Grazing intensity affects the environmental impact of dairy systems

Abstract

Dairy products are major components of the human diet but are also important contributors to global environmental impacts. This study evaluated greenhouse gas (GHG) emissions, net energy intensity (NEI), and land use of confined dairy systems with increasing levels of pasture in the diet. A Wisconsin farm was modeled to represent practices adopted by dairy operations in a humid continental climate typical in the Great Lakes region and other climates that have large differences in seasonal temperatures. Five grazing scenarios (all of which contained some portion of confinement) were modeled based on different concentrations of dry matter intake from pasture and feed supplementation from corn grain, corn silage, and soybean meal. Scenarios that incorporate grazing consisted of 5 mo of pasture feeding from May to September and 7 mo of confined feeding from October to April. Environmental impacts were compared within the 5 scenarios that incorporate grazing and across 2 entirely confined scenarios with and without on-farm electricity production through anaerobic digestion (AD). To conduct a fair comparison, all scenarios were evaluated based on the same total amount of milk produced per day where resource inputs were adjusted according to the characteristics of each scenario. A cradle-to-farm gate life cycle assessment evaluated the environmental burdens that were partitioned by allocation between milk and meat and by system expansion when biogas-based electricity was produced. Overall, results for all scenarios were comparable. Enteric methane was the greatest contributor to GHG emissions, and the production of crops was the most energy-intense process. For the confined scenario without AD, GHG emissions were 0.87 kg of CO2 equivalents, NEI was 1.59 MJ, and land use was 1.59 m2/kg of fat- and protein-corrected milk (FPCM). Anaerobic digestion significantly reduced emissions to 0.28 kg of CO2 equivalents/kg of FPCM and reduced NEI to −1.26 MJ/kg of FPCM, indicating a net energy producing system and highlighting the potential of AD to improve the sustainability of confined systems. For scenarios that combined confinement and grazing, GHG emissions ranged from 0.84 to 0.92 kg of CO2 equivalents, NEI ranged from 1.42 to 1.59 MJ, and land use ranged from 1.19 to 1.26 m2/kg of FPCM. All environmental impacts were minimized in scenarios that supplemented enough feed to increase milk yield but maintained dry matter intake from pasture at a level high enough to reduce material and energy use.