The main objective and focus of dairy effluent management to date has been on ensuring nutrients and pathogens remain on farm in the interests of nutrient utilisation, protecting water quality and biosecurity.
With an increasing focus on climate change, which is driven by the greenhouse effect, greenhouse gases from effluent may also need to be managed to reduce their losses into the future.
Greenhouse gases such as carbon dioxide (CO2), methane (CH4), water vapour and nitrous oxide (N2O) hold heat that enters the atmosphere from the sun and prevents this heat from escaping back into the atmosphere.
During the storage, management and application of dairy effluent, both methane and nitrous oxide are emitted.
The overall greenhouse gas contribution will vary between farms, however, a 'typical average' is provided in Figure 1, illustrating the breakdown of gases.
Methane can trap 28 times more heat than carbon dioxide, and nitrous oxide can trap 265 times more heat than carbon dioxide.
This makes reducing their emissions an important component to help address climate change.
Methane losses are overall the largest contribution of greenhouse gases from dairy farms.
Methane is produced from cows (burping), as well as from dairy effluent.
Methane emissions from ponds are the largest greenhouse gas contribution from dairy effluent systems (Laubach et al 2015).
Methane is produced when manure or organic material breaks down in an environment without oxygen (anaerobic conditions).
Situations that favour anaerobic conditions include deep effluent ponds and stockpiles of manure that may become waterlogged.
Higher temperatures and the volume of organic material (and its biodegradability) entering ponds, including milk waste, are also factors that can affect the production of methane.
Strategies that can be used to reduce methane production include:
Nitrous oxide emissions from dairy effluent are created through land application and before storage, either directly from urine and manure deposits onto concrete surfaces, or from stockpiles.
Laubach et al (2015) explains that nitrous oxide emissions from land applications of effluent are the second biggest source of greenhouse gas emissions from dairy effluent management.
Nitrous oxide is a greenhouse gas that can be produced when conditions are wet on-farm through a process called denitrification or when conditions are hot and windy through a process called volatilisation.
Strategies that can be used to reduce nitrous oxide include:
Practices that aim to maximise the benefit of the nutrients in the manure and effluent, such as regularly applying to paddocks, can also help to reduce greenhouse gas emissions.
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