Nitrogen loss partitioning and emissions in intensive subtropical hybrid dairy systems

Abstract

Dairy systems in South America's humid subtropics include grass-legume pastures in rotation with winter-summer double-cropping for silage, thus combining direct grazing with periods of effective confinement for concentrate and silage supplementation. The environmental impacts of these so-called hybrid systems remain unclear. We compiled detailed nitrogen (N) circulation budgets for four dairy systems in Uruguay stocked at 1,300 kg liveweight ha⁻¹ but with contrasting feeding strategies (lesser vs. greater use of maize silage) and cow genotypes (New Zealand vs. North American Holstein–Friesian) and then used a farm environmental model (Overseer® Science) to partition N surpluses into losses to water and air, and to estimate greenhouse gas (GHG) emissions. All systems exhibited substantial N surpluses (190–238 kg N ha⁻¹) and moderate whole-farm N use efficiencies (31–35%). Conversely, estimated GHG emission intensities were comparatively low: less than 9.9 kg CO₂e kg⁻¹ milk fat + protein, with N₂O representing less than 20% of total emissions. Nitrogen surpluses were predicted to be lost mainly through leaching (43%) and volatilisation (41%), not denitrification (10%). Loafing pads and fallow periods in pasture-crop transitions contributed the most to N losses. Feeding strategy and cow genotype effects on these patterns were minor. This study identified (i) specific spatiotemporal spots with a disproportionally large impact on potential losses of N –for instance, 12–18% of the farm area accounted for 87–90% of predicted N leaching losses, and (ii) a limited influence of feeding strategy and cow genotype. Such insights into where critical environmental impacts reside provide a quantitative foundation for future studies on intensified hybrid subtropical dairy mitigation strategies.