06 January 2025 Balancing nitrogen: science-backed solutions for dairy farmers

Collaborative research at Lincoln University’s Research Dairy Farm is supporting dairy farmers to farm productively into the future while continuing to reduce their nitrogen (N) losses and environmental footprint. 

The Low N Farmlet Trial, established in 2023 at the Lincoln University Research Dairy Farm, aims to help farmers achieve positive environmental outcomes by testing and demonstrating a profitable, practical system that combines strategies to mitigate on-farm N losses. Lowering surplus N leached on-farm can contribute to improving ground and surface water quality and benefit ecosystem health in N-sensitive catchments. Reducing greenhouse gas emissions can also result from improving N use efficiency. 

Researchers from Te Whare Wānaka o Aoraki Lincoln University are working with colleagues from DairyNZ, Bioeconomy Science Institute (BSI) – AgResearch Group, Fonterra and AbacusBio on the multi-year trial, which is the flagship of the DairyNZ-led Low N Systems research programme. The wider programme is providing dairy farmers with science-backed solutions by combining mitigation strategies and validating them through modelling, field trials and farmer case studies. Farmers fund the programme via DairyNZ in partnership with the Ministry of Business, Innovation and Employment, Fonterra and CRV.  

Lincoln University lead researcher in the Low N Farmlet Trial, Associate Professor Racheal Bryant, has completed extensive research on N excretion in dairy cows, with her expertise central to the project’s direction.

“Dairy cows play an important role in nitrogen recycling in the farm system,” says Associate Professor Bryant. “They consume pasture and supplements and redistribute nutrients back into the soil that can be used for forages to grow. To mitigate surplus nitrate leaching, we need a holistic approach—considering the animal, fertiliser use, plant uptake and gas losses.”

For context, when a dairy cow consumes crude protein, around 20 to 25 percent of the N in the protein ends up in milk. The excess N not used for growth and maintenance is returned to the soil through dung and urine. Though this supports further plant growth, it can be at risk of nitrate leaching or greenhouse gas losses.

The Low N Farmlet Trial integrates N-loss mitigation strategies across a farmlet featuring a milking area with a stocking rate of 3.2 cows per hectare and a separate wintering block. The results are then compared with those of a conventional dairy farmlet with a stocking rate of 3.4 cows per hectare and a wintering support area. With both farmlets located at the Dairy Futures Living Laboratory on the Lincoln University Research Dairy Farm, the trial began with best-practice methods, including catch crops after winter crops and a maximum fertiliser rate of 190 kg N per hectare per year. 

With the trial designed with input from Canterbury farmers and rural professionals, the research evolved to test combinations or ‘stacks’ of mitigations for compatibility and effectiveness. These include:

• Reduced N fertiliser use (80 kg of N per hectare per year)
• Diverse pasture species (Italian and hybrid ryegrass, red and white clover and plantain)
• Pasture-based wintering systems (as an alternative to kale crop wintering)
• Milk urea indicator tools to monitor herd dietary crude protein status.

After two years of data collection, the results estimated an average 30 to 40 percent reduction in nitrate leaching per year on the milking area, a 12 percent reduction in total greenhouse gas emissions and a five percent reduction in emissions intensity (per kg of milk). Preliminary results from BSI-AgResearch monitored ceramic suction cups, which measures N in leachate under the milking platform, also show cumulative reductions in nitrate leaching of 45 percent since the trial began. 

The reduced environmental footprint from stacking mitigation options aligns with pre-trial modelled predictions but came with some trade-offs, including an eight percent lower profit. This was an expected cost of achieving these significant reductions, but was minimised through careful selection of complementary mitigations suited to the farm’s soil and climate. 

“Collectively, mitigations like these can achieve a significant impact and this is what our pre-trial modelling predicted. It is great that farmers can have confidence in the models’ ability to estimate environmental impact and encourages them to adopt various options and technologies,” says Associate Professor Bryant. “However, every farm needs an approach tailored to its environmental and economic realities.”

The choice and number of mitigations that can be applied largely depend on an individual farm’s current operations and its unique soil/climate conditions, which affect both the farm’s environmental impact and its profitability.  

Winter is a higher-risk period for nitrate leaching due to cold temperatures, reduced plant growth and increased rainfall. The research explored pasture-based wintering systems utilising Italian ryegrass, clover and plantain. This reduces urinary N deposits and maintains an actively growing sward that can regrow and absorb N more effectively than systems based upon traditional winter crops, such as kale or fodder beet.

“We’ve estimated these approaches can reduce nitrate leaching risk from wintering systems by up to 70 percent,” says Associate Professor Bryant. “Catch crops like oats and annual ryegrass are proven strategies, though they are dependent on timely establishment following forage crop wintering. Permanent pasture systems offer more consistent plant nitrogen uptake.” 

Preliminary results from wintering studies at Lincoln University Research Dairy Farm are encouraging, indicating that pasture-based wintering is an effective alternative to mitigate N losses and improve cow comfort. Researchers are now considering the economics of these approaches relative to crop-based wintering and the amount of land required under different scenarios.

“It's been really pleasing to be a part of a team of researchers across the partner organisations who collectively have delivered tools and insights to help further mitigate nitrate leaching on dairy systems. Even a small reduction goes a long way towards reducing our nutrient losses.”

The project has been highly consultative, engaging farmers, industry stakeholders and policymakers, helping them continue to reduce the N footprint on farms.

“Farmers have been pleasantly surprised and encouraged by the reductions in nitrogen losses we’ve demonstrated while maintaining a viable, profitable system,” says Associate Professor Bryant. “They’re keen to see how the farmlets perform over this season and understand how these results would translate when applied to commercial farm scale.”

For media enquiries, please contact media@lincoln.ac.nz