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

To answer the question whether dairy cow nutrition could meaningfully reduce the carbon footprint of milk production, we have to first look at what constitutes the carbon footprint of milk (CFM) and what represents a “meaningful” reduction, and then evaluate the potential effect 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 effect 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 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 also decrease feed intake, and it may have no effect or increase methane emission yield (g/kg DMI) and emission intensity (g/kg milk or ECM, or ADG in growing animals), if DMI is substantially decreased. In contrast, a mitigation practice may not affect (or may nonsignificantly decrease) daily methane emission, but if it increases milk production (through increased DMI or other mechanisms) or milk components, it may decrease methane emission intensity.

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., 2020). 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 wk) 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., 2022).

Development of new antimethanogenic additives, both natural and synthetic, continues (with considerably more funding currently available in the United States) 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. Because 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 effect 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 or 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 50% to 60%, as in the following 2 scenarios: best case scenario (no adaptation of the rumen microbiome; additivity of mitigation practices)—up to 20% to 30% reduction by a feed additive; another 10% to 20% additive effect from a second feed additive; plus, perhaps, another 5% to 10% from improvements in forage quality and diet manipulation. This will result in total enteric methane mitigation of 35% to 60%, which would correspond to about 15% to 26% reduction in the CFM; and worst case scenario (adaptation of the rumen microbiome; no additivity of mitigation practices): 10% to 15% reduction by a feed additive; 5% additional effect from a second feed additive; no additional effect from improvements in forage quality and diet manipulation. This will result in total enteric methane mitigation of 15% to 20%, which will be about 7% to 9% reduction in CFM of milk in the United States. If manure mitigation practices (such as solid and liquid separation, anaerobic digesters, manure covers, acidification, and aeration, which reportedly can reduce manure methane emissions by 70% to 80%) are added to the equation, it might be possible to decrease the CFM in intensive dairy production systems, such as in the United States, by 35% to 42%.