rumen fermentation and methane production z Forage species Grass: timothy hay Legume: alfalfa hay DM (kg d 1 ) (kg d 1 ) (kg d 1 ) GE (Mcal d 1 ) kg d % intake kg d % intake Ruminal microbial efficiency (g N kg 1 F y ) Ruminal ph Total Acetate Propionate Butyrate Valerate Mcal d % GE intake % DE z Diet consisted of 100% timothy hay harvested at full inflorescence stage or alfalfa hay harvested at vegetative stage. y F = organic matter fermented in the rumen. were higher (+7% and +28%, respectively) for alfalfa hay than for timothy hay. However, when expressed relative to DE, replacing timothy hay with alfalfa hay substantially decreased ( 21%) methane production. Strategy 8: Processing of Forage (Long versus Pelleted) Results of simulation (Table 10) show that grinding and pelleting alfalfa hay decreased and degradation (as percent of intakes) in the rumen ( 19 and 22%, respectively). Physical treatment of alfalfa hay did not alter ruminal microbial efficiency. Ruminal ph was higher for the processed hay. Formation of end-products of fermentation in the rumen was also affected by processing of hay: pelleting alfalfa hay decreased VFA production. (Mcal d 1 ) was reduced ( 20%) by the physical treatment of hay. Similarly, methane losses reported as % of GE intake and DE were depressed by processing of alfalfa hay ( 21 and 13%, respectively). Strategy 9: Upgrading and Supplementation of Poor Quality Forages Non-treated versus Ammoniated Straw Results presented in Table 11 show that despite similar and intakes, ammoniation of straw greatly improved Table 10. Effect of forage processing on rumen fermentation and methane production z Alfalfa hay Long Pelleted DM (kg d 1 ) (kg d 1 ) (kg d 1 ) GE (Mcal d 1 ) kg d % intake kg d % intake Ruminal microbial efficiency (g N kg 1 F y ) Ruminal ph Total Acetate Propionate Butyrate Valerate Mcal d % GE intake % DE z Diet consisted of 100% long or pelleted alfalfa hay. y F = organic matter fermented in the rumen. digestion of and in the rumen. Ruminal microbial efficiency increased from 22 to 39 g of N kg 1 of fermented in the rumen. Predicted ph of rumen fluid was decreased by the chemical treatment of straw. Intensity of ruminal fermentation was increased by ammoniation of straw, which is illustrated by a greater VFA production. Total methane production increased from 1.07 to 5.35 Mcal d 1. Methane per unit of GE intake or DE was greater for treated straw. Supplementation of Straw Results of simulation presented in Table 11 indicate that supplementation of non-treated straw with soybean meal improved ruminal digestion of and. Ruminal microbial efficiency was greatly stimulated by the supplementation of straw. Production of VFA in the rumen was higher with supplemented straw. Finally, inclusion of soybean meal in the straw-based diet increased methane production. DISCUSSION Recently, Benchaar et al. (1998) used mechanistic models to predict methane production in dairy cows and concluded that these models were more appropriate and accurate than simple regression equations to predict ruminal methane emissions under a wide range of nutritional circumstances.

8 570 CANADIAN JOURNAL OF ANIMAL SCIENCE Table 11. Effect of chemical treatment or supplementation of cereal straw on rumen fermentation and methane production Untreated Ammoniated Supplemented straw straw straw z DM (kg d 1 ) (kg d 1 ) Starch (kg d 1 ) 0.14 (kg d 1 ) GE (Mcal d 1 ) Rumen digestion kg d % intake kg d % intake Ruminal microbial efficiency (g N kg 1 F y ) Ruminal ph Total Acetate Propionate Butyrate Valerate Mcal d % GE intake % DE z Straw supplemented with soybean meal (70:30). y F = organic matter fermented in the rumen. Several dietary strategies have been used to reduce methane emissions from ruminants. However, results and conclusions drawn from studies are variable and sometimes contradictory. This divergence can be explained at least partially by differences in experimental conditions. Hence, the utilization of mathematical models may be helpful to evaluate the effectiveness of different dietary manipulations to reduce methane production. Regarding methane predictions, the accuracy of model predictions depends on the hypothesis and assumptions made within the model, but also on the accuracy of the input values required to use the model. Benchaar et al. (1998) obtained a good agreement between observed and predicted values of methane production (r 2 = 0.71) by using a modified version of the mechanistic model of rumen digestion of Dijkstra et al. (1992). In general, the rumen model has been found to slightly underestimate ( 0.3 Mcal d 1 ) experimental observations of methane production. Reasons for underpredictions of methanogenesis by a modified version of the Dijkstra et al. (1992) model have been largely discussed by Benchaar et al. (1998). Similarly, in the present study, predictions of methane production (3 to 5% GE intake) seem lower than what is usually reported in the literature, although the latter shows extreme variation (1.7 to 14.9% of GE intake) of methane production (Holter and Young 1992). Moreover, the model used in the present study predicts the amount of methane produced in the rumen, whereas literature values correspond to the total (i.e. rumen and large intestine) methane emitted by an animal placed in a respiratory chamber. Murray et al. (1976) and Kennedy and Milligan (1978) reported that post-ruminal methane production accounted for 13 to 23% of total methane produced in sheep. Therefore, failure to account for post-ruminal methanogenesis might account for the underestimation of methane emissions by the model. Nevertheless, for the purpose of the present study, absolute values have less importance, since the objective was to assess if the model used could predict relative differences in methane production, when submitted to different nutritional strategies already known to impact on methane production. Control of methane production could be realized by shifting digestion of from the rumen to the intestines. One means to alter the extent of ruminal digestion is to increase DMI. It is well known that total methane production increases as DMI increases (Kriss 1930; Axelson 1949). Simulations of the present study clearly showed a linear increase in the total methane production when DMI increases. However, when expressed relative to daily GE or DE intakes, methane losses decreased as DMI intake increased. per unit of feed, energy consumed, or energy digested has also been found to decrease with the increased intakes in cows (Moe and Tyrrell 1979a) and sheep (Moss et al. 1995). Such a relationship between DMI and methane losses implies that in extensive milk production systems, the use of a large number of cows fed at restricted intakes (e.g. 12 kg of DM d 1 ) will result in more methane losses compared to a system in which a limited number of cows are fed at ad libitum intake (e.g. 17 kg of DM d 1 ). In the present simulation study, the reduction in methane production observed with increased intakes is the consequence of the reduction in ruminal digestion, the latter resulting from an acceleration of ruminal passage rates of particles and liquids. The effects of passage rates on methane production were well demonstrated by the experiment of Okine et al. (1989). In the present study, the effect of increasing the level of intake was more accentuated with the diet containing concentrates than with that consisting only of hay. Mathison [unpublished data, cited by McAllister et al. (1996)] also observed a greater decline in methane production when increasing the intake of concentrates than when increasing the intake of forages. The greater depression of methane production with increasing intake observed with the supplemented diet could be explained by the associative effects when the hay was supplemented with concentrates. Moss et al. (1995) also reported an associative effect of barley and silage in sheep fed at 1.5 maintenance. Associative effects were also observed between maize and silage (Blaxter and Wainman 1964). In fact, with the supplemented diet, factors other than the acceleration of rumen passage rates as a result of increased intake contribute to the reduction of methane production. These factors include reduced ruminal ph, decreased cellulolytic activity, depression of fiber degradation, high propionate production, and increased starch by-pass to the intestines. It can be concluded that

9 BENCHAAR REDUCTION OF METHANE EMISSIONS FR RUMINANTS 571 methane production will decrease as dietary intake increases and this decline will be greater for mixed diets (highly digestible) compared to forage diets (low digestibility). However, caution must be taken in the extrapolation of the simulated results to real situations. Indeed, depending on the nature of the diet, the contribution of the large intestine to feed digestion can be more or less important. If the that escapes ruminal digestion is not digested in the small intestine, or if the digestion is incomplete, it will be fermented in the hindgut resulting in the production of methane. For instance, ground forage diets and diets rich in corn starch can supply large quantities of digestible to the hindgut. It has been reported that 10 to 30% of digestible can be digested in the hindgut (Moss et al. 2000). Manipulation of methane production could also be achieved by altering rumen fermentation pattern. It is well known that the production of methane in the rumen is closely related to the production of VFA (Wolin and Miller 1988). Formation of both acetic and butyric acids is accompanied by the production of H 2 and CO 2, whereas propionic production involves a net uptake of H 2 (Czerkawski 1986). Thus, the relative formation of these three acids largely determines the amount of excess hydrogen available in the rumen and ultimately converted to methane by the methanogenic bacteria. The shift in VFA pattern from acetic towards propionic could be realized by replacing structural carbohydrates (forage) with easily fermented carbohydrates (concentrate) (stragtegy 2), or by a change in the nature of concentrates (strategy 3). The latter is possible by replacing structural carbohydrates (e.g., beet pulp) with starch (e.g., barley). Results of the present study indicate that there are two mechanisms by which increasing the amount of starch in the diet at the expense of fiber reduces methane losses. First, it redirects reducing equivalents (i.e. hydrogen) from methane to propionate. Second, it decreases digestibility of the fiber due to the associative effects described earlier. Inclusion of high amounts of starch in the diet also had a negative effect on the synthesis of microbial protein in the rumen. The same factors involved in the reduction of fiber degradation (substrate and ph effects) explain the alteration of ruminal microbial efficiency. In practice, generally the substitution of forage with concentrates in ruminant diets can be accompanied by an increase in feed intake and, therefore, the concentrate effect on methane reduction could be more accentuated than reported in the present study. Feeding starches that are less degradable in the rumen could also be used as a means to control ruminal methane production. It is well documented that the ruminal digestion of starch varies largely according to the species of cereal grain (Sauvant et al. 1994). For instance, wheat and barley are more rapidly fermented by ruminal microorganisms than are corn and sorghum (McAllister et al. 1990). Results from the present study show that the replacement of barley by corn grain in ruminant diets results in a reduction of the amount of methane generated during rumen fermentation. Johnson et al. (1996) also reported lower methane losses with diets containing corn compared to barley. In the present study, the depression of methane production observed with the corn diet was mainly related to the alteration of ruminal fiber degradation than to the reduction of starch fermentation. Indeed, the difference in rumen digestion of starch between corn and barley was not as large as expected (97 vs. 87). The degradation of starch in the rumen can vary according to many factors such as the level of intake and the amount and the type of roughage in the diet (Galyean et al. 1979; Galyean and Owens 1991). These results suggest that reducing methane production by modifying starch degradability must take into account the factors discussed above. Feeding starches that are more slowly degraded in the rumen increases starch escape to the small intestine. If the escaped starch is digested efficiently in the small intestine, and if it results in an improvement of animal productivity, this strategy may be very efficient in reducing the ruminant contribution to the atmospheric flux of methane. However, if the digestion of the starch in the small intestinal is not complete, the non digested starch will be fermented in the large intestine and contribute to methane production. With forage-based diets, different strategies could be applied to reduce ruminal methane production. In this study, methane production was severely depressed with the utilization of legume (alfalfa) instead of grass (timothy) hay (strategy 7). Varga et al. (1985) reported a decrease in methane production from cattle consuming alfalfa silage compared to orchardgrass silage. The utilization of less-mature herbage (strategy 5) has been shown to lower methane yields. Corbett et al. (1966) also observed greater methane losses for late-season compared to early-season herbage. The common factor between these strategies is the relative proportions of soluble and structural carbohydrates within the plant tissue. For instance, the ratio of non-structural to structural carbohydrates decreases with advancing maturity. This ratio is also lower for grasses than for legumes. Physical alteration of forage by pelleting (strategy 8) could help to reduce methane losses in ruminants fed forage-based diets. Hironaka et al. (1996) also observed a decrease in methane production in steers fed pelleted alfalfa hay. In the present study, the observed depression in methane production with pelleted hay was mainly due to the reduction in fiber degradation. Thomson (1972) reported that the extent of cellulose digestion in the rumen was reduced by pelleting alfalfa. It is important to note that the reduction in methane production observed in the present simulation study would be more accentuated in real situations. In fact, it has been reported that physical alteration of forage by pelleting is associated with an increase in feed intake (Vermorel et al. 1974). Therefore, the effect of physical treatment of forage on methane reduction could be even greater due to the associated increase in feed intake. The chemical treatment of cereal straws, e.g. ammoniation, could also be used to reduce methane production in ruminants. predicted in the current study for untreated straw (1.7% GE; 6.1% DE) is lower than that observed by Birkelo et al. (1986) for nontreated straw (5.9 % GE; 10.7% DE) and for ammoniated straw (6.5 % GE intake; 10.8% DE) diets supplemented with soybean meal. However, predicted values for ammoniated (8.3% GE intake; 12.6% DE) and supplemented straw (5.1% GE intake; 9.1% DE) diets are in the same range as those report-

10 572 CANADIAN JOURNAL OF ANIMAL SCIENCE ed by these authors. Moss et al. (1994) recorded methane losses of 6 and 9% of GE intake and DE, respectively, for cereal straws given alone to sheep. Values of methane production reported by Birkelo et al. (1986) and Moss et al. (1994) were obtained from animals placed in respiration calorimetry chambers, and thus, represent the total methane produced in the rumen and the large intestine, whereas the simulated values represent the amount of methane produced solely in the rumen. These comparisons between observed and predicted values might again suggest that with untreated (non supplemented) straw a significant part of methane emitted by the animal results from the fermentation of straw in the large intestine. The ruminal digestion of untreated straw was reported to be low (Fondevila et al. 1993), and therefore large amounts of straw can by-pass the rumen and be fermented post-ruminally, resulting in more methane production. In the present study, treating cereal straw with ammonia increased methane energy losses. This increase was related to the improvement of ruminal digestion of straw fiber. It is well established that ammonia treatment modifies cell wall structure and increases ruminal straw degradability (Fondevila et al. 1994). There are few data on the effect of upgrading straw on methane production. The values available in the literature are very variable and, generally, the straws have not been given as the sole feed, but supplemented with small quantities of high-protein supplements. Moss et al. (1994) showed that alkali treatment (ammonia or NaOH) increases the volume (L d 1 ) of methane produced by sheep, but decreases the methane generated per unit of digestible. Differences between the results of Moss et al. (1994) and those of the present study could be related to differences in feed intake. In fact, these authors observed a greater DMI with the treated than with the untreated straw, whereas in the present simulation, DMI was set equally for ammoniated and untreated straw. As discussed before, a higher DMI results in a reduction in rumen retention time of treated straw and, consequently, in a decrease in methane production. can also be decreased in ruminants fed straw-based diets by supplementing the diet with adequate sources of protein and/or energy. Results of this study demonstrate that supplementation of untreated straw with soybean meal increases the daily amount of energy lost as methane. However, when methane production was expressed relative to DE, differences between supplemented and non-supplemented straw diets were mitigated. Chemical treatment or supplementation of cereal straw improves the efficiency of rumen microbial synthesis and consequently, increases the amount of protein available for absorption in the small intestine. This can be beneficial for the animal and may improve production performance, and hence reduce methane produced per unit of product (milk, meat). The method of preservation of forages can also be used to manipulate methane production in ruminants fed foragebased diets. In the present study, methane production was lower with alfalfa silage than with alfalfa hay. Moss et al. (2000) reported that methane production decreases when forages are preserved in ensiled form. Ekern and Sundstøl (1974) also observed lower methane losses (% GE intake) Table 12. Summary of the effectiveness of the simulated strategies to mitigate methane production from ruminants Methane variation Methane variation Strategy (per unit of GE intake) (per unit of DE) Increasing DMI 9 to 23% 7 to 17% Increasing concentrate 31% 40% proportion in the diet Fibrous concentrate vs. 24% 22% starchy concentrate Rapidly vs. slowly 16% 17% degraded starch Forage maturity +15% 15% Forage species +28% 21% (legume vs. grass) Method of forage preservation 32% 28% (dried vs. ensiled) Forage processing 21% 13% Chemical treatment of poor 5 2 quality forage (straw) z Supplementation of poor quality forage (straw) z z Positive effects on efficiency of ruminal metabolism. for silages compared to hays made from the same material. The difference in methane production between silage and hay ( 1.7 percentage units, % GE intake) observed in the present simulation is larger than that ( 0.5 percentage units) observed by Ekern and Sundstøl (1974). In our simulation, methane depression was related to the lower rumen degradation, particularly the fraction of the alfalfa silage. Huhtanen and Jaakkola (1993) also observed a lower digestibility for ensiled grass than for dried grass. The higher degradation observed for the alfalfa hay compared to alfalfa silage might reflect changes in the cell wall fraction during silage fermentation. In fact, some fiber degradation occurs during silage fermentation. Hydrolysis occurs in the hemicellulose fraction during silage preservation. This hydrolysis seems to take place in the digestible fraction thereby, decreasing the potential digestibility of silage hemicellulose and also (McDonald et al. 1991). Results from strategies simulated in this study to mitigate methane production from forage-based diets suggest that the application of any methane reduction strategy must take into account factors such as stage of maturity, species, preservation, and forage processing. In conclusion, this work demonstrated the usefulness of a mechanistic model of rumen digestion to understand and to assess the effectiveness of different existing dietary strategies aiming at reducing methane emissions from ruminants. Many specific dietary interventions can currently be used to control methane production from ruminants. Methane emissions from ruminants can be reduced by 9 to 40%, depending on the nature of the intervention (Table 12). Ruminal

11 BENCHAAR REDUCTION OF METHANE EMISSIONS FR RUMINANTS 573 methane production can be decreased by shifting fermentation towards propionate production and (or) by reducing the extent of ruminal fermentation. However, in some circumstances, methane production cannot be reduced without negative effects on efficiency of digestion. For instance, shifting digestion from the rumen to the intestines could result in a depression of total tract digestion. The utilization of highstarch diets drains away reducing equivalents from methane production to propionate formation, but alters fiber digestion and microbial protein synthesis in the rumen. The best approach to reduce methane emissions from ruminants is the one that could result in improving the productivity of the animal. In this way, methane production per kilogram of milk or meat will be reduced. 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