A life cycle assessment of seasonal grass-based and confinement dairy farms

Life cycle assessment (LCA) is a holistic systems approach that aims to assess the environmental impacts (potential pollutants and resource use) of the production of goods and services. The aim of this study was to develop an LCA model to compare contrasting milk production systems, a seasonal pasture-based dairy farm and a confinement dairy farm. The environmental impacts considered were global warming, eutrophication, acidification, land use and non-renewable energy use. The LCA estimated on-farm, off-farm (pollutants and resources associated with the production and supply of purchased farm inputs) and total (on-farm and off-farm) environmental impacts. Environmental impacts were quantified per unit of milk and per unit area. The study only considered two research farms, because high quality data were unavailable for a large number of farms. Thus, this was not a representative LCA comparison. The genetic merit of cows modelled was similar for each system. A total mixed ration was fed in the confinement system and grazed grass was mainly fed in the grass-based system. Research data were used to confirm simulated dry matter intake (DMI) and predicted enteric CH4 output from simulated DMI. The study found that when expressed per unit of milk and per on-farm area, all total environmental impacts were greater for the confinement system compared to the grass-based system. Per total farm area (on-farm and off-farm area), all environmental impacts except global warming were lower for the grass-based system. The greater environmental impact of the confinement dairy system was due to the greater use of concentrate feed and the longer manure storage period. Scenario modelling demonstrated that there is potential to decrease the environmental impact of dairy systems, particularly the confinement system, by reducing the use of concentrate ingredients with a high environmental impact and by storing manure in solid systems. Scenario modelling also showed that assumptions regarding the carbon cycle should be clearly outlined when assessing milk production systems and that standardisation of LCA allocation procedures is required. This LCA study is one of the few to directly compare the environmental impact of a grass-based and a confinement dairy system. However direct comparisons are needed, using an LCA methodology such as described in this paper, and using data from a greater number of farms so that each system is better represented thereby ensuring a robust comparison of the two systems on a regional or national basis.

► Life cycle assessment assesses environmental impacts (pollutants and resource use).
► Generally, all environmental impacts were greater for the confinement system.
► Aerobically storing manure or feeding domestic concentrate reduced environmental impacts.
► Model results were strongly influenced by modelling assumptions and decisions.
► More direct comparisons, using a greater number of farms are required.