The impact of subclinical ketosis in dairy cows on greenhouse gas emissions of milk production

Highlights

• A dynamic stochastic simulation model was combined with life cycle assessment.

• Subclinical ketosis increases greenhouse gas emissions of milk production.

• Cows that die on farm have the highest impact on greenhouse gas emissions.

Abstract

The dairy sector is an important contributor to greenhouse gas (GHG) emissions. Subclinical ketosis (SCK), a metabolic disorder in dairy cows, increases the risk of other diseases. SCK can increase GHG emissions per kg of milk produced by reducing production efficiency of dairy herds. With an expected increase in milk consumption, and potential new policies to reduce GHG emissions from agriculture, producing efficiently and reducing GHG emissions becomes increasingly important. The objective of this study was to estimate the impact of SCK and related diseases (i.e. mastitis, metritis, displaced abomasum, lameness, and clinical ketosis) on GHG emissions of milk production. To this end, a dynamic stochastic simulation model was developed and combined with life cycle assessment (LCA). This model simulates the dynamics of SCK and related diseases, and the associated production losses (reduced milk production, discarded milk, a prolonged calving interval, and removal (culling or dying on the farm) per cow during one lactation. Subsequently, an LCA was performed to quantify the impact of SCK and related diseases on GHG emissions per ton fat-and-protein-corrected milk (kg CO₂equivalents/t FPCM) from cradle to farm gate. The emissions of GHGs increased on average by 20.9 kg CO₂e/t FPCM per case of SCK, with a range between 6.8 and 48.0 (5–95 percentiles). This increase in emissions was caused by a prolonged calving interval (31%), discarded milk (30%), reduced milk production (19%), and removal of cows (20%). Most cows had SCK only (61%); for these cows emissions increased by 7.9 kg CO₂e/t FPCM, whereas emissions of cows that died on farm increased by 463.0 kg CO₂e/t FPCM. Sensitivity analysis showed that the disease incidence, removal risk, relations of SCK with other diseases, and emission factors related to feed production, enteric fermentation, and manure resulted in a high variation of GHG emissions. This study is the first study that estimated the impact of SCK on GHG emissions and showed the relation between cow health and GHG emissions of milk production.

Introduction

Subclinical ketosis (SCK) in dairy cows is a metabolic disorder that occurs in the period around calving. In this period, the energy requirement of the cow can exceed her energy intake, resulting in a negative energy balance (NEB) (Grummer, 1995). An NEB results in an increase of non-esterified fatty acids and beta-hydroxybutyrate levels in the blood. A cow is considered to have SCK when the beta-hydroxybutyrate level is higher than 1.2–1.4 mmol/l blood, but shows no clinical signs of ketosis (Raboisson et al., 2014). The prevalence of SCK in European dairy cows varies between 11 and 49% (Berge and Vertenten, 2014, Suthar et al., 2013). SCK increases the risk of other diseases, e.g. displaced abomasum (DA), metritis, mastitis, lameness and clinical ketosis (Berge and Vertenten, 2014, Raboisson et al., 2014). SCK and these related diseases can reduce milk production, reproduction performance, longevity, and increase the amount of discarded milk (McArt et al., 2012, Raboisson et al., 2014). The economic impact of this reduction in production has been estimated at €257 (Raboisson et al., 2015) and $289 (McArt et al., 2015) per case of SCK. A common method to estimate the costs of diseases in dairy cows is dynamic stochastic simulation modeling (Bruijnis et al., 2010, Huijps and Hogeveen, 2007), which simulates diseases and production of a cow over time and shows the variation in results (Dijkhuizen et al., 1997). Studies on the environmental impact of SCK in dairy herds, however, are lacking.

One of the most urgent environmental issues is global warming, induced by greenhouse gas (GHG) emissions. The livestock sector is responsible for about 14.5% of anthropogenic GHG emissions globally (Gerber et al., 2013). In Western Europe, the annual emissions of the livestock sector is over 0.6 gigatonnes of CO₂equivalents (CO₂e), which is 9% of the global emissions of this sector. About one third of these emissions is ascribed to milk production (Gerber et al., 2013). Important GHGs related to milk production are carbon dioxide (CO₂), methane (CH₄), and nitrous oxide (N₂O), mainly emitted during enteric fermentation, feed production and manure management. With an expected increase in milk consumption of 58% in 2050 compared to 2010 (FAO, 2011), producing efficiently and reducing GHG emissions per kg milk becomes increasingly relevant.

To assess the environmental impact of SCK, life cycle assessment (LCA) can be used. LCA is a method that assesses the environmental impact of a product by taking into account its entire life cycle (Baumann and Tillmann, 2004). Many studies have used LCA to estimate GHG emissions of milk production, by summing emissions of GHGs along the production chain into carbon dioxide equivalents (CO₂e). Based on a literature review, De Vries and de Boer (2010) showed that GHG emissions of milk produced in western countries varies from 0.84 to 1.3 kg CO₂e per kg milk at the farm gate. LCA has also been used to compare conventional and organic dairy systems (Cederberg and Mattsson, 2000) and to evaluate the impact of strategies to reduce GHG emissions, including breeding (Van Middelaar et al., 2014a), and feeding strategies (Van Middelaar et al., 2014b). So far, no LCA study has focused on the impact of SCK on GHG emissions of dairy production. SCK reduces feed efficiency (kg feed intake/kg milk) of dairy cows, and therefore affects GHG emissions related to feed production (production of inputs, e.g. fertilizers, cultivation, and processing stages), enteric fermentation (i.e. digestion of the feed by the cow), and manure management per unit of milk produced. Furthermore, an increased removal rate, results in additional breeding of replacement heifers, which increases emissions related to non-productive animals. The increasing attention for climate change might result in policies or other incentives to reduce the impact of the dairy sector on GHG emissions. While other strategies to reduce GHG emissions might impair farmer income or cow welfare, preventing SCK, improves both. Therefore, reducing SCK is likely to not only reduce GHG emissions, but to improve the overall sustainability of the dairy sector.

The objective of this study was to estimate the impact of SCK in dairy cows on GHG emissions per unit of milk, including all processes from cradle to farm gate. This is the first study that combines environmental impact assessment using LCA, with production losses of diseases on cow level using dynamic stochastic simulation modeling. This approach enables decision making to improve dairy cow health, while reducing the environmental impact of milk production.