Farmers use Nitrogen (N) to increase forage yields and increase profitability, however, there has been increasing requirements to minimise environmental impact and for this reason N use on farms is under scrutiny. N that is not used in an animal must be excreted as, unlike with energy, there is no capacity to store N. The N excreted in urine represents a liability in terms of nitrate leaching and nitrous oxide. N however is important in forage for growth and reproduction and it is unlikely that the concentrations can be reduced without compromising growth and persistence of the forage; hence decreasing production of a farm. When excess N is applied, the pool of plant N that does not have a functional role increases, so proper management could lower total N by minimising this pool.
Increasing animal production is one way to improve the efficiency of N utilisation. This means either more animal product or increased protein content of products, thereby capturing more N with less excreted to the environment. Balancing carbohydrate supply to the rumen is another way to improve N utilisation as rumen microorganisms require energy to synthesise protein. Reducing protein degradability is another strategy which can shift excretion from urine to in the faeces.
Known principles of N partitioning tell us that, on average, it is possible to reduce N leakiness through reduction of N inputs. On average, such reduction may impact production. There are untapped sources of variation that, if understood, would add to farmers’ toolbox to manage their environmental impact.
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