Taking a few minutes to calculate a dairy herd’s nitrogen use efficiency is a good start to working out how to lower production costs.

Improving nitrogen use efficiency from 15% to 25% could be worth 2.2p/litre, according to Institute of Grassland and Environmental Research calculations.

The institute’s Richard Weller told Dairy Event visitors it was fairly easy to work this out (see examples).

“It will tell you how much of the nitrogen going into the cows is converted into milk for sale.”

Start by considering the nitrogen content of the concentrate fed, that used in forage production and any nitrogen fixed from the atmosphere or from rain on a kg/ha basis (see panel).

Any bedding bought in should also be added.

Then work out the nitrogen that goes off the farm on a kg/ha basis. For milk, this is the annual litres sold a hectare, multiplied by average milk protein during the year and divided by 6.25 to give the nitrogen content.

Culled livestock also need to be taken into account as an output.

To calculate this take the bodyweight of culls a hectare and multiply by 0.025.

The total inputs minus total outputs equals the farmgate nutrient balance in kg/ha.

Take the total outputs and divide it by the total inputs, then multiply by 100 to give the efficiency of nitrogen use as a percentage.

“From that you can see how efficient you are,” he said.

An average NUE will be 20-25%, below that is poor and more than 30% is very good.

IGER predictions show improving NUE from 15% to 20% could reduce production costs by 1.4p/litre, from 20 to 25% by 0.8p/litre and 25 to 30% by 0.5p/litre.

One way this could be achieved was by reducing fertiliser use on pastures to optimum levels.

This could lower costs and reduce pollution.

When making silage, rapid wilting silage would reduce losses and using a good silage inoculant to make better quality silage could help, too, said Mr Weller.

He also suggested drilling a grass mix with high sugar grasses and improving ration formulation to ensure protein was not overfed.

jessica.buss@rbi.co.uk