Metabolic and environmental implications of ketosis in dairy cows: effects on greenhouse gas emissions and productivity

Non-CO₂ greenhouse gas (GHG) emissions saw a rise of approximately 29% between 1990 and 2015, with methane (CH₄) emissions accounting for a 19% increase. In 2015, these emissions made up 25% of total GHG emissions, and nearly half (48%) of non-CO₂ emissions originated from the agricultural sector, with 25% coming specifically from livestock. CH₄ produced during enteric fermentation makes up about 80% of livestock-related emissions. It is projected that methane emissions from livestock will grow by approximately 10% from 2015 to 2030 [1]. Methane is primarily produced by methanogenic archaea during microbial fermentation in the reticulum and released into the atmosphere mainly through eructation. Given its potent greenhouse effect, CH₄ significantly contributes to climate change [2,3]. Hormonal changes associated with parturition and the onset of lactation drastically shift energy metabolism in mammals [4,5]. During the peripartum period, dairy cows experience immune dysfunction and systemic inflammation [6–9]. The metabolic and immunological challenges during this phase significantly impact a cow’s ability to achieve optimal performance and immune-metabolic balance. Inflammatory cytokines play a crucial role in the metabolic, immune, and inflammatory responses of transition dairy cows [10]. Ketosis is a metabolic disorder of dairy cows characterized by elevated levels ketone bodies, especially β-hydroxybutyrate (BHB). A BHB concentration of 1.0 mmol/L is the common cut-off for subclinical ketosis. The incidence of subclinical ketosis within the first month of lactation is estimated at 26–56% [11]. Ketotic cows show elevated CH4 substrates by metabolomics, suggesting a link between metabolic disease and CH4 production. The aim of the study was to assess the link between GHG emissions and ketosis through productive performances in dairy cows. Confirming an increase in GHG emissions due to ketosis might be of helpful for the international scientific community and society as a whole. Therefore, it allows to identify new critical points where interventions can be taken to limit GHG emission and improve environmental, human and animal health. Study procedures were approved by Ethical Committee for Animal Welfare of University of Padua (protocol n. 103549/2024). A total of 60 Holstein-Friesian multiparous dairy cows were enrolled from a single farm (CERZOO ltd, Piacenza, Italy). For each animal, the following data were recorded: anamnestic history, incidence of postpartum diseases, age, parity, type of calving (eutocic or dystocic), and both daily and lactation milk yield. To be included in the study, cows must have had calved between 5 and 9 days prior (5–9 days in milk, DIM), be free from ruminal acidosis, abomasal displacement, metritis, or mastitis, and must not be undergoing antimicrobial treatment. All animals were evaluated with blood BHB measurement on field at 7, 14, 21 and 28±2 DIM. The BHB field measurement were conducted using a portable digital meter (Abbott Precision Xtra™ meter, Oxon, UK) and blood ketone test strips (Abbott Precision Xtra™ Blood Ketone test strips, Oxon, UK). Animals were divided into two groups according to BHB: CTR or control (BHB<1.0 mmol/L; n=43); KET or subclinical ketosis (BHB≥1.0 mmol/L; n= 17). Moreover, GHG emissions (CH4, CO2 and H2) were measured daily for all the study period using the GreenFeed system (GreenFeed system; C-Lock Inc., Rapid City, SD, USA). This system distributes concentrates that are part of the animals' normal diet, allowing for the measurement of the primary GHG during ingestion. Statistical analysis of the data was performed using a mixed model to assess statistical differences among the groups. A Spearman correlation matrix was calculated to assess the relationships among parameters. Statistical significance was defined as a p-value ≤ 0.05. P-values greater than 0.05 but less than or equal to 0.10 were considered indicative of a trend towards significance. Differences in all time points were found in both groups regarding BHB evaluation (p-value< 0.001). Decreasing in milk production were recorded in KET animals (p-value=0,072), with no differences between groups in protein and fat contents. Over the study, control animals produced an average of 36.33 ± 1.41 L/day, whereas diseased animals produced 32.63±2.25 L/day of milk. Regarding CH4 emissions (KET=392±27.2 g/d; CTR=216±16.3 g/d; p-value< 0.001), diseased animals showed higher values. Significant effect on groups was detected in concentration of CO2 (KET=10838±355.03 g/d; CTR=12209±215.35 g/d; p-value= 0,006) and H2 (KET=0.83±0.52 g/d; CTR=2.01±0.51 g/d; p-value=0,023) in all time points. When milk production is taken into account, KET produced more grams of CH4 per litre of milk per day compared to healthy animals at +7, +14 and +21 (KET = 15.46, 10.96, 10.83, 12.02 g/L of milk per day; CTR = 6.37, 5.02, 6.37, 6.06 g/L of milk per day). Pearson’s correlations emphasize correlation between BHB and CH4 production (r=0.45; p-value= 0,02). This positive association confirms that higher levels of BHB are linked to increased methane production. Since methane is a by-product of ruminal fermentation, this finding highlights a potential connection between energy balance and enteric fermentation processes. A negative correlation between CO₂ production and BHB levels was observed (r=-0.41; p-value= 0.03), suggesting that as BHB levels increase, CO₂ production tends to decrease, as mentioned before. The relationship between CO₂ and H₂ production was found positive (r=-0.50; p-value<0.001). This indicates that the processes producing CO₂ might also be driving the production of H₂. This could reflect shared metabolic pathways, where both gases are produced as by-products of similar physiological or microbial activities within the animals. In conclusion, the findings of this study highlight the negative impact of subclinical ketosis on both animal performance and GHG emissions in dairy cows. KET animals exhibited higher CH4 production, while H2 and carbon dioxide CO2 emissions were lower compared to CTR. In conclusion, these results emphasize the need for ketosis management to improve both animal welfare and environmental impact of dairy farming.