Understanding the nitrogen cycle – the transformations that nitrogen undergoes as it moves between the atmosphere, the land and living things – is an important part of dairy farming. Balance is the key. Too little nitrogen and pasture plants do not thrive, but too much nitrogen can be harmful to the environment.
Dairy farming in Aotearoa New Zealand is unusual. It is one of the few places in the world where cows graze on pastures year round. Being able to grow and harvest high-quality pasture is important for animal welfare and farm profitability. Optimal pasture growth is dependent on many factors, including soil fertility, and this is where nitrogen and knowledge of the nitrogen cycle is important.
Nitrogen is a limited resource, so for farmers, it’s incredibly important to be able to manage the nitrogen cycle – from a business perspective to ensure we’re getting the most out of the resource and to make sure we are limiting our environmental footprint.Kieran McCahon, DairyNZ
Nitrogen inputs on farms
Nitrogen inputs on dairy farms come from a variety of sources.
Legumes fix atmospheric nitrogen and convert it to forms plants can use. Clover has long been a staple in New Zealand pastures. Clover is a dual-purpose pasture plant – it’s highly nutritious and it contributes to the growth of other pasture species.
Fertilisers supply plants with the elements that may be missing or in short supply in a form that can be used by the plants for faster growth. Nitrogen fertiliser requires careful use to optimise its efficiency and to minimise adverse environmental impacts.
Dairy effluent is the waste produced during the milking process. Dung, urine and milk are washed from the milking shed and surrounding yards and collected in special storage ponds. Effluent is a natural, diluted, liquid fertiliser. When it is well managed, it is a valuable resource for increasing pasture production.
When cows graze a large area, the nitrogen from the pasture plants becomes concentrated in their bodies. A good deal of it is returned to the land in urine and dung.
Nitrogen outputs and losses on a farms
Nitrogen leaves the farm as produce – as milk, meat or in crops. It is estimated that 40% of the world’s population is fed by food grown using nitrogen from fertilisers or nitrogen-fixing plants.
Other ways in which nitrogen leaves the farm are less desirable and are considered losses. Nitrate (a form of nitrogen) is highly mobile and is easily lost in soil water leached from the plant root zone. Nitrogen in fertiliser particles and dung can be carried away via run-off.
Nitrogen is lost to the atmosphere via volatilisation and denitrification. Volatilisation is a chemical process that occurs on the soil surface while denitrification is a biological process that occurs with the soil. Volatilisation and denitrification can both result in nitrous oxide emission.
Dairy farm management and the nitrogen cycle
Farmers and farm managers need to understand the nitrogen cycle to ensure nitrogen inputs target plant growth and produce desired outputs – increased pasture growth and milk production – while minimising nitrogen losses. Farmers need to be aware of the transformations that happen at atmospheric, soil, plant and animal levels and the interplay of seasonal conditions, soil type, pasture grazing management and various pasture species.
That’s a lot of information to process. To help dairy farmers make sound management decisions and meet environmental standards and goals, industry groups like DairyNZ provide extension services. They work closely with farmers via discussion groups and as advisors and ambassadors and provide management guides and on-farm tools.
Online software such as OverseerFM models nutrient flows on farms and enables users to understand the impacts of farm management changes before they are made.
Nature of science
Science knowledge underpins a wide range of professions. Knowledge of complex systems like nitrogen cycling is key to making good on-farm management decisions.
Read about the nitrogen cycle.
Watch as Dr Ross Monaghan talks about fertilisers – what they are and why they are used on farms.
Explore two of the chemical processes that occur within the nitrogen cycle – nitrification and denitrification – using a jar of sand to follow the transformation of ammonium to nitrate and its reduction to nitrogen gases.
Observe the colour inside the root nodules of clover (or another legume) to see if they are fixing nitrogen.
Soil microbes are responsible for some of the nitrogen transformations. They are too small to see, but building a habitat suitable for their growth enables us to view colourful colonies of them.
This resource has been produced with the support of DairyNZ.