Nitrogen loss mitigation using duration-controlled grazing: Field observations compared to modelled outputs

Abstract

Dairy farmers in New Zealand are encouraged to adopt a range of management strategies, both well established and emerging, to reduce nitrogen (N) losses to waterways. In most regions the OVERSEER® nutrient budgeting software (Version 6) (hereafter referred to as Overseer) is the tool of choice in the assessment of N losses for both regulatory and monitoring purposes. As part of these processes, Overseer is used to assess the impact of improved farm practices on N leaching and runoff from individual farms. In a 3-year dairy system field trial at Massey University, N losses in leaching and runoff under duration-controlled grazing (DC; 4 hours per grazing) were compared with those under standard grazing (SG; 7 hours per day-grazing, 13 hours per night-grazing). A 36% reduction in total nitrogen (TN) losses under DC grazing was measured (14 kg TN/ha) relative to standard grazing (22 kg TN/ha). Farmers adopting DC grazing as a mitigation strategy will only be able to claim the reduction in TN losses estimated by Overseer, and thus observations from the field trial were compared with outputs from Overseer. There was good agreement between the Overseer predictions of N leaching and values measured at the trial site for both the SG and DC grazing treatments. A second Overseer simulation of a DC system suggests that while Overseer is able to predict the reductions in N leaching under DC grazing reasonably well, some issues such as runoff losses and storage of effluent need further consideration. Keywords: Duration-controlled grazing; OVERSEER®; N leaching