Gaseous N losses from livestock production originate from the faeces (dung) and urine excreted by the livestock.
Decreasing the amount of animal excrement in animal housing systems through increased grazing is an effective measure to decrease NH3 emissions. Though emissions from grazing will increase when animals are kept outside, NH3 emissions from animal housing systems will decrease much more, provided surfaces in the house are clean while the animals are grazing outside.
Whilst reduction in NH3 emissions can be achieved by increasing the proportion of the year the cattle spend grazing outdoors, this will depend on the baseline (emission of ungrazed animals), the time the animals are grazed, and the N fertilizer level of the pasture. The potential to increase grazing is often limited by soil type, topography, farm size and structure (distances), climatic conditions, etc. It should be noted that grazing of animals may increase other forms of N emissions (e.g., nitrate-N leaching and N2O emissions). However, given the clear and well quantified effect on NH3 emissions, increasing the period that animals are grazing all day can be considered as a category 1 strategy to reduce emissions, but depending on grazing time.
The effect of changing the period of partial housing (e.g., grazed during daytime only) is less certain and is rated as a category 2 strategy. Changing from a fully housed period to grazing for part of the day is less effective in reducing NH3 emissions than switching to complete (24-hour) grazing, since buildings and stores remain dirty and continue to emit NH3. Grazing management (strip grazing, rotational grazing, continuous grazing) is expected to have little additional effect on NH3 losses and is considered a category 3 strategy.