NVZs – Dairy farms: Calculating the Whole Farm N Limit

Having established whether the farm can meet the storage requirements for animal manures in the closed period, the next step is to determine how much animal manure can be spread on the holding to keep within the 170kg/ha N limit.

“There’s still confusion over exactly what this means,” says Oliver Lee of consultant Andersons. “This 170kg/ha limit applies to nitrogen from livestock manures only. This is the maximum farmers can apply to the whole farm in terms of animal manure in each calendar year.”

So how do you work out whether you’re within the Whole Farm N Limit? As in our example below, the DEFRA guidance books provide standard values for the amount of nitrogen produced in livestock manures for animals of different species, types and ages.

Farmers Weekly has teamed up with Andersons to calculate the slurry storage requirement for ‘Friesian Farm’, a fictional, 100ha unit with 150 cows yielding 7500l a year, used by Andersons for budgeting purposes. 

“‘Friesian Farm’ comes out at a total of 18,990kg of nitrogen a year. Divided by the 100ha area of the farm, that leaves a Whole Farm N Loading of 189.9kg/ha – nearly 20kg/ha over the N limit,” says Mr Lee. “So the management of Friesian Farm has several options. One would be to export some manure to another holding – possible if there’s a nearby arable farmer who might want it, but more problematic in parts of the country where dairy farms are concentrated together.

“Another option is to take on more land – in this example, another 12ha would be sufficient to bring the farm onto the 170kg/ha limit. And again, this may not be strategically possible,” says Mr Lee.

An uncomfortable third option would be to reduce cow numbers by 20 head.