This paper reviews the small but growing literature on the water footprint (WF) of livestock production and provides an analysis of the strengths, weaknesses, opportunities and threats of this indicator. We identified 42 papers published in peer-reviewed international journals between 2000 and 2017, which covered the WF of dairy, meat and egg production using life-cycle assessment and WF network methodologies. The WF of livestock products decreases with the level of intensification of the farming system. In addition, the WF of meat is higher than that of either milk or eggs. The WF of beef is much larger than the WFs from sheep, goat, pork and chicken. The WF variation among different animal products is explained by the difference of the feed conversion ratio. Ruminants (cattle, sheep and goat) have a poor feed conversion ratio compared with monogastric animals (poultry and swine). Estimating the WF of livestock production and economic analysis of water use at different stages of production will help farmers and other stakeholders to identify the most demanding activities in term of water use, and implement strategies to improve water-use efficiency. Thus, feed production was identified as the largest contributor of the WF of livestock production. Options to reduce the WF of livestock production include the use of low-WF feeds, more efficient irrigation of crops used for livestock feed, and reduced consumption of animal-sourced protein in human diets through substitution with plant proteins. The strengths, weaknesses, opportunities and threats analysis highlighted the importance of combining WF with other environmental-footprint and sustainability indicators to provide more reliable information for decision makers.

Our previous research revealed the advantages of separate feeding (SF) systems compared to total mixed ration (TMR) in terms of ruminal methane (CH₄) production. The purpose of this experiment was to confirm the advantage of SF as a nutritional strategy for CH₄ mitigation, and to determine the effects of different feeding systems (TMR and SF) on the rumen microbiome and associated metagenome of two different breeds and on CH₄ emissions. We randomly allocated four Holstein (305 ± 29 kg) and four Hanwoo steers (292 ± 24 kg) to two groups; the steers were fed a commercial concentrate with tall fescue (75:25) as TMR or SF, in a crossover design (two successive 22-day periods). Neither feeding systems nor cattle breeds had an effect on the total tract digestibility of nutrients. The TMR feeding system and Hanwoo steers generated significantly more CH₄ (P < 0.05) and had a higher yield [g/d and g/kg dry matter intake (DMI)] compared to the SF system and Holstein steers. A larger rumen acetate:propionate ratio was observed for the TMR than the SF diet (P < 0.05), and for Hanwoo than Holstein steers (P < 0.001), clearly reflecting a shift in the ruminal H₂ sink toward CH₄ production. The linear discriminant analysis (LDA) effect size (LEfSe) revealed a greater abundance (α < 0.05 and LDA > 2.0) of operational taxonomic units (OTUs) related to methanogenesis for Hanwoo steers compared to Holstein steers. Kendall’s correlation analysis revealed wide variation of microbial co-occurrence patterns between feeding systems, indicating differential H₂ thermodynamics in the rumen. A metagenome analysis of rumen microbes revealed the presence of 430 differentially expressed genes, among which 17 and 27 genes exhibited positive and negative associations with CH₄ production, respectively (P < 0.001). A strong interaction between feeding system and breed was observed for microbial and metagenomic abundance. Overall, these results suggest that the TMR feeding system produces more CH₄, and that Hanwoo cattle are higher CH₄ emitters than SF diet and Holstein cattle, respectively. Interestingly, host-associated microbial interactions differed within each breed depending on the feeding system, which indicated that breed-specific feeding systems should be taken into account for farm management.

Dairy cattle farming contributes significantly to greenhouse gas (GHG) emissions through methane (CH₄) from enteric fermentation. To complement global efforts to mitigate climate change, there is a need for accurate estimations of GHG emissions using country-specific emission factors (EFs). The objective of this study was to develop national EFs for the estimation of CH₄ emissions from enteric fermentation in South Korean dairy cattle. Information on dairy cattle herd characteristics, diet, and management practices specific to South Korean dairy cattle farming was obtained. Enteric CH₄ EFs were estimated according to the 2019 refinement of the 2006 Intergovernmental Panel on Climate Change (IPCC) using the Tier 2 approach. Three animal subcategories were considered according to age: milking cows >2 years, 650 kg body weight (BW); heifers 1–2 years, 473 kg BW; and growing animals <1 year, 167 kg BW. The estimated enteric CH₄ EFs for milking cows, heifers, and growing animals, were 139, 83 and 33 kg/head/year, respectively. Currently, the Republic of Korea adopts the Tier 1 default enteric CH₄ EFs from the North America region for GHG inventory reporting. Compared with the generic Tier 1 default EF of 138 (kg CH₄/head/year) proposed by the 2019 refinement to the 2006 IPCC guidelines for high-milking cows, our suggested value for milking cows was very similar (139 kg CH₄ /head/year) and different to heifers and growing animals EFs. In addition, enteric CH₄ EFs were strongly correlated with the feed digestibility, level of milk production, and CH₄ conversion rate. The adoption of the newly developed EFs for dairy cattle in the next national GHG inventory would lead to a potential total GHG reduction from the South Korean dairy sector of 97,000 tons of carbon dioxide-equivalent per year (8%). The outcome of this study underscores the importance of obtaining country-specific EFs to estimate national enteric CH₄ emissions, which can further support the assessment of mitigation actions.