Land use change and the intensification of farming practices
are recognised as important contributors to a range of environmental
problems. In many parts of New Zealand economic drivers
continue to encourage a shift from dry stock to dairy farming.
This is particularly evident for the provinces of Otago and Southland
in southern New Zealand, where dairy cow numbers have
increased from 315,000 in the 1999/2000 season to more than
657,000 in 2009/2010 (LIC, 2010). This change in land use has
raised concerns about the environmental impacts of a growing
dairy industry. Effects on ground and surface water bodies are
issues that are of immediate concern to local regulatory agencies,
whilst potential increases in the emissions of greenhouse gases
such as nitrous oxide are of concern at a national level. Monitoring
by Environment Southland, a local regulatory agency, indicates
that N concentrations in rivers and in 36% of the region’s aquifer
monitoring sites are increasing, in some instances to levels close
to or above maximum permissible concentrations for safe drinking
water (Environment Southland, 2010). Unfortunately our quantitative
understanding of the relative importance of different farming
practices is poor, both when considering contrasting land use activities,
such as sheep v. dairy, and when comparing losses from
different components within a farm system. Dairy cow wintergrazing
on forage crops is one example of a farm system component
that could have a relatively large environmental footprint (e.g.
Smith et al., 2008), despite usually representing only a small part of
total farm area. This animal management practice normally occurs
over a 10 week period during winter and is seen as a cost-effective
strategy for providing required amounts of winter feed and avoiding
animal treading damage to soils and pastures during wet and
cool winters in southern New Zealand. Winter-grazed forage crops
such as swedes (Brassica napus) or kale (Brassica oleracea) may
account for between 10 to 20% of the total dairy farm area and
are typically sown in late spring following cultivation of old pastures
that are in need of renovation. Crops are then break-fed daily