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Perspective: Could dairy cow nutrition meaningfully reduce the carbon footprint of milk production?

2023; Elsevier BV; Volume: 106; Issue: 11 Linguagem: Inglês

10.3168/jds.2023-23461

1529-9066

A.N. Hristov,

Anaerobic Digestion and Biogas Production

To answer the above question, we have to first look at what constitutes the carbon footprint of milk (CFM), what represents a "meaningful" reduction, and then evaluate the potential impact on CFM of adopting available greenhouse gas (GHG) mitigation strategies related to animal nutrition. The major GHG contributing to CFM on a dairy farm are methane (from enteric fermentation and manure management) and nitrous oxide (from manure management and feed production). As the impact of animal nutrition is expected to primarily affect ruminal fermentation, the focus of this analysis will be on enteric methane mitigation as related to CFM. It should be noted here that the method used to estimate the global warming potential (GWP) of methane can have a large impact on the predicted effect of mitigation practices, but discussing the topic is beyond the scope of this paper. Metrics used to quantify livestock GHG emissions can also affect conclusions on mitigation practices. As an example, a feed additive may decrease daily methane emission (as g/d) but may decreased feed intake and have no effect or increase methane emission yield (g/kg DMI) and increase emission intensity (g/kg milk or ECM, or average daily gain in growing animals), if DMI is substantially decreased. In contrast, a mitigation practice may not affect (or non-significantly decrease) daily methane emission but increase milk production (through increased DMI or other mechanisms) and/or milk components and thus may decrease methane emission intensity. Conclusions from this analysis are mostly applicable to confined, intensive dairy productions systems but for comparative purposes, extensive dairy production systems will be also briefly discussed. Depending on the production system, CFM has been estimated at as low as 0.75 to as high as 1.21 and even over 5.0 kg of CO2-equivalents/kg energy-corrected (ECM) or fat- and protein-corrected milk (Mazzetto et al., 2022Mazzetto A.M. Falconer S. Ledgard S. Mapping the carbon footprint of milk production from cattle: A systematic review.J. Dairy Sci. 2022; 105 (36241436): 9713-9725https://doi.org/10.3168/jds.2022-22117Abstract Full Text Full Text PDF PubMed Scopus (0) Google Scholar; Uddin et al., 2022Uddin M.E. Tricarico J.M. Kebreab E. Impact of nitrate and 3-nitrooxypropanol on the carbon footprints of milk from cattle produced in confined-feeding systems across regions in the United States: A life cycle analysis.J. Dairy Sci. 2022; 105 (35346477): 5074-5083https://doi.org/10.3168/jds.2021-20988Abstract Full Text Full Text PDF PubMed Scopus (5) Google Scholar), with the general trend being for lower CFM in intensive vs. extensive production systems. The allocation of the major GHG to emission sources also depends on the production system and varies among and within regions. Consequently, the role of nutrition in GHG and CFM mitigation would, to a large extent, depend on the production system. It can be estimated, for example, that in extensive, pasture-based dairy production systems where ≥ 80% the GHG may be from enteric fermentation (i.e., Costa Rica, Colombia, Peru in the Mazzetto et al., 2022Mazzetto A.M. Falconer S. Ledgard S. Mapping the carbon footprint of milk production from cattle: A systematic review.J. Dairy Sci. 2022; 105 (36241436): 9713-9725https://doi.org/10.3168/jds.2022-22117Abstract Full Text Full Text PDF PubMed Scopus (0) Google Scholar analysis), a 30% reduction in enteric methane emissions would result in a 25% overall reduction in CFM. For comparison, in intensive dairy production systems (i.e., USA in Mazzetto et al., 2022Mazzetto A.M. Falconer S. Ledgard S. Mapping the carbon footprint of milk production from cattle: A systematic review.J. Dairy Sci. 2022; 105 (36241436): 9713-9725https://doi.org/10.3168/jds.2022-22117Abstract Full Text Full Text PDF PubMed Scopus (0) Google Scholar, based on Rotz et al., 2021Rotz A. Stout R. Leytem A. Feyereisen G. Waldrip H. Thoma G. Holly M. Bjorneberg D. Baker J. Vadas P. Kleinman P. Environmental assessment of United States dairy farms.J. Clean. Prod. 2021; 315128153 https://doi.org/10.1016/j.jclepro.2021.128153Crossref PubMed Scopus (22) Google Scholar), where the share of eneteric methane emissions may be < 50% of the total GHG, the effect of that same mitigation practice on CFM would be about 13%. It is also important to define what a meaningful reduction in CFM is. Considering the large uncertainty in livestock GHG inventories (Hristov et al., 2017Hristov A.N. Harper M. Meinen R. Day R. Lopes J. Ott T. Venkatesh A. Randles C.A. Discrepancies and uncertainties in bottom-up gridded inventories of livestock methane emissions for the contiguous United States.Environ. Sci. Technol. 2017; 51 (29094590): 13668-13677https://doi.org/10.1021/acs.est.7b03332Crossref PubMed Scopus (25) Google Scholar) and variability in the efficacy of mitigation practices (Hristov et al., 2013Hristov A.N. Oh J. Lee C. Meinen R. Montes F. Ott T. Firkins J. Rotz A. Dell C. Adesogan A. Yang W. Tricarico J. Kebreab E. Waghorn G. Dijkstra J. Oosting S. Mitigation of greenhouse gas emissions in livestock production – A review of technical options for non-CO2 emissions.in: Gerber Pierre J. Henderson Benjamin Makkar Harinder P.S. FAO Animal Production and Health Paper No. 177. FAO, Rome, Italy2013Google Scholar), it is safe to assume that a meaningful reduction in CFM should exceed 10% and preferably be > 20%. Are there effective enteric methane mitigation practices that could be implemented on US dairy farms? A recent meta-analysis of global data carried out by an international group of scientists (Arndt et al., 2022Arndt C. Hristov A.N. Price W.J. McClelland S.C. Pelaez A.M. Cueva S.F. Oh J. Bannink A. Bayat A.R. Crompton L.A. Dijkstra J. Eugène M.A. Enahoro D. Kebreab E. Kreuzer M. McGee M. Martin C. Newbold C.J. Reynolds C.K. Schwarm A. Shingfield K.J. Veneman J.B. Yáñez-Ruiz D.R. Yu Z. Full Adoption of Strategies to Mitigate Enteric Methane Emissions by Ruminants and How They Can Help to Meet the 1.5°C Climate Target by 2030 but Not 2050.Proc. Natl. Acad. Sci. USA. 2022; 119 (35537050)e2111294119 https://doi.org/10.1073/pnas.2111294119Crossref Scopus (32) Google Scholar), recommended several strategies that can reduce absolute or relative (per unit of product) enteric methane emissions in ruminants by 12 to 32%. Two of the 3 so-called "product-based" mitigation strategies, improving forage quality (i.e., digestibility) and inclusion of more concentrate feeds in the diet, may be applicable to the US dairy industry. There is a good body of data on the effects of forage type and quality on enteric methane emissions, although life cycle assessments (LCA) accounting for soil (from growing the forages) and manure (from feeding diets based on the forages) emissions are scarce or lacking. In the case of corn silage vs. alfalfa silage (or haylage), for example, the carbon footprints of growing the 2 forages are similar (Rotz et al., 2021Rotz A. Stout R. Leytem A. Feyereisen G. Waldrip H. Thoma G. Holly M. Bjorneberg D. Baker J. Vadas P. Kleinman P. Environmental assessment of United States dairy farms.J. Clean. Prod. 2021; 315128153 https://doi.org/10.1016/j.jclepro.2021.128153Crossref PubMed Scopus (22) Google Scholar), but enteric methane emissions are decreased when corn silage (starch content of the silage will also have an effect) replaces alfalfa silage (see Hristov et al., 2022Hristov A.N. Melgar A. Wasson D. Arndt C. Symposium invited review: Effective nutritional strategies to mitigate enteric methane in dairy cattle.J. Dairy Sci. 2022; 105 (35863922): 8543-8557https://doi.org/10.3168/jds.2021-21398Abstract Full Text Full Text PDF PubMed Scopus (0) Google Scholar). There are some indications that improved forage genetics, such as increased concentration of metabolizable energy (i.e., increased lipid content), could decrease methane emissions, but it is uncertain how these transgenic forages will compare with conventional forages as part of a complex diet and what their digestible organic matter yields per acre would be. Rising temperatures may result in decreased digestibility/nutritive value of typical dairy forages, which would likely increase methane yield. To compensate for lowered forage digestibility, higher inclusion of concentrate feeds in the diet may be necessary, which may or may not change absolute methane emissions, yield, and intensity, but will increase feed costs. Increased intake of digestible organic matter promotes methane production, but the type of organic matter can influence emissions. Increasing proportion of starch/non-fiber carbohydrates in the total dietary carbohydrates would decrease methane emissions. Increased concentrate-to-forage ratio in dairy diets, on the other hand, is typically associated with decreased NDF degradability and may decrease milk fat content and yield and cause milk fat depression. As an example, a 66% increase in dietary concentrate decreased methane emission by 17%, emission yield by 19%, and emission intensity (per unit of ECM) by 20% (Aguerre et al., 2011Aguerre M.J. Wattiaux M.A. Powell J.M. Broderick G.A. Arndt C. Effect of forage-to-concentrate ratio in dairy cow diets on emission of methane, carbon dioxide, and ammonia, lactation performance, and manure excretion.J. Dairy Sci. 2011; 94 (21605777): 3081-3093https://doi.org/10.3168/jds.2010-4011Abstract Full Text Full Text PDF PubMed Scopus (172) Google Scholar). Increases in dietary concentrate proportion of up to 91%, which is not typical for lactating dairy cow diets, have resulted in over 50% reduction in methane yield and 40% reduction in methane intensity, but lower ECM production due to milk fat depression (Olijhoek et al., 2021Olijhoek D.W. Hellwing A.L.F. Noel S.J. Lund P. Larsen M. Weisbjerg M.R. Børsting C.F. Feeding up to 91% concentrate to Holstein and Jersey dairy cows: Effects on enteric methane emission, rumen fermentation and bacterial community, digestibility, production, and feeding behavior.J. Dairy Sci. 2021; 105 (36207184): 9523-9541https://doi.org/10.3168/jds.2021-21676Abstract Full Text Full Text PDF Scopus (6) Google Scholar), although no statistical effect on ECM was reported in some studies (Aguerre et al., 2011Aguerre M.J. Wattiaux M.A. Powell J.M. Broderick G.A. Arndt C. Effect of forage-to-concentrate ratio in dairy cow diets on emission of methane, carbon dioxide, and ammonia, lactation performance, and manure excretion.J. Dairy Sci. 2011; 94 (21605777): 3081-3093https://doi.org/10.3168/jds.2010-4011Abstract Full Text Full Text PDF PubMed Scopus (172) Google Scholar). Diets with concentrate proportion as high as in the Olijhoek et al., 2021Olijhoek D.W. Hellwing A.L.F. Noel S.J. Lund P. Larsen M. Weisbjerg M.R. Børsting C.F. Feeding up to 91% concentrate to Holstein and Jersey dairy cows: Effects on enteric methane emission, rumen fermentation and bacterial community, digestibility, production, and feeding behavior.J. Dairy Sci. 2021; 105 (36207184): 9523-9541https://doi.org/10.3168/jds.2021-21676Abstract Full Text Full Text PDF Scopus (6) Google Scholar study are unlikely to be a viable mitigation practice for the dairy industry due to negative impacts on milk components and farm economics and risks for animal health, but moderate increases in concentrate inclusion may have a sizeable impact on absolute enteric methane emission or emission intensity without negative effects on feed intake and lactational performance. In such cases, however, feed cost will likely also increase. It should be kept in mind that diet manipulations are likely to result in changes in manure composition, which may result in increased manure methane emissions due to an increase in fecal organic matter excretion (i.e., from decreased rumen or total-tract fiber digestibility due to increased concentrate inclusion). The Arndt et al., 2022Arndt C. Hristov A.N. Price W.J. McClelland S.C. Pelaez A.M. Cueva S.F. Oh J. Bannink A. Bayat A.R. Crompton L.A. Dijkstra J. Eugène M.A. Enahoro D. Kebreab E. Kreuzer M. McGee M. Martin C. Newbold C.J. Reynolds C.K. Schwarm A. Shingfield K.J. Veneman J.B. Yáñez-Ruiz D.R. Yu Z. Full Adoption of Strategies to Mitigate Enteric Methane Emissions by Ruminants and How They Can Help to Meet the 1.5°C Climate Target by 2030 but Not 2050.Proc. Natl. Acad. Sci. USA. 2022; 119 (35537050)e2111294119 https://doi.org/10.1073/pnas.2111294119Crossref Scopus (32) Google Scholar meta-analysis identified chemical inhibitors, electron sinks, lipids, and tannins (specifically, tanniferous forages) as having sizeable and, excluding tannins, consistent mitigation effect on enteric methane. These mitigation practices have also been reported as effective in other reviews (Hegarty et al., 2021Hegarty, R. S., R. A. Cortez Passetti, K. M. Dittmer, Y. Wang, S. Shelton, J. Emmet-Booth, E. Wollenberg, T. McAllister, S. Leahy, K. Beauchemin, and N. Gurwick. 2021. An evaluation of emerging feed additives to reduce methane emissions from livestock. Edition 1. A report coordinated by Climate Change, Agriculture and Food Security (CCAFS) and the New Zealand Agricultural Greenhouse Gas Research Centre (NZAGRC) initiative of the Global Research Alliance (GRA).Google Scholar). More recently, the red algae Asparagopsis spp. have been shown to have strong anti-methanogenic effect through the halogenated compounds they accumulate, primarily bromoform. A key requirement with all methane-mitigating additives is to not depress or affect in a negative way feed DMI, milk production and composition, animal health, and reproduction. A small decrease in DMI may improve feed efficiency and not necessarily affect milk production but a substantial drop in DMI (e.g., 10% or more) would certainly result in decreased milk production, which has been the case in studies with A. taxiformis conducted at The Pennsylvania State University (Stefenoni et al., 2021Stefenoni H.A. Räisänen S.E. Welchez S.F. Wasson D.E. Lage C.F.A. Melgar A. Fetter M.E. Smith P. Hennessy M. Vecchiarelli B. Bender J. Pitta D. Cantrell C.L. Yarish C. Hristov A.N. Effects of the macroalga Asparagopsis taxiformis and oregano leaves on methane emission, rumen fermentation, and lactational performance of dairy cows.J. Dairy Sci. 2021; 104 (33516546): 4157-4173https://doi.org/10.3168/jds.2020-19686Abstract Full Text Full Text PDF PubMed Scopus (44) Google Scholar). Progress has been made in aquaculture cultivation of Asparagopsis spp. but concerns with rumen adaptation and side effects on animal health and milk quality with this additive remain. Algae are typically high in iodine and our studies with A. taxiformis have shown increased concentrations of iodine and bromide in milk. Clearly, more research is needed to better document long-term efficacy of novel feed additives such as macroalgae and effects on animal health, reproduction, and product quality. Adaptability of the rumen ecosystem to fermentation modifiers, including methane mitigants, has been a long-standing challenge to develop mitigation strategies through ruminant nutrition. The rumen microbiome has a tremendous ability to adapt, over time, to exogenous compounds or microbial cultures, which is the primary reason methane mitigants need to be tested in long-term animal experiments. Our experiments with the methane inhibitor 3-nitrooxypropanol (3-NOP) and bromoform (in pure form or through A. taxiformis supplementation) have shown that it is possible that the mitigation effect of these inhibitors may be diminishing over time, which is concerning given the fact that, currently, those are among the few nutritional practices with repeatable ≥ 30% mitigation effect. Information from long-term experiments may help design an effective application scheme for these additives. Clearly, research on the long-term effect of feed additives is needed before they are recommended to the livestock industries. In this context, it is also worth mentioning that some additives have to enter the rumen continuously for achieving maximum efficacy. This is the case with 3-NOP, for which we demonstrated that its efficacy is highest (up to 40% reduction in methane emissions) immediately after feeding (i.e., greatest 3-NOP intake), diminishes to about 20% 10 h after feeding, and is undetectable 2 h before feeding (Hristov and Melgar, 2020Hristov A.N. Melgar A. Short communication: Relationship of dry matter intake with enteric methane emission measured with the GreenFeed system in dairy cows receiving a diet without or with 3-nitrooxypropanol.Animal. 2020; 14 (32720629): s484-s490https://doi.org/10.1017/S1751731120001731Crossref PubMed Scopus (0) Google Scholar). Diet composition may also affect the efficacy of anti-methanogenic additives. It has been demonstrated, for example, that the mitigation effect of 3-NOP decreases with increasing diet NDF concentration (Kebreab et al., 2022Kebreab E. Bannink A. Pressman E.M. Walker N. Karagiannis A. van Gastelen S. Dijkstra J. A meta-analysis of effects of 3-nitrooxypropanol on methane production, yield, and intensity in dairy cattle.J. Dairy Sci. 2022; 106 (36494226): 927-936https://doi.org/10.3168/jds.2022-22211Abstract Full Text Full Text PDF Scopus (2) Google Scholar), likely due to higher hydrogen production with acetate-type rumen fermentation. The question of cost of methane mitigation cannot be ignored. As a business, a dairy farm cannot (or should not) lose money and should be either somehow compensated for increased feed cost because of using these additives (premium for "low-carbon footprint" milk, carbon credits, etc.) or there must be co-benefits of the practice, such as increased milk production or components, body weight gain, or improved animal health. The last 2 co-benefits, in addition to reproductive performance, can only be investigated in long-term studies and typically with large number of animals, which puts an additional strain on researchers and companies developing these products. To achieve a 10 to 20% or greater reduction in CFM, a suite of mitigation practices, specific for each dairy operation, may have to be implemented. Research on additivity of the mitigation effects of nutritional strategies, however, is limited. Some studies have shown that simultaneous implementation of 2 practices resulted in quantitatively greater reduction in enteric methane emission. Ideally, mitigation practices with divergent modes of action will be combined – for example, inhibitors acting on the methyl coenzyme-M reductase and electron acceptors, or lipids. In one such example with beef cattle fed a high-forage diet, lipid (canola oil) alone decreased methane yield by 25% and 3-NOP decreased it by 28%; when the 2 treatments were combined, methane yield was decreased by 51% (Zhang et al., 2021Zhang X.M. Smith M.L. Gruninger R.J. Kung Jr., L. Vyas D. McGinn S.M. Kindermann M. Wang M. Tan Z.L. Beauchemin K.A. Combined effects of 3-nitrooxypropanol and canola oil supplementation on methane emissions, rumen fermentation and biohydrogenation, and total tract digestibility in beef cattle.J. Anim. Sci. 2021; 99 (33755112): 1-10https://doi.org/10.1093/jas/skab081Crossref Scopus (13) Google Scholar). The lipid treatment, however, had a large and negative effect on fiber digestibility, which might have resulted in decreased animal performance, if it was measured and reported in the study. Another area of scientific interest that requires further investigation is the fate of hydrogen in the rumen when methanogenesis is inhibited. Currently accepted models of rumen stoichiometry outlined in the scientific literature are simple. The rumen ecosystem, however, is not simple and it is likely there are biochemical pathways and intermediary metabolites that are not considered by current stoichiometry models. To the best of our knowledge, published studies (including all experiments conducted at The Pennsylvania State University) have not been able to account for the fate of hydrogen resulting from inhibited methane synthesis (see discussion in Melgar et al., 2020Melgar A. Harper M.T. Oh J. Giallongo F. Young M.E. Ott T.L. Duval S. Hristov A.N. Effects of 3-nitrooxypropanol on rumen fermentation, lactational performance, and the resumption of ovarian cyclicity in dairy cows.J. Dairy Sci. 2020; 103 (31733848): 410-432https://doi.org/10.3168/jds.2019-17085Abstract Full Text Full Text PDF PubMed Google Scholar). Even in vitro, where mass balance is more feasible, this has not been possible. Apart from increased gaseous hydrogen emissions, increased concentration of minor hydrogen sinks, decreased hydrogen production, and perhaps changes in microbial synthesis (which has not been studied or documented well), the most plausible explanation for the missing hydrogen is changes in fermentation pathways. In our long-term (up to 15 weeks) studies with 3-NOP we clearly observed a decreased emission of gaseous hydrogen over time, which would indicate shifts in fermentation pathways and perhaps adaptation of the rumen ecosystem. A similar challenge exists with feed energy "spared" from going into methane. In theory, it is expected that a measurable decrease in methane emission would provide additional energy for production purposes, but this has not been conclusively demonstrated in practice, except perhaps a statistically significant increase in milk fat in studies with 3-NOP (Hristov et al., 2022Hristov A.N. Melgar A. Wasson D. Arndt C. Symposium invited review: Effective nutritional strategies to mitigate enteric methane in dairy cattle.J. Dairy Sci. 2022; 105 (35863922): 8543-8557https://doi.org/10.3168/jds.2021-21398Abstract Full Text Full Text PDF PubMed Scopus (0) Google Scholar). Development of new antimethanogenic additives, both natural and synthetic, continues (with considerably more funding currently available in the US) and rapid progress can be expected. Novel methane inhibitors are under investigation but the road from a laboratory proof-of-concept, through animal experimentation, to practical application is long and unforgiving. Numerous factors have to be considered before a mitigation practice is recommended to livestock producers, as discussed elsewhere in this text. In addition to nutritional strategies, progress is expected in areas such as methanogen vaccines and genetic selection for low-methane emitters. For the latter approach, however, reliable methane emission measurement techniques, allowing screening of large number of animals (such as milk mid-infrared spectra, for example), are needed but currently not well developed. It is noted that reliable methane emission measurement techniques are also needed to reduce the cost and accelerate testing of nutritional strategies across different animal types and a wide variety of diets. Since most ruminants on the planet are predominantly on pasture and may not have access to feed on a regular basis, it is also important to develop technologies for delivering mitigants directly into the rumen, such as slow-release devices. This work is underway, and progress is plausible. Important questions related to nutritional GHG mitigation strategies that have not been adequately addressed are the persistence of the effect over full lactation or multiple lactations in dairy cows and the effects of diet ingredient and nutrient composition on additive efficacy, manure composition, and manure GHG emissions. Nutritional approaches alone can have a significant mitigation impact on CFM, but that impact can be considerably greater if they are integrated, particularly in intensive dairy production systems, with manure- and animal management-related mitigation practices. If nutrition and animal management (animal health, longevity/lifetime productivity, genetics) practices are combined, and assuming a high adoption rate (an important condition of which is to have no side effects or unintentional selection for undesirable performance, have production co-benefits, or carbon market incentives for the producer), it is not unreasonable to expect that enteric methane emissions from intensive dairy production systems can be reduced by 40 to 50% (20–30% by nutritional means/feed additives, perhaps 10% from genetic selection for low-methane emitters, and perhaps another 10% from improving animal management – i.e., health and productive life). If manure mitigation practices (such as solid/liquid separation, anaerobic digesters, manure covers, acidification, aeration – which, reportedly, can reduce manure methane emissions by 70 to 80%) are added to the equation, it might be possible to decrease methane emissions from dairy farms by 60 to 70%, which for intensive dairy production systems, such as in the US, would represent a meaningful reduction in the CFM of 35 to 40%.