Tackling N mitigation head-on

Farmers can choose from various nitrogen (N) mitigation strategies and technologies, but combining them in a farm system doesn’t always result in additive reductions, as their effectiveness depends on how they interact within the N cycle (see Figure 1). It can also be challenging to assess which options are best suited to a specific farm and its conditions, including which measures are the most cost-effective.

The DairyNZ-led Low N Systems research programme is exploring how different mitigation strategies can be combined to help farmers significantly reduce N losses while achieving their farm business goals.

The findings will offer practical solutions to support farmers in lowering their N footprint, benefiting both ground and surface water quality and improving ecosystem health in N-sensitive catchments.

The programme uses a range of approaches (such as modelling, detailed experiments, larger-scale farm system trials and farmer case studies) to investigate reducing N losses.

The N loss reduction targets some farmers face can feel challenging, particularly when considering the potential implications for their farms’ profitability. This research is about tackling those challenges head-on — applying and testing solutions, pushing the boundaries and uncovering practical insights for integrating mitigations into a farm system. The goal is to provide farmers with reliable information and confidence as they explore their options.

The research seeks to understand the potential short- and long-term outcomes of implementing stacked low N systems and the range of likely on-farm results across different farm systems.

Trial work combined with modelling and case studies from commercial farms offers a clearer understanding of the variability in responses under practical farming conditions.

This multi-year research programme, running from mid-2021 to late 2025, is funded by New Zealand dairy farmers through DairyNZ and by the Ministry for Business, Innovation and Employment (MBIE), with additional co-funding and in-kind support from Fonterra and CRV. DairyNZ collaborates on this research with Lincoln University, Fonterra, AgResearch and AbacusBio.

To find further information and the most recent results, check out dairynz.co.nz/low-n

Low N farmlet trial

A farmlet trial began at the Lincoln University Research Dairy Farm (LURDF) in July 2023 to test and demonstrate a profitable and practical stacked low nitrogen (N) system.

The study is examining how different farm systems influence N losses by comparing a typical Canterbury dairy farm system with an alternative approach that integrates multiple N mitigation strategies.

The Control farmlet represents a standard system, while the Stacked farmlet incorporates diverse pastures, including Italian ryegrass and plantain, applies over 50% less N fertiliser, operates with a 6% lower stocking rate, and modifies wintering practices by using grass and baleage instead of kale and baleage.

It also includes tactical mitigations, using bulk milk urea as an indicator to manage herd dietary N surplus through grazing management, nitrogen fertiliser application and supplementary feeding.

The Stacked farmlet aims to demonstrate a system that helps farmers significantly reduce N leaching while also delivering co-benefits, such as lowering greenhouse gas (GHG) emissions and minimising any potential impacts on farm profitability.

The research team is monitoring the performance of both farmlets to see how they align with modelled expectations.

Early results from the first year of the research trial show the Stacked farmlet delivers promising environmental benefits while identifying opportunities to refine the system. Despite using significantly less N fertiliser, pasture growth was only 5% lower than the Control farmlet, and milk production per cow remained the same – though total milksolids per hectare were 7% lower.

Encouragingly, the suite of mitigations on the Stacked farmlet reduced N leaching by ~40 to 50% on the milking platform, with total GHG emissions reduced by 12% per hectare and emissions intensity by 6% per kilogram of milksolids. A further ~10% reduction is expected across the whole farm system by switching from a kale-baleage wintering system (followed by an oats catch crop) to pasture-baleage wintering.

The significant reductions in N loss and GHG emissions came with some trade-offs, including an 8% lower profit per hectare, partly due to increased pasture conservation and re-grassing expenses and lower milk revenue.

With further refinements to pasture management, there is potential to enhance both environmental gains and farm profitability in future seasons.

The farmlet trial validates previous modelling and, while based in Canterbury, its findings — particularly on the core principles of stacked mitigations — will be relevant across regions.

Bulk milk urea tool

A new bulk milk urea (BMU) indicator tool is being developed to help farmers track and manage their herd’s dietary N surplus throughout the season. By monitoring BMU alongside other farm data, farmers can identify when mitigations could be utilised to reduce the risk of higher urinary N loading onto soils at critical times of the year.

This on-farm management tool is based on new research showing that BMU can act as a near realtime indicator of herd dietary N surplus and the risk of urinary N loss in pasture-fed cows. It could help farmers make both tactical and strategic decisions to improve N use efficiency and reduce N loss risk.

When cows consume more protein than needed, the excess dietary N is converted to urea N and excreted in urine, milk and dung. Elevated urinary N increases the risk of higher urinary N loading onto soils, which is a key contributor to N losses to the environment.

A milk urea level above ~30 milligrams per decilitre (mg/ dL) suggests an increasing risk of dietary N surplus and higher urinary N excretion, while a BMU below 20 mg/dL may indicate a risk of protein deficiency in the diet. Considering these levels alongside other information — such as diet, animals and management factors — is important for understanding the full context of these risks.

To develop the BMU tool and management guidelines, researchers are using multiple datasets and approaches, including an observational study of 38 farms across Waikato and Canterbury. This study highlighted differences in grazing management between farms with low and high BMU levels. Herds with low BMU tended to graze pastures with a higher pre-graze cover and a more advanced leaf stage and used slower grazing rotations.

How these farms managed their pastures resulted in lower protein levels and a lower N-to-energy ratio, which likely meant the cows had less excess N in their diet relative to their requirements. Studies also identified that farmers could manipulate their herd dietary N surplus and risk of N loss through altered N fertiliser and supplementary feed management practices.

The tool is currently in the prototype stage and is being tested by farmers across several regions. It’s an online dashboard app that allows farmers to monitor their BMU levels in near real-time, and see how they compare to an optimal zone and benchmark against district averages, similar farm systems and previous years. The tool also includes data on prior annual metrics like purchased N surplus and fertiliser use, with links to guidance on managing dietary N surplus and N loss risks.