Intensification, nitrogen use and recovery in grazing-based dairy systems

There is increased international interest in the intensification of grazing-based dairy systems such as those that occur in Australia. However, associated with increased milk production on a per ha basis is the potential for decreasing nitrogen (N) recovery and increased N losses to the environment. In this study we produced a 22-year (1990-2012) time series of N recovery measures, for the entire Australian dairy industry and largest dairy producing State, Victoria, using a farm-gate N balance method and long-term farm survey data. Nitrogen recovery measures included whole-farm N surplus (kg N ha(-1)), N use efficiency (%), milk production N surplus (g N l(-1) milk) and total industry-wide N surplus (t N). On-going intensification in dairy production at both the national and state level led to fewer and larger dairy farms, with increased stocking rates, reliance on imported feed, nitrogen fertiliser use and milk production per cow and per hectare. All N recovery measures deteriorated markedly over the 22 year period, although the adverse trend moderated somewhat since 2006. The Victorian industry was higher-performing in terms of N recovery compared to the national dairy industry as a whole, though there was some convergence during the last decade. The whole-farm N surplus for the 'industry' average Australian dairy farm increased from 54 to 158 kg N ha(-1) and 38 to 136 kg N ha(-1) yr.(-1) for the average Victorian dairy farm. Nitrogen use efficiency declined from 40 to 26% and from 51 to 29% for the average Australian and Victorian farm, respectively. Milk production N surplus increased from 10.2 to 17.3 (Australian farm) and 6.9 to 15.2 g N l(-1) milk (Victorian farm). Total N surplus increased from 63,076 to 164,621 t N for the Australian dairy industry as a whole, despite a decline of 470,000 ha in land used in dairying, suggesting a growing problem in terms of higher losses of reactive N. Looking to the future, we examined a scenario whereby N use efficiency for Victorian dairy farms increases to 35% by 2030 due to higher milk yields per cow and per hectare, increased forage yields, improved bovine genetics and feed conversion efficiencies. Given the trends over 22 years and current N use efficiency, this goal will be difficult to achieve within current grazing-based dairy farming operations. Improvements in N recovery will likely depend on significant on-farm mitigation incentivised by cost-effective policy measures, as well as future technological advances stemming from strong public and industry investment in research and development. Crown Copyright (C) 2016 Published by Elsevier Ltd. All rights reserved.